. ..

C()(/

NIT

IH P

SYC

HO

LOG

Y 18

,86 (1986)

0010

-028

5/86

$7.

50C

o"yr

i~hl

~ 1

9II~

hy

Acad

emic

Pre

... In

c,A

ll ri~

h.. o

r r."r

oduc

lion

in a

ny ro

rm re

""ry

ed.

MC

CLE

LLAN

D A

ND

ELM

AN

Cm"'I

'1?i

.f!-

Mel

ltm U

"i,'er

,f;/Y

the

abili

ty to

trad

e cu

es o

ff ag

ains

t eac

h ot

her i

n ph

onem

e id

entif

icat

ion.

At t

hew

ord

leve

l. th

e m

odel

cap

ture

s Ihe

ma.

ior p

ositi

ve fe

atur

e of

Mar

slen

-Wils

onC

OH

OR

T m

odel

of s

peec

h pe

rcep

tion.

in th

at it

sho

ws

imm

edia

te s

ensi

tivity

toin

form

atio

n fa

vorin

g on

e w

ord

or s

et o

f wor

ds o

ver o

ther

s. A

t the

sam

e tim

e, it

over

com

es a

diff

icul

ty w

ith th

e CO

HO

RT m

odel

: it c

an re

cove

r fro

m u

nder

spec

-, i

fical

ion

or m

ispro

nunc

iatio

n of

a w

ord'

s beg

inni

ng. T

RACE

II a

lso u

ses l

exic

alin

form

atio

n to

segm

ent a

stre

am o

f spe

ech

into

a se

quen

ce o

f wor

ds a

nd to

find

wor

d be

ginn

ings

and

end

ings

, and

it s

imul

ates

a n

umbe

r of r

ecen

t fin

ding

s re

late

dto

thes

e po

ints

. The

TR

ACE

mod

el h

as s

ome

limita

tions

, but

we

belie

ve it

is a

step

tow

ard

a ps

ycho

logi

cally

and

com

puta

tiona

lly a

dequ

ate

mod

el o

f the

pro

cess

of speech perception. ~ 1

986

Acad

emic

Pre

.., In

c,

The

TRAC

E M

odel

of S

peec

h Pe

rcep

tion

JAM

ES L

. MC

CLE

LLAN

D

AN

DC

onsi

der t

he p

erce

ptio

n of

the

phon

eme

/g/ i

n th

e se

nten

ce "

She

re-

ceiv

ed a

val

uabl

e gi

ft; "

Ther

e ar

e a

larg

e nu

mbe

r of c

ues

in th

is se

nten

ceto

the

iden

tity

of th

is ph

onem

e, F

irst,

ther

e ar

e th

e ac

ousti

c cu

es to

the

iden

tity

of th

e /g

/ itse

lf, S

econ

d. th

e ot

her p

hone

mes

in th

e sa

me

wor

dpr

ovid

e an

othe

r sou

rce

of c

ues.

for i

f we

know

the'

rest

of t

he p

hone

mes

in th

is w

ord.

ther

e ar

e on

ly a

few

pho

nem

es th

at c

an fo

rm a

wor

d w

ith, them, Third. the semantic and syntactic context further constrain the

poss

ible

wor

ds w

hich

mig

ht o

ccur

. and

thus

lint

it sti

ll fu

rther

the

poss

ible

inte

rpre

taJio

n of

the

first

phon

eme

in "

gift,

Ther

e is

ampl

e ev

iden

ce th

at a

ll of

thes

e di

ffere

nt s

ourc

es o

f inf

or-

mat

ion

are

used

in re

cogn

izin

g w

ords

and

the

phon

emes

they

con

tain

,In

~eed

. as

Col

e an

d R

udni

cky (1983) have recently noted. these basic

fact

s w

ere

desc

ribed

in e

arly

.exp

erim

ents

by

Bagl

ey (1

900)

ove

r 80

year

sag

o, C

ole

and

Rudn

icky

poi

nt o

ut th

at re

cent

wor

k (w

hich

we

c~m

sider

in d

etai

l bel

ow) h

as a

dded

cla

rity

and

deta

il to

thes

e ba

sic fi

ndin

gs b

utha

s not

lead

to a

theo

retic

al sy

nthe

sis th

at p

rovi

des a

satis

fact

ory

acco

unt

of th

ese

and

man

y ot

her b

asic

asp

ects

of sp

eech

per

cept

ion,

In th

is pa

per.

we

desc

ribe

a m

Ode

l who

se p

rimar

y pu

rpos

e is

to a

ccou

ntfo

r the

inte

grat

ion

of m

ultip

le so

urce

s of i

nfor

mat

ion.

or c

onst

rain

t, in

spee

ch p

erce

ptio

n, T

he m

odel

is c

onstr

ucte

d w

ithin

a fr

amew

ork

whi

chap

pear

s to

beid

eal f

or th

e ex

ploi

tatio

n of

sim',l

tane

ous.

and

ofte

n m

utua

l,co

nstra

ints

. Thi

s fra

mew

ork

is th

e in

tera

ctiv

e ac

tivat

ion

fram

ewor

k(M

cCle

lland

& R

umel

hart.

198

1; R

umel

hart

& M

cCle

lland

. 1981. 1982),

This

appr

oach

gre

w o

ut o

f a n

umbe

r of e

arlie

r ide

as. s

ome

com

ing

first

from

rese

arch

on

spok

en la

ngua

ge re

cogn

ition

(Mar

slen-

Wils

on &

Wel

sh.

1978

; Mor

ton.

196

9; R

eddy

. 1976) and others arising from m

ore

gene

ral

cons

ider

atio

ns o

f int

erac

tive

para

llel p

roce

ssin

g (A

nder

son.

197

7; G

ross

-be

rg. 1

978;

McC

lella

nd. 1

979)

,Ac

cord

ing

to th

e in

tera

ctiv

e-ac

tivat

ion

appr

oach

. inf

orm

atio

n pr

o-ce

ssin

g ta

kes p

lace

thro

ugh

the

exci

tato

ry a

nd in

hibi

tory

inte

ract

ions

amon

g a

larg

e nu

mbe

r of p

roce

ssin

g el

emen

ts ca

lled

units

, Eac

h un

it is

a ve

ry si

mpl

e pr

oces

sing

devi

ce, I

t sta

nds f

or a

hyp

othe

sis a

bout

the

inpu

t bei

ng p

roce

ssed

. The

act

ivat

ion

of a

uni

t is

mon

oton

ical

ly re

late

d

,. q, ~

JEFF

REY

L. E

LMAN

U"i

t'er.fi/y of C(/l((omi(/. Stili Dif!1!O

We

desc

ribe

a m

odel

cal

led

the

TRA

CE m

odel

of s

peec

h pe

rcep

tion.

The

mod

elis

base

d on

the

prin

cipl

es o

f int

erac

tive

activ

atio

n. In

form

atio

n pr

oces

sing

take

spl

ace

thro

ugh

the

e xci

tato

ry a

nd in

hibi

tory

inte

ract

ions

of a

larg

e nu

mbe

r of

simple processing u

rtits,

eac

h w

orki

ng c

ontin

uous

ly to

upd

ate

its o

wn

activ

atio

non

the

basis

of t

he a

ctiv

atio

ns o

f oth

er u

nits

to w

hich

it is

con

nect

ed. T

he m

odel

is ca

lled

the

TRA

CE m

odel

bec

ause

the

netw

ork

of u

nits

form

s a d

ynam

ic p

ro-

cessing structure called "the Trace." w

hich

serv

es a

t onc

e as

the

perc

eptu

alpr

oces

sing

mec

hani

~m a

nd a

s th

e sy

stem

s w

orki

ng m

emor

y. T

he m

odel

is in

-sta

ntia

ted

in tw

o sim

ulat

ion

prog

ram

s. TR

ACE

I, d

escr

ibed

in d

etai

l else

whe

re,

deals with short seg~ents orreal speech. and suggests a m

echa

nism

for c

opin

gw

ith th

e fa

ct th

at th

e cu

es to

the

iden

tity,

of p

hone

mes

var

y as

a fu

nctio

n of

cont

ext.

TRAC

E II_

the

focu

s of t

his a

rticl

e, si

mul

ates

a la

rge

num

ber o

f em

piric

alfin

ding

s on

the

perc

:ept

ion

of p

hone

mes

and

wor

ds a

nd o

n th

e in

tera

ctio

ns o

fph

onem

e ;m

d w

ord

perc

eptio

n. A

t the

pho

nem

e le

vel,

TRA

CE II

sim

ulat

es th

ein

nuen

ce o

f lex

ical

jnfo

rmat

ion

on th

e id

entif

icat

ion

of p

hone

mes

and

acc

ount

sfor Ihe fact that le

xic:

ial e

ffect

s ar

e fo

und

unde

r cer

tain

con

ditio

ns b

ut n

ot o

ther

s.Th

e m

odel

als

o sh

ows

how

kno

wle

dge

of p

hono

logi

cal c

onst

rain

ts c

an b

e em

-bo

died

in p

artic

ular

lexi

cal i

tem

s but

can

still

be

used

to in

nuen

ce p

roce

ssin

g of

nove

l. no

nwor

d ul

lera

nces

. The

mod

el a

lso e

xhib

its c

ateg

oric

al p

erce

ptio

n an

d

The

wor

k re

porte

d he

re w

as s

uppo

rted

in p

art b

y a

cont

rdct

from

the

Offi

ce o

f Nav

alResearch CN-

tKK

II4-!

l2-

'13741. in part by a grant from the National Science Foundation

IHN

S-79

-240

621.

and

in p

art b

y a

.Res

earc

h Sc

ient

ists C

aree

r Dev

elop

men

t Aw

ard

to th

efir

st au

thor

from

the

Nat

jona

llnsti

tute

of M

enta

l Hea

lth (5

-KO

I-MHOO3851. We thank

Dr.

Joan

ne M

iller f

or a

ver

y u5

eful

dis

cuss

ion

whi

ch in

spire

d us

to w

rite

this

arti

cle

in it

s pr

esen

tfo

rm. D

avid

Piso

ni w

as e

~tre

mel

y he

lpfu

l in

mak

ing

us d

eal m

ore

fully

with

seve

ral i

mpo

rtant

issu

es, a

nd in

.Ier

ling

us to

a la

rge

num

ber o

f use

ful p

aper

s in

the

liter

atur

e, W

e al

so th

ank

Dav

id R

umel

hart

for u

se~u

l dis

cuss

ions

dur

ing

the

deve

lopm

ent o

f the

bas

ic a

rchi

tect

ure

ofTR

ACE

and

Eile

en C

onw

ay, M

ark

John

son.

Dav

e Pa

re, a

nd p

odul

Sm

ith fo

r the

ir as

sista

nce

in p

rogr

amin

g an

d gr

aphi

cs. S

end

requ

esls

for r

eprin

ts to

Jam

es L

McC

lella

nd. D

epar

tmen

tof

I)sy

chol

ogy.

Car

negi

e..'.

Mel

lon

Uni

vers

ity. S

chen

ley

Park

. Pills

burg

h, P

A 15

213.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

to th

e str

engt

h of

the

hypo

thes

is fo

r whi

ch th

e un

it sta

nds,

Cons

train

tsam

ong

hypo

thes

es a

re re

pres

ente

d by

con

nect

ions

. UnU

s w

hich

are

mu-

tual

ly c

onsi

sten

t are

mut

ually

exc

itato

ry, a

nd u

nits

that

are

mut

ually

in-

cons

iste

nt a

re m

utua

lly in

hibi

tory

. Thu

s, th

e un

it fo

r /g/

has

mut

ually

exci

tato

ry c

onne

ctio

ns w

ith u

nits

for w

ords

con

tain

ing

/g/,

and

has

mu-

tual

ly in

hibi

tory

con

nect

ions

with

uni

ts fo

r oth

er p

hone

mes

, Whe

n th

eac

tivat

ion

of a

uni

t exc

eeds

som

e th

resh

old

activ

atio

n va

lue,

it b

egin

s to

influence the

activ

atio

n of

oth

er u

nits

via

its

outg

oing

con

nect

ions

; the

stren

gth

of th

~se

signa

ls de

pend

s on

the

degr

ee o

f the

send

ers a

ctiv

atio

n,Th

e st

ate

of th

e sy

stem

at a

giv

en p

oint

in ti

me represents the current

stat

us o

f the

var

ious

pos

sibl

e hy

poth

eses

abo

ut th

e in

put;

info

rmat

ion

proc

essin

g am

ount

s to

the

evol

utio

n of

that

stat

e, o

ver t

ime,

Thr

ough

out

the course

of

proc

essi

ng, e

ach

unit

is c

ontin

ually

rece

ivin

g in

put f

rom

othe

r uni

ts, c

ontin

ually

upd

atin

g its

act

ivat

ion

on th

e ba

sis o

f the

se in

puts,

and,

if it

is o

ver t

hres

hold

, it i

s con

tinua

lly se

ndin

g ex

cita

tory

and

inhi

b-ito

ry si

gnal

s to

othe

r uni

ts. T

his "

inte

ract

ive-

activ

atio

n " ,p

roce

ss a

llow

sea

ch h

ypot

he~i

s bo

th to

con

stra

in a

nd b

e co

nstra

ined

by

othe

r mut

ually

cons

isten

t or i

ncon

siste

nt h

ypot

hese

s,

Crite

rill l

Ind

Colls

tmill

ts O

il M

odel

Dev

elop

men

tTh

ere

are

gene

rally

two

kind

s of m

odel

s of t

he sp

eech

per

cept

ion

pro-

cess

, One

kin

d of

mod

el, w

hich

gro

ws o

ut o

f speech engineering and

artif

ical

inte

llige

nce,

atte

mpt

s to

prov

ide

a m

achi

ne so

lutio

n to

the

prob

lem

of s

~eec

h re

cogn

ition

, Exa

mpl

es o

f thi

s kin

d of

mod

el a

reH

EAR

SAY

(Erm

an &

Les

ser,

1980

; Red

dy, E

rman

, Fen

nell.

& N

eely

,19

73) H

WIM

(Wol

f & W

oods

, 197

8), H

ARP

Y (L

ower

re, 1

976)

, and

LAFS

/SCR

IBER

(Kla

tt, 1

980)

, A se

cond

kin

d of

mod

el, g

row

ing

out o

fex

perim

enta

l ~sy

chol

ogy,

atte

mpt

s to

acc

ount

for a

spec

ts o

f psy

chol

og-

ical

dat

a on

the

perc

eptio

n of

spee

ch. E

xam

ples

of t

his c

lass

of m

odel

sin

clud

e M

arsl

~n- W

ilson

s C

OH

OR

T M

odel

(Mar

slen

- Wils

on &

Tyl

er,

1980

; Mar

slen

~ Wils

on &

Wel

sh, 1

978;

Nus

baum

& S

iow

iacz

ek, 1

982)

;M

assa

ros

feat

ure

inte

grat

ion

mod

el (M

assa

ro, 1

981;

Mas

saro

& O

den,

I 980

a, 1

980b

; Ode

n &

Mas

saro

, 197

8); C

ole

and

Jaki

mik

' s (1978, 1980)

mod

el o

f aud

itory

wor

d pr

oces

sing,

and

the

mod

el o

f aud

itory

and

pho

-ne

tic m

emor

y es

pous

ed b

y Fu

jisak

i and

Kaw

ashi

ma

(196

8) a

nd P

ison

i(1973, 1975).

Each

app

roac

h ho

nors

a d

iffer

ent c

riter

ion

for s

ucce

ss. M

achi

nemodels are ju4ged in terms of actual performance in

reco

gniz

ing

real

spee

ch. P

sych

olog

ical

mod

els a

re ju

dged

in te

rms o

f the

ir ab

ility

toac

-co

unt f

or d

etai

ls of

hum

an p

erfo

rman

ce in

spe

ech

reco

gniti

on, W

e ca

llthese two criteria

ompi

llllti

mw

i an

d pj

iych

olog

;cal

ad

equa

cy.

In e

xten

ding

the

inte

ract

ive

activ

atio

n ap

proa

ch to

spee

ch p

erce

ptio

n,w

e ha

d es

sent

ially

two

ques

tions

: Firs

t, co

uld

the

inte

ract

ive-

activ

atio

n

appr

oach

con

tribu

te to

war

d th

e de

velo

pmen

t of a

com

puta

tiona

lly s

uffi-

cien

t fra

mew

ork

for s

peec

h pe

rcep

tion?

Sec

ond ,

cou

ld it

acc

ount

for w

hat

is kn

own

abou

t the

psy

chol

ogy

of sp

eech

per

cept

ion?

In sh

ort,

we

wan

ted

to k

now

, was

the

appr

oach

frui

tful,

both

on

com

puta

tiona

l and

psy

cho-

logi

cal g

roun

ds.

Two

fact

s im

med

iate

ly b

ecam

e ap

pare

nt. F

irst,

spok

en la

ngua

ge in

tro-

duce

s man

y ch

alle

nges

that

mak

e it

far f

rom

cle

ar h

ow w

ell t

he in

tera

c-tiv

e-ac

tivat

ion

appr

oach

will

serv

e w

hen

exte

nded

from

prin

t to

spee

ch.

Seco

nd, t

he a

ppro

ach

itsel

f is

too

broa

d to

pro

vide

a c

oncr

ete

mod

el,

with

out f

urth

er a

ssum

ptio

ns, H

ere

we

revi

ew se

vera

l fac

ts ab

out s

peec

hth

at p

laye

d a

role

in sh

apin

g th

e sp

ecifi

c as

sum

ptio

ns e

mbo

died

inTR

ACE

.

Som

e Im

porta

nt F

acts

abou

t Spe

ech

Our

inte

ntio

n he

re is

not

to p

rovi

de a

n ex

tens

ive

surv

ey o

f the

nat

ure

of sp

eech

and

its p

erce

ptio

n, b

ut ra

ther

to p

oint

to se

vera

l fun

dam

enta

las

pect

s of s

peec

h th

at h

ave

play

ed im

porta

nt ro

les i

n th

e de

velo

pmen

tof

the

mod

el w

e de

scrib

e 'h

ere,

A v

ery

usef

ul d

iscus

sion

of se

vera

l of

thes

e po

ints

is av

aila

ble

in K

latt

(198

0).

Tem

pora

l nat

ure

of th

e speec'h stimulus,

It do

es n

ot, o

f cou

rse,

take

asc

ient

ist to

obs

erve

one

fund

amen

tal d

iffer

ence

bet

wee

n sp

eech

and

prin

t: sp

eech

is a

sign

al w

hich

is e

xten

ded

in ti

me,

whe

reas

prin

t is . a

stim

ulus

whi

ch is

ext

ende

d in

spa

ce, T

he se

quen

tial n

atur

e of

spee

chpo

ses p

robl

ems f

or a

mod

eler

, in

that

to a

ccou

nt fo

r con

text

effe

cts,

one

need

s to

kee

p a

reco

rd o

f the

con

text

, It w

ould

be

a si

mpl

e m

atte

r to

proc

ess s

peec

h if

each

succ

essiv

e po

rtion

of t

he sp

eech

inpu

t wer

e pr

o-ce

ssed

inde

pend

ently

of a

ll of

the

othe

rs, b

ut in

fact

, thi

s is c

lear

ly rio

tth

e ca

se. T

he p

rese

nce

of c

onte

xt e

ffect

s in

spee

ch p

erce

ptio

n re

quire

sa

mec

hani

sm th

at k

eeps

som

e re

cord

of t

hat c

onte

xt, i

n a

form

that

allo

ws

it to

influ

ence

th~

inte

rpre

tatio

n of

subs

eque

nt in

put.

A fu

rther

poi

nt, a

nd o

ne th

at h

as b

een

muc

h ne

glec

ted

in c

erta

in. m

odel

s, is

that

it is

not

onl

y pr

ior c

onte

xt b

ut a

lso su

bseq

uent

con

text

that

influ

ence

s per

cept

ion,

(Thi

s and

rela

ted

poin

ts ha

ve re

cent

ly b

een

mad

e by

Gro

sjean

& G

ee, 1

984;

Sal

asoo

& P

isoni

, 198

5; a

nd T

hom

pson

,19

84).

For e

xam

ple,

Gan

ong

(198

0) re

porte

d th

at th

e id

entif

icat

ion of a

sylla

ble-

initi

al sp

eech

soun

d th

at w

as c

onstr

ucte

d to

be

betw

een

/gI a

nd/k

/ was

;ptlu

ence

d by

whe

ther

the

rest

of t

he s

ylla

ble

was

Iisl

(as

inki

ss) o

r /lft

l (as

in "

gift"

), Su

ch' "

right

con

text

efte

cts "

(Thompson,

1984) indicate that the pe

rcep

tion

of w

hat c

omes

in n

ow b

oth

influ

ence

san

d is

influ

ence

d by

the

perc

eptio

n of

wha

t com

es in

late

r. Th

is fa

cisu

gges

ts th

at th

e re

cord

of w

hat h

as a

lread

y be

en p

rese

nted

can

not n

otbe

a st

atic

repr

esen

tatio

n , b

ut sh

ould

rem

ain

in a

mal

leab

le fo

rm. s

ubje

ctto

alte

ratio

n as

a re

sult

of in

fluen

ces

aris

ing

from

sub

sequ

ent c

on/e

x!.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

::.-

LLI

LLI

0::

:.....

..

wor

d se

gmen

tatio

n (B

ond

& G

arne

s, 19

80).

and

certa

in se

gmen

tatio

nde

cisio

ns a

re e

asily

influ

ence

d by

con

text

ual f

acto

rs (C

ole

& Ja

kiin

ik.

1980

), Th

us. i

t is c

lear

that

wor

d re

cogn

ition

can

not c

ount

on

an a

ccur

ate

segm

enta

tion

of th

e ph

onem

e str

eam

into

sepa

rate

wor

d un

its. a

nd

man

y ca

ses s

uch

a se

gmen

tatio

n w

ould

per

forc

e ex

clud

e fro

m o

ne o

f the

wor

ds a

shar

ed se

gmen

t tha

t is d

oing

dou

ble

duty

in e

ach

of tw

o su

cces

-siv

e w

ords

, .C

Oflt

ext-s

emiti

t'ity

of m

e.f,

A th

ird m

ajor

fact

abo

ut sp

eech

is th

at th

ecu

es fo

r a p

artic

ular

uni

t var

y co

nsid

erab

ly w

ith th

e co

ntex

t in

whi

chth

ey o

ccur

. For

exa

mpl

e, th

e tra

nsiti

on oftht second formant carries a

great deal of information about the identity of the stop consonant

Ibl

Fig,

I. b

ut th

at fo

rman

t wou

ld lo

ok q

uite

diff

eren

t had

the

sylla

ble

been

big"

or "

bog"

instead of "

bag.

" Thu

s the

con

text

.in

whi

ch a

pho

nem

eoc

curs

restr

uctu

res t

he c

ues t

o th

e id

entit

y of

that

pho

nem

e (L

iber

man

,19

70).

The

exte

nt o

f the

restr

uctu

ring

depe

..ds o

n th

e un

it se

lect

ed a

ndon

the

parti

cula

r cue

invo

lved

. But

the

prob

lem

is u

biqu

itous

in sp

eech

,N

ot o

nly

are

the

cues

for e

ach

phon

eme

dram

atic

ally

affe

cted

by

prec

edin

g an

d fo

llow

ing

cont

ext.

they

are

als

o al

tere

d by

. mor

e gl

obal

fact

ors s

uch

as ra

te o

f spe

ech

(Mill

er. 1

981)

. by

mor

phol

ogic

al a

nd p

ro-

sodi

c fa

ctor

s suc

h as

pos

ition

in w

ord

and

in th

e str

ess c

onto

ur o

f the

utte

ranc

e. a

nd b

y ch

arac

teris

tics o

f the

spea

ker s

uch

as si

ze a

nd sh

ape

of th

e vo

cal t

ract

. fun

dam

enta

l fre

quen

cy o

f the

spea

king

voi

ce. a

nddialectical variations (see Klatt, 1980. a

nd R

epp

& L

iber

man

. 1984. for

disc

ussi

ons)

, .A

num

ber o

f diff

eren

t app

roac

hes t

o th

e pr

oble

m h

ave

been

trie

d by

diffe

rent

inve

stiga

tors

. One

app

roac

h is

to tr

y to find relatively invar-

iant

-generally relational-

feat

ures

(e.g

.. St

even

s & B

lum

stein

. 198

1),

Ano

ther

appr

oach

has

bee

n to

rede

fine

the

unit

so th

at it

enco

mpa

sses

the

cont

ext a

nd th

eref

ore

beco

mes

mor

e in

varia

nt (F

ujim

ura

& L

ovin

s.19

82; K

latt.

198

0; W

icke

lgre

n, 1

969)

, Whi

le th

ese

are

both

sen

sibl

e an

dus

eful

app

roac

hes,

the

first

has n

ot y

et su

ccee

ded

in e

stab

lishi

ng a

suf

-fic

ient

ly in

varia

nt se

t of c

ues,

and

the

seco

nd m

ay a

llevi

ate

but d

oes n

otel

imin

ate

the

prob

lem

; eve

n un

its s

uch

as d

emis

ylla

bles

(Fuj

imur

a &

Lovi

ns, .

1982

), co

ntex

t-sen

sitiv

e al

loph

ones

(Wic

kelg

ren,

196

9). o

r eve

nw

hole

wor

ds (K

latt,

198

0) a

re st

ill in

fluen

ced

by c

onte

xt. W

e ha

ve c

hose

nto

focu

s ins

tead

on

a th

ird p

ossib

ility

: tha

t the

per

cept

ual s

yste

m u

ses

info

rmat

ion

from

the

cont

ext i

n w

hich

an

utte

ranc

e oc

curs

to a

lter c

on-

nect

ions

, the

reby

effe

ctiv

ely

allo

win

g th

e co

ntex

t to

retu

ne th

e pe

rcep

tual

mec

hani

sm o

n th

e fly

.Noise and indeterminacy

in

the speech signal.

To c

ompo

und

all t

hepr

oble

ms

allu

ded

to a

bove

, the

re is

the

addi

tiona

l fac

t tha

t spe

ech

is o

fteil

perc

eive

d un

der l

ess

than

idea

l circ

umst

ance

s. W

hile

a s

low

and

car

eful

spea

ker i

n a

quie

t roo

m m

ay p

rodu

ce s

uffic

ient

cue

s to

allo

w c

orre

ct

Lack

(~r h

Ollllc!a/";es alld te

mpo

/"al O

I'C'r/

ap.

A se

cond

fund

amen

tal p

oint

abou

t spe

ech

.is th

at th

e cu

es to

successive units of speech frequently

over

lap

in ti

me.

The

pro

blem

is p

artic

ular

ly se

vere

at t

he p

hone

me

leve

l. A glance at a schematic speech spectrogram (Liberman. 19

70; F

ig. I

)cl

early

illu

strat

es th

is pr

oble

m. T

here

oar

e no

sep

arab

le p

acke

ts o

f inf

or-

mat

ion

in th

e sp

ectro

gram

like

the

sepa

rate

feat

ure

bund

les t

hat m

ake

uple

tlt~r

s in

prin

ted

wor

ds,

Beca

use

of th

e ov

erla

p of

succ

essiv

e ph

onem

es. i

t is d

iffic

ult a

nd. w

ebelieve. counterproductive to try to divide the speech s

tream

up

into

sepa

rate

pho

nem

e un

its in

adv

ance

of i

dent

ifyin

g th

e un

its. A

num

ber o

fot

her r

esea

rche

rs (e

.g., Fowler, 1984: Klatt, 1980) have m

ade

muc

h th

esa

me

poin

t. A

supe

rior a

ppro

ach

seem

s to

be to

allo

w th

e ph

onem

e id

e;n-

tific

atio

n pr

oces

s to

dam

ine

the

spee

ch s

tream

for c

hara

cter

istic

pat

-te

rns.

with

out f

irst s

egm

entin

g th

e str

eam

into

sepa

rate

u!1i

ts,Th

e pr

oble

m o

f ove

rlap

is le

ss se

vere

for w

ords

than

for p

hone

mes

.bu

t it d

oes n

ot g

o aw

ay c

ompl

ete.

ly, I

nra

pid

spee

ch. w

ords

run

into

eac

hot

her.

and

ther

e ar

e no

pau

ses b

etw

e~n

wor

ds in

runn

ing

spee

ch, T

o be

sure

. the

re a

re o

ften

cues

that

sig

nal t

he lo

catio

ns o

f bou

ndar

ies

betw

een

words-stop consona

hts

are

gene

rally

asp

irate

d at

the

begi

nnin

gs o

fstr

esse

d w

ords

in E

nglis

h. a

nd w

ord

initi

al v

owel

s are

gen

eral

ly p

rece

ded,

by g

lotta

l sto

ps. f

or e

xam

ple.

The

se c

ues h

ave

been

stud

ied

by a

num

ber

of in

vesti

gato

rs. p

artic

tllar

ly L

ehist

e (e

,g,. Lehiste, 1960, 1964) an

d N

ak-

atan

i and

col

labo

rato

rs, N

akat

ani a

nd D

ukes

(197

7) .d

emon

stra

ted

that

perc

eive

rs e

xplo

it so

me

of th

ese

cues

but

foun

d th

at c

erta

in u

ttera

nces

do n

ot p

rovi

de su

ffici

ent c

ues t

o w

ord

boun

darie

s to

perm

it re

liabl

e pe

r-ce

ptio

n of

the

inte

nded

utte

ranc

e, S

peec

h er

rors

ofte

n in

volv

e er

rors

of

TIM

EFI

G, I

, A ~

chcm

alic

spcc

lrngr

am ro

r the

~yJlab'e "

bag,

" indicating the overtap or the

inro

rmat

ion

~pec

iryin

g th

e di

ffere

nt p

hone

mes

. Rep

rinte

d w

ith p

erm

issi

on rr

om L

iber

man

1197

01,

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

perc

eptio

n of

all

of th

e ph

onem

es in

an

utte

ranc

e w

ithou

t the

aid

of l

exic

alor

oth

er h

ighe

r lev

el c

onstr

aint

s, th

ese

cond

ition

s do

not a

lway

s obt

ain,

People can correctly perceive speech under quite impoverished condi-

tions

, if i

t is s

eman

tical

ly c

oher

ent a

nd s

ynta

ctic

ally

wel

l for

med

(G,

Mill

er, H

eise

, & L

icht

en, 1

951)

, Thi

s m

eans

that

the

spee

ch m

echa

nism

sm

ust b

e ab

le to

func

tion,

eve

n w

ith a

hig

hly

degr

aded

stim

ulus

, In

par-

ticul

ar, a

s Tho

mps

on (1

984)

, Nor

ris (1

982)

, and

Gro

sjea

n an

d G

ee (1

984)

have

poi

nted

out,

the

mec

hani

sms o

f spe

ech

perc

eptio

n ca

nnot

cou

nt o

nac

cura

te in

form

atio

n ab

out a

ny p

art o

f a w

ord,

As w

e sh

all s

ee, t

his f

act

pose

s a

serio

us p

robl

em fo

r one

of t

he b

est c

urre

nt p

sych

olog

ical

mod

els

of th

e pr

oces

s of s

poke

n w

ord recognition (Marslen-Wilson & Welsh,

1978

),M

any

of th

e ch

arac

teris

tics t

hat w

e ha

ve re

view

ed d

iffer

entia

te sp

eech

from print-

at le

ast,

from

ver

y hi

gh q

ualit

y pr

int o

n w

hite

pap

er-b

utit

wou

ld b

e a:

mist

ake

to th

ink

that

sim

ilar p

robl

ems a

re n

ot e

ncou

nter

edin

oth

er d

omai

ns. C

erta

inly

, the

sequ

entia

l nat

ure

of sp

oken

inpu

t set

s. speech apart from vi

sion,

in w

hich

ther

e ca

n be

som

e de

gree

of s

imul

-ta

neity

of p

erce

ptio

n, H

owev

er, t

he p

robl

ems

of il

l-defined boundaries,

cont

ext s

ellsi

tivity

of c

ues,

and

noise

and

inde

term

inac

y ar

e ce

ntra

lpr

oble

ms i

n vi

sion

just

as m

uch

as th

ey a

re in

spee

ch (c

f. Ba

llard

, Hin

ton,

an

d Se

jnow

ski,

1983

; Bar

row

& T

enen

baum

, 1978; Marr, 19

82),

Thus

,th

ough

the

mod

el w

e pr

esen

t her

e is

focu

ssed

on

spee

ch p

erce

ptio

n, w

ew

ould

hop

e th

at th

e w

ays i

n w

hich

it d

eals

with

the

chal

leng

es p

osed

by

the

spee

ch s

igna

l are

app

licab

le in

oth

er d

omai

ns.

activ

atio

n m

odel

s th

an w

ith m

odel

s in

any

oth

er computational frame-

wor

k, s

uch

as e

xper

t sys

tem

s or

pro

duct

ion

syst

ems.

.0

-

THE

TRAC

E M

OD

EL

The

Impo

rltw

ce o

f the

Rig

ht A

rchi

tect

ure

All

four

of t

he c

onsid

erat

ions

liste

d ab

ove

play

ed a

n im

porta

nt ro

le in

the

form

ulat

ion

of th

e TR

ACE

mod

el. T

he m

odel

is a

n in

stanc

e of

an

inte

ract

ive

activ

atio

n m

odel

, but

it is

. by

no m

eans

the

only

insta

nce

ofsu

ch a

mod

el th

at w

e ha

ve c

onsid

ered

or t

hat c

ould

be

cons

ider

ed, O

ther

form

ulat

ions

,we

cons

ider

ed si

mpl

y di

d no

t app

ear t

o of

fer a

satis

fact

ory

fram

ewor

k fo

r dea

ling

with

thes

e fo

ur a

spec

ts of

spee

ch (s

ee E

lman

&M

cCle

lland

, ,19

84, f

or d

iscus

sion)

. Thu

s, th

e TR

ACE

mod

el h

inge

s as

muc

h on

the

parti

cula

r pro

cess

ing

arch

itect

ure

it pr

opos

es fo

r spe

ech

perc

eptio

n as

it d

oes

on th

e in

tera

ctiv

e ac

tivat

ion

proc

esse

s tha

t occ

urw

ithin

this

arch

itect

ure.

Inte

ract

ive~

activ

atio

n m

echa

nism

s are

a c

lass

too

broa

d to

stan

d or

fall

on th

e m

erits

of a

sin

gle

mod

el, T

o th

e ex

tent

that

com

puta

tiona

lly a

ndps

ycho

logi

cally

ade

quat

e m

odel

s can

be

built

with

in th

e fra

mew

ork,

the

attra

ctiv

enes

s of t

he fr

amew

ork

as a

who

le is

, of c

ours

e, in

crea

sed,

but

the

adeq

uacy

of a

ny p

artic

ular

mod

el w

ill g

ener

ally

dep

end

on th

e pa

r-tic

ular

ass

umpt

ions

that

mod

el e

mbo

dies

. It i

s no

diff

eren

t with

inte

ract

ive-

Ove

rvie

wTh

e TR

ACE

mod

el c

onsis

ts pr

imar

ily o

f a v

ery

larg

e nu

mbe

r of u

nits

organized into three levels,

thefealllre. phoneme,

and

It'or

d le

vels,

Eac

hun

it sta

nds f

or a

hyp

othe

sis a

bout

a p

artic

ular

per

cept

ual o

bjec

t occ

urrin

gat

!a p

artic

ular

poi

nt in

tim

e de

fined

rela

tive

to th

e be

ginn

ing

of th

e ut

- .te

ranc

e.A

smal

l sub

set o

f the

uni

ts in

TRA

CE II

, the

ver

sion

of th

e m

odel

we

focu

s on

in th

is pa

per,

is ill

ustra

ted

in F

igs,

2,3,

and

4, E

ach

of th

e th

ree

figur

es re

plic

ates

the

sam

e se

t of u

nits,

illu

strat

ing

a di

ffere

nt p

rope

rtyof

the

mod

el in

eac

h ca

se. I

n th

e fig

ures

, eac

h re

ctan

gle

corre

spon

ds to

a se

para

te p

roce

ssin

g un

it, T

he la

belS

on

the

units

and

alo

ng th

e si

dein

dica

te th

e sp

oken

obj

ect (

feat

ure,

pho

nem

e, o

r wor

d) fo

r whi

ch e

ach

unit

stand

s, Th

e le

fland

righ

t edg

es o

f eac

h re

ctan

gle

indi

cate

the

porti

onof

the

inpu

t the

uni

t spa

ns,

At t

he fe

atur

e le

vel,

ther

e ar

e se

vera

l ban

ks o

f fea

ture

det

ecto

rs, o

nefo

r eac

h of

seve

ral d

imen

sions

of s

peec

h so

unds

. Eac

h ba

nk is

repl

icat

edfo

r' ea

ch o

f sev

erdl

succ

essiv

e m

omen

ts in

tim

e, o

r tim

e sli

ces,

At.

the

phon

eme

leve

l, th

ere

are

dete

ctor

s for

eac

h of

the

phon

emes

, The

re is

one

copy

of e

ach

phon

eme

dete

ctor

cen

tere

d ov

er e

very

thre

e tim

e sli

ces.

Each

uni

t spa

ns si

x tim

e sli

ces,

so u

nits

with

adj

acen

t cen

ters

span

ove

r-la

ppin

g ra

nges

of s

lices

, At t

he w

ord

leve

l, th

ere

are

dete

ctor

s for

eac

hw

ord.

The

re is

one

cop

y of

eac

h w

ord

dete

ctor

cen

tere

d ov

er e

very

thre

efe

atur

e sl

ices

. Her

e ea

ch d

etec

tor s

pans

a s

tretc

h of

feat

ure

slic

es c

or-

resp

ondi

ng to

the

entir

e le

ngth

of '

the

wor

d. A

gain

, the

n, u

nits

with

ad-

jace

nt c

ente

rs s

pan

over

lapp

ing

rang

es o

f slic

es,

Inpu

t to

the

mod

el, i

n th

e fo

rm o

f a p

atte

rn o

f act

ivat

ion

to b

e ap

plie

dto

the

units

at t

he fe

atur

e le

vel,

is pr

esen

ted

sequ

entia

lly to

the

feat

urc-

leve

l uni

ts in

succ

essiv

e sli

Ces,

as it

wou

l(J if

it w

ere

a re

al s

peec

h st

ream

,un

fold

ing

in ti

me.

Moc

k-sp

eech

inpu

ts on

the

thre

e ill

ustra

ted

dim

ensio

nsfo

r the

phr

ase

"tea

cup

" (/tik"p/

)' ar

e sh

own

in F

ig. 2

, At a

ny in

stant

,in

put i

s arri

ving

onl

y at

the

units

in o

ne sl

ice

at th

e fe

atur

e le

vel.

In te

rms

of th

e di

spla

y in

Fig

, 2, t

hen,

we

can

visu

aliz

e th

e in

put b

eing

app

lied

tosu

cces

sive

slice

s of t

he n

etw

ork

at ~

ucce

ssiv

e m

omen

ts in

tim

e. H

owev

er,

it is

impo

rtant

to re

mem

ber t

hat a

ll th

e un

its a

re c

ontin

ually

invo

lved

inpr

oces

sing,

and

pro

cess

ing

of th

e in

put a

rrivi

ng a

t one

tim

e is

just

begi

n-ni

ng a

s th

e in

put i

s m

oved

alo

ng to

the

next

tim

e sl

ice.

. The

ent

ire n

etw

ork

of u

nits

is ca

lled

"the

Tra

ce,"

bec

ause

the

patte

rnof

act

ivat

ion

left

by a

spok

en in

put i

s a tr

ace

of th

e an

alys

is of

the

inpu

tat

eac

h of

the

thre

e pr

oces

sing

leve

ls. T

his

trace

is u

nlik

e m

~i\y

trac

es

TRAC

E M

OD

EL

kAp

i~i~

i~i~

i~i~

i~i

. p

rlnr~

t~t~

t~t~

t'11

11: ,

: : :

I : I

II1'

rnn:

l:!t!t

!,!t'

1111

1: , III

IIII I

'r~

r~r~

r~r~

I~.r

~I'

.L:- a..

i~i~

i~i~

i~i~

i~i'

, - 11-

I:h;

ctl~

A. P

-Fl

o. 2

, A su

bsel

of t

he u

nits

in T

RACE

II. E

ach

rect

angl

e re

pres

ents

a di

ffere

nt u

nit,

The

labe

ls in

dica

te th

e ite

m fo

r whi

ch th

e un

it sta

nds,

and

the

horiz

onta

l edg

es o

f the

rect

angl

ein

dica

le th

e po

rtion

of t

he T

race

span

ned

by e

ach

unit.

The

inpu

t fea

ture

spec

ifica

tions

for

the

phra

se "

tea

cup.

" pre

cede

d an

d fo

llow

ed b

y sil

ence

. are

indi

cate

d fo

r the

thre

e iII

us-

trate

ddi

men

sions

by

the

blac

keni

ng o

f the

cor

resp

ondi

ng fe

atur

e un

its.

thou

gh, i

n th

at it

is d

ynam

ic, s

ince

it c

onsis

ts of

act

ivat

ions

of p

roce

ssin

gel

emen

ts. a

nd th

ese

pro c

essi

ng e

lem

ents

' con

tinue

to in

tera

ct a

s tim

e go

eson

. The

dis

tinct

ion

bet!w

een

perc

eptio

n an

d (p

rimar

y) m

emor

y is

com

-pl

etel

y bl

urre

d~.si

nce

the

perc

ept i

s unf

oldi

ng in

the

sam

e str

uctu

res t

hat

serv

e as

wor

king

mem

ory,

and

per

cept

ual p

roce

ssin

g of

old

er p

ortio

ns o

fth

e in

put c

ontin

ues e

ven

as n

ewer

por

tions

are

com

ing

into

the

syste

m,

Thes

e co

ntin

uing

inte

ract

ions

per

mit

the

mod

el to

inco

rpor

ate

right

con

-te

xt e

ffect

s, an

d al

low

~he

mod

el to

acc

ount

dire

ctly

for c

erta

in a

spec

ts

MC

CLE

LLAN

D A

ND

ELM

AN

kAp

i~i~

i~i~

i~i~

i~i'

.L:- a..

rlrA

r~'A

rArA

rlr'

I :11

1: II

I.: I

: I :

I',t'

ttt'l,

!,t'lt

'11111: III! I :

1 : I

'II

r:flf~

ilr:I:

I:I'

i~i~

i~T~

i!i!i!

i'

~ ~

I~ :,

.

JI-

FIG. 3. The connections orthe unit for the phoneme

Ik/,

cent

ered

ove

r Tim

e Sl

ice

:!4. T

herectangle for this unit is highlighted with a bold outline. The

Ikl

unit

has m

utua

lly e

xcilu

tory

conn

ectio

ns to

all

the

wor

d- a

nd fe

atur

e-le

vel u

nits

col

ored

eith

er p

urtly

or w

holly

in b

luck

;Th

e m

ore

colo

ring

on a

uni

ts' r

ecta

ngle

. the

gre

ater

the

stre

ngth

of t

he c

onne

ctio

n. T

heIk

l un

it ha

s mut

ually

inhi

bito

ry c

onne

ctio

ns to

all

of th

e ph

onem

e-le

vel u

nits

col

ored

pur

tlyor

who

lly in

gre

y. A

gain

. the

rela

tive

amou

nt o

f inh

ibiti

on is

indi

cate

d by

the

exte

nt o

f the

colo

ring

of th

e un

it: it

is d

irect

ly p

ropo

rtion

al to

the

exte

nt o

f the

tem

pora

l ove

rlap

of th

eun

its.

of short-te

rm m

emor

y, s

uch

as th

e fa

ct th

at m

ore

info

rmat

ion

can

bere

tain

ed fo

r sho

rt pe

riods

of t

ime

if it

hang

s tog

ethe

r to

form

a c

oher

ent

who

le,

Proc

essin

g ta

kes p

lace

thro

ugh

the

exci

tato

ry a

nd in

hibitory interac-

tions

of t

he u

nits

in th

e Tr

ace.

Uni

ts o

n di

ffere

nt le

vels

that

are

mut

ually

cons

isten

t hav

e m

utua

lly e

xcita

tory

con

nect

ions

. whi

le u

nits

on th

e sa

me

TRAC

E M

OD

ELMCCLELLAND AND ELMAN

. kl\p

vi

sual

mo.

del e

limin

ate

thes

e be

twee

n-le

vel i

nhib

ito.ry

co.

nnec

tions

. sin

ceth

ese

co.n

nect

io.n

s ca

n in

terfe

re w

ith s

ucc~

ssfu

l use

o.f

parti

~1 in

form

atio

n(McClelland, 1985; McClelland

, 198

6), L

ike

thes

e ne

wer

ver

sio.

ns o

.f th

evi

sual

mo.

del,

TRA

CE li

kew

ise c

o.nt

ains

, no.

bet

wee

n-le

vel i

nhib

itio.

n. W

ew

ill se

e th

at th

is fe

atur

e o.

f TR

ACE

play

s a

very

impo

.rtan

t rol

e in

its

abili

ty to

. sim

ulat

e a

num

ber o

.f em

piric

al p

heno

.men

a.SO

Im:e

s of T

RACE

' s lIrL'hitectllre,

The

insp

iratio

.n fo

.r th

e ar

chite

ctur

eo.

f TRA

CE g

o.es

bac

k to

. the

HEA

RSA

Y S

peec

h un

ders

tand

ing

syste

m(E

rman

& L

esse

r, 19

80; R

eddy

et a

l.. 1

973)

, HEA

RSA

Y in

trodu

ced

the

no.ti

o.n

o.f a

Bla

ckbo

.ard

, a s

truct

ure

sim

ilar t

o. th

e Tr

ace

in th

e TR

ACE

mo.

del.

The

mai

n di

ffere

nce

is th

at th

e Tr

ace

is a

dyn

amic

pro

cess

ing

stru

ctur

e th

at is

sel

f-upd

atin

g, w

hile

the

Blac

kbo.

ard

in H

EARS

AY

was

a pa

ssiv

e da

ta s

truct

ure

thro

.ugh

whi

ch a

nto.

no.m

o.us

pro

cess

es sh

ared

info

.rmat

io.n

. Th

e ar

chite

ctur

e o.

fTRA

CE b

ears

a st

ro.n

g re

sem

blan

ce to

. the

"ne

ural

spec

trogr

am" propo.sed by Cro.wder (1978, 1981) to

. acc

o.un

t fo.

r int

erfe

r-en

ce e

ffect

s bet

wee

n su

cces

sive

item

s in sho.rt-term memo.ry.

. Lik

e ~U

I'Tr

ace,

Cro

wde

rs neural spectro.gram pro.vides a d

ynam

ic w

o.rk

ing

mem

o.ry

repr

esen

tatio

.n o

.f a

spok

en in

put.

Ther

e ar

e tw

o. im

po.rt

ant d

if-fe

renc

es b

etw

een

the

Trac

e an

d C

ro.w

der s

neu

ral s

pect

rogr

am, h

o.w

ever

.Fi

rst o

.f al

l, th

e ne

ural

spe

ctro

.gra

m w

as a

ssum

ed o

.nly

to. r

epre

sent

the

frequ

ency

spec

trum

o.ft

he sp

eech

wav

e o.

ver t

ime;

the

Trac

e , o

.n th

e ot

her

hand

, rep

rese

nts

the

spee

ch w

ave

in:te

rms

o.fa

larg

e nu

mbe

r o.f

ditfe

rent

feature dimensio.ns, as well as in terms of the p

ho.n

emes

and

wo.

rds

co.n

-si

sten

t with

the

patte

rn o

.f activatio.n at the feature level, In this r

egar

dTR

ACE

mig

ht b

e se

en a

s an

exte

nsio

.n o

.f th

e ne

ural

spe

ctro

gram

idea

,Th

e se

co.n

d di

ffere

nce

is th

at C

row

der p

o.stu

late

s inh

ibito

.ry in

tera

ctio

.ns

betw

een

dete

cto.

rs fo

.r sp

ectra

l co.

mpo

.nen

ts s

pace

d up

to. s

ever

al h

undr

edm

illise

co.n

ds a

part.

The

se in

hibi

to.ry

Inte

ract

io.n

s ex

tend

co.

nsid

erab

ly fa

r-th

er th

an th

o.se

we

have

incl

uded

in th

e fe

atur

e le

vel o

.f th

e Tr

ace.

Thi

sdi

ffere

nce

do.e

s no.

t ref

lect

a' d

isagr

eem

ent w

ith C

row

der

sass

umpt

io.n

s.Th

o.ug

h w

e ha

ve n

o.t ,

fo.u

nd it

nec

essa

ry , t

o. a

do.p

t thi

s as

sum

ptio

.n to

. ac-

coun

t fo.

r the

phe

no.m

ena

we fo.cus on in this article, lateral extensio.n o.f

inhi

bitio

.n in

the

time

do.m

ain

mig

ht w

ell a

llo.w

the

TRAC

E fra

mew

o.rk

to.

inco

.rpo.

rate

man

y o.

f the

find

ings

Cro

.wde

r dis

cuss

es in

the

two.

arti

cles

cite

d.

i~i~

i~i~

;~;~

i~i'

Q.)

Q.)

..c:. a..

~ 10

ID S

hi

j i '?

.

FI(j,

4. T

he c

onne

clio

ns o

f Ihe

hig

hlig

hled

uni

l for

Ihe

high

val

ue o

n Ih

e V

ocal

ic fe

lllur

edi

men

sion

in T

ime'

Slic

e 9

and

for I

he h

ighl

ighl

ed u

nil f

or Ih

e w

ord

Ik' p

l starling in Slice

24. E

xcila

lory

con

necl

ions

are

rf(re

senl

ed in

bla

ck. i

nhib

itory

con

nect

ions

in g

rey.

as

inFig. 3.

leve

l tha

t are

inco

.nsi

sten

t hav

e m

utua

lly in

hibi

to.ry

co.nnectio.ns, All co.n-

nect

io.n

s ar

e bi

dire

ctio

.nal

. Bid

irect

io.n

al e

xcita

to.ry

and

inhi

bito

.ry' c

o.n-

nect

io.n

s o.

f the

unit fo.r

Ikl

cent

ered

o.v

er F

eatu

re-s

lice

24 (c

o.un

ting

from

. 0) a

re sh

o.wn ,n Fig. 3; co.nnectio.ns far the high

valu

e o.

f the feature

Vo.c

alic

in S

lice

9.an

d fo

.r th

ewo.

rd Ik

" pl with the

Ikl

cent

ered

o.v

er S

lice

24 a

re sh

o.w

n i~

Fig

. 4.

The

inte

ract

ive

activ

atio

.n mo

.del o.f visual w

ard

reco

.gni

tio.n

(Mc-

Clel

land

& R

umel

hart,

198

1) in

clud

ed in

hibi

to.ry

co.

nnec

tio.n

s be

twee

nea

ch u

nit o

.n th

e fe

atur

e le

vel a

nd le

tters

that

did

no.

t co.

ntai

n th

e fe

atur

e,an

d be

twee

n ea

ch le

tter u

nit a

nd th

e w

o.rd

s th

at d

id n

o.t c

o.nt

ain

the

lette

r.Th

us th

e un

its fo

.r T

in th

e fir

st le

tter p

o.sit

io.n

inhi

bite

d th

e un

its fo

.r al

lwo.rds that did no

.t be

gin

with

T. H

o.wever, mo.re recent versio.ns

o.f

the

Cont

ext-S

ensit

ive

TlIn

ingo

f Pho

nem

e U

nits

The

co.n

nect

io.n

s, b

etw

een

the

feat

ure

and

pho.

nem

e le

vel d

eter

min

ewhat pattern

of activations o.ver the feature units will mo.st slrongly ac-

tivate the detecto.r fo

.r ea

ch p

hone

me.

To. co.pe with the fact that the

feat

ures

repr

esen

ting

each

pho

.nem

e va

ry a

cco.

rdin

g to

. the

pho

.nem

es su

r-ro

undi

ng th

em, t

he m

odel

adj

usts

the

conn

ectio

ns fr

om u

nits

al th

e fe

a-tu

re le

vel t

o. u

nits

at th

e nh

onem

e le

vel a

s a fu

nctio

n nf

Iu'tiv:llinn" al 'hI"

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

TRACE land TRACE

In d

evel

opin

g TR

ACE

, and

i(l t

ryin

g to

test

its c

ompu

tatio

nal a

nd p

sy-

chol

ogic

al a

dequ

acy,

we

foun

d th

at w

e w

ere

som

etim

es le

d in

rath

erdi

ffere

nt d

irect

ions

, We

wan

ted

to sh

ow th

at T

RACE

cou

ld p

roce

ss re

alsp

eech

, but

to b

uild

a m

odel

that

did

so it

was

nec

essa

ry to

wor

ry a

bout

exac

tly w

hat f

eatu

res m

ust b

e ex

tract

ed fr

om th

e sp

eech

sign

al, a

bout

diffe

renc

es in

dur

atio

n of

diff

eren

t fea

ture

s of d

iffer

ent p

hone

mes

, and

abou

t how

to c

ope

with

the

way

s in

whi

ch fe

atur

es a

nd fe

atur

e du

ratio

nsvary as a function of context. Obviously, these are important p

robl

ems,

wor

thy

of c

onsid

erab

le a

ttent

ion.

How

ever

, con

cern

with

thes

e iss

ues

tend

ed to

obs

cure

atte

ntio

n to

the

fund

amen

tal p

rope

rties

of t

he m

odel

and

the

mod

el's

abi

lity

to a

ccou

nt fo

r bas

ic a

spec

ts of

the

psyc

h(jlo

gica

lda

ta o

btai

ned

in m

any

expe

rimen

ts.

To c

ope

with

thes

e co

nflic

ting

goal

s, w

e ha

ve d

evel

oped

two

diffe

rent

vers

ions

of t

he m

odel

, cal

led

TRA

CE 1

and

TRA

CE II

, Bot

h m

odel

ssp

ring

from

the

sam

e ba

sic a

ssum

ptio

ns, b

ut fo

cus o

n di

ffere

nt a

spec

tsof

spee

ch p

erce

ptio

n. T

RACE

1 w

as d

esig

ned

to a

ddre

ss s

ome

of th

ech

alle

nges

pos

ed b

y th

e ta

sk o

f rec

ogni

zing

pho

nem

es fr

om re

al sp

eech

,Th

is v

ersi

on o

f the

mod

el is

des

crib

ed in

det

ail i

n El

man

and

McC

lella

nd(in

pre

ss).

With

this

vers

ion

of th

e m

odel

, we

wer

e ab

le to

show

that

the

TRA

CE fr

amew

ork

coul

d in

deed

be

used

to p

roce

ss re

al sp

eech

-alb

eit

from

a si

ngle

spea

ker u

tterin

g iso

late

d m

onos

ylla

bles

at t

his p

oint

. We

wer

e al

so a

ble

to d

emon

stra

te th

e ef

ficac

y of

the

idea

of a

djus

ting

feat

ure

to p

hone

me

conn

ectio

ns o

n th

e ba

sis o

f act

ivat

ions

pro

duce

d by

sur-

roun

ding

con

text

. With

con

nect

ion

stren

gth

adju

stmen

t in

plac

e' t

hem

odel

was

abl

e to

iden

tify

the

stop

cons

onan

t in

90%

of a

set b

f iso

late

dm

onos

ylla

bles

cor

rect

ly, u

p fro

m 7

9% w

ith a

n in

varia

nt se

t of c

onne

c-tio

ns. T

his l

evel

of p

erfo

rman

ce is

com

para

ble

to w

hat h

as b

een

achi

eved

by other machine-ba

sed

phon

eme

iden

tific

atio

n sc

hem

es (e

,g.,

Kope

c,19

84) a

nd il

lust

rate

s th

e pr

omise

of t

he c

onne

ctio

n str

engt

h ad

justm

ent

sche

me

for c

opin

g w

ith v

aria

bilit

y du

e to

loca

l pho

netic

con

text

. Ide

asfo

r ext

endi

qg th

e co

nnec

tion

stren

gth

adju

stmen

t sch

eme

to d

eal w

ith th

ew

ays

in w

hich

cue

s to

pho

nem

e id

entif

icat

ion

vary

with

glo

bal v

aria

bles

(rate

, spe

aker

cha

ract

erist

ics,

eJc,

) are

con

sider

ed in

the

gene

ral d

iscus

-sio

n. TRA

CE II

, the

ver

sion

desc

ribed

in th

e pr

esen

t pap

er, w

as d

esig

ned

to a

ccou

nt p

rimar

ily fo

r lex

ical

influ

ence

s on

phon

eme

perc

eptio

n an

d

for w

hat i

s kno

wn

abou

t on-

line

reco

gniti

on o

f wor

ds, t

houg

h w

e us

e it

to il

lustr

ate

how

cer

tain

oth

er a

spec

ts of

pho

nem

e pe

rcep

tion

fall

out o

fth

e TR

ACE

fram

ewor

k, T

his v

ersio

n of

the

mod

el is

act

ually

a si

mpl

ified

vers

ion

of T

RACE

1. M

ost i

mpo

rtant

ly, w

e el

imin

ated

the

conn

ectio

n-st

reng

th a

djus

tmen

t fac

ility,

and

we

repl

aced

the

real

spe

ech

inpu

ts to

TRA

CE I

with

moc

k sp

eech

, Thi

s moc

k sp

eech

inpu

t con

siste

d of

ove

r-la

ppin

g bu

t con

text

ually

inva

riant

spec

ifica

tions

of t

he fe

atur

es o

f suc

-ce

ssiv

e ph

onem

es, O

bvio

usly

, the

n, T

RACE

II si

deste

ps m

any

fund

a-mental issues about speech, But it makes "it

muc

h ea

sier t

o se

e ho

w th

em

echa

nism

can

acc

ount

for a

num

ber o

f asp

ects

of p

hone

me

and

wor

dre

cogn

ition

, A n

umbe

r of f

urth

er s

impl

ifyin

g as

sum

ptio

ns w

ere

mad

e to

faci

litat

e ex

amin

atio

n of

bas

ic p

rope

rties

of t

he in

tera

ctiv

e ac

tivat

ion

pro-

cess

es ta

king

pla

ce w

ithin

the

mod

el,

The

foll9

win

g se

ctio

ns d

escr

ibe

TRA

CE II

in m

ore

deta

il, F

irst w

eco

nsid

er th

e sp

ecifi

catio

ns o

f the

moc

k-sp

eech

in()u

t to

the

mod

el, a

ndth

en w

e co

nsid

er th

e un

its a

nd c

onne

ctio

ns th

at m

ake

up th

e Tr

ace

atea

ch o

f the

thre

e le

vels

,

Moc

k-Sp

eech

Inpu

tsTh

e in

put t

o TR

ACE

II w

as a

serie

s of s

peci

ficat

ions

for i

nput

s to

units

at th

e fe

atur

e le

vel,

one

for e

ach

25-m

s tim

e sl

ice

of th

e m

ock

utte

ranc

e,Th

ese

spec

ifica

tions

wer

e ge

nera

ted

by a

sim

ple

com

pute

r pro

gram

from

a se

quen

ce o

f to-

be-p

rese

nted

segm

ents

prov

ided

by

the

hum

an u

ser o

fth

e sim

ulat

ion

prog

ram

. The

allo

wed

segm

ents

cons

isted

of t

he st

op c

on-

sona

nts

Ibl,

Ipl,

Idl,

It/,lg

I, an

d Ik

l, the fricatives

Isl

and

ISI

sh" as in

ship

), the liquids

III

and

Irl,

and the vowels

Ia!

(as i

n "p

ot"),

IiI

(as

inbe

et

),

lul

(as i

n "b

oot"

), and

rI (a

s in

"but

).

rI w

as a

lso

used

tore

pres

ent r

educ

ed v

owel

s suc

h as

the

seco

nd v

owel

in "

targ

et." There

was

also

a "

silen

ce" segment represented by

I-I.

Spec

ial s

egm

ents,

such

as a segment halfway between

Ibl

and

Ipl,

wer

e al

so u

sed;

thei

r pro

perti

esar

e de

scrib

ed in

des

crip

tions

of t

he re

leva

nt s

imul

atio

ns,

A se

t of s

even

dim

ensio

ns w

as u

sed

in T

RACE

lIto represent the

feat

ure-

leve

l inp

uts,

Five

of t

he' d

imen

sion

s (C

onso

nant

al, V

ocal

ic, D

if-fu

sene

ss, A

cute

ness

, and

Voi

cing

) wer

e ta

ken

from

cla

ssic

al w

ork

inph

onol

ogy

(Jak

obso

n, F

ant,

& H

alle

, 195

2), t

houg

h w

e tre

at e

ach

of th

ese

dim

ensio

ns a

s con

tinua

, in

the

spiri

t ofO

den

and

Mas

saro

(197

8), r

athe

rth

an a

s bin

ary

feat

ures

, A si

xth

dim

ensio

n, P

ower

, was

incl

uded

bec

ause

it ha

s be

en fo

und

usef

ul fo

r pho

nem

e id

entif

icat

ion

in v

ario

us m

achi

nesy

stem

s (e

,g.,

Redd

y, 1

976)

, and

it w

as in

corp

orat

ed h

ere

to a

dd a

n ad

-di

tiona

l dim

ensio

n to

incr

ease

the

diffe

rent

iatio

n of

the

vow

els a

nd c

on-

sona

nts,

The

seve

nth

dim

ensio

n, th

e am

plild

de o

f the

bur

st of

noi

se th

atoc

curs

at t

he b

egin

ning

of w

ord

initi

al st

ops,

was

incl

uded

to p

rovi

de a

nad

ditio

nal b

asis

for d

istin

guis

hing

the

stop

con

sona

nts,

whi

ch o

ther

wis

edi

ffere

d fro

m e

ach

othe

r on

only

one

or t

wo

dim

ensio

ns, O

f cou

rse,

thes

e

phon

eme

leve

l in

prec

edin

g an

d fo

llow

ing

time

slice

s, Fo

r exa

mpl

e, w

hen

the phoneme

It I

is preceded or followed by the vowel

Iii,

the feature

pattern corresponding to the

It I

is very different than it is when the

It I

preceded or followed by another vowel, such as

la/.

Acc

ordi

ngly

, whe

nthe unit for

Iii in a particular sl

ice:

is a

ctiv

e, it

cha

nges

the pattern of

connections for units for

It I

in p

rece

ding

and

follo

win

g sl

ices

.

TRAC

E M

OD

EL

dim

ensio

ns a

n;: i

nten

tiona

l sim

plifi

catio

ns o

f the

real

aco

ustid

stru

ctur

eof

spe

ech,

in m

uch

the

sam

e w

ay th

at th

e fo

nt u

sed

by M

cCle

lland

and

Rum

elha

rt (1

981)

in th

e in

tera

ctiv

e-ac

tivat

ion

mod

el o

f visu

al w

ord

rec-

ogni

tion

Was

an

inte

ntio

nal s

impl

ifica

tion.

of t

he re

al s

truct

ure

of p

rint.

Each

dim

ensio

n w

as d

ivid

ed in

to e

ight

val

ue ra

nges

. Eac

h ph

onem

ew

as a

ssig

ned

a va

lue

on e

ach

dim

ensio

n; th

e va

lues

on

the

Voc

alic

, Oif-

fuse

ness

, and

Acu

tene

ss d

imen

sions

for t

he p

hone

mes

in th

e ut

tera

nce

!tik.

p! a

re sh

own

in F

ig. 2

, The

full

set o

f val

ues a

re sh

own

in T

able

I,N

umbe

rs in

the

cells

of t

he ta

ble

indi

cate

whi

ch v

alue

on

the

indi

cate

ddi

men

sion

was

mos

t stro

ngly

act

ivat

ed b

y th

e fe

atur

e pa

ttern

for t

hein

dica

ted

phon

eme,

Val

ues r

ange

from

I very low

to 8 =

very high,

The

last

two

dim

ensio

ns w

ere

alte

red

for t

he c

ateg

oric

al p

erce

ptio

n an

dtra

ding

rela

tions

sim

ulat

ions

,Va

lues

wer

e as

sign

ed to

app

roxi

mat

e th

e va

lues

real

pho

nem

es w

ould

have

on

thes

e di

men

sion

s an

d to

mak

e ph

onem

es th

at fa

ll in

to th

e . s

ame

phon

etic

cat

egor

y ha

ve id

entic

al v

alue

s on

man

y of

the

dim

ensio

ns, T

hus,

.fo

r exa

mpl

e, a

ll sto

p co

nson

ants

wer

e as

signe

d th

e sa

me

valu

es o

n th

ePo

wer

, Voc

alic

, and

Con

sona

ntal

dim

ensio

ns, W

e do

not

cla

im to

hav

eca

ptur

ed th

e de

tails

of p

hone

me

sim

ilarit

y ex

actly

, Ind

eed,

one

can

not

do s

o in

a fi

xed

feat

ure

set b

ecau

se th

e si

mila

ritie

s va

ry a

s a

func

tion

ofco

ntex

t. H

owev

er, t

he fe

atur

e se

ts do

hav

e th

e pr

oper

ty th

at th

e fe

atur

epa

ttern

for o

ne p

hone

me

is m

ore

simila

r to

the

feat

ure

patte

rn fo

r oth

erph

onem

es in

the

sam

e ph

gnet

ic c

ateg

ory

(sto

p, fr

icat

ive,

liqu

id, o

r vow

el)

than

it is

to th

e pa

ttern

s for

pho

nem

es in

oth

er c

ateg

orie

s. A

mon

g th

esto

ps, t

hose

pho

nem

es sh

arin

g pl

ace

of a

rticu

latio

n or

voi

cing

are

mor

esi

mila

r tha

n th

ose

shar

ing

neith

er a

ttrib

ute.

The

corre

latio

ns o

f the

feat

ure

patte

rns f

or th

e 15

pho

nem

es u

sed

are

show

n in

Tab

le 2

. It i

s the

se c

orre

latio

ns o

f the

pat

tern

s ass

igne

d to

the

~ -

!II :J , !II

II:)

I:!

1.1:

1 is '"

:;; '")

~ 1!

"" ~~

~~~

~ ~

~~~~

I~

....

-D - -D

..., ~

"0 "1

r-; r-; I ~

0 - ;

:::~

I~~~

~TA

BLE

IPh

onem

e Fe

alur

e V

alue

s Use

d in

TRA

CE II

Phun

eme

Pow

Voc

alic

Diff

use

ACU

leCO

Ins.

Voi

ced

Our

s.

11.

Co,r)

- "

0 ~

"" on

II:)

.. 01 ,- :J,.

,r)

~ I~

;:::~

~

I~;::

:~~~

:;;

~8~~

~ 00

....

"! '")

~~ I ~

~ I ~

~

~8 I

~~~

~ 18

I ~~

~ I

I ~"'!"',

....

'") r--

'") '"

).:.

:;;.,

,r) "ii .

,r) :0-

..c !II..!!

.... !II

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

diffe

rent

pho

nem

es. r

athe

r tha

n th

e ac

tual

val

ues a

ssig

ned

to p

artic

ular

phon

emes

or e

ven

the

labe

ls at

tach

ed to

the

diffe

rent

moc

k-sp

eech

di-

men

sion

s. th

at d

eter

min

e th

e be

havi

or o

f the

sim

ulat

ion

mod

el. s

ince

itis

thes

e co

rrela

tions

that

det

erm

ine

how

muc

h an

inst

ance

of o

ne p

ho-

nem

e w

ill te

nd to

exc

ite th

e de

tect

or fo

r ano

ther

,Th

e fe

atur

e pa

ttern

s w

ere

cons

tru~t

ed in

such

a w

ay th

at it

was

pos

sible

to c

reat

e fe

atur

e pa

ttern

s th

at w

ould

act

ivat

e tw

o di

ffere

nt p

hone

mes

inth

e sa

me

cate

gory

(sto

p. li

quid

. fric

ativ

e. o

r vowel) to an equal extent

by a

vera

ging

the

valu

es o

f the

two

phon

emes

on

one

or m

ore

dim

ensio

ns,

In th

is w

ay, i

t was

a si

mpl

e m

atte

r to

mak

e up

am

bigu

ous

inpu

ts. h

alfw

aybe

twee

n tw

o ph

ontim

es. o

r to

cons

truct

con

tinua

var

ying

bet

wee

n ' tw

oph

onem

es o

n on

e or

mor

e di

men

sion

s.Th

e fe

atur

e sp

ecifi

catio

n of

eac

h ph

onem

e in

the

inpu

t stre

am e

xten

ded

over

II ti

me

slice

s of t

he in

put.

The

stren

gth

of th

e pa

ttern

gre

w to

ape

ak a

t the

6th

slic

~ an

d fe

ll of

f aga

in. a

s illu

strat

ed in

Fig

, 2, P

eaks

of

succ

essiv

e ph

onem

es w

ere

sepa

rate

d by

6 sl

ices

, Thu

s. sp

ecifi

catio

ns o

fsu

cces

sive

phon

emes

ove

rlapp

ed. a

s the

y do

in re

al sp

eech

(Fow

ler.

1984

;Li

berm

an, 1

970)

.G

ener

ally

. the

re w

ere

no c

ues t

o w

ord boundaries in the speech

strea

m- th

e fe

atU

lie s

peci

ficat

ion

for t

he la

st p

hone

me

of o

ne w

ord

over

lapp

ed w

ith th

e , fi

rst p

hone

me

of th

e ne

xt in

just

the

sam

e w

ay fe

atur

esp

ecifi

catio

ns o

f adj

~\ce

nt p

hone

mes

ove

rlap

with

in w

ords

, How

ever

. en-

tire

utte

ranc

es p

rese

nted

to th

e m

odel

for p

roce

ssin

g-w

heth

er th

ey w

ere

indi

vidu

al s

ylla

bles

., w

ords

. or s

tring

s of

wor

ds-w

ere

prec

eded

and

fol-

low

ed b

y sil

ence

, Sile

nce

was

nol

sim

ply

the

abse

nce

ofan

y in

put;

rath

er.

it w

as a

pat

tern

of f

eatu

re v

alue

s. ju

st lik

e th

e ph

onem

es, T

hus.

a n

inth

valu

e on

eac

h of

the

isev

en d

imen

sion

s w

as a

ssoc

iate

d w

ith s

ilenc

e, T

hese

valu

es w

ere

actu

ally

out

side

the

rang

e of

val

ues

whi

ch o

ccur

red

in th

eph

onem

es th

emse

lves

. so

that

the

feat

ures

of s

ilenc

e w

ere

com

plet

ely

unco

rrela

ted

with

the

feat

ures

of a

ny o

f the

pho

nem

es u

sed,

The

Phon

eme

Leve

l and

Fea

ture

- Pho

nem

e C

onne

ctio

nsA

t the

pho

nem

e le

vel.

ther

e is

a se

t of d

etectors for each of the 15

phon

emes

liste

d ab

ove,

In a

dditi

on. t

here

is a

set o

f detectors for the

pres

ence

of s

ilenc

e, T

hese

sile

nce

dete

ctor

s are

trea

ted

like

all o

ther

phon

eme

dete

ctor

s. Ea

ch m

embe

r of t

he se

t of d

etec

tors

for a

par

ticul

arph

onem

e is

cen

tere

d ov

er a

diff

eren

t tim

e ~

;ce

at th

e fe

atur

e le

vel,

and

the

cent

ers a

re sp

aced

thre

e tim

e sli

ces a

part.

The

uni

t cen

tere

d ov

er a

parti

cula

r slic

e re

ceiv

ed e

xcita

tory

inpu

t fro

m fe

atur

e un

its in

a ra

nge

ofsli

ces,

exte

ndin

g bo

th fo

rwar

d an

d ba

ckw

ard

from

the

slice

in w

hich

the

phon

eme

unit

is lo

cate

d, It

also

send

s exc

itato

ry fe

edba

ck d

own

to th

esame feature units in the same range of slices.

The

conn

ectio

n st

reng

ths

betw

een

the

feat

ure-

leve

l uni

ts a

nd a

par

tic-

ular

pho

nem

e-le

vel u

nit e

xact

ly m

atch

the

feat

ure

patte

rn th

e ph

onem

eis

give

n in

its i

nput

spec

ifica

tion,

Thu

s. as

illu

strat

ed in

Fig

. 3. t

hestrengths of the connections between the node for

Ikl

cent

ered

ove

r Tim

eSl

ice

24 a

nd th

e no

des a

t the

feat

ure

leve

l are

exa

ctly

pro

porti

onal

to th

epa

ttern

of i

nput

to th

e fe

atur

e le

vel p

rodu

ced

by a

n in

put s

peci

ficat

ion

containing the featur~s of

Ikl

cent

ered

in th

e sa

me

time

slic

e.Th

ere

are

inhi

bito

ry c

onne

ctio

ns b

etw

een

units

at t

he p

hone

me

leve

l.U

nits

inhi

bit e

ach

othe

r to

the

exte

nt th

at th

e sp

eech

obj

ects

they

stan

dfo

r rep

rese

nt a

ltern

ativ

e in

terp

reta

tions

of t

he c

onte

nt o

f the

spee

chstr

eam

at t

he sa

me

poin

t in

the

utte

ranc

e. N

ote

that

. alth

ough

the

feat

ure

spec

ifica

tion

of a

pho

nem

e is

spre

ad o

ver a

win

dow

of I

I slic

es, s

ucce

s-si

ve p

hone

mes

in th

e in

put h

ave

thei

r ,ce

nter

s 6 sl

ices

apa

rt. T

hus e

ach

phon

eme-

leve

l uni

t is

thou

ght o

f as

span

ning

6 fe

atur

e-level slices. as

illust

rate

d in

Fig

. 3. E

ach

unit

inhi

bits

oth

ers

in p

ropo

rtion

to th

eir

over

lap,

Thu

s, a

pho

nem

e de

tect

or in

hibi

ts o

ther

pho

nem

e de

tect

orsc

en-

tere

d ov

er th

e sa

me

slice

twic

e as

muc

h as

it in

hibi

ts de

tect

ors c

ente

red

3 sl

ices

aw

ay, a

nd in

hibi

ts d

etec

tors

cen

tere

d 6

or m

ore

slic

es a

way

not

at all, (

Fc'lIl

IIre

Lel'e

l Vni

t. f a

nd C

o""e

ctio

n.Th

e un

its a

t the

feat

ure

leve

l are

det

ecto

rs fo

r fea

ture

s of t

he sp

eech

strea

m a

t par

ticul

ar , m

omen

ts in

tim

e. In

TRA

CE II

. the

re w

as a

uni

t for

each of the nine values on each of the seven dimensions in each time

slice

of t

he T

race

, The

figu

res s

how

thre

e se

ts of

feat

ure

units

in se

vera

ltim

e sli

ces,

Uni

ts fo

r fea

ture

s on

the

sam

e di

men

sion

with

in th

e sa

me

time

slice

are

mut

ually

inhi

bito

ry, T

hus,

the

unit

for t

he h

igh

valu

e of

the

Voc

alic

dim

ensio

n in

Tim

e Sl

ice

9 in

hibi

ts th

e un

its fo

r oth

er v

alue

s on

the

sam

e di

lJlen

sion

in th

e sa

me

time

slice

. as i

llustr

ated

in F

ig. 4

. Thi

sfig

ure

also

illu

stra

tes

the

mut

ually

exc

itato

ry c

onne

ctio

ns o

f thi

s sam

efe

atur

e un

il w

ith u

nits

at th

e ph

onem

e le

vel.

In th

e ne

xt se

ctio

n w

e re

-de

scrib

e th

ese

conn

ectio

ns fr

om th

e po

int o

f vie

w o

f the

pho

nem

~ le

vel,

Wor

d Vn

its a

nd W

ord-

Phon

eme

Con

nec'

tions

Ther

e is

a un

it fo

r eve

ry w

ord

in e

very

tim

e sli

ce, E

ach

of th

ese

units

repr

esen

ts a

diffe

rent

hyp

othe

sis a

bout

a w

ord

iden

tity

and

starti

ng lo

-cation in the 'Irace. For example, the unit for the word

Ik.pl

in Slice 24

. (hi

ghlig

hted

in F

ig, 4

) rep

rese

nts t

he h

ypot

hesis

that

the

inpu

t con

tain

sth

e w

ord

" cup

" sta

rting

in S

lice

24, M

ore

exac

tly, i

t rep

rese

nts t

he h

y-po

thes

is th

at th

e in

put c

onta

ins t

he w

ord

" cup

" with

its f

irst p

hone

me

centered in Time Slice 24.

Wor

d un

its re

ceiv

e ex

cita

tion

from

the

units

for t

he p

hone

mes

they

cont

ain

in a

serie

s of o

verla

ppin

g w

indo

ws.

Thus

, the

uni

t for

"cu

p" in

Time Slice 24 will receive excitation from

Ikl

in s

lices

nei

ghbo

ring

Slic

e

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

24, from

rI in slices neighboring Slice 30, and from

Ipl

in slices neigh-

borin

g Sl

ice

36. A

s with

the

feat

ure-

phon

eme

conn

ectio

ns, t

hese

con

-ne

ctio

ns a

re s

trong

est a

t the

cen

ter o

f the

win

dow

and

fall

off l

inea

rly o

neither side,

The

inhi

bito

ry c

onne

ctio

ns a

t the

wor

d le

vel a

re si

mila

r to

thos

e at

the

phon

eme

leve

l, A

gain

, the

stre

ngth

of t

he in

hibi

tion

betw

een

two

wor

dun

its d

epen

ds o

n th

e nu

mbe

rof t

ime

slice

s in

whi

ch th

ey o

verla

p. T

hus,

units

repr

esen

ting

alte

rnat

ive

inte

rpre

tatio

ns o

f the

sam

e str

etch

of p

ho-

nem

e un

its a

re st

rong

ly c

ompe

titiv

e, b

ut u

nits

repr

esen

ting

inte

rpre

ta-

tions

of n

onov

erla

ppin

g se

quen

ces o

f pho

nem

es ~

o no

t com

pete

at a

ll.TR

ACE

II h

as d

etec

tors

for t

he 21

1 words found in a computerized

phon

etic

wor

d lis

t tha

t met

all

of th

e fo

llow

ing

cons

train

ts: (

a) th

e w

ord

cons

iste

d on

ly o

f the

pho

nem

es li

sted

abo

ve; (

b) it

was

not

an

infle

ctio

nof

som

e ot

her w

ord

that

cou

ld b

e m

ade

by a

ddin

g "-

ed," "

os, " or

ing

; (c)

the

wor

d to

geth

er w

ith it

s "-e

d,

" "

-s," and "

ing"

inflections

occu

rred

wiih

a fr

eque

ncy

of 2

0 or

mor

e pe

r mill

ion

in th

e K

ucer

a an

dFr

anci

s (19

67) w

ord

coun

t. It

is no

t cla

imed

that

the

mod

el's

lexi

con

is an

exha

ustiv

e lis

t of w

ords

mee

ting

this

crite

rion,

sinc

e th

e co

mpu

teriz

edph

onet

ic le

xico

n w

as n

ot c

ompl

ete,

but

it is

reas

onab

ly c

lose

to th

is, T

om

ake

spec

ific

poin

ts ab

out t

he b

ehav

ior o

f the

mod

el, d

etec

tors

for t

hefo

llow

ing

thre

e w

ords

not

in th

e m

ain

list w

ere

adde

d: "

blus

h,

" "

rega

l,an

d "

sleet

." T

he m

odel

also

had

det

ecto

rs a

t the

wor

d le

vel f

or si

lenc

e(I-

I),

whi

ch w

as tr

eate

d lik

e it

one-

phon

eme

wor

d,

inde

finite

ly, t

houg

h fo

r pra

ctic

al p

urpo

ses i

t is a

lway

s ter

min

ated

. afte

rso

me

pred

eter

min

ed n

umbe

r of t

ime

cycl

es h

as ~

Iaps

ed,

...

Pres

enta

tion

tlnd

Proc

e!jti

'ing

of tm

Ulle

rtlnc

'Be

fore

pro

cess

ing

of a

n ut

tera

nce

begi

ns, t

he a

ctiv

atio

ns o

f all

of th

eun

its a

re se

t at t

heir

resti

ng v

alue

s. A

t the

star

t of p

rocessing, the input

to th

e in

itial

slic

e of

feat

ure

units

is a

pplie

d, A

ctiv

atio

ns a

re th

en u

pdat

ed,

endi

ng th

e in

itial

tim

e cy

cle,

On

the

next

tim

e cy

cle,

the

inpu

t to

the

next

slice

of f

eatu

re u

nits

is ap

plie

d, a

nd e

xcita

tory

and

inhi

bito

ry in

puts

toea

ch u

nit r

esul

ting

from

the

patte

rn o

f act

ivat

ion

left

at th

e en

d of

the

prev

ious

tim

e sli

ce a

re c

ompu

ted,

It is

impo

rtant

to re

mem

ber t

hat t

he in

put i

s ap

plie

d, o

ne sl

ice

at a

time,

pro

ceed

ing

from

left

to ri

ght a

s tho

ugh

it w

ere

an on

going stream

of sp

eech

"w

ritin

g on

" the successive time sl

ices

of t

he T

race

, The

in-

tera

ctiv

e-ac

tivat

ion

proc

ess

is o

ccur

ring

thrc

ugho

ut th

e Trace on each

time

slic

e, e

ven

thou

gh th

e ex

tern

al b

otto

m-u

p in

put i

s on

ly c

omin

g in

toth

e fe

atur

e un

its o

ne sl

ice

at a

tim

e. P

roce

ssin

g in

tera

ctio

ns c

an c

ontin

ueev

en a

fter t

he le

ft to

righ

t sw

eep

thro

ugh

the

inpu

t rea

ches

the

end

of th

eTr

ace.

Onc

e th

is ha

ppen

s, th

ere

are

simpl

y no

new

inpu

t spe

cific

atio

nsap

plie

d to

the

Trac

e; th

e co

ntin

uing

inte

ract

ions

are

bas

ed o

n w

hat h

asal

read

y be

en p

rese

nted

. Thi

s int

erac

tion

proc

ess i

s ass

umed

to c

ontin

ue

Det

ails

of P

roce

ssin

g D

ynam

kti,

The

inte

ract

ive

activ

atio

n pr

oces

s in

the

Trac

e m

odel

follo

ws

thed

y-na

mic

ass

umpt

ions

laid

out

in M

cCle

lland

and

Rum

elha

rt (1

981)

, Eac

hun

it ha

s a re

sting

act

ivat

ion

valu

e ar

bitra

rily

set a

t 0, a

max

imum

act

ivat

ion

value arbitrarily set at 1,0,

and

a m

inim

um a

ctiv

atio

n se

t at -

.3. O

nev

ery

time

cycl

e of

pro

cess

ing,

all

the

wei

ghte

d ex

cita

tory

and

inhi

bito

rysi

gnal

s im

ping

ing

upon

a u

nit a

re a

dded

toge

ther

, The

sig

nal f

rom

one

unit

to a

noth

er is

just

the

exte

nt to

whi

ch it

s ac

tivat

ion

exce

eds 0

; if i

tsac

tivat

ion

is le

ss th

an 0

, the

sig

nal '

is 0.

1 G

loba

l lev

el-s

peci

fic e

xcita

tory

,in

hibi

tory

, and

dec

ay p

aram

eter

s sca

le th

e re

lativ

e m

agni

tude

s of d

ilfer

ent

types of influences on the activation of each unit. Values for these pa-

ram

eter

s ar

e gi

ven

belo

w,

Afte

r the

net

inpu

t to

each

uni

t has

bee

n de

term

ined

bas

ed o

n th

e pr

ior

activ

atio

ns o

f the

uni

ts, th

e ac

tivat

ions

of t

he u

nits

arc

all u

pdat

ed fo

rth

e ne

xt p

roce

ssin

g cy

cle,

The

new

val

ue o

f the

act

ivat

ion

of th

e un

it is

a fu

nctio

n of

its n

et in

put f

rom

oth

er u

nits

and

its p

revi

ous

activ

atio

nva

lue,

The

exa

ct fu

nctio

n us

ed (s

ee M

cCle

lland

& R

umel

hart,

198

1) k

eeps

unit

activ

atio

ns b

ound

ed b

etw

een

thei

r max

imum

and

min

imum

val

ues.

Giv

en a

con

stant

inpu

t, th

e ac

tivat

ion

of a

uni

t will

sta

biliz

e at

a p

oint

betw

een

its m

axim

um a

nd m

inim

um th

at d

epen

ds o

n th

e str

engt

h an

dsi

gn (e

xcita

tory

or i

nhib

itory

) of t

he in

put.

With

a n

et in

put o

f 0, the

activ

atio

n of

the

unit

will

gra

dual

ly re

turn

to it

s res

ting

leve

l.Ea

ch p

roce

ssin

g tim

e cy

cle

corre

spon

ds to

a si

ngle

tim

e sli

ce a

t the

feat

ure

leve

l. Th

is is

actu

ally

a p

aram

eter

of t

he m

odel

-there is no

intri

nsic

reas

on w

hy th

ere

shou

ld b

e a

singl

e cy

cle

of th

e in

tera

ctiv

e-ac

tivat

ion

proc

ess s

ynch

roni

zed

with

the

arriv

al o

f eac

h su

cces

sive

slice

of th

e in

put.

A h

ighe

r rat

e of

cyc

ling

wou

ld sp

eed

the

perc

olat

ion

ofef

fect

s of n

ew in

put t

hrou

gh th

e ne

twor

k re

lativ

e to

the

rate

of p

rese

n-ta

tion,

Out

put A

ssum

ptio

nsA

ctiv

atio

ns o

f uni

ts in

the

Trac

e ris

e an

d fa

ll as

the

inpu

t sw

eeps

acr

oss

the

feat

ure

leve

l. A

t any

tim

e, a

dec

ision

can

be

mad

e ba

sed

on th

e pa

ttern

of a

ctiv

atio

n as

it st

ands

at t

hat m

omen

t. Th

e de

cisio

n m

echa

nism

can

,w

e as

sum

e, b

e di

rect

ed to

con

side

r the

set

of u

nits

loca

ted

with

in a

sm

all

win

dow

of a

djac

ent s

lices

with

in a

ny le

vel.

The

units

in th

is se

t the

nI A

t the

wor

d le

vel,

the

inhi

bito

ry si

gnal

from

one

wor

d to

ano

ther

is ju

st. th

e sq

uare

of

the

exte

nt to

whi

ch th

e se

nder

s act

ivat

ion

exce

eds z

ero.

Thi

s ten

ds to

smoo

th Ih

e ef

li:.:h

of m

any

units

sudd

enly

bec

omin

g sli

ghtly

act

ivat

ed. a

nd o

f cou

rse

il al

so in

crea

ses I

hedo

min

ance

of o

ne a

ctiv

e w

ord

over

man

y w

eakl

y ac

tivat

ed o

nes,

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

Sj =

kaj,

Even

with

all

the

sim

plifi

catio

ns d

escr

ibed

abo

ve. t

he T

RAC

E m

odel

still

has a

num

ber o

ffree

par

amet

ers.

Thes

e pa

ram

eter

s are

liste

d in

Thb

le3,

It sh

ould

be

note

d th

at p

aram

eter

s are

not

if'! g

ener

al d

irect

ly c

ompa

-ra

blea

cros

s le

vels

, For

exa

mpl

e. p

hone

me-

to-p

hone

me

and

wor

d-to

-w

ord

inhi

bitio

n ar

e no

t dire

ctly

com

para

ble

to e

ach

othe

r or t

o fe

atur

e-to

-pho

nem

e in

hibi

tion.

sinc

e fe

atur

e-le

vel u

nits

conl

pete

onl

y w

ithin

asin

gle

slice

. whi

le p

hone

me

and

wor

d un

its c

ompe

te in

pro

porti

on to

thei

rov

erla

p,Th

ere

was

som

e tri

al a

od e

rror i

n fin

ding

'he

set

of p

aram

eter

s us

ed in

the

repo

rted

sim

ulat

ions

. but

. in

gene

ral.

the

qual

itativ

e be

havi

or o

f the

mod

el w

as re

mar

kabl

y ro

bust

und

er p

aram

eter

var

iatio

ns. a

nd n

o sy

s~te

mat

ic s

earc

h of

the

spac

e of

par

amet

ers

was

nec

essa

ry, G

ener

ally

. ma-

nipu

latio

ns o

f par

amet

ers s

impl

y in

fluen

ce t h

emag

nitu

de o

r the

tim

ing

of o

ne e

ffect

or a

noth

er w

ithou

t cha

ngin

g th

e ba

sic n

atur

e of

the

effe

cts

obse

rved

, For

exa

mpl

e. st

rong

er b

otto

m-u

p ex

cita

tion

spee

ds th

ings

up

and

can

indi

rect

ly in

fluen

ce th

e si

~e o

f top

-dow

n ef

fects. since. for ex-

ampl

e. st

rong

er w

ord

leve

l act

ivat

ions

pro

duce

stro

nger

feed

back

to th

e. p

hone

me

leve

l. St

rong

er to

p-do

wn

exci

tatio

n. o

f cou

rse.

dire

ctly

influ

-en

ces t

he m

agni

tude

of l

exic

al e

ffect

s, Th

e on

e pa

ram

eter

that

app

eare

dto

influ

ence

the

qual

itativ

e be

havi

or o

f the

mod

el w

as th

e st

reng

th o

fw

ithin

- leve

l inh

ibiti

on, S

trong

er w

ithin

-leve

l inh

ibiti

on m

ake

the

mod

elco

mm

it its

elf m

ore

stron

gly

to sl

ight

ear

ly d

iffer

ence

s in

activ

atio

n am

ong

com

petin

g al

tern

ativ

es. T

here

was

. the

refo

re. s

ome

tuni

ng o

f thi

s pa

ram

-et

er to

avo

id e

arly

ove

rcom

mitm

ent t

hat w

ould

pre

vent

righ

t con

text

from

exer

ting

an in

fluen

ce u

nder

som

e ci

rcum

stanc

es, F

inal

ly. a

low

rate

of

feat

ure-

leve

l dec

ay w

as u

sed

toal

low

feat

ure-

level activations to persist

, afte

r the

inpu

t mov

ed o

n to

late

r slic

es,

The

para

met

er v

alue

s w

ere

held

con

stan

t at t

he v

alue

s sh

own

in th

e

cons

titut

e th

e se

t of r

espo

nse

alte

rnat

ives

. des

igna

ted

by th

e id

entit

y of

the

item

for w

hich

the

unit

stand

s (no

te th

at w

ith se

vera

l adj

acen

t slic

esin

clud

ed in

the

set.

seve

ral u

nits

in th

e al

tern

ativ

e se

t may

cor

r~sp

ond

toth

e sa

me

over

t res

pons

e), W

ord

iden

tific

atio

n re

spon

ses

are

assu

m~d

tobe based on

. rea

dout

from

the

wor

d le

vel.

and

phon

eme

iden

tific

atio

nre

spon

ses a

re a

ssum

ed to

be

base

d. o

n re

adou

t fro

m th

e ph

onem

e le

vel.

As f

ar a

s pho

nem

e id

entif

icat

ion

is co

ncer

ned,

then

. it h

Ol:n

~gen

eous

mec

hani

sm is

ass

umed

to b

e us

ed w

ith b

oth

wor

d an

d no

nwor

d sti

mul

i.Th

e de

cisio

n m

echa

nism

can

be

aske

d to

mak

e a

resp

onse

eith

er (a

) at a

criterialtime during processirig

or (b

) whe

n a

unit

in th

e al

tern

ativ

e se

tre

ache

s a c

riter

ial s

treng

th re

lativ

e to

the

activ

atio

n of

oth

er a

ltern

ativ

eun

its. O

nce

a de

cisio

n ha

s bee

n m

ade

to m

ake

a re

spon

se. o

ne o

f the

alte

rnat

ives

is c

hose

n fro

m th

e m

embe

rs o

f the

set,

The

prob

abili

ty o

fch

oosin

g a

parti

cula

r alte

rnat

ive

i is t

hen given by the Luce (1959)

choi

ce ru

le:

peRi

~ S

whe

n in

dexe

s the

mem

bers

of t

he a

ltern

ativ

e se

t. an

d

The

expo

nent

ial t

rans

form

atio

n en

sure

s th

at a

ll ac

tivat

ions

are

pos

itive

and

give

s gre

at w

eigh

t to

stron

ger a

ctiv

atio

ns. a

nd th

e Lu

ce ru

le e

nsur

esth

at th

e su

m o

f all

of th

e re

spon

se p

roba

bilit

ies a

dds u

p to

1, 0

. Sub

stan-

tially

the

sam

e assumptions were used by McClelland and Rumelhart

(198

1),

Mil1

imiz

il1R

the

Num

ber o

f Par

amet

ers

At t

he e

xpen

se o

Ccon

sider

able

real

ism. w

e ha

ve tr

ied

to k

eep

TRA

CEII

simpl

e by

usin

g ho

mog

eneo

us p

aram

eter

s whe

reve

r pos

sible

. Thu

s, as

alre

ady

note

d. th

e fe

atur

e sp

ecifi

catio

ns o

f all

phon

emes

wer

e sp

read

out

over

the

sam

e nu

mbe

r of t

ime

slic

es. e

ffect

ivel

y gi

ving

all p

hone

mes

the

sam

e du

ratio

n, T

he s

treng

th o

f the

tota

l exc

itatio

n co

min

g in

to a

par

tic-

ular

pho

nem

e un

it ~r

om th

e fe

atur

e un

its w

as n

orm

aliz

ed to

, the

sam

evalue for all phonelT1es. thus m

akin

g ea

ch p

hone

me

equa

lly e

xcita

ble

byits

ow

n ca

noni

cal p

atte

rn, O

ther

sim

plify

ing

assu

mpt

ions

shou

ld b

e no

ted

as w

ell.

For e

xam

ple.

ther

e w

ere

no d

iffer

ence

s in

con

nect

ions

or r

estin

gle

vels

for w

ords

of d

~ffe

rent

freq

uenc

y, It

wou

ld h

ave

been

a s

impl

e m

atte

rto

inco

rpor

ate

frequ

ency

as

McC

lella

nd a

nd R

umel

hart

(198

1) d

id. a

nd a

com

plet

e m

ltdel

wou

ld. o

f cou

rse.

incl

ude

som

e ac

coun

t for

the

ubiq

uito

usef

fect

s of w

ord

freq~

ency

. We

left

it ou

t her

e to

faci

litat

e an

exa

min

atio

nof

the

man

y ot

her f

~cto

rs th

at a

ppea

r to

influ

ence

the

proc

ess

of w

ord

reco

gniti

on in

spee

ch p

erce

ptio

n,

TABL

E 3

Para

met

ers o

f TRA

CE II

Para

met

erV

alue

Feat

ure-

phon

eme

exci

tatio

nPh

onem

e-w

ord

exci

tatio

nW

ord-

phon

eme

exci

tatio

nPh

onem

e-feature excitation

Feat

ure-

leve

l inh

ibiti

onPh

onem

e-le

vel i

nhib

ition

"W

ord-

leve

l inh

ibiti

on"

Feat

ure-

leve

l dec

ayPh

onem

e-le

vel d

ecay

Wor

d-le

vel d

ecay

" Per

thre

e tim

e-sl

ices

of o

verla

p.

TRAC

E M

OD

EL

tabl

e th

roug

hdut

the

sim

ulat

ions

, exc

ept i

n th

e si

mul

atio

ns o

f cat

egor

ical

perc

eptio

n an

d tra

di' n

g re

latio

ns, S

ince

we

wer

e no

t exp

licitl

y co

ncer

ned

with

the

effe

ds o

f fee

dbac

k to

the

feat

ure

leve

l in

any

of th

e ot

her s

im-

ulat

ions

, we

st h th

e fe

edba

ck fr

om th

e ph

onem

e le

vel t

o th

e fe

atur

e le

vel

to z

ero

to s

peed

up

the

sim

ulat

ions

in a

ll ot

her c

ases. In the categorical

perc

eptio

n an

d tra

ding

rela

tions

sim

ulat

ions

this

par

amet

er w

as s

etat

, OS,

Phon

eme-

to- f

eatu

re fe

edba

ck te

nded

to sl

ow th

e ef

fect

ive

rate

of d

ecay

at th

e fe

atur

e le

vel a

nd to

incr

ease

the

effe

ctiv

e di

stinc

tiven

ess o

f diff

eren

tfe

atur

e pa

tterri

s, R

ate

of d

ecay

of f

eatu

re- le

vel a

ctiv

atio

ns a

nd st

reng

thof

pho

nem

e-to

'-ph

onem

e co

mpe

titio

n w

ete

set t

o , 0

3 an

d .0

5 to

com

pen-

sate

for t

hese

effe

cts,

No

lexi

con

was

use

d in

the

cate

goric

al p

erce

ptio

nan

d tra

ding

relii

tions

sim

ulat

ions

, whi

ch is

equ

ival

ent t

o se

tting

the

pho-

nem

e to

wor

d ~x

dtat

ion

para

met

er to

zer

o,

't! C 0.::

g~+

-4;

QJ

.c:

Il.

MC

CLE

LLAN

D A

ND

ELM

AN

045+

II'! .

1.1 Ii!

! ~

~I

1IliI

~.: r

'fl!

ftlllf

l!I:

.~t!1

I -

B I

a- b

g.. b~ b

I' FI

G. S

. Pho

nem

e- a

nd w

ordc

levet activiltions at several points in Ihe un'tllding of iI ~cgl1lCl1t

ambiguous between

Ibl

and

Ipl

followed by

III, 0

and

Ig!.

See

texl

Itlr

ilful

l cxp

lal1

aliu

l1,

THE

DY

NA

MIC

S O

F PH

ON

EME

PERC

EPTI

ON

In th

e in

trodu

ctio

n, w

e m

otiv

ated

the

appr

oach

take

n in

the

TRA

CEm

odel

in g

ener

al te

rms.

In th

is se

ctio

n, w

e se

e . th

at th

e si

mpl

e co

ncep

tsth

at le

ad to

TRA

CE p

rovi

de a

coh

eren

t and

synt

hetic

acc

ount

of a

larg

enu

mbe

r of d

iffer

ent k

inds

of f

indi

ngs

on th

e pe

rcep

tion

of p

hone

mes

,Pr

evio

us m

ode:

ls ha

ve b

een

able

to p

rovi

de fa

irly

accu

rate

acc

ount

s of a

num

ber o

f the

se p

heno

men

a, F

or e

xam

ple,

Mas

saro

and

Ode

ns

feat

ure

inte

grat

ion

mod

el (M

assa

ro, 1

981;

Mas

saro

& O

den,

198

0a, 1

980b

; Ode

n&

Mas

saro

, 197

8) a

ccou

nts i

n de

tail

for a

larg

e bo

dy o

f dat

a on

the

influ

ence

s of m

ultip

le c

ues t

o ph

onem

e id

entit

y, a

nd th

e Pi

soni

/Fuj

isaki

-Kawashima m

bdel

of c

ateg

oric

al p

erce

ptio

n (F

ujisa

ki &

Kaw

ashi

ma,

1968; Pisoni, 1973, 1975) accounts for a large bo

dy o

f dat

a on

the

con-

ditio

ns u

nder

whi

ch su

bjec

ts ca

n di

scrim

inat

e so

unds

with

in th

e sa

me

phon

etic

cat

egor

y. M

arsl

er, W

ilson

s C

OH

OR

T m

odel

can

acc

ount

for

the

time

cour

se o

f lex

ical

influ

ence

s on

pho

nem

e id

entif

icat

ion.

Wha

t we

hope

to sh

ow h

ere

is th

at T

RACE

brin

gs th

ese

phen

omen

a, a

nd se

vera

lot

hers

not

con

side

red

by e

ither

mod

el, t

oget

her i

nto

a co

here

nt p

ictu

reof

the

proc

ess

of p

hone

me

perc

eptio

n as

it u

nfol

ds in

tim

e,Th

e pr

esen

t sec

tion

cons

ists o

f thr

ee m

ain

parts

. The

firs

t foc

uses

on

lexi

cal e

ffeC

ts O

n ph

onem

e id

entif

icat

ion

and

the

cond

ition

s un

der w

hich

thes

e ef

fect

s are

obt

aine

d. H

ere,

we

see

how

TRA

CE c

an a

ccou

nt fo

rth

e ba

sic

lexi

cal e

ffect

, and

we

mak

e it

clea

r why

lexi

cal e

ffect

s ar

e on

lyob

tain

ed u

nder

som

e co

nditi

ons,

The

seco

nd p

art o

f thi

s sec

tion

focu

ses

on th

e qu

estio

n of

the

role

of p

hono

tact

ic ru

les-

that

is, r

ules

spe

cify

ing

whi

ch p

hone

mes

can

occ

ur to

geth

er in

Eng

lish-

in p

hone

me

iden

tific

a-tio

n, H

ere,

we

see

how

TRA

CE m

imic

s , th

e ap

pare

ntly

rule

- gov

erne

dbe

havi

or o

f hum

an su

bjec

tf. in

term

s of a

"co

nspi

racy

" of the le

xica

lite

ms t

hat i

nsta

ntia

te th

e ru

le. T

he. th

ird p

art f

ocus

es o

n tw

o as

pect

s of

phon

eme

iden

tific

atio

n of

ten

cons

ider

ed q

uite

sepa

rate

ly fr

om le

xica

l ef-

fect

s-na

mel

y, th

e co

ntra

sting

phe

nom

ena

of c

ue tr

adeo

fTs

in p

hone

me

perc

eptio

n an

d ca

tego

rical

per

cept

ion,

Her

e w

e se

e th

at T

RACE

pro

vide

san

acc

ount

of b

oth

effe

cts a

s wel

l as d

etai

ls of

thei

r tim

e co

urse

, All

thre

epa

rts o

f thi

s se

ctio

n illu

stra

te 'h

ow th

e si

mpl

e m

echa

nism

s of m

utua

l ex-

cita

tion

and

inhi

bitio

n am

ong

the

proc

essin

g un

its o

f the

Tra

ce p

rovi

de a

natu

ral w

ay o

f acc

ount

ing

for t

he re

leva

nt p

heno

men

a, T

he se

ctio

n en

dsw

ith a

brie

f con

sider

atio

n of

the

way

s in

whi

ch T

RACE

mig

ht b

e ex

-te

nded

to c

ope

with

sev

eral

oth

er a

spec

ts o

f pho

nem

e id

entif

icat

ion

and

perc

eptio

n,

Lexical EffeL'

YOll

can

tell

a ph

onem

e by

Ihe

com

pany

Ihm

il ke

epj'

2 In

this

sect

ion,

we describe a simple simulation of the basic lexical effect

on

phon

eme

iden

tific

atio

n re

porte

d by

Gan

ong

(198

0), W

e st

art w

ith th

is p

heno

men

onbe

caus

e it.

and

the

rela

ted

phon

emic

rest

orat

ion

effe

ct, w

ere

amon

g th

epr

imar

y re

ason

s why

we

felt

that

the

inte

ract

ive-

activ

atio

n ap

proa

chw

ould

be

appr

opria

te fo

r spe

ech

perc

eptio

n as

wel

l as v

isual

wor

d re

c-og

nitio

n an

d re

adin

g,Fo

r the

firs

t sim

ulat

ion,

the

inpu

t to

the

mod

el c

onsi

sted

of a

feat

ure

specification which activated

Ibl

and

Ipl

equa

lly, f

ollo

wed

by

(am

I par

tially

overlapping with) the feature specifications for

IIIth

en

n,

then

Ig

!. Fi

gure

5 sh

ows

phon

eme

and

wor

d-level activations at several points in the

unfo

ldin

g of

this

inpu

t spe

cific

atio

n. E

ach

pane

l of t

he fi

gure

repr

esen

ts2

This

titte

is a

dapJ

ed fr

om th

e til

le o

f a ta

lk b

y D

-dvi

d E,

Rum

elha

rl on

relill

cd p

heno

l1lc

nil

in te

ller p

erce

ptio

n. T

hese

find

ings

are

des

crib

ed in

Rom

elha

rt an

d M

cCle

lland

(198

2/, W

eIh

ank

Dav

e fo

r his

per

mis

sion

to a

dapt

the

title

TRAC

E M

OD

EL

a di

ffere

nt p

oint

in ti

me

durin

g th

e pr

esen

tatio

n an

d co

ncom

itant

pro

-ce

ssin

g of

the

inpu

t. Th

e up

per p

ortio

n of

eac

h pa

nel i

s use

d to

disp

lay

activ

atio

ns a

t the

wor

d le

vel;

the

low

er p

anel

is u

sed

for a

ctiv

atio

ns a

tth

e ph

onem

e le

vel.

Each

uni

t is

repr

esen

ted

by a

rect

angl

e, la

bele

d w

ithth

e id

entit

y of

the

item

the

unit

stand

s for

, The

hor

izon

tal e

xten

sion

ofth

e re

ctan

gle

indi

cate

s the

por

tion

of th

e in

put s

pann

ed b

y th

e un

it. T

heve

rtica

l pos

ition

of t

he re

ctan

gle

indi

cate

s the

deg

ree

of a

ctiv

atio

n of

the

unit.

In th

is an

d su

bseq

uent

figu

res,

activ

atio

ns o

f the

pho

nem

e un

itslo

cate

d be

twee

n th

e pe

aks o

f the

inpu

t spe

cific

atio

ns o

f the

pho

nem

es(at Slices 3, 9, 15. etc,) h

ave

been

del

eted

from

the

disp

lay

for c

larit

y(the activations of these units genera1ly get suppressed by the model,

sinc

e th

e un

its o

n th

e pe

aks

tend

to d

omin

ate

them

), Th

e in

put i

tsel

f is

indi

cate

d be

low

eac

h pa

nel,

with

the

succ

essiv

e ph

onem

es p

ositi

oned

at

the

tem

pora

l pos

ition

s of t

he c

ente

rs o

f the

ir in

put s

peci

ficat

ions

, The

rt along the

axis represents the point in the presentation of the input

stre

am a

t whi

ch th

e sn

apsh

ot w

as t!

lken

,Th

e fig

ure

illust

rate

s th

e gradual' buildup of activation of the tw

o in

-te

rpre

tatio

ns o

f the

firs

t pho

nem

e, fo

llow

ed b

y gr

adua

l bui

ldup

s in

acti-

vatio

n fo

r sub

sequ

ent p

hone

mes

, As t

hese

pro

cess

es u

nfol

d, th

ey b

egin

to p

rodu

ce w

ord-

leve

l act

ivat

ions

, It i

s diff

icul

t to

reso

lve

any

wor

d-le

v~1

activ

atio

ns in

the

first

few

fram

es,.

how

ever

, sin

ce in

these frames, the

info

rmat

ion

at th

e p~

onem

e le

vel s

impl

y ha

s no

t evo

lved

to th

e po

int

whe

re it

pro

vide

s eno

ugh

cons

train

t to

sele

ct a

nyon

e pa

rticu

lar w

ord.

In this case, it is only after the

Igl

has

com

e in

that

the

mod

el h

as in

for-

mat

ion

tellin

g it

whe

~her

the

inpu

t is

clos

er to

"pl

ug," "

plus

," "

blus

h,or "bl

ood"

(TR

ACE'

s le

xico

n co

ntai

ns n

o ot

her w

ords

beginning with

Iprt

or fbr/), After that point, as ill

ustra

ted

in th

e fo

urth

pan

el, "

plug

win

s the

com

petit

ion

at th

e w

ord

leve

l and

, thr

ough

feed

back

supp

ort t

oIp

/, ca

uses

Ip

l to dominate

Ibl

at the ph

onem

e le

vel.

The

mod

el, t

hen,

prov

ides

an

expl

icit

acco

unt f

or th

e w

ay in

whi

ch le

xica

l inf

orm

atio

n ca

nin

fluen

ce p

hone

me

iden

tific

atio

n,Tw

o th

ings

abo

ut th

e le

xica

l effe

ct o

bser

ved

in th

is ca

se a

re w

orth

y of

note

. Firs

t. th

e ef

fect

is ra

ther

sm

all,

Seco

nd, i

t doe

s no

t em

erge

unt

ilw

ell a

fter t

he a

mbi

guou

s seg

men

t itse

lf ha

s com

e an

d go

ne, T

here

is a

slight advantage of

Ipl

over

Ib

l in

Fra

mes

2 a

nd 3

of t

he fi

gure

. In

thes

eca

ses,

how

ever

, the

adv

anta

ge is

not

due

to th

e sp

ecifi

c in

form

atio

n th

atth

is ite

m is

the

wor

d "p

lug

the

mod

el c

an h

ave

no w

ay o

f kno

win

gthis at these points in processing, The slight advantage for

Ipl

at these

early

poi

nts i

s due

to th

e fa

ct th

at th

ere

are

mor

e w

ords

beg

inni

ng w

ithIp

ll than Ibltin the model's

lexi

con,

and

in p

artic

ular

, the

re a

re m

ore

beginning with

Iprl

than

Ib

rt.

So, when the input is

nrd/

, w

ith th

e?standing for the ambiguous

Ib/-/

pl

segm

ent,

the

mod

el m

ust a

ctua

lly o

ver-

com

e th

is sli

ght I

p/-w

ard

bias

, Eve

ntua

lly, i

t doe

s so,

. Fig

ure

6 sh

ows t

he te

mpo

ral c

ours

e of

bui

ldup

of t

he st

reng

th o

f the

MCCLELLAND AND ELMAN

:-.. 1.00

::: :0 0.

!II 6: 0.

1/1 C

0.1/

1 ~ 0.

00

12 18 24 30 38 42 48 54 80 88 72

Processing Cycles

FIG. 6. The time course or the buitdup in the strength or the

Ipl

resp

onse

bas

ed o

n ac

ti-va

tions

or p

hone

me

units

in S

lice

12. i

n proces1iing an ambiguous

Ibl-l

pl

segm

ent i

n '-I

'g/,

and the same segment in

I'SI.

The

ambi

guou

s seg

men

t is i

ndic

ated

by

the

"?,' .

AI~o

show

n is

the

build

up o

r res

pons

e str

engt

h ro

r pro

cess

ing

an un

ambi

guou

~ Ip

l se

gmen

t in

Ipl'g

/. Th

e ve

rtica

l lin

e to

pped

with

"?" indicates the point in time c

orre

spon

ding

to th

ece

nter

or t

he in

itial

segm

ent i

n th

e in

put s

tream

. Suc

cess

ive

verti

cal l

ines

indi

cate

cen

ters

or s

ucce

ssiv

e ph

onem

es.

Ipl

resp

onse

bas

ed o

n ac

tivat

ions

of t

he p

hone

me

units

. in Slice 12 for

two cases in which the initial segment is ambiguous between

Ipl

and

Ib/.

In one case, the ambiguous segment is followed by

Irgl

(as i

n "p

lug

in th

e ot

her,

it is

followed by

IrSI

(as i

n "b

lush

"), G

iven

the

mod

el's

restr

icte

d le

xico

n, w

hich

doe

s not

con

tain

the

~ord

"pl

ush,

" the lexical

effect should lead to eventual dominance of the

Ipl

resp

onse

in th

e fir

stcase, but a suppression of the

Ipl

resp

onse

in th

e se

cond

cas

e, T

he d

if-fe

renc

es b

etw

een

the

cont

exts

do n

ot b

egin

to sh

ow u

p un

til a

fter t

hece

nter

of t

he fi

nal p

hone

me,

whi

ch o

ccur

s at S

lice

30. T

he re

ason

for t

his

is s

impl

y th

at th

e in

form

atio

n is

. not

ava

ilabl

e un

til th

at p

oint

, bec

ause

the

phon

eme

that

sign

als w

hat t

he w

ord

will

be

com

es a

t the

ver

y en

d of

the

wor

d. T

he e

ffect

take

s an

othe

r few

tim

e sl

ices

to b

egin

to in

fluen

cethe activation of the

' initi

al p

hone

me,

bec

ause

it p

erco

lat~

s to

the

first

phon

eme

by w

ay o

f the

feed

back

from

the

wor

d or

wor

ds th

at c

on-

tain

it, .

Elimination of the lexical effect by time pressure,

Fox

(198

2) h

as re

-po

rted

that

the

lexi

cal e

ffect

on

wor

d in

itial

segm

ents

is el

imin

ated

ifsu

bjec

ts a

re g

iven

a d

eadl

ine

to re

spon

d w

ithin

500

ms

of th

e am

bigu

ous

segm

ent.

Thou

gh th

ey c

an c

orre

ctly

iden

tify

unam

bigu

ous

segm

ents

inre

spon

ses

mad

e be

fore

the

dead

line,

thes

e ea

rly re

spon

ses

show

no

sen-

sit

ivity

to th

e le

xica

l sta

tus o

f the

alte

rnat

ives

, Sim

ilar f

indings are also

repo

rted

by F

ox (1

984)

.O

ur m

odel

is c

ompl

etel

y co

nsist

ent w

ith F

oxs

resu

lts, I

ndee

d, w

e ha

ve

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

alre

ady

seen

that

the

activ

atio

ns in

the 1i"ace only begin to reflect the

lexi

cal e

ffect

abo

ut o

ne p

hone

me

or so

afte

r the

pho

nem

e th

at e

stabl

ishes

the

lexi

cal i

dent

ity o

f the

item

, Giv

en th

at th

is se

gmen

t doe

s not

occ

ur,

in F

oxs experiments, until the second or third segment after the ambig-

uous

seg

men

t, th

ere

is n

o w

ay th

at a

lexi

cal e

ffect

cou

ld b

e ob

serv

ed in

early

resp

onse

s.Bu

t wha

t abq

ut th

e fa

ct th

at e

arly

resp

onse

s to

unam

bigu

ous s

egm

ents

can

be a

ccur

ate?

TRA

CE a

ccou

nts f

or th

is to

o. In

Fig

ure

7 w

e sh

ow th

esta

te o

f the

Tra

ce a

t var

ious

diff

eren

t poi

nts a

fter t

he un

ambi

guou

s Ib

l

Ibrgl, Here, the

Ibl

dominates the

Ipl

from

the

earli

est p

oint

. The

ana

l-og

ous

resu

lt is

!obtained, when the stimulus is

Ipl

in Iprgl, and the actio

vatio

n fo

r the

ihiti

al p

hone

me

is qu

ite in

depe

nden

t of w

heth

er o

r not

the

item

is a

wor

d. T

he re

spon

se st

reng

th fo

r the

cas

e w

hen

Iprg

l is p

rese

nttd

in F

ig, 6

sho

ws

that

the

prob

abilit

y of

choo

sing

Ipl

is n

ear u

nity

with

in12 processing cycles, or 300 ms of the i

nitia

, seg

men

t, w

ell b

efor

e th

ede

adlin

e w

ould

be

reac

hed-

and

wel

l bef

ore

wor

d id

entit

y sp

ecify

ing

info

rmat

ion

is I/

.vai

labl

e,Lexictll effec'ts Itlte in tI word.

In th

e m

odel

, lex

ical

effe

cts

on w

ord-

initi

al se

gmen

ts! d

evel

op ra

ther

late

, at l

east

in th

e ca

se w

here

ther

e is

no

cont

ext p

rece

dIng

the

wor

d, O

f cou

rse,

the

exac

t tim

ing

of th

e de

velo

p-m

ent o

f any

lexi

cal e

ffect

wou

ld b

e de

pend

ent u

pon

the

set o

f wor

dsac

tivat

ed b

y th

b sti

mul

us; i

f one

wor

d pr

edom

inat

ed e

arly

on,

a le

xica

lef

fect

cou

ld d

evel

op ra

ther

. ear

lier,

In g

ener

al, t

houg

h, w

ord-

initi

al a

m-

bigu

ities

will

requ

ire ti

me

to re

solv

e on

the

basis

of l

exic

al in

form

atio

n,

How

ever

, whe

n th

e am

bigu

ous s

egm

ent c

omes

late

in th

e w

ord ,

and

the

info

rmat

ion

that

pre

cede

s the

am

bigu

ous s

egm

ent h

as a

lread

y es

tabl

ished

whi

ch o

f the

two

alte

rnat

ives

for t

he a

mbi

guou

s seg

men

t is c

orre

ctTR

ACE

show

s a

lexi

cal e

ffect

that

dev

elop

s as

the

dire

ct p

erce

ptua

lin

form

atio

n re

leva

nt to

the

iden

tity

of th

e ta

rget

seg

men

t is

bein

g pr

o-ce

ssed

. Thi

s ph

enom

enon

is il

luSt

rate

d in

Fig

, 8, w

hich

show

s the

stat

eof

the

1i"a

ceat

sev

eral

poi

nts

in ti

me

rela

tive

to a

n am

bigu

ous

final

seg-

ment tha~ could be a

It I

or a

Idl,

at th

e en

d of

the

cont

ext I

targ

I, W

ithin

the

dura

tion

of a

sing

le p

hone

me

afte

r the

cen

ter o

f the

am

bigu

ous s

eg-

men

t, It!

already has an advantage over

Idl,

We

ther

efor

e pr

edic

t tha

tFo

xs

resu

lts w

ould

com

e ou

t diff

eren

tly, w

ere

he to

use

wor

d-fin

al, a

sop

pose

d to

wor

d-in

itial

, am

bigu

ous s

egm

ents,

In su

ch a

cas

e w

e w

ould

expe

ct th

e le

xica

l effe

ct to

show

up

wel

l with

in th

e 50

0-m

s de

adlin

e,D

epen

denc

'e o

f the

lexi

CtlI

effe

ct o

n ph

onol

ogic

' (11

tlmbi

gllit

y,

One

fur-

ther

asp

ect o

fthe

lexi

cal e

ffect

that

was

not

ed b

y G

anon

g (1

980)

des

erve

sco

mm

ent,

This

is th

e fa

ct th

at th

e le

xica

l effe

ct o

n th

e id

entit

y of

a. p

hone

me

only

occ

urs w

ith se

gmen

ts w

hich

fall

in th

e bo

unda

ry re

gion

b~tw

eent

wo

phon

emes

. For

segm

ents

whi

ch a

re u

nam

bigu

ous e

xam

ples

of o

ne c

ateg

ory

or th

e ot

her,

the

effe

ct is

not

obt

aine

d, T

RACE

is e

ntire

lyco

nsis

tent

with

this

asp

ect o

f the

dat

a, T

he in

fluen

ce o

f the

lexi

con

issim

ply

anot

her s

ourc

e of

evi

denc

e, li

ke th

at c

omin

g fro

m th

e fe

atur

e

- b

- b

Har

ll-lf

Eli . FLIl

8 ff:

Wff:

WiI:

W!il

W

- t- t

a r 1

a I'

a 1"1

I k

klSI

-

I 1

I' k

- f- B .

-tar

-targ

--ta

ra-

- t a

r a -

t -

-bl-a-

-bl-I- -

bl- I- -

bl-

FIG

, 7. T

he st

ate

of th

e Tr

ace

at v

ario

us st

ages

of p

roce

ssin

g th

e str

eam

Ibrg

l.

FIG

. 8. T

he st

ale

of th

e Tr

ace

al se

vera

l sta

ges o

f pro

cess

ing

Ihe

strea

m c

onsis

ling

ofIta

rg' followed by Ii segment ambiguous bel ween

III

and

Id/.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

leve

l, in

flucn

cing

the

activ

atio

n of

one

pho

nem

e un

it or

ano

ther

. Whe

nth

e bo

ttom

-up

inpu

t is d

ecisi

ve, i

t can

pre

empt

any

lexi

cal b

ias e

ffect

s.W

e ha

ve v

erifi

ed th

is in

sim

ulat

ions

pre

sent

ing

unam

bigu

ous t

oken

s of

Ipl

or

fbi,

followed either by Irgl or

IrS/.

In th

ese

sim

ulat

ions

, the

uni

tfo

r the

pre

sent

ed in

itial

segm

ent r

each

es a

ver

y hi

gh le

vel o

f act

ivat

ion,

inde

pend

ent o

f the

follo

win

g co

ntex

t. W

hen

the

segm

ent c

omes

at t

heen

d of

the

wor

d, th

e co

ntex

t exe

rts st

rong

er e

ffect

s, th

us a

ccou

ntin

g fo

rth

e fa

ct th

at s

peec

h di

stor

tions

are

eas

ier t

o de

tect

whe

n th

ey c

ome

early

in a

wor

d th

an w

hen

they

com

e la

te (M

arsl

en-W

ilson

& W

elsh

, 197

8),

How

ever

, eve

n th

ere,

it is

pos

sible

to o

verri

de le

xica

lly b

ased

act

ivat

ions

with

cle

ar b

otto

m-u

p sig

nals,

alth

ough

ther

e m

ay b

e so

me

slow

ing

of th

eac

tivat

ion

proc

ess w

hich

wou

ld p

roba

bly

show

up

in re

actio

n tim

es.

H s

houl

d be

not

ed th

at T

RAC

E's

acco

unt o

f lex

ical

effe

cts

is q

uite

simila

r to

the

acco

unt o

ffere

d by

the

feat

ure

inte

grat

ion

theo

ry o

f Mas

saro

and

Ode

n (1

980a

). In

deed

, Mas

saro

and

Ode

ns

mod

el p

rovi

des

quan

ti-ta

tive

fits t

o G

anon

gs f

indi

ngs.

We

will

mak

e so

me

men

tion

of th

e sli

ght

diffe

renc

es in

qua

ntita

tive

assu

mpt

ions

bet

wee

n th

e m

odel

s bel

ow. F

or. n

ow, w

e no

te a

mo~

e cr

ucia

l diff

eren

ce: T

RACE

inco

rpor

ates

spec

ific

assu

mpt

ions

abo

ut th

e tim

e co

urse

of p

roce

ssin

g w

hich

allo

ws i

t to

ac-

coun

t for

the

cond

ition

s un

der w

hich

lexi

cal e

ffect

s w

ill be

obtained, as

wel

l as f

or th

e in

fluen

ce (o

r a la

ck th

ereo

O o

f lex

ical

effe

cts o

n re

actio

ntimes, to which we now turn,

Ah.

H'IIn' (~(lexicaleffed

ill some reaction-time studies,

Foss

and

Bla

nk(1

980)

pre

sent

ed s

ome

resu

lts w

hich

see

med

to p

ose

a ch

alle

nge

to in

-te

ract

ive

mod

els o

f pho

nem

e id

entif

icat

ion

in sp

eech

per

cept

ion,

The

yga

ve su

bjec

ts th

e ta

~k o

f list

enin

g to

spok

en se

nten

ces f

or o

ccur

renc

esof

a p

artic

ular

pho

nem

e in

wor

d-in

itial

pos

ition

, Rea

ctio

n tim

e to

pre

ssa

resp

onse

key

from

the

onse

t of t

he ta

rget

pho

nem

e w

as th

e de

pend

ent

varia

ble.

In o

ne e

xam

ple,

the

targ

et w

as Ig

I and the sentence was,

At t

heen

d (~

"'a.~t

year, the Rovemment,

, , .

The

subj

ect's

task

was

sim

ply

topr

ess

the

resp

onse

key

upo

n he

arin

g th

e Ig

I at t

he b

egin

ning

of t

he w

ord

ROl'e

rllm

ellt.

. The pr

inci

ple

findi

ng o

f Fos

s and

Bla

nk's

stud

y w

as th

at it

mad

e no

diffe

renc

e w

heth

er th

e ta

rget

cam

e at

the

begi

nnin

g of

a w

ord

or a

non

-word. Later studies by Foss and Gernsbacher (1983) in

dica

te th

at o

ther

expe

rimen

ts w

hich

hav

e fo

und

lexi

cal o

r eve

n se

man

tic a

nd s

ynta

ctic

ntex

t effe

cts

on m

onito

ring

late

ncie

s ar

e fla

wed

, and

that

mon

itorin

gtim

es fo

r wor

d-in

itial

pho

nem

es a

re p

rimar

ily in

fluen

ced

by a

cous

ticfa

ctor

s affe

ctin

g ph

onem

e de

tect

abili

ty, r

athe

r tha

n le

xica

l, se

man

tic, o

rsyntactic fact

ors,

The

conc

lusi

on th

at p

hone

me

mon

itorin

g is

una

ffect

ed b

y th

e le

xica

lstatus of the target-be

arin

g ph

onem

e str

ing

seem

s at v

aria

nce

with

the

spiri

t of t

he T

RACE

mod

el, s

ince

in T

RACE

, the

lexi

cal l

evel

is a

lway

sin

volv

ed in

the

perc

eptu

al p

roce

ss, H

owev

er. w

e ha

ve a

lread

y se

en th

atth

ere

are

cond

ition

s un

der w

hich

the

lexi

cal l

evel

doe

s no

t get

muc

h of

a c~

ance

to e

xert

an e

ffect

, In

the

prev

ious

sec

tion

we

saw

that

ther

e is

no le

xica

l effe

ct o

n id

entif

icat

ion

of a

mbi

guou

s wor

d-in

itial

targ

ets w

hen

the

subj

ect ~

s und

er ti

me

pres

sure

to re

spon

d qu

ickl

y; si

mpl

y be

caus

e th

esu

bjec

t mus

t res

pond

bef

ore

info

rmat

ion

is e

ven

avai

labl

e th

at w

ould

allo

w th

e m

odel

-or a

ny o

ther

mec

hani

sm-to

pro

duce

a le

xica

l effe

ct.

In th

e Fo

ss a

nd B

lank

situ

atio

n, th

ere

is ev

en le

ss re

ason

to e

xpec

t ale

xica

l effe

ct, s

ince

the

targ

et is

not

an

ambi

guou

s se

gmen

t. W

e al

read

ysa

w th

at a

ctiv

atio

n cu

rves

rise

rapi

dly

for u

nam

bigu

ous s

egm

ents;

in th

epresent case, they can reach near-pe

ak le

vels

wel

l bef

ore

the

acou

stic

info

rmat

ion

that

indi

cate

s whe

ther

the

targ

et is

in a

wor

d or

non

wor

d ha

sre

ache

d th

e s~

bjec

t's e

ar.

The'

resu

lts o

f a s

imul

atio

n ru

n illu

stra

ting

thes

e po

ints

are

sho

wn

inFi

g. 9

. For

this

exam

ple,

we

imag

ine

that

the

targ

et is

Itl,

Not

e ho

w d

urin

gth

e in

itial

sylla

ble

of b

oth

strea

ms,

little

act

ivat

ion

at th

e w

ord

leve

l has

been

esta

blish

ed, E

ven

tow

ard

the

end

of th

e str

eam

, whe

re th

e in

for-

. mation is

just

com

ing

in w

hich

det

erm

ines

thai

"tru

gus"

is n

ot a

wor

d,th

ere

is lit

tle d

iffer

ence

, bec

ause

in b

oth

case

s, th

ere

are

seve

ral a

ctiv

ew

ord-

leve

l can

dida

tes,

all

supp

ortin

g th

e w

ord-

initi

al It

/. It

is on

ly a

fter

the

end

of th

e st

ream

that

a re

al c

hanc

e fo

r a d

iffer

ence

has

occ

urre

d, W

ell

befo

re th

is tim

e ar

rives

, the

subj

ect w

ill h

ave

mad

e a

resp

onse

, sin

ce th

estrength of the

It!

resp

onse

reac

hes a

leve

l suf

ficie

nt to

gua

rant

ee a

hig

hac

cura

cy b

y ab

out C

ycle

30,

wel

l bef

ore

the

end

of th

e w

ord,

as

illust

rate

d:in

Fig

, 10,

Ev

en th

ough

act

ivat

ions

are

qui

te ra

pid

for u

nam

bigu

ous s

egm

ents,

thes

e ca

n sti

ll be

influ

ence

d by

lexi

cal e

ffect

s, pr

ovid

ed th

at th

e le

xica

l.in

form

atio

n is

avai

labl

e in

tim

e, In

Fig

, II,

we

illus

trate

this

poin

t for

the

phon

eme

It I i

'n th

e st

ream

s Is

ikrt

l (th

e w

ord

"sec

ret"

) and

Igld

A

guld

ut,"

a n

onw

ord)

, The

figu

re sh

ows t

he st

reng

th o

f the

It I

resp

onse

as a

func

tion

of p

roce

ssin

g cy

cles

, rel

ativ

e to

all

othe

r res

pons

es b

ased

on a

ctiv

atio

ns o

f pho

nem

e un

its a

t Cyc

le 4

2, th

e pe

ak o

f the

inpu

t spe

c-ifi

catio

n fo

r the

It/.

Clea

rly, r

espo

nse

stren

~' h

gro

ws

fast

er fo

r the

It! i

nIs

ikrt

l tha

n fo

r the

It! i

n Ig

ldAt!;

pic

king

an

arbi

trary

thre

shol

d of

.9 fo

rresponse initiation, we find that the

It!

in Is

ikrt

l rea

ches

crit

erio

n ab

out

3 cy

cles

or 7

5 m

s so

oner

than

the

It I i

n Ig

ld"t

/.Studies showing lexical effects in reaction times.

Mar

slen-

Wils

on(1

980)

has

repo

rted

an e

xper

imen

t tha

t dem

onst

rate

s th

e ex

iste

nce

ofle

xica

l effe

cts i

n ph

onem

e m

onito

ring

for p

hone

mes

com

ing

at la

ter p

oi~t

sin

wor

ds. F

or p

hone

mes

com

ing

at th

e be

ginn

ing

of a

wor

d or

at t

he e

ndof

the

first

sylla

ble,

he

foun

d no

faci

litat

ion

for p

hone

mes

in w

ords

rel-

TRAC

E M

OD

EL

IN.l*

,,,~

U

-1,1

II I

;.., 1.

:=:

:.0 0.

111

!:

40-

Q.. ~ 0.

MC

CLE

LLAN

D A

ND

ELM

AN

00

12 1

6 24

30

36 4

2 48

54

Processing Cycles

FIG, 10. Time course

of growlh in Ihe probabililY

of

Ihe

III

resp

onse

bas

ed o

n ilc

livill

illns

of

phoneme unils in Slice 12, during processing

of

Ilarg

ll ilnd II

(gs/.

Th

e v.

:rlic

al li

n.:s

indi

cale

the

peak

s on

Ihe

feal

ure

paue

rns

corre

spon

ding

10

!he

succ

essiv

.: ph

on.:m

.:s o

f Ihe

pres

eRle

d w

ord.

-Lar

l-L-

-Lar

. I-L-

-Lar

l-L-

ms

adva

ntag

e co

mpa

red

to c

orre

spon

ding

pos

ition

s in

non

words. This

com

pare

s qui

te c

lose

ly w

ith th

e va

lue

of a

bout

75

ms w

e ob

tain

ed fo

r the

Isik

r t/-

ld'tl

exa

mpl

e. A

t the

end

s of e

ven

long

er w

ords

, the

wor

d ad

-va

ntag

e in

crea

sed

in si

ze to

185 ms. Marslen-

Wils

ons

resu

lt Ih

us c

on-

firm

s tha

t the

re a

re' in

deed

lexi

cal e

ffect

s in

pho

nem

e m

onilo

ring-

even

for unambiguous inputs-

but u

nder

scor

es th

e ta

ct th

at th

ere

is n

o w

ord

adva

ntag

e fo

r pho

nem

es w

hose

pro

cess

ing

can

be c

ompl

eted

long

bef

ore

lexi

cal i

nflu

ence

s wou

ld h

a\:,e

a c

hanc

e to

show

up,

_d__

000 _

LJ..J

L1

12 16 24

30

36 4

2 48

54 60 66 72

Processing Cycles

FIG. t t. ProbabililY

of

the

It I response as a funclion

of

proc

essi

ng c

ycle

s. b

ased

on

acli-

vatio

n of

pho

nem

e un

its a

t Cyc

l.:A

2. fo

r the

stre

am Is

ik..

.1 ("

secr

e., and

Id'

guld

ut" )

. Ver

tical

line

s in

dica

te th

e pe

aks

of th

e in

put p

atte

rns

corre

spon

ding

10

Ihe

succ

essiv

e ph

onem

es in

eith

er sl

ream

.

IIIm

m

B I

100

.!:.... ~ 0.

III 1.0

.... en

0.

III g 0.

III III02

0

-Lr-

s- -Lr-a-

s- -L

r-a-

e-FIG. 9. Stale of Ihe ltace al three di

ffere

nt p

ainl

s dur

ing

the

proc

essin

g of

the

wor

dta

rgel

(/tar

gt/!

an

d th

e no

nwor

d "t

rugu

s " (/

trsf

),

ativ

e to

pho

nem

es in

non

wor

ds (i

n fa

ct th

ere

was

a n

on w

ord

adva

ntag

efo

r the

se e

arly

targ

et c

ondi

tions

). Fo

r tar

gets

occu

rring

at t

he e

nd o

f the

seco

nd sy

llabl

e of

a tw

o-sy

llabl

e w

ord

(like

"se

cret

thou

gh th

e st

imul

iin

this

par

ticul

ar e

xper

imen

t wer

e D

utch

) Mar

slen

- Wilson found an 85-

- -- ----

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

The

TRA

CE m

odel

and

Mar

slen-

Wils

ons C

OH

ORT

mod

el (M

arsle

n-W

ilson

& T

yler

, 198

0; M

arsle

n-W

ilson

& W

elsh

, 1978) offer fairly similar

inte

rpre

tatio

ns o

f lex

ical

effe

cts i

n ph

onem

e m

onito

ring,

Bot

h m

odel

sac

coun

t for

the

grow

th in

the

effe

ct a

s a

func

tion

of p

ositi

on in

the

wor

d,A

s in

COH

ORT

, lex

ieal

effe

cts i

n TR

ACE

dep

end

on th

e po

int a

t whi

chth

e pa

ttern

of a

ctiv

atio

n at

the

wor

d le

vel b

egin

s to

spec

ify th

e id

entit

ies

of the ph

onem

es, I

nC

OH

OR

T, th

ere

is a

dis

cret

e m

omen

t whe

n th

isoc

curs

-whe

n th

e co

hort

of it

ems c

onsis

tent

with

the

inpu

t is r

educ

edto a single item. In T~R

ACE,

thin

gs a

re n

ot q

uite

so

disc

rete

, How

ever

,it

will

still

gen

eral

ly b

e th

e ca

se in

TR

ACE

that

the

size

of t

he le

xica

lef

fect

will

var

y w

ith th

e lo

catio

n of

the

"uni

que

poin

t," th

e po

int a

t whi

chth

e bo

ttom

-up

inpu

t rem

ains

con

siste

nt w

ith o

nly

a si

ngle

wor

d, H

ow-

ever, since Marslen-

Wils

ons

expe

rimen

ts w

ere

perfo

rmed

with

Dut

chw

ords

, we

have

not

bee

n ab

le to

sim

ulat

e hi

s exp

erim

enta

l dem

onstr

atio

nof

this

effe

ct in

det

ail.

TRA

CE a

nd C

OH

ORT

mak

e sim

ilar p

redi

ctio

ns in

som

e sit

uatio

ns,

but n

ot in

all.

In th

e ne

xt se

ctio

n, w

e co

nsid

er a

phe

nom

enon

whi

chTR

ACE

acc

ount

s for

,via

the

sam

e m

echa

nism

s it u

ses t

o ac

coun

t for

the

lexi

cal e

ffect

s we

have

bee

n co

nsid

erin

g. H

ere,

the

grad

ed fe

edba

ck fr

omth

e w

ord

leve

l to

the,

pho

nem

e le

vel a

llow

s TRA

CE to

acc

ount

for a

nef

fect

that

wou

ld n

ot b

e pr

edic

ted

by C

OH

ORT

, unl

ess a

dditi

onal

as-

sum

ptio

ns w

ere

mad

e,

Are

PlrO

llOto

ctic

Rille

Ejfe

ct.f

the Res"lt of a Co1lspiracy?

Rec

ently

, Mas

saro

;;tn

d C

ohen

(198

3) h

ave

repo

rted

evid

ence

they

take

as s

uppo

rt fo

r the

use

of p

hono

tact

ic ru

les

in p

hone

me

iden

tific

atio

n. In

one

expe

rimen

t, M

assa

ro a

nd C

ohen

s st

imul

i con

sist

ed o

f pho

nolo

gica

lsegments ambiguous between

Irl

and

III

in d

iffer

ent c

onte

xts.

In o

ne c

on-

text (fLi/)

Irl

is permissible in English, but

III

is no

t. In

ano

ther

con

text

(/5-i/) /11

is permissible in English but

Irl

is not. In a third context

(/Li/)

both are permissible, and in a fourth

(/v_ i

/) ne

ither

is p

erm

issi

ble.

Mas

saro

and

Coh

en fo

und

a bi

as to

per

ceiv

e ambiguous segments as

Irl

whe

n Irl

was permissible or as

III

whe

n 11

/ w

as p

erm

issib

le. N

o bi

as a

ppea

red

inei

ther

of t

he o

ther

two

cond

ition

s.W

ith m

ost o

f the

se st

imul

i, ph

onot

actic

acc

epta

bilit

y is

conf

ound

edwith the actual lexica1 status of the item; thus

/nil

and

ffril

flee"

and

free

) are both words, as is

Itril

bul not

Illil,

In the

Is_il

co

ntex

t, ho

w-

ever. neither Islil or

Isril

ar

e w

ords

, yet

Mas

saro

and

Coh

en fo

und

a bi

asto hear the ambiguous segment as

III,

in accordance with phonotactic

rule

s. It tu

rns o

ut th

at T

RACE

pro

duce

s the

sam

e ef

fect

, eve

n th

ough

it la

cks

phon

otac

tlc ru

les,

The

reas

on is

that

the

ambi

guou

s stim

ulus

pro

duce

s

parti

al a

ctiv

atio

ns o

f a n

umbe

r of w

ords

('~s

leep" and "sleet" in the

mod

el' s

lexi

con;

it w

ould

also

act

ivat

e "s

leev

e,

" "

sleek

,an

d ot

hers

ina

mod

el w

ith a

fulle

r lex

icon

). N

one

of th

ese

wor

d un

its g

ets a

s act

ive

as it

wou

ld if

the

entir

e w

ord

had

been

pre

sent

ed, H

owev

er, a

ll of

them

(in th

e sim

ulat

ion,

ther

e ar

e on

y tw

o, b

ut th

e pr

inci

ple

still

appl

ies)

are

parti

ally

act

ivat

ed, a

nd a

ll co

nspi

re to

geth

er a

nd c

ontri

bute

to th

e ac

ti-vation of

III.

This feedback support for the

III

allo

ws i

t to

dom

inat

e th

eIrl

, just as it would if

Islil

w

ere

an a

c~ua

l wor

d, a

s sho

wn

in F

ig, 1

2,Th

e hy

poth

esis

that

pho

nota

ctic

rule

effe

cts a

re re

ally

bas

ed o

n w

ord

activ

atio

ns le

ads t

o a

pred

ictio

n: th

at w

e sh

ould

be

able

to re

vers

e th

ese

effe

cts i

f we

pres

ent i

tem

s tha

t are

supp

orte

d str

ongl

y by

one

or m

ore

lexi

cal i

tem

s eve

n if

they

vio

late

pho

nota

ctic

rule

s, A

rece

nt e

xper

imen

tby

Elm

an (1

983)

con

firm

s thi

s pre

dict

ion,

In th

is ex

perim

ent,

ambi

guou

sphonemes (for example, halfway between

Ibl

and

Id/)

wer

e pr

esen

ted

inth

ree

diffe

rent

type

s of

con

text

s, In

all

thre

e ty

pes,

one

of t

he tw

o (in

this

case, the

Id/)

was phonotactically acceptable, while the other (the

Ib/)

was

not

. How

ever

, the

con

text

s di

ffere

d in

thei

l' re

latio

n to

wor

ds. I

n on

eca

se, t

he le

gal i

tem

act

ually

occ

urre

d in

a w

ord

("bw

indl

edw

indl

eIn

a s

econ

d ca

se, n

eith

er it

em m

ade

a w

ord,

but

the

illega

l ite

m w

as v

ery

cI()s

e to

a w

ord

("bw

acel

et"

dwac

elet

"). I

n a

third

cas

e, n

eith

er it

em

EiIT

llJIIT

rntil

ltil

l

9tIJ

JE!

l. 1

;..1

- 5

S J

1- -51- -81- -5

FIG

. 12.

Sta

te (I

f the

1i'a

ce a

t sev

eral

poi

nts i

n pr

oces

sing

a se

gmen

t am

bigu

ous b

etw

een

111

and

Irl,

in the context

Is_i

/. Th

e un

its fo

r "sle

ep" I

Islip

/) an

d "s

leet

" (/sl

it/) a

re b

oxed

toge

ther

sinc

e th

ey ta

ke o

n id

entic

al a

ctiv

atio

n va

lues

.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

was

par

ticul

arly

clo

se to

a w

ord

("bw

ime

dwim

eRe

sults

of

th

eex

perim

ent a

re s

how

n in

Thble 4. The existence

of

a w

ord

iden

tical

toon

e of

the two alternatives or differing from one

of

the

alte

rnat

ives

by

asingle phonetic feature

of

one

phon

eme

stro

ngly

influ

ence

d th

e su

bjec

t'ch

oice

s bet

wee

n th

e tw

o al

tern

ativ

es, I

ndee

d, in

the

case

whe

re th

e ph

o-notactically irregular alternative ("bw

acel

et) w

as o

ne fe

atur

e aw

ay fr

oma

parti

cula

r lex

ical

item

("br

acel

et" )

, sub

ject

s te

nded

to h

ear t

he a

mbi

g-uo

us it

em in

acc

ord

with

the

sim

ilar l

exic

al it

em (t

hat i

s, a

s a

Ibl)

even

thou

gh it

was

pho

nota

ctic

ally

inco

rrect

.. To determine whether the model w

ould

also

pro

duce

such

are

vers

alof

the phonotactic rule effects with the appropriate kinds

of

stim

uli,

ran a simulation using a simulated inputambiguous between

Ipl

and

It I i

nthe context I-Iuli/,

Ipl

is ph

onot

actic

ally

acc

epta

ble

in th

is co

ntex

t, bu

tIt

I in

this

con

text

mak

es a

n ite

m th

at is

ver

y cl

ose

to th

e w

ord

" tru

ly,

The results

of

this

run,

at t

wo

diffe

rent

poi

nts d

urin

g pr

oces

sing,

are

show

n in

Fig

. 13,

Ear

ly o

n in

pro

cess

ing,

ther

e is

a s

light

bia

s in

favo

rof

th

e Ip

l over the

Itl,

because at first a large number

of

Ipll

wor

ds a

reslightly more activated than any words beginning with

Itl,

Late

r, th

ough

,th

e It

I get

s th

e up

per h

and

as th

e w

ord

" tru

ly" c

omes

to d

omin

ate

at th

eword level. Thus, by the end

of

the

wor

d or

shor

tly th

erea

fter,

the

clos

est

wor

d ha

s be

gun

to p

laya

dom

inat

ing

role

, cau

sing

the

mod

el to

pre

fer

the phonotactically inappropriate interpretation

of

the

ambi

guou

s in

itial

segm

ent.

Of c

ours

e, a

t the

sam

e tim

e th

e w

ord

" tru

ly" tends to support

Irl

rath

erth

an

III for the second segment.. Thus, even. though th

is se

gmen

t is n

otambiguous, and the III would suppress the

Irl

inte

rpre

tatio

n in

a m

ore

neutral context, the

Irl

stay

s qu

ite a

ctiv

e,

Trad

ing

Rel

atio

ns a

nd C

ateg

oriL

'al P

erc'

eptio

nIn

the

sim

ulat

ions

con

side

red

thus

far,

phon

eme

iden

tific

atio

~ is

intlu

-enced by two different kinds

of

fact

ors,

feat

ural

and

lexi

cal.

Whe

n on

eso

rt of

in

form

atio

n is

lack

in~,

the

othe

r can

com

pens

ate

for i

t. Th

e im

age

tha, emerges from these kinds

of

findings is

of

a sy

stem

that

exh

ibits

grea

t fle

xibi

lity

by b

eing

abl

e to

bas

e id

entif

icat

ion'

dec

isio

ns o

n di

ffere

ntso

urce

s of

information, It is.

of course, well established il1

at w

ithin

the

feat

ura

l dom

ain

each

pho

nem

e is

generally signaled by a number

of

dif-

ferent cues, and that human subjects can trade these cues

off

agai

nst e

ach

othe

r, Th

e TR

ACE

mod

el e

xhib

its th

i-s sa

me

flexi

bilit

y, a

s we

deta

ilsh

ortly

.But there is something

of

a pa

rado

x, W

hile

the

p(:rc

eptu

al m

echa

nism

sex

hibi

t gre

at fl

exib

ility

in th

e cu

es th

at th

ey re

ly o

n fo

r pho

nem

e id

enti-

ficat

ion,

they

also

app

ear t

o be

qui

te "

cate

goric

al" i

n na

ture

. Tha

t is

they

pro

duce

muc

h sh

arpe

r bou

ndar

ies b

etw

een

phon

etic

cat

egor

ies t

han

we

mig

ht e

xpec

t bas

ed o

n th

eir s

ensit

ivity

to m

ultip

le cu

es; and they

appe

ar to

trea

t aco

ustic

ally

dis

tinct

feat

ure patterns as perceptually

equi

vale

nt, a

s lo

ng a

s th

ey a

re id

entif

ied

as ins

tanc

es

of

the

sam

e ph

o-ne

me. In th

is se

ctio

n, w

e ill

ustra

te th

at in

TRA

CE, j

ust a

s in

hum

an sp

eech

. per

cept

ion,

flex

ibili

ty in

feat

ure

inte

rpre

tatio

n-sp

ecifi

cally

, the

abi

lity

to trade one feature

of

a ph

onem

e of

f aga

inst

ano

ther

-coe

xist

s w

ith a

stron

g te

nden

cy to

war

d ca

tego

rical

per

cept

ion.

For t

hese

sim

ulat

ions

, the

mod

el w

as st

rippe

d do

wn

to th

e es

sent

ial

min

imum

nec

essa

ry, s

o th

at th

e ba

sic

mec

hani

sms

prod

ucin

g cu

e tra

dc-

n:::u

u:n

JIT!

iI:TI

!a:it

Lnu:

:r::il

l

fillJ

8iam

u I

I U I

i -f I

I I

P r

a r i

P r

,,

TABL

E 4

Perc

enla

ge C

hoic

e of

Pho

nola

clic

ally

Irre

gula

r Con

sona

nt

Stim

ulus

type

Lega

l wor

d/ill

egal

non

wor

d Le

gal n

on w

ord/

illeg

al n

onw

ord

Lega

l non

wor

d/ill

egal

nea

rwor

d

Exam

ple

dwin

dle/

bwin

dle

dwim

e/bw

ime

dwac

elel

/bw

acel

el

Perc

enta

ge o

f ide

nlifi

clili

ons

as "

illeg

al" p

hone

me

F(2,34) = 26.414, p

c:: .

001.

luli- _

lull- _

luli- _

luli-

FIG. 13. Slate of the Trace al several points in processing an ambiguous

Ipl- /

11 se

gmen

lfollowed by Ilulil

TRAC

E M

OD

EL. M

C C

LELL

AND

AN

D E

LMAN

100

150 300

seve

ral c

ateg

o~ic

al p

erce

ptio

n stu

dies

of V

OT

cont

inua

(usin

g Ig

/-/k/

.Id

/-/tl.

or

Ib

/-/pl

sti

mul

i) ha

ve c

ovar

ied

both

VO

T an

d FI

OF.

if o

nly

beca

use

FIO

F te

nds t

o co

vary

with

VO

T W

hl.ll

real

istic

stim

uli a

re u

sed

(e.g

.. Pi

soni

& L

azar

us. 1

974: Samuel. 1977), Though the simulations us

eIg

/-/kl

co

ntin

uum

. we

cons

ider

seve

ral c

ateg

oric

al p

erce

ptio

n ex

peri-

ments using

Id/-/

tl an

d Ib

l-lpl

co

ntin

ua. s

ince

the

sam

e di

men

sions

can

diffe

rent

iate

the

two

mem

bers

of b

oth

of th

ese

othe

r pai

rs, W

e al

so c

on-

sider

dat

a ob

tain

ed in

exp

erim

ents

on o

ther

con

tinua

. usin

g ot

her c

ues,

We

coul

d ea

sily

have

repe

ated

the

simul

atio

ns w

ith o

ther

sets

of c

ontin

ua; .

how

ever

. the

gen

eral

qua

litat

ive

form

of t

he re

sults

wou

ld b

e th

e sa

me,

Wha

t wou

ld v

ary

from

cas

e to

cas

e w

ould

be

the

mag

nitu

de o

f the

effe

ctof

a s

tep

alon

g a

give

n di

men

sion

,Th

e pa

ttern

of e

xcita

tory

inpu

t to

the

VO

T an

d FI

OF

dete

ctor

s pro

-duced by the canonical mock speech

Igl

and

Ikl

used

in th

e si

mul

atio

nsar

e illu

stra

ted

in F

ig. 1

5,

Trad;"g relat;OIu,

TRAC

E qu

ite n

atur

ally

tend

s to

pro

duce

trad

ing

rela

tions

bet

wee

n fe

atur

es. s

ince

it re

lies o

n th

e w

eigh

ted

sum

of t

heex

cita

tory

inpu

ts to

det

erm

ine

how

stro

ngly

the

inpu

t will

act

ivat

e a

par-

ticul

ar p

hone

me

unit.

All

else

bei

ng e

qual

. the

pho

nem

e un

it re

ceiv

ing

the

larg

est s

um b

otto

m-u

p ex

cita

tion

will

be

mor

e str

ongl

y ac

tivat

ed th

anan

y ot

her,

and

will

ther

efor

e be

the

mos

t lik

ely

resp

onse

whe

n a

choi

cem

ust b

e m

ade

betw

een

one

phon

eme

and

anot

her,

Sinc

e th

e ne

t bot

tom

-up input is just the sum of all of the inputs, no one input is necessarily

deci

sive

in th

is re

gard

,G

ener

ally

, exp

erim

ents

dem

onstr

atin

g tra

ding

rela

tions

bet

wee

n tw

o or

.m

ore

cues

man

ipul

ate

each

of t

he c

ues

over

a n

umbe

r of v

alue

s ra

ngin

gbe

twee

n a

valu

e m

ore

typi

cal o

f one

of t

wo

phon

emes

and

a v

alue

mor

ety

pica

l of t

he o

ther

, Sum

mer

field

and

Hag

gard

did

this

for V

OT

and

FIO

F, a

nd fo

und

the

typi

cal r

esul

t. na

mel

y th

at th

e va

lue

of o

ne c

ue th

atgives rise to 50% choices of

Ikl

was

affe

cted

by'

the

valu

e of

the

othe

rcu

e: th

e hi

gher

the

valu

e of

FIO

F, th

e sh

orte

r the

val

ue o

f VQ

T ne

eded

.fo

r 50%

cho

ices

of I

k/. U

nfor

tuna

tely

; the

y di

d no

t pre

sent

full

curv

esre

latin

g ph

onem

e id

entif

icat

ion

to th

e va

lues

use

d on

eac

h of

the

two

dim

ensio

ns, I

n lie

u of

this,

we

pres

ent c

urve

s in

Fig,

16

from

a c

lass

ictra

ding

rela

tions

exp

erim

ent,

by D

enes

(195

5), S

imila

r pat

tern

s of

resu

ltsha

ve b

een

repo

rted

in o

ther

stud

ies,

usin

g ot

her c

ues (

e,g" Massaro.

1981

, Fig

s. 4

and

5), t

houg

h th

e tra

nsiti

ons a

re o

ften

som

ewha

t ste

eper

(see

bel

ow fo

r a d

iscus

sion

of th

e is~

ue o

f ste

epne

ss).

We

have

cho

sen

to p

rese

nt th

e sh

allo

wer

cur

ves r

epor

ted

by D

enes

bec

ause

in th

em w

ese

e cl

early

.that

ther

e ar

e ca

ses i

n w

hich

a c

ue th

at fa

vors

one

of t

he tw

oph

onem

es to

a m

oder

ate

degr

ee w

ill g

ive

rise

to th

e pe

rcep

tion

of th

eot

her p

hone

me

whe

n pa

ired

up w

ith a

stro

ng c

ue th

at fa

vors

the

othe

r

offs

and

cat

egod

cal p

erce

ptio

n co

uld

be b

roug

ht to

the

fore

. The

wor

dle

vel w

as e

limin

ated

alto

geth

er. a

nd a

t the

rhon

emel

evel

ther

e w

ere

only

three phonemes.

tal. Ig/.

and

Ik/,

plus silence

(I-I).

Fr

om th

ese

four

item

s.inputs and percepts of the form

ga-

and

ka-

coul

d be

con

struc

ted.

. The

follo

win

g ad

ditio

nal c

onst

rain

ts w

ere

impo

sed

on th

e fe

atur

e sp

eci-

fications of each of the phonemes: (I) the

Ial

and

I-I had no overlap with

eith

er

Igl

or

Ik/,

so that neither

Ial

nor

I-I

wou

ld b

ias t

he a

ctiv

atio

ns o

fth

e Ig

l an

d Ik

l ph

onem

e un

its w

here

they

ove

rlapp

ed w

ith th

e co

nson

ant:

(2)

Igl

and

Ikl

wer

e id

entic

al o

n fiv

e of

the

seve

n di

men

sions

. and

diff

ered

only

on

the

rem

aini

ng tw

o di

men

sion

s,

The two dimensjons which differentiated

Igl

and

Ikl

wer

e vo

ice

onse

ttim

e (V

OT)

and

the

()nse

t fre

quen

cy o

f the

fitst

form

ant (

FIO

F), T

hese

dim

ensi

ons

repl

aced

the

voic

ing

and

burs

t am

plitu

de d

imen

sion

s us

ed in

all o

f the

oth

er si

mul

atio

ns, F

igur

e 14

illu

strat

es h

ow F

IOF

tend

s to

incr

ease

as

voic

e on

set t

ime

is d

elay

ed.

Sum

mer

field

and

Hag

gard

(197

7) h

ave

show

n th

at su

bjec

ts ar

e se

nsiti

vebo

th to

VaT

and

to F

IOF

and

that

it is

pos

sible

to tr

ade

one

of th

ese

cues off against the other. Thus. the boundary between

Igal

an

d Ik

al

shift

sto

long

er V

OTs

whe

n FI

star

ts of

f low

er ra

ther

than

hig

her,

Cat

egor

ical

per

cept

ion

and

tradi

ng re

latio

ns a

mon

g cu

es h

ave

been

stud

ied

on a

var

iety

of d

iffer

ent c

ontin

ua b

y a

varie

ty o

f diff

eren

t inv

es-

tigat

ors,

We

have

chi

osen

to fo

cus o

n th

e V

OT

and

FIO

F fe

atur

es. a

sexemplified by the

Igal

-/kal

co

ntin

uum

. bec

ause

ther

e is

data

on

trade

-of

fs b

etw

een

thes

e cu

es (S

umm

erfie

ld &

Hag

gard

, 197

7). a

nd b

ecau

se

2000

1500

i'510

00IiI

. 0: lL 500

TIM

E

FlU

, 14,

Sch

emat

ic d

iag~

am of a syllable that will be heard as

Igal

or

Ik

al,

depe

ndin

g on

the

poin

t in

the

sylla

ble

at w

hich

voi

cing

beg

ins,

Prio

r to

the

onse

t of v

oici

ng. F

2 '(lO

Pcu

rvel

is e

nerg

ized

by

aper

iodi

c no

ise so

urce

s. an

d FI

is "

cut b

ack"

(the

noi

se so

urce

has

liUle

or n

o en

ergy

in th

is ra

ngel

. Bec

ause

of t

he fa

ct th

at F

t ris

es o

ver t

ime

afte

r syl

labl

eon

set (

as th

e vo

cattr

uct m

oves

from

a sh

ape

cons

isten

t with

the

cons

onan

t int

o a.

sha

peco

nsis

tent

with

...he

vow

ell.

its fr

eque

ncy

at th

e on

set o

f voi

cing

is h

ighe

r for

late

r val

ues

of V

OT.

Par

amet

ers u

sed

in c

onstr

uctin

g th

is sc

hem

atic

sylla

ble

are

deriv

ed fr

om K

ewle

y-!'o

rt's

(198

21 a

naly

sis

of th

e pa

ram

eter

s of

form

ants

. in

natu

ral s

peec

h. a

nd a

re s

imila

r to

thos

e us

ed in

man

y pe

rcep

tual

exp

erim

ents

,

....

..5 0.

:5 0,

I.. U) 0

.

....

..5 075

:5 0.

till

s.. en 025

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

Ioo

~80

I!i 4

9

, ~20

r 0

III 12

345 Ikl

I .

Voic

e O

nset

Tim

e10

0 20

0FR

ICA

TIO

N D

URA

TIO

N (M

SEC)

FIG

. .16

. Res

ulls

of a

n ex

perim

enl d

emon

slra

ling

Ihe

Irade

-off

belw

een

IWO

cue

s 10

Ihe

identity of

Isl

and

Iii.

Dala from Denes, 1955

, fill

ed b

y th

e m

odel

of M

assa

ro a

nd C

ohen

,1977. e. so ms; 0, 100 ms; 8, ISO ms; A

, 200

ms,

Repr

inle

d w

ilh p

erm

issio

n fro

m M

assa

roan

d C

ohen

(197

7).

1.00

1&1

1234

5 Ik

l

trade

-off

curv

es a

s ha

ve b

een

gene

rally

repo

rted,

we

gene

rate

d a

set o

f25

inte

rmed

iate

pho

netic

segm

ents

mad

e up

by

pairi

ng e

ach

of fi

ve d

if-fe

rent

inte

rmed

iate

pat

tern

s on

the

VO

T di

men

sion

with

eac

h of

live

diffe

rent

inte

rmed

iate

pat

tern

s on

the

FIO

F di

men

sion

. The

dilf

cren

tfe

atur

e pa

ttern

s use

d on

eac

h di

men

sion

are

show

n in

Fig

. 15,

alo

ng w

iththe canonical feature patterns for

IgJ

and

Ikl

on e

ach

of th

e Iw

o di

men

-si

ons,

On

the

rem

aini

ng fi

ve d

imen

sions

, the

inte

rmed

iate

segm

ents

all

had the common canonical feature values for

Ig/

and

Ik/.

The

mod

el w

as te

sted

with

eac

h of

the

25 st

imul

i. pr

eced

ed b

y sil

ence

(I-I)

and followed by

la-I.

In th

is an

d al

l sub

sequ

ent s

imul

atio

ns w

e re

port

in th

is pa

per.

the

peak

of t

he in

itial

sile

nce

phon

eme

occu

rred

at T

ime

Slic

e 6

in th

e in

put.

and

the

peak

s of s

ucce

ssiv

e ph

onem

e se

gmen

ts o

c-curred at six sli

ce in

terv

als.

Thus

. for

thes

e sti

mul

i. th

e pe

ak o

n th

ein

term

edia

te p

hone

tic se

gmen

t occ

urre

d at

Slic

e 12

. the

pea

k of

the

fol-

low

ing

vow

el o

ccur

red

at S

lice

18. a

nd ~

he p

eak

of th

e fin

al si

lenc

e oc

-cu

rred

at S

lice

24. F

or e

ach

inpu

t 'pr

esen

ted.

the

inte

ract

ive

activ

atio

npr

oces

s w

as a

llow

ed to

con

tinue

thro

ugh

a to

tal o

f 60

time

slic

es. w

ell

past

the

end

of th

e in

put.

The

stat

e of

the

Trac

e at

var

ious

poi

nts

inpr

oces

sing.

for t

he m

ost/g

/~Iik

e of

the

25 st

imul

i, is

show

n in

Fig

. 17.

At

the

end

of th

e 60

th ti

me

slice

. we

reco

rded

the

activ

atio

n of

the

units

for

Ig/

and

Ikl

in T

ime

Slic

e 12

and

the .

probability of choosing

Igl

base

d on

thes

e ac

tivat

ions

, (It

mak

es n

o di

ffere

nce

to th

e qu

alita

tive

appe

aran

ceof

the

resu

lts if

a d

iffer

ent d

ecis

ion

time

is us

ed; e

arlie

r dec

ision

tim

esar

e as

soci

ated

with

smal

ler d

iffer

ence

s in

rela

tive

activ

atio

n be

twee

n th

eIg

I an

d Ik

l ph

onem

e un

its. a

nd la

ter o

nes w

ith la

rger

dift

eren

ces.

but t

hege

nera

l pat

tern

is th

e sa

me,

Ft Onset Frequency

FIG

. 15.

Can

onic

al fe

atur

e-level inpul for Ig! and I

kI,

on Ih

elw

o di

men

sions

Ihal

disl

ingu

ish .

them

. and

Ihe

palle

rns

used

for t

he li

ve in

lerm

edia

le v

alue

s us

ed in

the

Iradi

ng re

lalio

nssi

mul

atio

n, A

long

Ihe

absc

issa

of e

ach

dim

ensi

on Ih

e ni

ne u

nils

for t

he n

ine

dil1

eren

l val

uerdnges of the dimension are armyed. The curves labeled Ig! and

Ikl

jndi

cale

Ihe

rela

live

slren

glh

of th

e ex

cita

tory

inpu

llo e

ach

i,)f I

hese

uni

ls, p

rodu

ced

by Ih

e in

dica

led

phon

eme.

The

cano

nica

l cur

ves a

lso in

dica

le Ih

e slr

engl

hs o

f the

feat

ure-

la- p

hone

me

conn

eclio

ns fo

rIg

!. an

d Ik

l on

Ihes

e di

men

sions

. Tha

i is.

Ihe

cano

nica

l inp

ut p

alle

rn fo

r eac

h ph

onem

eex

aclly

mal

ches

Ihe

slren

glhs

of I

he c

orre

spon

ding

feat

ure-

phon

eme

conn

eclio

ns. N

um-

bere

d cu

rves

on

each

dim

ensi

on s

how

Ihe

feat

ure

palle

rns

used

in Ih

e Ira

din.

: rel

alio

nssim

ulal

ion.

phon

eme,

An

addi

tiona

l fin

ding

is th

e bo

win

g of

the

curv

es; t

hey

tend

tobe

app

roxi

mat

ely

linea

r thr

ough

the

mid

dle

of th

eir r

ange

, but

to le

vel

off a

t bot

h en

ds. w

here

' the

val

ues

on b

oth

dim

ensi

ons

agre

e in

poi

ntin

gto

one

alte

rnat

ive

or th

e ot

her,

To se

e if

TRA

CE w

ould

sim

ulat

e th

e ba

sic tr

ade-

off e

ffect

obt

aine

d by

Sum

mer

field

and

Hag

gard

. and

to se

e if

it w

ould

prd

duce

the

sam

e sh

ape

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

~ 1,

00"-

060

-..c

Ik/-

like

valu

es o

n bo

th d

imen

sion

s, In

term

s of

Sum

mer

field

and Hag-

gard

's m

easu

re. t

he v

alue

of V

aT n

eede

d to

ach

ieve

50%

pro

babi

lity

ofre

porti

ng Ik

/. w

e ca

n se

e th

at th

e V

aT n

eede

d in

crea

ses a

s the

FIO

Fde

crea

ses,

just

as th

ese

inve

stiga

tors

foun

d.

Cue

trade

-offs

in p

hone

me

iden

tific

atio

n ar

e ac

coun

ted

for i

n de

tail

byth

e fe

atur

e in

tegr

atio

n m

odel

ofO

den

and

Mas

saro

(197

8; M

assa

ro. 1

981;

Mas

sarQ

and

Ode

n. I

980a

. 198

0b);

Whi

le w

e ha

ve sh

own

how

TRA

CEca

n ac

coun

t for

the

basic

trad

e-of

f effe

ct a

nd th

e ge

nera

l for

m o

f the

trade

-off

curv

es. w

e ha

ve n

ot y

et a

ttem

pted

the

kind

s of

det

aile

d fit

s th

atM

assa

ro, O

den,

and

col

labo

rato

rs h

ave

repo

rted

in a

num

ber o

f stu

dies

,H

owev

er. t

he m

odel

s are

qui

te si

mila

r. so

it se

ems r

athe

r unl

ikel

y th

atcu

e tra

de-o

ff da

ta w

ould

be

able

to d

iScr

imin

ate

betw

een

them

, And

bot

hm

ake

spec

ial a

ssum

ptio

ns a

bout

lack

of i

nvar

ianc

e of

cue

s to

phon

eme

iden

tity

acro

ss c

onte

xts,

O

ne a

ppar

ent d

issim

ilarit

y be

twee

n th

e m

odel

s des

erve

s com

men

t.W

here

as c

ue st

reng

ths a

re c

ombi

ned

mul

tiplic

ativ

ely

in th

e de

term

inat

ion

of re

spon

se st

reng

ths i

n th

e fe

atur

e in

tegr

atio

n m

odel

. the

y ar

e co

mbi

ned

addi

tivei

y in

the

botto

m-u

p in

puts

to th

e un

its in

TR

ACE,

How

ever

, in

TRA

CE, t

wo

furth

er c

ompu

tatio

nal s

teps

take

pla

ce b

efor

e th

ese

inpu

tsresult in resp

onse

stre

ngth

s. Fi

rst,

the

inte

ract

ive-

activ

atio

n process en-

hanc

es d

iffer

ence

s bet

wee

n co

mpe

ting

units

, Sec

ond,

the

resu

lting

uni

tac

tivat

ions

are

sub

ject

ed to

an

expo

nent

ial t

rans

form

atio

n. J

ust t

his

. second step by i

tself

wou

ld tr

ansf

orm

influ

ence

s tha

t hav

e ad

ditiv

e ef

fect

son

uni

t act

ivat

ions

into

influ

ence

s tha

t. ha

ve m

ultip

licat

ive

effe

cts o

n re

-sp

onse

stre

ngth

. Thu

s, th

e m

odel

s w

ould

be

mat

hem

atic

ally

equ

ival

ent

if th

e in

tera

ctiv

e ac

tivat

ion

proc

ess w

ere

sim

ply

repl

aced

by

a lin

ear,

addi

tive

com

bina

tion

of in

puts

to th

e un

its. I

n qu

antit

ativ

e fo

rmul

atio

nsof

the

inte

ract

ive

activ

atio

n pr

oces

s clo

sely

rela

ted

to th

e on

es w

e us

e(G

ross

berg

, 197

8), w

hat t

he in

tera

ctiv

e ac

tivat

ion

proc

ess d

oes i

s sim

ply

resc

ale

the

unit

activ

atio

ns, p

rese

rvin

g th

e ra

tios

of th

eir b

otto

m-u

pac

tivat

ion

but k

eepi

ng th

em b

ound

ed. T

houg

h ou

r ver

sion

of th

ese

equa

-tio

ns d

oes n

ot d

o th

is ex

actly

, the

way

s in

whi

ch it

dev

iate

s fro

m th

isw

ould

be

diffi

cult

to u

se a

s the

bas

is fo

r an

empi

rical

dist

inct

ion

betw

een

the

TRA

CE a

ppro

ach

and

the

feat

ure

inte

grat

ion

mod

el" T

hus,

up

to a

poin

t, w

e ca

n se

e TR

ACE

as (a

ppro

xim

atel

y) im

plem

entin

g th

e co

mpu

-ta

tions

spec

ified

in a

den

and

Mas

saro

s m

odel

. The

mod

els

diffe

r,th

ough

, in

that

TRA

CE is

dyn

amic

and

in th

at it

inco

rpor

ates

feed

back

to th

e ph

onem

e le

vel.

This

allo

ws T

RACE

to a

ccou

nt fo

r cat

egor

ical

perc

eptio

n in

a d

iffer

ent w

ay.

Categorical perception.

In s

pite

ofth

e fa

ct th

at T

RAC

E is

qui

te fl

exib

lein

the

way

it c

ombi

nes i

nfor

mat

ion

from

diff

eren

t fea

ture

s to

dete

rmin

ethe identity of a ph

onem

e, th

e m

odel

is q

uite

categorical in its overt

resp

onse

s, Th

is is

illus

trate

d in

two

way

s: fir

st. th

e m

odel

show

s a m

uch

shar

per t

rans

ition

in it

s cho

ices

of r

espo

nses

as w

e m

ove

from

Ig/ to

Ikl

07+

- g

II..

g 04

~ 03

5;!.

0 I

Xn"

-X.

.- -X

a- +

2 -X

..- +.

1'10

. 17,

The

sta

te o

f the

'/i'a

ce a

t var

ious

poi

nts

durin

g an

d af

ter t

he p

rese

ntat

ion

of a

sylla

hle

cuns

istil

fg (,

f the

mos

t IgI

-lik

e ot

the

2S in

term

edia

te se

gmen

ts us

ed in

the

tradi

ngrelations experiment. represented by

I'!(/.

preceded by silence and followed by

lal,

then

anot

her s

ilenc

e.

Resp

onse

pro

babi

litie

s wer

e co

mpu

ted

usin

g th

e fo

rmul

as g

iven

ear

lier

for c

onve

rting

act

ivat

ions

to re

spon

se st

reng

ths a

nd st

reng

ths i

nto

prob

-ab

ilitie

s, Th

e re

sulti

ng re

spon

se p

roba

bilit

ies,

for e

ach

of th

e 25

con

di-

tions

of t

he e

xper

imen

t, ar

e sh

own'

in F

ig, 1

8. T

he p

atte

rn o

f res

ults

isqu

ite si

mila

r to

that

obt

aine

d in

Den

es (1

955) experiment on the

1st-

cont

inuu

m. T

he c

ontri

butio

n of

eac

h cu

e is

app

roxi

mat

ely

linea

r and

addi

tive

in th

e m

iddl

e of

the

rang

e; b

ut th

e cu

rves

flat

ten

out a

t the

extre

mes

, as i

n th

e D

enes

(195

5) e

xper

imen

t. M

ore

impo

rtant

ly, t

he m

od-

el's

beh

avio

r exh

ibits

the

abili

ty to

trad

e on

e cu

e of

f aga

inst

anot

her.

For

exam

ple,

ther

e ar

e th

ree

diffe

rent

com

bina

tions

of f

eatu

re v

alue

s w

hkh

lead

to a

pro

babi

lity

betw

een

.82

and

,85 of choosing

Ik/:

(I) th

e ne

utra

lva

lue

of th

e VO

T di

men

sion

cou

pled

with

the

mos

t Ik/

-like

val

ue o

n th

e1'

101'

dim

ensi

on: (

2) th

e ne

utra

l val

ue o

n th

e FI

OF

dim

ensi

on c

oupl

edw

ith th

e m

ost I

k/-li

ke v

alue

of t

he V

aT d

imen

sion;

and

(3) t

he so

mew

hat

...;' '- 060-

Yoke Olisel.Tirne

FI(L III, Simulated prob;lbility of choosing

Ik/ll

t Ti

me

Slic

e 1i

0, fo

r eac

h of

the

25 s

timul

ius

ed in

'he

tradi

ng re

latio

ns si

mul

atio

n ex

perim

ent.

Num

bers

nex

t to

each

cur

ve re

fer t

oth

e in

term

edia

te p

;llIe

rn o

n,th

e FI

OF

cont

inuu

m u

sed

in th

e fiv

e st

imul

i con

tribu

ting

toea

ch c

urve

, Uig

her n

umbe

rs c

orre

spon

d to

hig

her v

alue

s of

FIO

F.

:.., ::=

040

-

020

a.. 0,

0 I

3, 4

6

7 8

\I 10 II 12

Stim

ulus

Num

ber

FIG

. 20.

Effe

cts o

f com

petit

ion

on p

hone

me

activ

iltio

ns. T

he fi

rst p

anel

show

s rel

ativ

eamounts of bollom-up excitatory input to

Igl

and

IkJ

produced by each of the II sti

mul

ius

ed in

the

cate

goric

al p

erce

ptio

n si

mul

atio

n, T

he s

econ

d pa

nel s

how

s th

e ac

tivat

ions

ul"

units for Ig/ and

Ikl

at T

ime

Cycl

e 60

, Stim

uli 3

and

9 c

orre

spun

d 10

Ihe

cano

nica

l/gl a

ndIk

/, re

spec

tivel

y.

of th

e un

its fo

r /g/

and

/k/ a

t the

end

of 6

0 cy

cles

of p

roce

ssin

g. T

he sl

ight

diffe

renc

es in

net

inpu

t hav

e be

en g

reat

ly a

mpl

ified

. and

the

activ

atio

ncu

rves

exh

ibit

a m

uch

stee

per t

rans

ition

than

the

rela

tive

botto

m-u

p ex

-ci

tatio

n cu

rves

. Th

ere

are

two

reas

ons w

hy th

e ac

tivat

ion

curv

es ,a

re s

o m

uch

shar

per

than

the

initi

al b

otto

m-u

p ex

cita

tion

func

tions

. The

prim

ary

reas

on is

competitive inhibition.

The

effe

ct o

f the

com

petit

ive

inhi

bitio

n at

the

pho-

nem

e le

vel i

s to

grea

tly m

agni

fy th

e sl

ight

diff

eren

ce in

the

exci

tato

ryin

puts

to th

e tw

o ph

onem

es. I

t is e

asy

to se

e w

hy th

is ha

ppen

s. O

nce

one

phon

eme

is sli

ghtly

mor

e str

ongl

y ac

tivat

ed th

an th

e ot

her.

it ex

erts

a st

rong

er in

hibi

tory

influ

ence

on

the

othe

r tha

n th

e ot

her c

an e

xert

onit.

The

net

resu

lt is

that

"th

e ric

h ge

t ric

her," This general property of

com

petit

ive

inhi

bitio

n m

echa

nism

s was

disc

usse

d by

McC

lella

nd a

nd R

u-m

elha

rt (1

981)

, fol

low

ing

earli

er o

bser

vatio

ns b

y G

ross

berg

(see

Gro

ss-

berg, 1978, f

or a

disc

ussio

n) a

nd L

evin

(197

6); i

t is a

lso w

ell k

now

n as

one

poss

ible

bas

is' o

f edg

e en

hanc

emen

t effe

cts i

n lo

w le

vels

of v

isual

TRAC

E M

OD

EL

alon

g th

e V

aT a

nd F

IOF

dim

ensio

ns th

an w

e w

ould

exp

ect f

rom

the

sligh

t cha

nges

in th

e re

lativ

e ex

cita

tion

of th

e /g

/ and

/kI u

nits.

Sec

ond,

.th

e m

odel

tend

s to

obl

itera

te d

iffer

ence

s be

twee

n di

ffere

ntin

puts

whi

chit

iden

tifie

s as t

he sa

me

phon

eme,

whi

le sh

arpe

ning

diff

eren

ces b

etw

een

inpu

ts as

signe

d to

diff

eren

t cat

egor

ies,

We

will

con

sider

eac

h of

thes

etw

o po

ints

in tu

rn, a

fter w

e de

scrib

ethe

stim

uli u

sed

in th

e simulations.

Elev

en d

iffer

ent c

onso

nant

feat

ure

patte

rns w

ere

used

, em

bedd

ed in

the

sam

e sim

ulat

ed /-

/ context as in the trading relations simulation,

The

stim

uli v

arie

d fro

m v

ery

low

val

ues

of b

oth

VaT

and FIOF, more

extre

me

than

the

cano

nica

l /g/

, thr

ough

ver

y hi

gh v

alue

s on

bot

h di

men

-sions, more extreme than the canonical

/kl.

All

the

stim

uli w

ere

spac

edeq

ual d

ista

nces

apa

rt on

the

VaT

and

FIO

F di

men

sion

s, T

he lo

catio

nsof

the

peak

act

ivat

ion

valu

es o

n ea

ch o

f the

se tw

o co

ntin

ua a

re sh

own

in F

ig, 1

9,

Figu

re 2

0 in

dica

tes

the

rela

tive

initi

al b

otto

m-u

p ac

tivat

ion

of th

e /g

/, a

nd /k

/ pho

nem

e un

its fo

r eac

h of

the

II stimuli used in the simulation.

The

first

thin

g to

not

e is

that

the

rela

tive

botto

m-u

p ex

cita

tion

of th

e tw

oph

onem

e un

its d

iffer

onl

y sli

ghtly

, For

exa

mpl

e, th

e ca

noni

cal f

eatl,

Jre

patte

rn fo

r /g/

send

s 75%

as m

uch

exci

tatio

n (0

/g/ a

s it s

ends

to /k

/, Th

efe

atur

e pa

ttern

two

step

s to

war

d/g/

from

/k/ (

Stim

ulus

5),

send

s 88%

as

muc

h ac

tivat

ion

to /g

/ as

to /k

/.Th

e fig

ure

also

indi

cate

s, in

the

seco

nd p

anel

, the

resu

lting

act

ivat

ions

r:: 0 1.

::;.... 0 0.

c.J

iii 0.

::;

026

r:: !:! 1.

....

::; 0. 0:

10

5 0.

.r:: Il.

/s//k

/1 2 3 4 5 6 7 8 91011

Voic

e O

nsel

Tim

e

/s//k

/1

2 3

4 5

6 7

8 9

1011

0 2 F

t Onsel Frequency

FtG

. t9.

Loc

atio

ns o

f pea

k ac

tivat

ions

alo

ng Ih

e V

aT a

nd F

tOF

dim

ensio

ns. l

or e

ach

ofth

e II

stim

uli u

sed

in th

e ca

tego

rical

per

cept

ion

simul

atio

n.

MC

CLE

LLAN

D A

ND

ELM

AN

/8/

/k/

...

1'2

Stim

ulus

Num

ber

/s//k

/

TRAC

E M

OD

EL:4

6M

C C

LELL

AND

AN

D E

LMAN

/81

Ikl

This

obvi

ously

brin

gs o

ut th

e fa

ct th

at th

e ap

pare

nt st

eepn

ess o

f the

iden

tific

atio

n fu

nctio

n de

pend

s on

the

grai

n of

the

sam

plin

g of

diff

eren

tpo

ints

alo

ng th

e co

ntin

uum

bet

wee

n tw

o st

imul

i, as

wel

l as

a ho

st

othe

r fac

tors

(Lan

e, 1

965)

, Whe

ther

an

empi

rical

or s

imul

ated

iden

tifi-

catio

n fu

nctio

n lo

oks

stee

p or

not

dep

ends

on

the

sele

ctio

n of

stim

uli b

yth

e ex

perim

ente

r or m

odel

er, H

owev

er, i

t is w

orth

not

ing

that

the

steep

-ne

ss o

f the

iden

tific

atio

n fu

nctio

n is

inde

pend

ent o

f the

pre

senc

e of

tradi

ng re

latio

ns, a

t lea

st in

the

simul

atio

n m

odel

, Tha

t is,

if w

e ha

d us

edm

ore

wid

ely

sepa

rate

d ste

ps a

long

the

VO

T an

d FI

OF

dim

ensio

n, w

ew

ould

hav

e ob

tain

ed m

uch

steep

er id

entif

icat

ion funct1Qns. The additivity

of e

xcita

tory

inpu

ts w

ould

still

app

ly, a

nd th

us it

wou

ld st

ill b

e po

ssib

leto trade cues off against e~ch other,

In T

RACE

, the

cat

egor

ical

out

put o

f the

mod

el c

omes

abo

ut o

nly

afte

ran interactive competition process that greatly sharpens the differences

in the activation of the detectors for the relevant units, This interactive

proc

ess t

akes

tim

e. In

the

simul

atio

n re

sults

repo

rted

here

, we

assu

med

that

subj

ects

wai

ted

a fix

ed ti

me

befo

re re

spon

ding

. But

, if w

e as

sum

e, t

hat s

ubje

cts a

re a

ble

to re

spon

d as

soon

as t

he re

spon

se st

reng

th ra

tiore

ache

s som

e cr

iteria

lleve

l, w

e w

ould

find

that

subj

ects

wou

ld b

e ab

leto

resp

ond

mor

e qu

ickl

y to

stim

uli n

ear t

he p

roto

type

of e

ach

cate

gory

than

they

can

to s

timul

i nea

r the

bou

ndar

y, T

his

is ex

actly

wha

t was

foun

d by

Piso

~i a

nd T

ash

(197

4).

The

shar

peni

ng th

e m

odel

impo

ses o

n th

e id

entif

icat

ion

func

tion,

co

njun

ctio

n w

ith th

e fa

Ct th

at it

can

trad

e on

e fe

atur

e of

f aga

inst

anot

her,

show

s how

the

mod

el, l

ike

hum

an p

erce

iver

s of

spe

ech,

can

be

both

flexi

ble

and

deci

sive

at th

e sa

me

time,

The

se a

spec

ts of

TRA

CE a

resh

ared

with

the

feat

ure

inte

grat

ion

mod

el (M

assa

ro, 1

981)

,. How

ever

, the

TRA

CE m

odel

's de

cisiv

enes

s ext

ends

eve

.1 fu

rther

than

we

have

ob-

serv

ed th

us fa

r: fe

edba

ck fr

om th

e ph

onem

e to

the

feat

ure

leve

l ten

ds to

.ca

use

the

mod

el to

obl

itera

te th

e di

ffere

nces

bet

wee

n in

put f

eatu

re p

at-

tern

s tha

t res

ult i

n th

e id

entif

icat

ion

of th

e sa

me

phon

eme,

thus

allo

win

gth

e m

odel

to p

rovi

de a

n ac

coun

t not

onl

y fo

r sha

rp id

entif

icat

ion

func

-tio

ns, b

ut a

lso

for t

he fa

ct th

at d

iscr

imin

abilit

y of

spee

ch so

unds

is fa

rpo

orer

with

in c

ateg

orie

s tha

n it

is be

twee

n ca

tego

ries.

Stric

tly s

peak

ing,

at l

east

as

defin

ed b

y Liberman, Cooper, Shank-

wei

ler,

and

Stud

dert-

Kenn

edy

(196

7), t

rue

cate

goric

al p

erce

ptio

n is

onl

yex

hibi

ted

whe

n th

e ab

ility

to d

iscrim

inat

e di

ffere

nt so

unds

is n

o be

tter

than

. cou

ld b

e ex

pect

ed 'b

ased

on

the

assu

mpt

ion

that

the

only

bas

is a

liste

ner h

as fo

r disc

rimin

atio

n is

the

cate

goric

al a

ssig

nmen

t of t

he st

im-

ulus

to a

par

ticul

ar p

hone

tic c

ateg

ory,

How

ever

, it i

s con

cede

d th

attru

e" c

ateg

oric

al p

erce

ptio

n in

this

sem

o: is

nev

er in

fact

obs

erve

d(S

tudd

ert-

Ken

nedy

, Lib

erm

an, H

arris

, & C

oope

r, 19

70),

Whi

le it

is tr

ueth

at th

e di

scrim

inat

ion

of so

unds

is m

uch

bette

r for

soun

ds w

hich

per

-

info

rmat

ion

proc

essin

g, A

seco

nd c

ause

of t

he sh

arpe

ning

of t

he a

ctiv

a-tio

n cu

rves

is th

e ph

onem

e-to

-feat

ure

feed

back

, whi

ch w

e co

nsid

er in

deta

il in

a m

omen

t. Th

e id

entif

icat

ion

func

tions

that

resu

lt fro

m a

pply

ing

the

Luce

cho

ice

rule

to th

e ac

tivat

ion

valu

es sh

own

in th

e se

cond

pan

el o

f Fig

, 20

are

show

n in

Fig

. 21 along with the

lBX

di

scrim

inat

ion

func

tion,

whi

ch is

disc

usse

d be

low

. The

iden

tific

atio

n fu

nctio

ns a

re e

ven

shar

per t

han

the

activ

atio

n cu

rves

; the

re is

onl

y a

4% c

hanc

e th

at th

e m

odel

will

choo

se/k

/ ins

tead

of /

g/ fo

r Stim

ulus

5, f

or w

hich

/k/ r

ecei

ves

88%

as

muc

hbo

ttom

-up

supp

ort a

s /g/

, The

in~r

ease

d sh

arpn

ess i

s due

to th

~ pr

oper

ties

of th

e re

spon

se s

treng

th a

ssum

ptio

ns. T

hese

ass

umpt

ions

ess

entia

lly im

-pl

emen

t the

not

ion

that

the

sens

itivi

ty o

f the

deci

sion

mec

hani

sm, i

nterms of

d'

for c

hoos

ing

the

mos

t stro

ngly

. act

ivat

ed o

f tw

o un

its, i

s alin

ear f

unct

ion

of th

e di

ffere

nce

in a

ctiv

atio

n of

the

two

units

, Whe

n th

eactivations are far enough apart,

d'

)Viii

be s

uffic

ient

to e

nsur

e ne

ar-lO

O%

corre

ct p

elfo

rman

ce, e

ven

thou

gh b

oth

units

hav

e gr

eate

r tha

n 0

activ

a-tio

n, O

f cou

rse,

the

.am

ount

of s

epar

atio

n in

the

activ

atio

ns th

at is

nec

-es

sary

for a

ny g

iven

leve

l of p

erfo

rman

ce is

a m

atte

r of p

aram

eter

s; th

ere

leva

nt p

aram

eter

her

e is

the

scal

e fa

ctor

use

d in

the

expo

nent

ial t

rans

-fo

rmat

ion

of a

ctiv

atio

ns. T

he v

alue

use

d fo

r thi

s par

amet

er in

the

pres

ent

sim

ulat

ions

(10)

was

the

sam

e as

that

use

d in

all

othe

r cas

es w

here

we

trans

late

act

ivat

ion

into

resp

onse

pro

babi

lity,

incl

udin

g th

e tra

ding

:rela

-tio

ns si

mul

atio

n.

Som

e re

ader

s may

be

puzz

led

as to

why

TRA

CE II

exh

ibits

a sh

arp

iden

tific

atio

n fu

nctio

n in

the

cate

goric

al p

erce

ptio

n ex

perim

ent,

but

show

s a

muc

h m

ore

grad

ual t

rans

ition

bet

wee

n /g

/ and

/k/ i

n th

e tra

ding

rela

tions

sim

ulat

ion,

The

reas

on is

sim

ply

that

fine

r ste

ps a

long

the

VO

Tan

d FI

OF

cont

inua

wer

e us

ed in

the

tradi

ng re

latio

ns si

mul

atio

n, A

ll of

the stimuli for the tl1ading relations s

imul

atio

n lie

bet

wee

n St

imul

i 6 a

nd4

in th

e ca

tego

rical

per

cept

ion

simul

atio

n.

~ 1. 10

0

~ o~-

~ 0.

'-.. 0.

.. 6: 0. 00

0

020

10 II

Stim

ulus

F"L 21. Simulated identilicalion functions and forced-choice accuracy in the

ABX

ta

sk.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

ceiv

ers a

ssig

n to

diff

eren

t cat

egor

ies t

han

for s

ound

s the

y as

sign

to th

esa

me

cate

gory

, the

re is

als

o at

leas

t a te

nden

cy fo

r dis

crim

inat

ion

to b

eso

mew

hat b

ette

r tha

n pr

edic

ted

by th

e id

entif

icat

ion

func

tion,

eve

n be

-tw

een

stim

uli w

hich

are

alw

ays a

ssig

ned

to th

e. s

ame

cate

gory

. TRA

CEII

prod

uces

this

kind

of a

ppro

xim

ate

cate

goric

al p

erce

ptio

n.Th

e w

ay it

wor

ks is

this,

Whe

n a

feat

ure

patte

rn c

omes

in, i

t sen

dsm

ore

exci

tatio

n to

som

e ph

onem

e un

its th

an o

ther

s; as

they

bec

ome

activ

e, th

ey b

egin

to c

ompe

te, a

nd o

ne g

radu

ally

com

es to

dom

inat

e th

eot

hers

, Thi

s muc

h w

e ha

ve. a

lread

y ob

serv

ed, B

ut a

s th

is c

ompe

titio

npr

oces

s is

goi

ng o

n, th

ere

is a

lso

feed

bpck

from

the

phon

eme

leve

l to

the

feat

ure

leve

l. Th

us, a

s a p

artic

ular

pho

nem

e be

com

es a

ctiv

e, it

tend

s to

impo

se it

s ca

noni

cal p

atte

rn o

f act

ivat

ion

on th

e fe

atur

e le

vel,

The

effe

ctof

the

feed

back

bec

omes

par

ticul

arly

stro

ng a

s tim

e go

es o

n, si

nce

the

feat

ure

inpu

t onl

y ex

cite

s th

e fe

atur

e un

its v

ery

brie

fly; t

he o

rigin

al p

at-

tern

of a

ctiv

atio

n pr

oduc

ed b

y th

e ph

onem

e un

its is

, the

refo

re, g

radu

ally

repl

aced

by

the

cano

nica

l pat

tern

impo

sed

by th

e fe

edba

ck fr

om th

e ph

o-" n

eme

leve

l, Th

e re

sult

is th

a~ th

e pa

ttern

of a

ctiv

atio

n re

mai

ning

at t

hefe

atur

e le

vel a

fter 6

0 cy

cles

of p

roce

ssin

g ha

s bec

ome

assim

ilate

d to

the

prot

otyp

e, In

this

way

, fea

ture

pat

tern

s for

diff

eren

t inp

uts a

ssig

ned

toth

e sa

me

cate

gory

are

rend

ered

nea

rly in

dist

ingu

isha

ble,

An

impr

essio

n of

the

mag

nitu

de o

f thi

s effe

ct is

illu

strat

ed in

Fig

, 22,

whi

ch sh

ows h

ow d

iffer

ent t

he fe

atur

e pa

ttern

s of a

djac

ent s

timul

i are

at

the

end

of 6

0 cy

cles

of p

roce

ssin

g. T

he m

easu

re o

f diff

eren

ce is

sim

ply

I -

"b'

whe

re

"b

stand

s for

the

corre

latio

n of

the

patte

rns p

rodu

ced

bysti

mul

i an

d b,

O

nly

the

two

dim

ensio

ns w

hich

act

ually

diff

er b

etw

een

the canonical

IgI

and

Ikl

are

cons

ider

ed in

the

diffe

renc

e m

easu

re, F

ur-

ther

mor

e, th

e co

rrela

tion

cons

ider

s onl

y th

e fe

atur

e pa

ttern

on

the

feat

ure

units

in T

ime

Slic

e 12

, rig

ht a

t the

cen

ter o

f the

inpu

t spe

cific

atio

n. If

all

dim

ensio

ns a

re c

onsid

ered

, the

val

ues o

f the

diff

eren

ce m

easu

re a

re re

-du

ced

over

all,

but t

he p

atte

rn is

the

sam

e, In

clus

ion

of ft

!'dtu

re p

atte

rns

from

sur

roun

ding

slic

es li

kew

ise

mak

es li

ttle

diffe

renc

e.To

rela

te th

e di

ffere

nce

betw

een

two

stim

uli ~

o pr

obab

ility

corre

ctchoice performance in the

ABX

ta

sk g

ener

ally

use

d in

cat

egor

ical

per

-ce

ptio

n ex

perim

ents

, we

once

aga

in u

se th

e Lu

ce (1

959)

choice model.

The

prob

abilit

y of

iden

tifyi

ng sti

mul

us

with alternative

in is given

by Ix

=,,)

II.(

whe

re S

ax is the "st

reng

th" o

f the

sim

ilarit

y between (/ and

x.

This

isgiven simply by the exponential of the correlation of

and

"x = e

k,r"

x

g 0.

is 0.

and similarly for

Sbx'

(T

he e

xpon

entia

l tra

nsfo

rmat

ion

is re

quire

d to

tran

s-la

te c

orre

latio

ns, r

angi

ng fr

om +

Ito

- I, i

nto

posit

ive

valu

es, s

o th

atLu

ces

ratio

rule

can

be

used

. The

sam

e tra

nsfo

rmat

ion

is u

sed

for t

rans

-la

ting

activ

atio

ns in

to re

spon

se st

reng

ths i

n id

entif

icat

ion

task

s.) Here

is th

e pa

ram

eter

that

scal

es th

e re

latio

n be

twee

n co

rrela

tions

and

stre

ngth

s, T

hese

ass

umpt

ions

are

con

sist

ent w

ith th

e ch

oice

ass

umpt

ions

mad

e fo

r ide

ntifi

catio

n re

spon

ses,

The

resu

lting

resp

onse

pro

babi

litie

s,for one choice of the parameter

,.

(5) a

re sh

own

in F

ig; 2

1 (th

e ex

pone

n-tiation parameter

,.

is different than the parameter

used

in g

ener

atin

gid

entif

icat

ion

prob

abili

ties f

rom

act

ivat

ions

bec

ause

cor

rela

tions

and

ac-

tivations are not on equivalent scales),

Basic

ally

, the

figu

re sh

ows t

hat t

he e

ffect

of f

eedb

ack

is to

mak

e th

efe

atur

e pa

ttern

s for

inpu

ts w

ell w

ithin

eac

h ca

tego

ry m

ore

simila

r tha

nth

ose

for i

nput

s nea

r the

bou

ndar

y be

t~ee

n categories. Differences be-

twee

n sti

mul

i nea

r the

pro

toty

pe o

f the

' sam

e ph

onem

e ar

e al

mos

t obl

i-te

rate

d. W

hen

two

stim

uli s

tradd

le th

e bo

unda

ry, t

he fe

atur

e-level pat-

tern

s ar

e m

uch

mor

e di

stin

ct. A

s a

resu

lt, th

e pr

obab

ility

of c

orre

clly

disc

rimin

atin

g sti

mul

i with

in a

pho

nem

e ca

tego

ry is

muc

h lo

wer

Ihan

Ihe

prob

abili

ty o

f disc

rimin

atin

g sti

mul

i in

diffe

rent

cat

egor

ies.

The

proc

ess o

f "ca

noni

caliz

atio

n" of the representation of a speech

soun

d vi

a th

e fe

edba

ck m

echa

nism

take

s tim

e. O

urin

g Ih

is lim

e, Iw

oth

ings

are

hap

peni

ng: o

ne is

that

the

activ

atio

ns in

itial

ly p

rodu

ced

by th

esp

eech

inpu

t are

dec

ayin

g; a

noth

er is

that

the

feed

back

, whi

ch d

rives

the

repr

esen

tatio

n to

war

d th

e pr

otot

ype,

is b

uild

ing

up. I

n th

e sim

ulat

ions

,w

e al

low

ed a

con

sider

able

am

ount

of t

ime

for t

hese

pro

cess

es b

efor

e

r:::

030

I 2

7 8

8 10. II 12

Stim

ulus

Num

ber

FIG

. 22.

Diff

eren

ces

betw

een

palle

rns

of a

ctiv

atio

n at

the

feat

ure

leve

l at C

ycle

60.

I()r

pairs of stimuli one step apart along the

Ir/-ik

i co

ntin

uum

use

d fo

r pro

duci

ng th

e id

entif

i-ca

tion

func

tions

show

n pr

evio

usly

in F

ig. 2

1. T

he d

iffer

ence

mea

sure

is th

e co

rrela

tion

ofth

e tw

o pa

ttern

s, su

btra

cted

from

1.0

; thu

s, if

the

two

patte

rns c

orre

late

d pe

rfect

ly, t

heir

difference would be O.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

L~ '

com

putin

g sim

ilarit

ies o

f diff

eren

t act

ivat

ion

patte

rns t

o ea

ch o

ther

, Ob-

viou

sly. i

f we

had

left

less

tim

e. th

ere

wou

ld n

ot h

ave

been

as m

uch

ofan

opp

ortu

nity

for t

hese

forc

es to

ope

rate

. Thu

s, TR

ACE

is in

agr

eem

ent

with the finding that there tends to be an increase in within-category

disc

rimin

atio

n w

hen

n ta

sk is

use

d w

hich

allo

ws

subj

ects

to b

ase

thei

rre

spon

ses o

n ju

dgm

~nts

of th

e sim

ilarit

y of

stim

uli s

pace

d cl

osel

y to

-ge

ther

in ti

me

(Piso

nl&

Laz

arus

, 197

4).

It sh

ould

be

note

d th

at it

wou

ld b

e po

ssib

le to

acc

ount

for c

ateg

oric

alpe

rcep

tion

in T

RACE

with

out i

nvok

ing

feed

back

from

the

phon

eme

leve

lto

the

feat

ure

leve

l, A

ll w

e w

ould

nee

d to

~o

is as

sum

e th

at th

e fe

atur

ein

form

atio

n th

at g

ives

rise

to p

hone

me

iden

tific

atio

n is

inac

cess

ible

, as

. pro

pose

d by

the

mot

or th

eory

of s

peec

h pe

rcep

tion

(Lib

erm

an e

t ai"

1967

). or

is ra

pidl

y lo

st a

s pr

opos

ed b

y th

e "d

ual-c

ode"

mod

el (F

ujisa

ki&

Kaw

ashi

ma,

196

8; M

assa

ro. 1

975, 1981; Pisoni, 1973, 1975,) T

he d

ual-

code

mod

el, w

hich

has

had

con

sider

able

succ

ess a

ccou

ntin

g fo

r cat

egor

-ic

al p

erce

ptio

n da

ta, a

ssum

es th

at p

hone

me

iden

tific

atio

n ca

n be

bas

edei

ther

on

prec

ateg

oric

al in

form

atio

n or

on

the

resu

lts o

f the

pho

nem

eid

entif

icat

ion

proc

ess,

Sinc

e it

is as

sum

ed th

at fe

atur

e in

form

atio

n de

cays

rapi

dly

(esp

ecia

lly fo

r con

sona

nt fe

atur

es-s

ee b

elow

), re

spon

ses

mus

tof

ten

be b

ased

sole

ly o

n th

e ou

tput

of t

he p

hone

me

iden

tific

atio

n pr

oces

s,w

hich

is a

ssum

ed to

"rov

ide

a di

scre

te c

ode

ofth

e se

quen

ce o

f pho

nem

es.

This

inte

rpre

tatio

n ac

coun

ts fo

r muc

h of

the

data

on

cate

goric

al p

erce

p-tio

n qu

ite w

ell.

Inde

ed, i

t is

fairl

y di

fficu

lt to

find

way

s of d

istin

guish

ing

betw

een

a fe

edba

ck f!

1ode

l and

one

that

attr

ibut

es c

ateg

oric

al p

erC

eptio

nto

a lo

ss o

f inf

orm

atio

n fro

m th

e fe

atur

e le

vel c

oupl

ed w

ith a

relia

nce

ona

mor

e ab

stra

ct c

ode,

Bot

h fe

edba

ck m

odel

s an

d du

al c

ode

mod

els

can

acco

mm

odat

e th

e fa

ct th

at v

owel

s sho

w le

ss o

f a te

nden

cy to

war

d ca

t-eg

oric

al p

erce

ptio

n th

an c

onso

nant

s (Fr

y, A

bram

son,

Eim

as, &

Lib

-er

man

, 196

2; P

ison

i, 19

73).

It is

sim

ply

nece

ssar

y to

ass

ume

that

vow

elfe

atur

es a

re m

ore

pers

isten

t tha

n co

nson

ant f

eatu

res (

Crow

der,

1978

,19

81; F

ujisa

ki &

Kaw

ashi

ma,

196

8; P

isoni

, 197

3, 1

975)

, How

ever

, the

two

clas

ses o

f int

erpr

teta

tions

do

diffe

r in

one

way

, The

feed

back

acc

ount

seem

s to

diffe

r mos

t cle

arly

from

a li

mite

d fe

atur

e ac

cess

acc

Qun

t in

itspr

edic

tions

of p

erfo

rman

ce in

dis

crim

inat

ing

two

stim

uli,

both

aw

ay fr

omth

e ce

nter

of a

cat

egor

y, b

ut st

ill w

ithin

it H

ere,

TRA

CE te

nds t

o sh

owgr

eate

r disc

rimin

atio

n th

an it

show

s bet

wee

n sti

mul

i squ

arel

y in

the

mid

dle

of a

cat

egor

y,

Stan

dard

inte

rpre

tatio

ns o

f cat

egor

ical

per

cept

ion

can

acco

unt f

or in

-cr

ease

s in

~lo

;crim

inab

ility

near

the

boun

dary

bet

wee

n tw

o ca

tego

ries

(whe

re id

entif

icat

ion

.may

in fa

ct b

e so

mew

hat v

aria

ble)

, sim

ply

in te

rms

of th

e fa

ct th

at m

argi

nal s

timul

i are

mor

e lik

ely

to g

ive

rise

to d

iffer

ent

cate

gory

labe

ls, B

ut T

RAC

E ca

n ac

coun

t for

incr

ease

s in

dis

crim

inab

ility

at e

xtre

me

valu

es o

f fea

ture

con

tinua

whi

ch w

ould

not

giv

e ris

e to

dif-

fere

nt c

ateg

ory

labe

ls. In

TRA

CE, t

he re

ason

for t

his i

ncre

ase

in d

iscrim

-in

abilit

y is

that

the

activ

atio

n of

the

appr

opria

te it

em a

t the

pho

nem

ele

vel i

s ,w

eake

r, an

d th

eref

ore

the

feed

back

sign

al is

wea

ker,

than

it is

whe

n th

e in

put o

ccur

s nea

r the

cen

ter o

f the

category. For example,

Stim

ulus

I in

our

sim

ulat

ions

falls

bel

ow th

e ca

noni

cal/g

/ stim

ulus

, and

ther

efor

e ac

tivat

es th

e Ig

/ pho

nem

e de

tect

or le

ss s

trong

ly th

an s

timul

icloser to the canonical

Ig/,

A similar thing happens with the

Ik/.

This

resu

lts in

less

"ca

noni

caliz

atio

n" o

f the

ext

rem

e sti

mul

i, an

d pr

oduc

es a

-sha

ped

disc

rimin

atio

n fu

nctio

n, a

s sho

wn

in F

ig. 2

2,Th

ere

is so

me

evid

ence

bea

ring

on th

is as

pect

of T

RACE

's a

ccou

nt o

fca

tego

rical

per

cept

ion,

Sam

uel (

1977) has reported

ABX

di

scrim

inat

ion

data

that

sho

w n

otic

eabl

e m

inim

a in

the

disc

: im

inat

ion

func

tion

near

the

canonical stim~1i within each category on a

Id/-/

tl co

ntin

uum

, Ind

eed,

Sam

uel's

acc

ount

of t

his e

ffect

, tho

ugh

not c

ouch

ed in

term

s of i

nter

ac-

tive

activ

atio

n pr

oces

ses,

has a

gre

at d

eal o

f sim

ilarit

y to

wha

t we

see

inTR

ACE

; he

sugg

ests

that

nea

r-can

onic

al it

ems a

re m

ore

stron

gly

assim

-ila

ted

to th

e ca

noni

cal p

atte

rn, U

nfor

tuna

tely

the

effe

ct w

e se

ek is

fairl

ysu

btle

, and

so it

will

be

diffi

cult

to se

para

te fr

om n

oise

. In

Sam

uel'

expe

rimen

t., th

e ef

fect

is fa

irly

clea

r-cut

at b

oth

extre

mes

of t

he V

OT

cont

inuu

m in

thre

e ob

serv

ers a

t the

end

of e

xten

sive

train

ing,

as s

how

nin

Fig

, 23,

and

eve

n un

prac

ticed

subj

ects

tend

to sh

ow th

e ef

fect

tow

ard

the high end of the VOT continuum, well past the prototype for

It/.

In su

mm

ary,

TRA

CE a

ppea

rs to

pro

vide

a fa

irly

accu

rate

acc

ount

of

the

phen

omen

a of

cue

trad

e-of

fs a

nd c

ateg

oric

al p

erce

ptio

n of

spee

chso

unds

, It a

ccou

nts

for c

ateg

oric

al p

erce

ptio

n w

ithou

t rel

ying

on

the

no-

tion

that

the

phen

omen

on d

epen

ds o

n re

adou

t fro

m a

n ab

strac

t lev

el o

fpr

oces

sing;

it a

ssum

es in

stead

that

the

feat

ure

leve

l, lik

e ot

her l

evel

s of

the

syste

m, i

s sub

ject

to fe

edba

ck fr

om h

ighe

r lev

els

whi

ch a

ctua

llych

ange

s the

repr

esen

tatio

n as

it is

bei

ng re

tain

ed in

mem

ory,

pus

hing

to

war

d a

cano

nica

l rep

rese

ntat

ion

of th

e ph

onem

e m

ost s

trong

ly a

ctiv

ated

by th

e in

put.

Oth

er P

heno

men

a at

the

Phon

eme

Leve

lTh

e lit

erat

ure

on p

hone

me

perc

eptio

n in

clud

es s

ever

al fu

rther

find

ings

we

have

not

yet

bee

n ab

le to

con

side

r in

deta

il. T

he n

ext f

ew. p

arag

raph

sco

nsid

er o

ne o

f the

se fi

ndin

gs a

nd h

ow it

mig

ht b

e ac

com

mod

ated

in th

eTRACE model.

Effects of global and local context on phoneme identification.

In o

ursim

ulat

ions

of t

radi

ng re

latio

ns, w

e ha

ve sh

own

that

the

crite

rial v

alue

need

ed o

n on

e di

men

sion

of st

imul

us v

aria

tion

can

be a

ffect

ed b

y ot

~er

dim

ensio

ns. T

hus,

whe

n th

e on

set o

f FI i

s rel

ativ

ely

high

, sho

rter v

oici

ngla

tenc

ies a

re n

eede

d to

per

ceiv

e a

soun

d as

unv

oice

d, O

ther

fact

ors a

lsoin

nuen

ce th

e ph

onem

e pe

rcei

ved

as a

resu

lt of

a p

artic

ular

feat

ural

inpu

t.

TRAC

E M

OD

EL

TRAINED SUBJECTS

100

LZ:

un .

. '

.0 "

/ -

20 '

AG

S

.....

I&J

..J ..J I&J

III ..J ... 100

c( 0

I&J

......

MD

S

100

....

80 ..

.40

\. 20 MCB

0102

0504

0501

0101

0

'"'

MC

CLE

LLAN

D A

ND

ELM

AN

CONTROL GROUP

spea

ker p

aram

eter

s, O

ur m

ain

poin

t her

e is

that

con

nect

ion

mod

ulat

ion

is q

uite

a d

iffer

ent m

echa

nism

than

the

sim

ple

addi

tive

,com

bina

tion

ofex

cita

tory

influ

ence

s th

at u

nder

lies

the

way

TR

ACE

acco

O'nt

s for trade-

offs

am

ong

the

cues

to a

sin

gle

phon

eme

or fo

r the

effe

cts o

f top

-dow

nin

fluen

ces o

n th

e ph

onem

e bo

unda

ry,

Sum

mar

y of

Pho

nem

e Id

entif

icat

ion

Sim

ulat

iolls

We

have

con

sider

ed a

num

ber o

f phe

nom

ena

conc

erni

ng th

e id

entit

i-ca

tion

and

perc

eptio

n of

pho

nem

es, T

hese

incl

ude

lexi

cal i

nflu

ence

s on

phpn

eme

iden

tific

atio

n, a

nd th

e la

ck th

ereo

f, bo

th in

reac

tion

time

and

in re

spon

se c

hoic

e m

easu

res;

"pho

nota

ctic

rule" effects on ph

onem

eid

entif

icat

ion

and

the

role

of s

peci

fic le

xica

l ite

ms i

n in

fluen

cing

thes

eef

fect

s; th

e in

tegr

atio

n of

mul

tiple

cue

s to

phon

eme

iden

tity

and

the

cat-

egor

ical

nat

ure

of th

e pe

rcep

t tha

t res

ults

from

this

inte

grat

ion.

TR

ACE

inte

grat

es a

ll of

thes

e ph

enom

ena

into

a si

ngle

acc

ount

that

inco

rpor

ates

aspe

cts o

f the

acc

ount

s offe

red

for p

artic

ular

asp

ects

of th

ese

resu

lts b

yot

her m

odel

s, In

the

next

sec

tion,

we

show

how

TR

ACE

can

also en-

com

pass

a n

umbe

r of p

heno

men

a co

ncer

ning

the

reco

gniti

on o

f spo

ken

wor

ds,

. ,""

." \

. CGI

/ "

, ,..' "

. .. "

CG2

CG3

0 10 20 50 40 50 IiO

10

.0

THE

TIM

E C

OU

RSE

OF

WO

RD

REC

OG

NIT

ION

The

study

of s

poke

n w

ord

reco

gniti

on h

as a

long

his

tory

, and

man

ym

odel

s ha

ve b

een

prop

osed

, Mor

ton

s no

w-c

lass

ic lo

goge

n m

odel

(Mor

ton,

196

9) w

as th

e fir

st to

pro

vide

an

expl

icit

acco

unt o

f the

inte

-gr

atio

n of

con

text

ual a

nd se

nsor

y in

form

atio

n in

wor

d re

cogn

ition

. Oth

erm

odel

s of t

his p

erio

d (e

,g.,

Broa

dben

t, 1967) concentrated primarily on

effe

cts o

f wor

d fre

quen

cy, U

ntil

the

mid

197

0s, h

owev

er, t

here

was

littl

eex

plic

it co

nsid

erat

ion

of th

e tim

e co

urse

of s

poke

n w

ord

reco

gniti

on.

Seve

ral s

tudi

es b

y M

arsl

en-W

ilson

and

his

col

labo

rato

rs (M

arsl

en-

Wilson, 1973; Marslen-W

ilson

& T

yler

, 1975) and by Cole and hiscollab-

orat

ors (

Cole

, 197

3; C

ole

& Ja

kim

ik, 1

978, 1980) pioneered the in

vesti

-ga

tion

of th

is p

robl

em,

Mar

slen-

Wils

ons

CO

HO

RT

mod

el (M

arsl

en~W

ilson

& T

yler

, 198

0;M

!irsle

n-W

ilson

& W

elsh

, 197

8) o

f spe

ech

perc

eptio

n w

as b

ased

on

this

early

wor

k on

the

time

cour

se o

f spo

ken

wor

d re

cogn

ition

. The

CO

HO

RTm

odel

was

one

of t

he so

urce

s of i

nspi

ratio

n fo

r TRA

CE, f

or tw

o m

ain

reas

ons"

Firs

t, it

prov

ided

an

expl

icit

acco

unt o

f the

way

top-

dow

n an

dbo

ttom

-up

info

rmat

ion

coul

d be

com

bine

d to

pro

duce

a w

ord

reco

gniti

onm

echa

nism

that

act

ually

wor

ked

in re

al ti

me.

Sec

ond,

it a

ccou

nted

for

the

findi

ngs

of a

num

ber o

f im

porta

nt e

xper

imen

ts d

emon

stra

ting

the

" on-

line"

character of the speech recognition process. H

owev

er, s

ever

al d

e-fic

ienc

ies o

f the

CO

HO

RT m

odel

hav

e be

en p

oint

ed o

ut, a

s we

shal

l see

,Be

caus

e TR

ACE

was

mot

ivat

ed in

larg

e pa

rt by

a d

esire

to k

eep

wha

tis

good

abo

ut C

OH

ORT

and

impr

ove

upon

its w

eakn

esse

s, w

e be

gin

this

VO

ICE

ON

SET

TIM

E (m

ile)

FIG, 23. Identification (solid curves) and

ABX

di

scrim

inat

ion

data

(das

hed

curv

es) f

rom

Ihre

e pr

actic

ed a

nd th

ree

naiv

e su

bjec

ts. S

impl

ified

and

repr

inte

d, w

ith p

erm

issi

on, f

rom

Sam

uel( 1

977)

. Th

e id

entit

y of

pho

nem

es su

rroun

ding

a ta

rget

pho

nem

e, th

e ra

te o

fsp

eech

of a

, syl

labl

e in

whi

ch a

par

ticul

ar fe

atur

e va

lue

occu

rs, a

s w

ell a

sch

arac

teris

tics o

f the

spea

ker a

nd th

e la

ngua

ge b

eing

spok

en a

ll in

fluen

ceth

e in

terp

reta

tions

of f

eatu

res.

See

Repp

and

Lib

erm

an (1

984) for a dis-

cuss

ion

of a

ll of

thes

e so

rts o

f inf

luen

ces o

n th

e boundaries between

phon

emes

, It

has b

een

sugg

este

d by

Mill

er, G

reen

, and

Sch

erm

er (1

984)

and

by

Rep

p an

d Li

berm

an (1

984)

that

thes

e di

ffere

nt e

ffect

s m

ay h

ave

diffe

rent

sour

ces,

In p

artic

ular

, Mill

er e

t al.

(198

4) su

gges

t tha

t lex

ical

effe

cts a

nd, s

eman

tic a

nd s

ynta

ctic

influ

ence

s on

the

one

hand

may

be

due

to a

dif-

fere

nt m

echa

nis~

than

influ

ence

s suc

h as

spee

ch ra

te a

nd c

oarti

cula

tory

influ

ence

s due

to lo

cal p

hone

tic c

onte

xt.

The

assu

mpt

ions

we

have

inco

rpor

ated

into

TRA

CE m

ake

a si

mila

rdi

stinc

tion,

In T

RACE

I, w

e ha

ve a

ccou

nted

for e

ffect

s of p

hone

tic c

on-

text

by

allo

win

g ac

tivat

ions

of u

nits

to in

fluen

ce th

e fe

atur

e-to

- pho

nem

eco

nnec

tions

in a

djac

ent t

ime

slice

s (se

e El

man

& M

cCle

lland

, in

pres

s,fo

r det

ails)

. In

the

disc

ussio

n,w

e ' c

onsid

er w

ays o

f ext

endi

ng th

e co

n-ne

ctio

n m

odul

atio

n id

ea to

acc

omm

odat

e ef

fect

s of v

aria

tions

in ra

te a

nd

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

sect

ion

by c

onsi

derin

g th

e C

OH

OR

T m

odel

in s

ome

deta

il. F

irst w

e re

-vi

ew th

e ba

sic a

ssum

ptio

ns o

f the

mod

el, t

hen

cons

ider

its s

treng

ths a

ndw

eakn

esse

s. T

here

app

ear t

o be

four

bas

ic a

ssum

ptio

ns o

f the

CO

HO

RT

mod

el.

J. T

he m

odel

use

s th

e fir

st s

ound

(in

Mar

slen

-Wils

on &

Tyl

er, 1

980,

the

initi

al c

onso

nant

clu

ster-

plus

-vow

el) o

f the

wor

d to

det

erm

ine

whi

chw

ords

will

be

in a

n in

itial

coh

ort o

r can

dida

te se

t,2.

Onc

e th

e ca

ndid

ate

set i

s est

ablis

hed,

the

mod

el e

limin

ates

wor

dsfro

m th

e co

hort

imm

edia

tely

, as"

each

succ

essiv

e ph

onem

e ar

rives

, if t

hene

w p

hone

me

fails

to m

atch

the

next

pho

nem

e in

the

wor

d, W

ords

can

also

be

elim

inat

ed o

n th

e ba

sis o

f sem

antic

con

strai

nts,

alth

ough

the

initi

alco

hoI'I

is a

ssum

ed to

'be determined by acoustic input alone.

3. W

ord

recognition occurs immediately, as

. soo

n as

the cohort has

been

redu

ced

to a

sin

gle

mem

ber;

in a

n au

dito

ry le

xica

l dec

isio

n ta

sk,

the

deci

sion

that

an

item

is a

non

wor

d ca

n be

mad

e as

soon

as t

here

are

no re

mai

ning

mem

bers

in th

e co

hort,

4. W

ord

reco

gniti

on c

an in

fluen

ce th

e id

entif

icat

ion

of ph

onem

es in

aw

ord

only

afte

r the

wor

d ha

s bee

n re

cogn

ized

,Th

ere

is a

cons

ide~

able

bod

y of

dat

a th

at su

ppor

ts va

rious

pre

dict

ions

of th

e CO

HO

RT m

odel

, It h

as b

een

obse

rved

in a

var

iety

of pa

radi

gms

thai

lexi

cal i

nflu

ence

s on

phon

eme

iden

tific

atio

n re

spon

ses a

re m

uch

grea

ler l

aler

in w

ords

than

at t

heir

begi

nnin

gs (B

agle

y, 1

900;

Col

e an

dJakimik, 1978, 1980; Marslen-Wilson, 1980; Marslen-Wilson and Welsh,

1978

). W

e co

nsid

ered

som

e of

this

evid

ence

in ea

rlier section~.. A

noth

er

impo

rtant

find

ing

suJi)

porti

ng C

OH

ORT

is th

e fa

ct th

at th

e re

attio

n tim

eto

dec

ide

that

an

item

is a

non

wor

d is

cons

tant

, whe

n m

easu

red

from

the

occu

rrenc

e of

the

first

pho

nem

e th

at ru

les

out t

he la

st re

mai

ning

wor

d in

the

coho

rt (M

arsl

en-W

ilson

, 198

0),

Perh

aps t

he m

ost d

irect

supp

ort f

or th

e ba

sic w

ord

reco

gniti

on a

s-su

mpt

ions

of C

OH

ORT

com

es fr

om th

e ga

ting

para

digm

, int

rodu

ced

first

by G

rosje

an (1

980)

, In

this

para

digm

, sub

ject

s are

requ

ired

to g

uess

the

iden

t it y

of a

wor

d af

ter h

earin

g su

cces

sive

pres

enta

tions

of t

he w

ord,

The

first

pre

sent

atio

n is

cut

ofT

so that the subject hears only the first

(N

= 30

to 5

0 in

diff

eren

t stu

dies

). La

ter p

rese

ntat

ions

are

succ

essiv

ely

leng

then

ed in

N-m

s inc

rem

ents

until

eve

ntua

lly th

e w

hole

wor

d is

pre-

sent

ed. T

he d

urat

ion

at w

hich

hal

fthe

subj

ects

corre

ctly

iden

tify

the

wor

dis

calle

d th

e "i

sola

tiQn

poin

t. " C

onsi

dera

bly

mor

e in

put i

s re

quire

d be

fore

subjects are. reason~bly sure of the identity of the w

ord;

that

poi

nt is

tenn

ed th

e "a

ccep

tanc

e po

int." Grosjean

s ini

tial s

tudy

con

firm

ed m

any

basic

pre

dict

ions

of C

OH

ORT

, tho

ugh

it al

so ra

ised

a fe

w d

iffic

ultie

s for

it (s

ee b

elow

). In

a m

ore

rece

nt st

udy

usin

g th

e sa

me

met

hod,

Tyl

er an

dW

esse

ls (1

983)

car

ried

out a

ver

y cl

ose

anal

ysis

of th

e re

latio

n be

twee

nth

e em

piric

ally

det

erm

ined

isol

atio

n po

int a

nd th

e po

int a

t whi

ch th

e in

put

the

subj

ect h

as re

ceiv

ed is

con

sist

ent w

ith o

ne a

nd o

nly

one

rem

aini

ngite

m, t

he p

oint

at w

hich

reco

gniti

on w

ould

be

exep

ecte

d to

occ

ur in

the

CO

HO

RT

mod

el. T

hey

repo

rt th

at th

e is

olat

ion

poin

t fal

ls v

ery

clos

e to

this

theo

retic

ally

der

ived

reco

gniti

on p

oint

, stro

ngly

supp

ortin

g th

e ba

sicimmediacy assumptions of the COHORT model,

It sh

ould

be

note

d th

at th

e ga

ting

task

is n

ot a

tim

ed ta

sk, a

nd so

itdo

es n

ot. p

rovi

de a

dire

ct me

asure of what the su

bjec

t kno

ws a

s the

spee

ch in

put i

s unf

oldi

ng, H

owev

er, i

t is n

ow in

fairl

y w

ide

use,

and

Cotton and Grosjean (1984) have established that the basic patterns of

resu

lts o

btai

ned

in G

rosje

ans (

1980

) pio

neer

ing

gatin

g ex

perim

ent d

o no

tde

pend

on

the

pres

enta

tion

of s

ucce

ssiv

ely

long

er a

nd lo

nger

pres

enta

-

tions

of t

he sa

me

stim

ulus

.A dilemma for COHORT,

Thou

gh th

e CO

HO

RT m

odel

acc

ount

s for

ala

rge

body

of d

ata,

ther

e ar

e se

vera

l diff

icul

t:"s w

ith it

. We

cons

ider

firs

tth

e on

e th

at se

ems t

he m

ost s

erio

us: a

s st

ated

, CO

HO

RT

requ

ires

ac-

cura

te, u

ndist

orte

d in

form

atio

n ab

out t

he id

entit

y of

the

phon

emes

in a

wor

d up

to th

e is

olat

ion

poin

t. W

ords

can

not e

nter

into

cons

ider

atio

nun

less

the

initi

al c

onso

nant

clu

ster p

lus v

owel

is h

eard

. and

they

ar~

discarded from it as soon as a ph

onem

e co

mes

alo

ng th

at th

ey fa

il to

mat

ch. N

o ex

plic

it pr

oced

ure

is de

scrib

ed fo

r rec

over

ing

wor

ds in

to th

eco

hort

once

they

hav

e be

en e

xclu

ded

from

it, o

r whe

n th

e beginning of

the

wor

d is

not

acc

urat

ely

perc

eive

d du

e to

noi

se o

r elis

ion,

Thes

e as

pect

s of C

OH

ORT

mak

e it

very

diff

icul

t for

the

mod

el to

explain recognition of words with distorted be

ginn

ings

. suc

h as

dwib

ble"

(Norris, 1982), or w

ords

who

se b

egin

ning

s hav

e be

en re

plac

edby

noi

se (S

aJas

so &

Piso

ni, 1

985)

. Fro

m a

com

puta

tiona

l poi

nt o

f vie

w,

this

mak

es th

e m

odel

an

extre

mel

y br

ittle

one

; in

parti

cula

r it f

ails

to d

eal

with

the

prob

lem

of n

oise

and

und

ersp

ecifi

catio

n w

hich

is so

cru

cial

for

reco

gniti

on o

f rea

l spe

ech

(Tho

mps

on. 19

84),

The

reco

gniz

abili

ty o

f dist

orte

d ite

ms l

ike

"dwib

ble"

might be taken

as su

gges

ting

that

wha

t we

need

to d

o is

liber

aliz

e th

e cr

iterio

n fo

r en-

terin

g an

d re

tain

ing

wor

ds in

the

coho

rt. T

hus,

the

coho

rt co

uld

be d

e-fin

ed a

s th

e se

t of w

ords

con

sist

ent w

ith w

hat h

as b

een

hear

d or

mild

(e,g" one or two fe

atur

es) d

evia

tions

from

wha

t has

bee

n he

ard.

Thi

swould allow mild distortions like replacing

Irl

with

Iw

l not to disqualify

a w

ord

from

the

coho

rt. It

wou

ld a

lso a

llow

the

mod

el to

cop

e w

ith c

ases

whe

re th

e be

ginn

ing

of th

e w

ord

is un

ders

peci

fied;

in- t

hese

cas

es. t

hein

itial

coh

ort w

ould

sim

ply

be la

rger

than

in th

e ca

se w

here

the

inpu

tde

arly

spec

ified

the

initi

al p

hone

mes

.H

owev

er. t

here

is st

ill a

pro

blem

. Som

etim

es w

e ne

ed to

be

able

toru

le o

ut it

ems w

hich

mism

atch

the

inpu

t on

one

or tw

o di

men

sion

s an

dso

met

imes

we

do n

ot. C

onsid

er th

e ite

ms "

plea

sant" and "

blac

elet

." In

the

first

case

. we

need

to ex

clude "

pres

ent" from the cohort, so the

TRAC

E M

OD

ELMC CLELLAND AND ELMAN

sligh

t diff

eren

ce b

etw

een

III a

nd Ir

l mus

t be

suffi

cien

t to

rule

it o

ut; i

n th

ese

cond

cas

e, w

e do

not

wan

t to

lose

the

wor

d " b

race

let, "

sinc

e it

pro-

vides the best fit overall to the input. Thus, in this case, the difference

betw

een

III a

nd Ir

l mus

t not

be

allo

wed

to ru

le a

wor

d ca

ndid

ate

out.

Thus

the

dile

mm

a: o

n th

e on

e ha

nd, w

e w

ant a

mec

hani

sm th

at w

ill be

able

to s

elec

t the

cor

rect

wor

das

soo

n as

an

undi

stor

ted

inpu

t spe

cifie

sit

uniq

uely

, to

acco

unt f

or th

e Ty

ler a

nd W

esse

ls re

sults

. On

the

othe

rha

nd, w

e do

not

wan

t the

mod

el to

com

plet

ely

elim

inat

e po

ssib

ilitie

sw

hich

mig

ht la

ter t

urn

out t

o . b

e co

rrect

, We

shal

l sho

rtly

see

that

TR

ACE

prov

ides

... w

ay o

ut o

f thi

s dile

mm

a.

Another problem for COHORT.

Gro

sjean

(198

5) h

as re

cent

ly p

oint

edou

t ano

ther

pro

blem

for C

OH

ORT

, nam

ely,

the

poss

ibili

ty th

at th

e su

b-je

ct m

ay b

e un

certa

in a

bout

the location of the beginning of each suc-

cess

ive

wor

d, A

taci

t ass

umpt

ion

of th

e m

odel

is th

at th

e su

bjec

t goe

sin

to th

e be

ginn

ing

of e

ach

wor

d kn

owin

g th

at it

is' t

he b

egin

ning

. In

the

rela

ted

mod

el' o

f Col

e an

d Ja

kim

ik (

1980) this as

sum

ptio

n is

mad

e ex

plic

it.U

nfor

tuna

tely

, it i

s not

alw

ays p

ossib

le to

kno

w in

adv

ance

whe

re o

new

ord

star

ts a

nd th

e ne

xt w

ord

ends

. As

we

disc

usse

d in

the

intro

duct

ion,

acou

stic

cue

s to

junc

ture

are

not

alw

ays

relia

ble,

' and

in th

e ab

senc

e of

acou

stic

cues

, eve

n an

opt

imal

ly e

ffici

ent m

echa

nism

~an

not a

lway

s, know th

at it

has

hea

rd th

e en

d of

one

wor

d un

til it

hea

rs en

ough

of t

hene

xt to

rule

out

the

poss

ible

con

tinua

tions

of t

he fi

rst w

ord.

Wha

t is

need

ed, t

hen.

is a

mod

el th

at c

an a

ccou

nt fo

r CO

HO

RT'

succ

esse

s, an

d ov

erco

me

thes

e tw

o im

porta

nt d

efic

ienc

ies,

The

next

two

sect

ions

show

that

TRA

CE d

oes q

uite

wel

l on

both

cou

nts.

The

first

ofth

ese

sect

ions

exa

min

es T

RACE

' s b

ehav

ior i

n pr

oces

sing

wor

ds w

hose

begi

nnin

gs a

nd e

ndin

gs a

re c

lear

ly d

elin

iate

d fo

r it b

y th

e pr

esen

ce o

fsil

ence

, The

seco

nd c

onsid

ers t

he p

roce

ssin

g of

mul

tiwor

d in

puts,

whi

chth

e m

odel

mus

t par

se fo

r its

elf.

One

Wor

d tit

(t T

ime

In th

is se

ctio

n w

e se

e ho

w T

RACE

reso

lves

the

dile

mm

a fa

cing

CO

-H

ORT

, in

that

it is

imm

edia

tely

sens

itive

to n

ew in

form

atio

n bu

t is

still

able

to c

ope

with

und

ersp

ecifi

ed o

r dist

orte

d w

ord

begi

nnin

gs, W

e al

soco

nsid

er h

ow th

e m

odel

acc

ount

s for

the

pref

eren

ce fo

r sho

rt-w

ord

re-

spon

ses e

arly

in p

roce

ssin

g a

long

wor

d, T

he se

ctio

n co

nclu

des w

ith a

disc

ussio

n of

way

s the

mod

el c

ould

be

exte

nded

to a

ccou

nt fo

r wor

dfre

quen

cy a

nd c

onte

xtua

l inf

luen

ces,

Com

petit

ion

~'S

bottom-lip inhibition.

TRA

CE d

eals

with

CO

HO

Rrs

dile

mm

a by

usin

g co

mpe

titio

n. ra

ther

than

pho

nem

e-to

-wor

d in

hibi

tion.

The

esse

nce

of th

e id

ea is

sim

ply

this,

Pho

nem

e un

its h

ave

exci

tato

ryco

nnec

tions

to a

ll th

e w

ord

units

they

are

con

sist

ent w

ith. T

hus,

whe

n-ev

er a

pho

nem

e be

com

es a

ctiv

e in

a p

artic

ular

slic

e of

the

Trac

e , it

send

s.

exci

tatio

n to

all

the

wor

d un

its c

onsis

tent

with

that

pho

nem

e in

that

slic

e.Th

e w

ord

units

then

com

pete

with

eac

h ot

her;

item

s tha

t con

tain

eac

hsu

cces

sive

phon

eme

dom

inat

e al

l oth

ers,

but i

f no

wor

d "m

atch

es p

er-

fect

ly, a

wor

d th

at p

rovi

des a

clo

se ti

t to

the phoneme sequence can

even

taul

ly w

in o

ut o

ver w

ords

that

pro

vide

less

ade

quat

e m

atch

es. T

heex

act m

etric

of "

clos

enes

s of f

it " d

epen

ds. o

f cou

rse.

on

a la

rge

num

ber

of d

etai

ls, In

the

abse

nce

of su

ch a

met

ric, a

sim

ple

coun

t of t

he n

umbe

rof

aco

ustic

feat

ures

diff

erin

g be

twee

n a

lexi

cal i

tem

. and

a p

rese

nted

stim

-ul

us c

an p

rovi

de a

use

ful f

irst a

ppro

xim

atio

n. b

ut o

ther

fact

ors s

uch

asst

ress

, loc

atio

n of

diff

eren

ces

with

in th

e w

ord,

and

dis

crim

inab

ility

of th

edi

fferin

g fe

atur

e~. w

ill of

cou

rse

com

e into play.

Cons

ider

, fro

m th

is po

int o

f vie

w, o

ur tw

o ite

ms "

plea

sant

" and

"bl

ace-

let"

aga

in, I

n th

e fir

st in

stanc

e. "

plea

sant

" will

rece

ive

mor

e bo

ttom

-up

exci

tatio

n th

an "

pres

ent,"

and so w

ill w

in o

ut in

the

com

petit

ion.

We

have

alre

ady

seen

, in

our a

naly

sis

of c

ateg

oric

al p

erce

ptio

n at

the

pho-

nem

elev

el, h

ow e

ven

slig

ht d

iffer

ence

s in

initi

al b

otto

m-u

p ex

cita

tion

can

be m

agni

fied

by th

e jo

int e

ffect

s of c

ompe

titio

n an

d fe

edba

ck. B

ut th

ere

al b

eaut

y of

the

com

petit

ion

mec

hani

sm is

that

this

actio

n is

cont

inge

nton

the

activ

atio

n of

oth

er w

ord

cand

idat

es, T

hus,

in th

e ca

se o

f "bl

ace-

let"

, sin

ce th

ere

is no

wor

d "b

lace

let

" "

brac

el~t

" will

not

be

SlIp

-pr

esse

d, In

itial

ly, i

t is t

rue,

wor

ds li

ke "

blam

e " and "

blat

ant"

will tend

t~ d

omin

ate

"bra

cele

t," but since the input matches "

brac

elet

" bet

ter

than

any

oth

er w

ord,

"br

acel

et" wjll eventually co

me

to d

omin

ate

the

other possibilities,

This

beha

vior

of t

he m

odel

is . i

llustr

ated

usin

g ~x

ampl

es fr

om it

s re-

stric

ted

lexi

con

in F

ig. 2

4. In

one

cas

e. th

e in

put i

s "le

gal,"

and

the

wor

dre

gal"

is c

ompl

etel

y do

min

ated

by

"leg

al."

In the other case. the input

is "l

ugge

d," a

nd th

e w

ord

"rug

ged"

eve

ntua

lly d

omin

ates

, bec

ause

ther

eis

no w

ord

" lug

ged"

(pro

noun

ced

to rh

yme

with

"ru

gged

"th

e w

ord

lug"

is n

ot in

the

mod

el' s

lexi

con)

, Her

e "r

ugge

d~' m

ust c

ompe

te w

ithot

her p

artia

l mat

ches

of "

lugg

ed,"

of c

ours

e, a

nd it

is le

ss e

ffect

ive

inth

is re

gard

than

it w

ould

be

if th

e in

put ,

exac

tly m

atch

ed it

, but

it d

oes

win

out

in th

e en

d.

It sh

ould

be

note

d th

at th

e de

tails

of w

hat w

ord

will

be

mos

t stro

ngly

activ

ated

in su

ch c

ases

dep

end

on a

num

ber o

f fac

tors

, inc

ludi

ng, i

npa

rticu

lar,

the

disti

nctiv

enes

s of m

ismat

chin

g ph

onem

es. A

lso, i

t is p

os-

sible

to fi

nd c

ases

in w

hich

a w

ord

that

cor

rect

ly sp

ans a

par

t of a

long

erst

ring

dom

inat

es a

long

er w

ord

that

spa

ns th

e w

hole

strin

g bu

t miss

esou

t on

a ph

onem

e in

one

pla

ce o

r ano

ther

. An

item

like

"vi

gore

tte" may

or m

ay n

ot b

e a

case

in p

oint

. In

such

cas

es, t

houg

h, th

e m

ost i

mpo

rtant

thin

g m

ight

not

turn

out

to b

e w

inni

ng a

nd lo

sing,

but

rath

er th

e fa

ct th

atbo

th te

nd to

stay

in th

e ga

me,

Suc

h ne

olog

ism

s ca

n su

gges

t a p

oetic

...

TRAC

E M

OD

ELM

C C

LELI

,-AN

D A

ND

ELM

AN

~Jj

fITCl

J

tEH

'.=-f1

tITiI

q;-

EliT

Ia:

:w

- Jig

~ I

- I

i g

I' a

. . I i g - 1-

- I I

g - I

-llg~l- +2

phon

emes

, In

this

sect

ion,

we

exam

ine

how

wel

l TRA

CE e

mul

ates

the

COH

ORT

mod

el, i

n ca

ses w

here

the

inpu

t is a

n un

disto

rted

repr

esen

ta-

tion

of so

me

parti

cula

r wor

d. In

par

ticul

ar, w

e w

ante

d to

see

how

clo

se.

TRAC

E w

ould

com

e to

beh

avin

g in

acc

ord

with

CO

HO

RT'

s ass

umpt

ion

that

inco

rrect

wor

ds a

re d

ropp

ed fr

om th

e co

hort

of a

ctiv

e ca

ndid

ates

as

soon

as

the

inpu

t div

erge

s fro

m th

em,

To e

xam

ine

this

pro

cess

, we

cons

ider

ed th

e pr

oces

sing

of th

ew

ord

"pro

duct

" (/p

rad"

ct/).

Fig

ure

25 s

how

s th

e st

ate

of th

e Th

ace

atva

rious

poi

nts i

n pr

oces

sing

this

' wor

d, a

nd F

ig. 2

6 sh

ows

the

resp

onse

stren

gths

of s

ever

al u

nits

rela

tive

to th

e str

engt

h of

the

wor

d "p

rodu

ct"

itsel

f, as

a fu

nctio

n of

tim

e re

lativ

e to

the

arriv

al o

f the

succ

essiv

e ph

o-ne

mes

in th

e in

put.

In th

is fig

ure,

the

resp

onse

stre

ngth

of "

prod

uct"

issim

ply

set t

o 1,

0 at

eac

h tim

esl

ice

and

the

resp

onse

stre

ngth

s of

uni

tsfo

r oth

er w

ords

are

plo

tted

in te

rms o

f the

ratio

of t

heir

stren

gth.

div

ided

by th

e str

engt

h of

"pr

oduc

t." T

he c

urve

s sho

wn

are

for t

he w

ords

!'tro

t,po

ssib

le," priest,

" "

prog

ress

," and "

prod

uce

; the

se w

ords

diff

erfro

m th

e w

ord

"pro

duct

" (ac

cord

ing

to th

e sim

ulat

ion

prog

ram

s st

ress

-le

ss e

ncod

ing

of th

em!)

ir, th

e 1s

t, 2n

d, 3

d, 4

th, a

nd 5

th p

hone

mes

, re-

spec

tivel

y, F

igur

e 26

show

s tha

t the

se it

ems b

egin

to d

rop

out o

f "co

n-te

ntio

n" ju

st af

ter e

ach

succ

essiv

e ph

onem

e co

mes

in, O

f cou

rse,

ther

eis

noth

ing

hard

and

fast

or a

bsol

ute

abou

t dro

ppin

g a

cand

idat

e in

TRAC

E. W

hat w

e se

e in

stea

d is

that

mis

mat

chin

g ca

ndid

ates

sim

ply

begi

n to

fade

as t

he in

put d

iver

ges f

rom

them

in fa

vor o

f som

e ot

her

cand

idat

e. T

his i

s jus

t the

kin

d of

beh

avio

r the

CO

HO

RT m

odel

wou

ld

fi' -j!

"...!!

;

G (E!J

If-;;-

'(E

!J

S - r I

lit I

.,;:!d

" If

tE.i.

.!t

~ d

- I -

g ~

d -

- I

'to

I..ra

k

II

i..u"

l8l8

tIl81

11tr

~ ~I

u 91

- p

r ad

- kr a

ull

I II u

DfI

g-d-

-g-

d- -

,;-d- +2

FIG, 24, SIBle of Ihe lrace al

lwo

poin

ls during processing of "'eg

a'" a

nd "

'ugg

ed.

conj

unct

ion

of m

eani

ngs,

if us

ed ju

st rig

ht: "

He

wal

ked

brisk

ly d

own

the

stre

et, p

uffin

g hi

s vi

gore

tte.

Tim

e C

Ollr

.fe of w( ird recognition in

TRA

CE.

So fa

r we

have

sho

wn

how

TRA

CE o

verc

omes

a d

ificu

lty w

ith th

e CO

HO

RT m

odel

in c

ases

whe

re th

e be

ginn

ing

of a

wor

d ha

s bee

n di

storte

d. In

ear

lier s

ectio

ns o

nph

onem

e pr

oces

sing

,som

e of

the

simul

atio

ns il

lustr

ate

that

the

mod

el is

capa

ble

of re

cogn

izin

g w

ords

with

und

ersp

ecifi

ed (i

, e" a

mbi

guou

s) in

itial

prad

~kl-

-pra

d-kl

- -pr

ad~k

l- -p

rad-

kl-

FIG, 25. State of th

elhl

ce a

t var

ious

poi

nts i

n pr

oces

sing

the

wor

d "p

rodu

ct" (/prad'kt

/'.

12 1

8 24

30

38 4

2 48

!i4

80 8

8 Pr

oces

sing

Cycl

es

FIG

. 26.

Res

pons

e st

reng

ths

of tt

~ u

nits

for s

ever

al w

ords

rela

tive

to th

e re

spon

se s

treng

thof

the

unit

for "

prod

uct"

(/pr

ad- k

t/). a

s a fu

nctio

n of

tim

e re

lativ

e to

the

peak

of t

he fi

rst

phon

eme

that

fails

to m

atch

the

wor

d, T

he s

ucce

ssiv

e cu

rves

com

ing

off o

f the

hor

izon

tal

line

repr

esen

ting

the

norm

aliz

ed re

spon

se st

reng

th o

f "pr

oduc

t" a

re fo

r the

wor

ds "

Irot." ,

poss

ible

," "

prie

sl,

" "

prog

ress

, " and "

prod

uce." respectively. In our lexicon they are

, rendered as

Itnil/

, Ip

asI/,

Ipris

t/, Ip

ragr

s/, a

nd Ip

radu

s/, r

espe

ctiv

ely.

.... ~

2.~

1.

.... VJ 1

.80

~ 1.

S 1. 10

0Q

) ,D::: 0.

Q) 0

.~

0.'ii

' 0.

D::: 0.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

We

wer

e at

firs

t som

ewha

t dist

urbe

d by

this

aspe

ct o

f the

mod

el's

beha

vior

, but

it tu

rns o

ut to

cor

resp

ond

quite

clo

sely

with

resu

lts o

btai

ned

in e

xper

imen

ts by

Gro

sjean

(198

0) a

nd C

otto

n an

d G

rosj

ean

(l9H

4) u

sing

the

gatin

g pa

radi

gm, B

oth

pape

rs fo

und

that

subj

ects

hear

ing

the

begi

n-ni

ngs o

f wor

ds li

ke "

capt

ain "

tended to report shorter words consistent

with

wha

t the

y ha

d he

ard

(e.g.

, "

cap

). H

owev

er, w

e sh

ould

obs

erve

that

in th

e ga

ting

para

digm

, whe

n th

e w

ord

" cap

tain

" is truncated just

after the

Ipl,

it w

ill so

und

quite

a b

it lik

e " c

ap" followed by silence, In

TRA

CE, t

his s

ilenc

e w

ould

act

ivat

e sil

ence

uni

ts at

the

phon

eme

and

wor

d le

vels

, and

the

wor

d-le

vel s

ilenc

e un

its w

ould

com

pete

with

uni

tsfo

r wor

ds th

at e

xten

d in

to th

e si

lenc

e. It

will

rein

forc

e th

e pr

efer

ence

of

the

mod

el fo

r sho

rt-w

ord

inte

rpre

tatio

ns, b

ecau

se th

e de

tect

ion

of th

esil

ence

will

inhi

bit t

he d

etec

tor f

or th

e lo

nger

wor

d, T

hus,

ther

e ar

e ac

-tu

ally

two

reas

ons w

hy T

RACE

mig

ht fa

vor s

hort-

wor

d in

terp

reta

tions

over

long

-wor

d in

terp

reta

tions

in a

gat

ing

expe

rimen

t. W

heth

er h

uman

subj

ects

show

a re

sidua

l pre

fere

nce

for s

horte

r int

erpr

etat

ions

ove

r lon

ger

ones

in th

e ab

senc

e of

a fo

llow

ing

sile

nce

durin

g th

e co

urse

of p

roce

ssin

gis

not y

et c

lear

from

ava

ilabl

e da

ta,

We

shou

ld p

oint

out

that

the

expe

rimen

tal l

itera

ture

indi

cate

s tha

t the

adva

ntag

e of

sho

rter w

ords

ove

r lon

ger o

nes

hold

s on

ly u

nder

the

spec

ial

circ

umsta

nces

of g

ated

pre

sent

atio

n an

d th

en o

nly

with

ear

ly g

ates

, whe

nsh

orte

r wor

ds a

re re

lativ

ely

mor

e co

mpl

ete

than

long

er o

nes w

ould

be.

It ha

s bee

n w

ell k

now

n fo

r a lo

ng ti

me

that

long

er w

ords

are

gen

eral

lym

ore

read

ily re

cogn

ized

than

sho

rter o

nes

whe

n th

e w

hole

wor

d is

pre

-se

nted

for i

dent

ifica

tion

agai

nst a

bac

kgro

und

of n

oise

(Lic

klid

er &

Mill

er, 1

951)

, Pre

sum

ably

, the

reas

on fo

r thi

s is s

impl

y th

at lo

nger

wor

dsge

nera

lly p

rovi

de a

larg

er n

umbe

r of c

ues t

han

shor

ter w

ords

do

and

henc

e ar

e sim

ply

less

con

fusa

ble.

Frequency a"d context effects,

Ther

e ,a

re, o

f cou

rse ,

oth

er fa

ctor

sw

hich

influ

ence

whe

n w

ord

reco

gniti

on w

ill o

ccur

bey

ond

thos

e w

e ha

veco

nsid

ered

thus

far,

Two

very

impo

rtant

one

s are

wor

d fre

quen

cy a

ndco

ntex

tual

pre

dict

abilit

y, T

he li

tera

ture

on

thes

e t)o

Vo

fact

ors g

oes b

ack

to the turn of the century (Bagley, 1900).\ M

orto

ns (

1969

) log

ogen

mod

elef

fect

ivel

y de

als w

ith se

vera

l im

porta

nt a

spec

ts of

this

huge

lite

ratu

rethough not with the time course of these effects,

We

have

not

yet

incl

uded

eith

er w

ord

frequ

ency

or h

ighe

r lev

el c

on-

text

ual i

nflu

ence

s in

TRA

CE, t

houg

h of

cou

rse

we

belie

ve th

ey a

re im

-po

rtant

.Wor

d fre

quen

cy e

ffect

s cou

ld b

e ac

com

mod

ated

, as t

hey

wer

ein

the

inte

ract

ive-

activ

atio

n m

odel

of w

ord

reco

gniti

on, i

n te

rms

of v

arat

ion

in th

e re

sting

act

ivat

ion

leve

l of w

ord

units

, or i

n te

rms o

f var

iatio

nin

the

stren

gth

of p

hone

me-

to-w

ord

conn

ectio

ns. C

onte

xtua

l inf

luen

ces

can

be th

ough

t of a

s sup

plyi

ng a

ctiv

atio

n to

wor

d un

its fr

om e

ven

high

erle

vels

of p

roce

ssin

g th

an th

e w

ord

leve

l. In

this

way

, bas

ic a

spec

ts of

prod

uce

in th

is c

ase,

thou

gh o

f cou

rse

the

drop

-off

wou

ld b

e as

sum

ed to

be a

n ab

rupt

, dis

cret

e ev

ent,

Ther

e is

one

asp

ect o

f TR

ACE'

s beh

avio

r whi

ch d

iffer

s fro

m th

aI o

fCO

HO

RT: a

mon

g th

ose

wor

ds th

at a

re c

onsis

t~nt

with

the

inpu

t up

to a

parti

cula

r poi

nt in

tim

e. T

RACE

show

s a b

ias i

n fa

vor o

f sho

rter w

ords

over

long

er w

ords

, Thu

s, " p

riest"

has

a sl

ight advantage before the

Ial

com

es in

, and

"pr

oduc

e" is well ahead of "pr

oduc

t" u

ntil

the

tlcom

esin

(in

phon

emes

, "pr

oduc

e." i

s one

shor

ter t

han

" pro

duct

"Th

is ad

vant

age

for s

horte

r wor

ds is

due

to th

e co

mpe

titio

n m

echa

nism

,Re

call

that

wor

d un

its c

ompe

te w

ith e

ach

othe

r in

prop

ortio

n to

the

over

lap

of th

e se

ts of

tim

e sli

ces s

pann

ed b

y ea

ch o

f the

wor

ds, O

verla

pis,

of c

ours

e, sy

mm

etric

al, s

o lo

ng a

nd sh

ort w

ords

inhi

bit e

ach

othe

r to

an e

qual

ext

ent.

But l

onge

r wor

ds s

uffe

r mor

e in

hibi

tion

from

oth

er lo

ngw

ords

than

shor

t wor

ds d

o. F

or e

xam

ple,

"pr

ogre

ss" and "

prob

able

inhi

bit "

prod

uct "

mor

e th

an th

ey in

hibi

t "pr

iest"

and

"pr

oduc

e." T

hus

units

for l

onge

r wor

ds a

re g

ener

ally

subj

ecte

d to

ext

ra in

hibi

tion,

par

tic-

ular

ly e

arly

on

whe

n m

any

cand

idat

es a

re a

ctiv

e, a

nd so

they

tend

tosu

ffer i

n co

mpa

rison

to sh

ort w

ords

as a

resu

lt.3

The

data

repo

rted

by T

yler

and

Wes

sels

actu

ally

app

ears

to in

dica

te a

n e,ven more

imm

edia

te d

rop-

off t

han

is se

en in

this

simul

atio

n. H

owev

er, i

l sho

uld

be re

mem

bere

d t h

atth

e cu

rves

show

n in

Fig

. 26

are

on- li

ne re

spon

se st

reng

th c

urve

s, an

d th

us re

nec t

the lags

inherent in the percolation of input from the feature to the word level. The gating task, on

the other hand, does not require subjects 10 respond on,lin

e. "I

f the

i npu

t is'

sim

ply

turn

edof

f at t

he p

eak

of e

ach

phon

eme

s inp

ut sp

ecifi

catio

n, a

nd th

en a

llow

ed to

run

free

for a

few

cyc

les,

the

drop

out p

oint

shift

s eve

n ea

rlier

.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

thes

e tw

o ki

nds o

f inn

uenc

es c

an b

e ca

ptur

ed. W

e le

ave

it to

futu

rere

sear

ch. h

owev

er. t

o de

term

ine

to w

hat e

xten

t the

se el

abor

atio

ns o

fTR

ACE

wou

ld p

rovi

de a

det

aile

d ac

coun

t of t

he d

ata

on th

e ro

les

ofth

ese

fact

ors,

For

now

. we

turn

to th

e pr

oble

m o

f det

erm

inin

g w

here

one

wor

d en

ds a

nd th

e ne

xt o

ne b

egin

s,

we

can

say

eith

er it

em in

a w

ay th

at m

akes

it so

und

like

a si

ngle

wor

dor

like

two

wor

ds, t

here

is a

n in

term

edia

te w

ay o

f say

ing

them

so th

atth

e fir

st se

ems t

o be

two

wor

ds a

nd th

e se

cond

seem

s lik

e on

ly o

ne.

To se

e w

hat T

RACE

II w

ould

do

with

sing

le- a

nd m

ultip

le-w

ord

inpu

ts,w

e ra

n sim

ulat

ion

expe

rimen

ts w

ith e

ach

indi

vidu

al w

ord

in th

e m

ain

2 I 1

-w

ord

lexi

con

prec

eded

and

follo

wed

by

sile

nce,

and

then

with

2 I

I pai

rsof

wor

ds, w

ith a

sile

nce

at th

e be

ginn

ing

and

at th

e en

d of

the

entir

est

ream

. The

pai

rs w

ere

mad

e by

sim

ply

perm

utin

g th

e le

xico

n tw

ice

and

then

abu

tting

the

two

perm

utat

ions

so

that

eac

h' w

ord

occu

rred

once

as

the

first

wor

d an

d on

ce a

s the

seco

nd w

ord

in th

e en

tire

set o

f 211

pai

rs,

We

stres

s, of

cou

rse,

that

real

spee

ch w

ould

tend

to c

onta

in c

ues t

hat

wou

ld m

ark

wor

d bo

unda

ries i

n m

any

case

s; th

e ex

peri

men

t is

sim

ply

desig

ned.

to sh

ow w

hat T

RACE

wou

ld d

o in

cas

es w

here

thes

e cu

es a

rela

ckin

g,W

ith th

e in

divi

dual

wor

ds, T

RAC

E m

ade

no m

ista

kes-

that

is, b

y a

few

slic

es a

fter t

he e

nd o

f the

wor

d, th

e w

ord

that

span

ned

the

entir

ein

put w

as m

ore

stron

gly

activ

ated

than

any

oth

er w

ord,

An

exam

ple

ofthis is shown using the item

Ipar

til

in Fig. 27. The stream

Ipar

til

mig

ht b

eei

ther

one

wor

d ("

party

) or t

wo

("par tea" or "par tee

the

mod

elknows of only one word pronounced

Iti/).

At

ear

ly p

oint

s in

'pro

cess

ing

the

wor

d, "

par" dominates over "

party

" and other longer words, for

reas

ons d

iscus

sed

in th

e pr

evio

us se

ctio

n, B

y th

e tim

e th

e m

odel

has

had

a ch

ance

to p

roce

ss th

e en

d of

the

wor

d, ho

wever, "

party

" comes to

dominate. .

Why

doe

s a si

ngle

long

er w

ord

even

tual

ly w

in o

ut o

ver t

wo

shor

ter

Lexi

cal B

as;. ~

(~r W

OI:d

Seg

men

tatio

nH

ow d

o w

e kn

ow w

hen

one

wor

d en

ds a

nd th

e ne

xt w

ord

begi

ns?

This

is b

y no

mea

ns a

n ea

sy ta

sk. a

s w

e no

ted

in th

e in

trodu

ctio

n, T

o re

cap

our e

arlie

r arg

umen

t. th

ere

are

som

e cu

es in

the

spee

ch st

ream

. but

as

seve

ral i

nves

tigat

ors h

ave

poin

ted

out (

Cole

& Ja

kim

ik. 1

980;

Gro

sjea

n& Gee. 19

84: T

hom

pson

. 198

4), t

hey

are

not a

lway

s suf

ficie

nt. p

artic

u-la

rly in

flue

nt sp

eech

, It w

ould

thus

app

ear t

hat t

here

is a

n im

porta

nt ro

lefo

r lex

ical

kno

wle

dge

to p

lay

in d

eter

min

ing

whe

re o

ne w

ord

ends

and

the

next

wor

d be

gins

; as

wel

l as

in id

entif

ying

the

obje

cts

that

resu

lt fro

mthe process of segmentation. Indeed. as Re

ddy

(197

6) h

as su

gges

ted,

segm

enta

tion

and

iden

tific

atio

n m

ay b

e jo

int r

esul

ts of

the

mec

hani

sms

of word recognition.

Col

e an

d Ja

kim

ik (1

980)

dis

cuss

thes

e po

ints

and

pre

sent

evi

denc

e th

atse

man

tic a

nd sy

ntac

tic c

onte

xt c

an g

uide

segm

enta

tion

in c

ases

whe

reth

e le

xico

n is

cons

is~en

t with

two

read

ings

("ca

r go"

vs "

carg

o).

Our

pres

ent m

odel

lack

s ~yn

tact

ic a

nd se

man

tic le

vels.

so it

can

not m

ake

use

of th

ese

high

er le

ver c

onstr

aint

s; bu

t it c

an m

ake

use

of it

s kn

owle

dge

about words, not only to identify individual w

ords

in is

olat

ion,

but

topi

ck o

ut a

sequ

ence

of w

ords

in c

ontin

uous

stre

ams o

f pho

nem

es. W

ord

iden

tific

atio

n an

d se

~men

tatio

nem

erge

toge

ther

from

the

i"te

riJct

ive~

ac-

tivat

ion

proc

ess,

as

part

and

parc

el o

f the

pro

cess

of w

ord

activ

atio

n,Th

is se

ctio

n co

nsid

ers s

ever

al a

spec

ts of

the

way

in w

hich

wor

d se

g-

men

tatio

n em

erge

s f~o

m th

e in

tera

ctiv

e-ac

tivat

ion

proc

ess,

as o

bser

ved

in si

mul

atio

ns w

ith T

RACE

II, B

efor

e 'w

e co

nsid

er th

ese,

it is

wor

thre

callin

g th

e de

tails

elf

som

e of

the

assu

mpt

ions

mad

e ab

out t

he b

otto

m-

up a

ctiv

atio

n of

wor

d' u

nits

and

abou

t com

petit

ive

inhi

bitio

n be

twee

n w

ord

units

. Firs

t, th

e ex

tent

to w

hich

a p

artic

ular

pho

nem

e ex

cite

s a p

artic

ular

wor

d un

it is

inde

pen d

ent o

f the

leng

th o

f the

wor

d, S

econ

d, th

e ex

tent

to w

hich

a p

artic

ular

wor

d un

it in

hibi

ts an

othe

r wor

d un

it is

prop

ortio

nal

to th

e te

mpo

ral o

verla

p of

the

two

wor

d un

its. T

his

mea

ns th

at w

ords

whi

ch d

o no

t ove

rlap

in ti

me

will

not

inhi

bit e

ach

othe

r, bu

t will

gan

g up

on o

ther

wor

ds th

at p

artia

lly o

verla

p ea

ch o

f the

m. T

hese

two

assu

mp-

tions

form

mos

t of t

h~ b

asis

of th

e ef

fect

s we

obse

rve

in th

e sim

ulat

ions

,

..,.

11/e bollndary

~ ill th

e ea

r of t

he "

behe

Ol'e

I",

Firs

t, w

e co

nsid

er th

eba

sic fa

ct th

at th

e nu

mbe

r of w

ords

we

hear

in a

sequ

ence

of p

hone

mes

can

depe

nd o

n ou

r kno

wle

dge

of th

e nu

mbe

r of w

ords

the

sequ

ence

mak

es. C

onsid

er th

e tw

o ut

tera

nces

, "sh

e ca

nt"

and

"se

cant

". T

houg

h

ili:IT

I:nili

:ITI:n

m.u

II r

aft I

ItP

~IT

JJ

iIJID

iIJID

II r t

II r

i -

u I P

u I

pllrti- -pB

rti- -

pBrl'

- -

pBrli

-Fl

o. 2

7. T

he s

tate

of t

he 'lh

tce

at v

ario

us p

oint

s du

ring

proc

essin

g of

Ipar

ti/,

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

m:IT

rnffi

:WilJ

Jim

B

m::!

LDa r l

m::u

:rni!!

IIDilJ

Ju:

:nliJ

ll -

l . -

b r l

i- P

r i- P

- a

and

"tea

do n

ot o

verla

p. T

hus,

"ar

t " receives in

hibi

tion

from

hot

hba

r " and "

tea," while "

bar " and "

tea "

each receive in

hibi

tion

only

from "

art." Thus two words th

at d

o no

t ove

rlap

with

eac

h ot

her c

anga

ng u

p on

a th

ird e

ach

over

laps

with

par

tly, a

nd d

rive

it ou

l.Th

ese

rem

arka

bly

simpl

e m

echa

nism

s of a

ctiv

atio

n an

d co

mpe

titio

n do

a ve

ry g

ood

job

of w

ord

segm

enta

tion,

with

out t

he a

id o

f any

sylla

bifi-

catio

n, st

ress

, pho

netic

wor

d bo

unda

ry c

ues,

or se

man

tic a

nd sy

ntac

ticconstraints. In 189 of the 211 w

ord

pairs

test

ed in

the

sim

ulat

ion

expe

r-im

ent,

the

mod

el c

ame

up w

ith th

e co

rrect

par

se, i

n th

e se

nse

that

no

othe

r wor

d w

as m

ore

activ

e th

an e

ither

of t

he tw

o w

ords

that

had

bee

npr

esen

ted,

Som

e of

the

failu

res o

f the

mod

el o

ccur

red

in c

ases

whe

re th

ein

put w

as a

ctua

lly c

onsis

tent

with

two

pars

es, e

ither

a lo

nger

span

ning

wor

d ra

ther

than

a si

ngle

wor

d (a

s in

" par

ty) o

r a d

iffer

ent p

arse

into

two

wor

ds, a

s in

" part rust" f

or "

par trusl." In such cases T

RACE

tend

sto

pre

fer p

arse

s in

whi

ch th

e lo

nger

wor

d co

mes

firs

t. Th

ere

wer

e, h

ow-

ever

, som

e ca

ses i

n w

hich

the

mod

el d

id n

ot c

ome

up w

ith a

val

id p

arse

,th

at is

, a p

atte

rn th

at re

pres

ents

com

plet

e co

vera

ge o

f the

inpu

t hy

a se

tof

non

over

lapp

ing

wor

ds. F

or e

xam

ple,

con

side

r the

inpu

t Ipa

rki/.

Though this makes the

two

wor

ds "

par " and "

key," the word "

park

"has a stronger activation than either "

par" or "

key." as illustrated in

Fig. 28,

This

aspe

ct o

f TRA

CE II

' s b

ehav

ior i

ndic

ates

that

the

pres

ent v

crsi

onof

the

mod

el is

far f

rom

the

final

wor

d on

wor

d se

gmen

tatio

n. A

com

plet

em

odel

wou

ld a

lso e

xplo

it sy

llabi

ficat

ion,

stre

ss, a

nd o

ther

cue

s to

wor

did

entit

y to

hel

p el

imin

ate

som

e of

the

poss

ible

inte

rpre

tatio

ns o

f TRA

CEII'

s si

mpl

e ph

onem

e st

ream

s, T

he' a

ctiv

atio

n an

d co

mpe

titio

n m

echa

-ni

sms i

n TR

ACE

II a

re s

uffic

ient

to d

o qu

ite a

bit

of th

e w

ord

segm

en-

tatio

n w

ork,

but

we

do n

ot e

xpec

t the

m to

do

this

perfe

ctly

in a

ll ca

ses

with

out t

he a

id ' o

f other cues.

Som

e re

ader

s may

be

troub

led

by a

mec

hani

srr t

hat d

oes

not i

nsis

tup

on a

par

se in

whi

ch e

ach

phon

eme

is co

vere

d by

one

and

onl

y on

ew

ord,

Act

ually

, tho

ugh,

this

char

acte

ristic

of t

he m

odel

is o

ften

a vi

rtuc,

since

in m

any

case

s the

last

phon

eme

ofh

wor

d m

ust d

o do

uble

dut

y as

the

first

phon

eme

of th

e ne

xt, a

s in

"hou

nd d

og" o

r "brush shop," Whilc

spea

kers

tend

to s

igna

l the

dou

blin

g in

car

eful

spe

ech,

the

cues

to s

ingl

evs

dou

ble

cons

onan

ts a

re n

ot a

lway

s su

ffici

ent f

or d

isam

bigu

atio

n, a

s is

clea

r whe

n str

ings

with

mul

tiple

inte

rpre

tatio

ns a

re u

sed

as st

imul

i. Fo

rex

ampl

e, a

n ut

tera

nce

inte

nded

as

'no

notio

n" w

ill so

met

imes

be

hear

das

"kn

own

notio

n" (N

akat

ani &

Duk

es, 1

977)

. The

mod

el is

not

incl

ined

to su

ppre

ss a

ctiv

atio

ns o

f par

tially

ove

rlapp

ing

wor

ds, e

ven

whe

n a

non-

over

lapp

ing

pars

e is

avai

labl

e. T

his b

ehav

ior o

f TRA

CE is

illu

strat

ed w

ithIb

Asta

pl ("

bus top" or "

bus

stop

) in

Fig.

29.

In th

is ca

se, h

ighe

r lev

els

coul

d pr

ovid

e an

add

ition

al so

urce

of i

nfor

mat

ion

that

wou

ld h

elp

the

mod

el c

hoos

e be

twee

n ov

erla

ppin

g an

d no

nove

rlapp

ing

inte

rpre

tatio

ns.

ones

in T

RAC

E? T

here

are

two

mai

n re

ason

s, F

irst o

f all,

a lo

nger

wor

dev

entu

ally

rece

ives

mor

e bo

ttom

-up

supp

ort t

han

eith

er s

horte

r wor

d,si

mpl

y be

caus

e th

ere

are

mor

e ph

onem

es a

ctiv

atin

g th

e lo

nger

wor

d th

anth

e sh

orte

r wor

d. T

he se

cond

reas

on h

as to

do

with

the

sequ

entia

l nat

ure

of th

e in

put,

In th

e ca

se o

f Ipa

rtil,

by th

e tim

e th

e Iti

l is

com

ing

in, t

heword "

party

" is w

ell e

noug

h es

tabl

ished

that

it k

eeps

Itil

from

get

ting

asst

rong

ly a

ctiv

ated

as

it w

ould

oth

erw

ise,

as

illus

trate

d in

Fig

. 27.

Thi

sbe

havi

or o

f the

mod

el le

ads

to th

e pr

edic

tion

that

sho

rt w

ords

em

bedd

edin

the

ends

of l

onge

r wor

ds sh

ould

not

get

as s

trong

ly a

ctiv

ated

as s

horte

rw

ords

com

ing

earli

er in

the

long

er w

ord.

Thi

s pre

dict

ion

coul

d be

teste

dus

ing

the

gatin

g pa

radi

gm, o

r a c

ross

-mod

al p

rimin

g pa

radi

gm su

ch a

sth

e on

e us

ed b

y Sw

inne

y (1

982)

.H

owev

er, i

t. sh

ould

be

note

d th

at th

is a

spec

t of t

he b

ehav

ior o

f the

mod

el c

an b

e ov

errid

den

if th

ere

is b

otto

m-u

p in

form

atio

n fa

vorin

e th

etw

o-w

ord

inte

rpre

tatio

n, C

urre

ntly

, thi

s ca

n on

ly h

app~

n in

TRA

CEth

roug

h th

e in

serti

on o

f a b

rief s

ilenc

e be

twee

n th

e "p

ar" and the "te

a.A

s sho

wn

in F

ig, 2

8, th

is re

sults

in "

par"

and

"te

a" d

omin

atin

g al

l oth

erword candidates,

Wha

t hap

pens

whe

n th

ere

is no

long

wor

d th

at sp

ans t

he e

ntire

stre

am,

as in

Ibar

til?

In th

is ca

se, t

he m

odel

settl

es o

n th

e tw

o-w

ord

inte

rpre

tatio

nba

r tea

," a

s sho

wn

in F

ig, 2

8. N

ote

that

oth

er w

ords

, suc

h as

"ar

t,th

at sp

an a

porti

on o

f the

inpu

t, ar

e le

ss su

ccessful than either "

bar " or

tea.

" The

reas

on is

that

the

inte

rpre

tatio

ns "

bar" and "

art" overlap

with

eac

h ot

her,

and

" art

" and

"te

a" o

verla

p w

ith e

ach

othe

r, bu

t "ba

r

part

i- ' +

3 -p

ar-t

1-+3

-bar

f j- +

3 -p

ark i- +3

FIG

. 28,

Sla

le o

f the

'lta

ce a

fter p

roce

ssin

a th

e str

eam

s /pa

rliJ,

/par

-tiJ,

/bar

liJ, u

nit I

park

i/.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

~!'

fl~P -

- b

esta

blish

whe

re a

wor

d w

ill e

nd e

ven

befo

re i.

act

ually

doe

s end

, par

tic-

ular

ly in

the

case

of l

onge

r wor

ds o

r whe

n ac

tivat

ions

at t

he w

ord

leve

lar

e ai

ded

by sy

ntac

tic a

nd se

man

tic c

onstr

aint

s, H

owev

er, i

t is m

uch

hard

er to

esta

blish

the

end

of a

non

wor

d, si

nce

the

fact

that

it is

a n

on-

w()r

d m

eans

that

we

cann

ot e

xplo

it an

y kn

owle

dge

of w

here

it sh

ould

end

to d

oso

,Th

is fa

ct m

ay a

ccou

nt fo

r the

find

ing

of Fo

ss and Blank (1

980)

that

subj

ects

are

muc

h sl

ower

to re

spon

d to

targ

et p

hone

mes

at t

he b

egin

ning

of a word preceded by a non w

ord

than

at t

he b

egin

ning

of a

wor

dpreceded by a word. For example, responses to detect word initial

Idl

wer

e fa

ster

in s

timul

i lik

e th

e fo

llow

ing:

At th

e en

d of

last

yea

r, th

e go

vern

men

t dec

ided

. . .

b-sl

ap- -

br-S

ap.

FIG. 29. State of the Trace at the end of the streams Ibustapl ("bus stop" o

r "bus top

and

Ibru

Sapl

("br

ush

shop

than

they

wer

e w

hen

the

wor

d pr

eced

ing

the

targ

et (i

n th

is ca

se g

over

n-m

ent)

was

repl

aced

by

a no

nwor

d su

ch a

s "gatabont." It should be noted

that

the

targ

ets

wer

e sp

ecifi

ed a

s w

ord-

initi

al se

gmen

ts, T

here

fore

, the

subj

ects

had

not o

nly

to id

entif

y th

e ta

rget

pho

nem

e, th

ey h

ad to

det

er-

min

e th

at it

fell

at th

e be

ginn

ing

of a

wor

d, a

s, w

ell,

The

faef

that

reac

tion

times

wer

e fa

ster w

hen

the

targ

et w

as p

rece

ded

by a

wor

d su

gges

ts th

atsu

bjec

ts w

ere

able

to u

se th

eir k

now

ledg

e of

whe

re th

e w

ord

"gov

ern-

men

t" e

nds t

o he

lp th

em d

eter

min

e w

here

the

next

wor

d be

gins

,A

n ex

ampl

e of

how

TRA

CE a

llow

s one

wor

d to

hel

p es

tabl

ish w

here

its su

cces

sor b

egin

s is i

llustr

ated

in F

ig, 3

0; In

the

exam

ple,

the.

mod

elre

ceiv

es th

e st

ream

"possible target" or "

pagu

sle ta

rget

." and we

imag

ine

that

the

targ

et is

wor

d-in

itial

/t/. I

n th

e fir

st ca

se. t

he w

ord

"pos

-, s

ible

is cl

early

esta

blish

ed a

nd c

ompe

titor

s und

erne

atIJ

it h

ave

been

completely crushed by the time the initial

It I

in "

targ

et" b

ecom

es a

ctiv

eat

the

phon

eme

leve

l (se

cond

pan

el in

the

uppe

r par

t of t

he fi

gure

). so

there is no ambiguity about the fact that this

It I

is a

t the

beg

inni

ng o

f the

next

wor

d, (T

he d

ecisi

on m

echa

nism

wou

ld. o

f cou

rse.

be

requ

ired

tono

te th

at th

e m

odel

had

established the location of the end of the

prec

edin

g w

ord,

We

have

not

yet

inco

rpor

ated

exp

licit

assu

mpt

ions

abo

utho

w th

is w

ould

be

done

.) In

the

seco

nd c

ase.

wor

ds b

egin

ning

and

end

ing

at a

num

ber o

f. di

ffere

nt p

lace

s. in

clud

ing

som

e th

at o

verla

p w

ith th

elocation of the

Itl,

are

partl

y ac

tivat

ed, T

hus.

the

subj

ect w

ould

hav

e to

wai

t unt

il he

is w

ell i

nto

the

wor

d "t

arge

t" b

efor

e it

beco

mes

cle

ar th

atthe first

It I

in ta

rget

is in

fact

a w

ord-

initi

al

It/.

In re

ality

, the

situ

atio

n is

prob

ably

not

as b

leak

for t

he p

erce

iver

as i

tap

pear

s in

this

exam

ple,

bec

ause

in m

any

case

s the

re w

ill b

e cu

es in

the

man

ner o

f pro

nunc

iatio

n an

d th

e sy

llabi

ficat

ion

of th

e in

put t

hat w

ill h

elp

to in

dica

te th

e lo

catio

n of

the

wor

d bo

unda

ry. H

owev

er. g

iven

'the

im-

prec

ision

and

freq

uent

abs

ence

of s

uch

cues

. it i

s not

surp

risin

g th

at th

e

The

sim

ulat

ions

we

have

repo

rted

show

that

the

wor

d ac

tivat

ion/

com

-pe

titio

n m

echa

nism

can

go

a lo

ng w

ay to

war

d pr

ovid

ing

a co

mpl

ete

in-

terpretation of the input stream as a seq,u

ence

of w

ords

, As a

wor

d is

begi

nnin

g to

com

e in

. the

mod

el te

nds

to p

refe

r sho

rter w

ords

con

sist

ent

with

the

inpu

t stre

am o

ver l

onge

r one

s. A

s the

input unfolds through

time.

how

ever

. the

mod

el te

nds t

o pr

efer

to in

terp

ret s

treams of pho-

nem

es a

s sin

gle

long

er w

ords

rath

er th

an a

s a se

quen

ce o

f sho

rt w

ords

;an

d it

tend

s to

find

pars

es th

at a

ccou

nt fo

r eac

h ph

onem

e on

ce. B

ut it

does

not

insi

st u

pon

this

. and

will

occa

sion

ally

pro

duce

an

inte

rpre

tatio

nth

at le

aves

par

t of t

he st

ream

of p

hone

mes

una

ccou

nted

for o

r whi

chac

coun

ts fo

r par

t of t

he s

tream

of p

hone

mes

twic

e, O

ften

enou

gh, i

t will

also

leav

e an

alte

rnat

ive

to it

s "pr

efer

red

paise

" in a strong position. so

that both the preferred parse and the al

tern

ativ

e w

ould

be

avai

labl

e to

high

er le

vels

and

sub

ject

to p

ossi

ble

rein

forc

emen

t by

them

.Th

us fa

r in

this

sect

ion,

we

have

con

sider

ed th

e ge

nera

l pro

perti

es o

fth

e w

ay in

whi

ch T

RACE

use

s lex

ical

info

rmat

ion

to se

gmen

t a sp

eech

stre

am in

to w

ords

. but

we

have

not

con

side

red

muc

h in

the

way

of e

m-

piric

al d

ata

that

thes

e as

pect

s of t

he m

odel

shed

ligh

t on,

How

ever

. the

rear

e tw

o fin

ding

s in

the

liter

atur

e w

hich

can

be

inte

rpre

ted

in a

ccor

danc

ew

ith T

RAC

E's h

andl

ing

of m

ultiw

ord

spee

ch st

ream

s,W

here

doe

s 1I 1I00lword elld?

A n

umbe

r of i

nves

tigat

ors (

e,g,

. Col

e &

Jaki

mik

. 198

0) h

ave

sugg

este

d th

at w

hen

one

wor

d is

iden

tifie

d. it

s ide

n-tit

y ca

n be

use

d to

det

erm

ine

whe

re it

end

s and

ther

efor

e w

here

the

next

wor

d be

gins

. In

TRA

CE. t

he in

tera

ctiv

e ac

tivat

ion

proc

ess c

an o

ften

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

a s -

as-

a s -

ElD

as-b

lifa

. '

tl!.!!

J!..:

tgi

iII2

ili 8

H,j,

.. -

II

- p

- b

u I

r -'I

:0 ~

r 8

or "

tarn

ished

" or o

ne o

f sev

eral

oth

er p

ossib

ilitie

s, It

is on

ly a

fter m

orc

time

has

pass

ed. a

nd w

e ha

ve p

erce

ived

eith

er a

sile

nce

or e

noug

h of

the

next

wor

d to

rule

out

any

of t

he c

ontin

uatio

ns o

f Ita

rl, th

at w

e ca

n de

cide

we

have

hea

rd th

e w

ord

"tar

, " T

his s

ituat

ion.

as i

t aris

es in

TRA

CEw

ith th

e sim

ple

utte

ranc

e Ita

rbak

sl ("

tar b

ox) is illustrated in Fig. 31.

Though "

tar"

is so

mew

hat m

ore

activ

e th

an th

e lo

nger

wor

d " t

arge

twhen the

Irl

is co

min

g in

. it i

s onl

y w

hen

the

wor

d "b

ox" emerges as

the

inte

rpre

tatio

n of

the

phon

emes

follo

win

g " t

ar" t

hat t

he ri

val "

targ

et"

final

ly fa

des a

s a se

rious

con

tend

er,

With

long

er w

ords

the

situa

tion

is di

ffere

nt. A

s we

have

alre

ady

seen

in a

noth

er e

xam

ple.

by

the

time

the

end

of a

long

er w

ord

is re

ache

d it

is

pas'b

-Ilar

l-~-

pas-

b-Ila

rl-l-

pas-

b-Ila

rl-l-

IITD

...-r!

i;-~I

COI

ar:w

!i:I!:

D!!J

!Dj

s a

iE!!l

lllI

a rb

. ra

k

Pili

Pil

II I

u I !

!~

IIII

- p' .

Elii.

'!Eb

Enlli

!!i!::

iEE

TIl

W!i.

!- s

t:-

. -

fii.i4

P:~=

:. '-

II -

. . 1111

i2ili

9L__

_.Ci

Ll1

- s

- P a

. -

r d j

a S

a

-larb

aka-

-Iarb

,ka-

-Iarb

,",-

-I,rb

ak,-

pal-s

-llar

,-I- -

P"I-s-

Ilarg

-t- -

pal-

s-lls

rl-l-

FIG

. 30,

Sta

te o

f the

Tra

ce a

t sev

eral

poi

nts d

urin

g Ih

e pr

oces

sing

of "

poss

ible

targ

et" a

ndpa

gusl

e ta

rget

.

rjb- ;

' ., S

)fC

ITiI

u::L

fi

11m

iITID

I, ~i

liliE

!!I a k

a r

s -a

k

u I

u I P

II I

.iad

-

. lex

ical

stat

us o

f one

par

t of a

spee

ch st

ream

pla

ys a

n im

porta

nt ro

le in

dete

rmin

ing

whe

re th

e be

ginn

ing

of th

e ne

xt w

ord

mus

t be.

The

lollg

tlllJ

~'horl of lI'ort! ideillifi('tllioll,

One

pro

blem

atic

feat

ure

ofsp

eech

is th

e fa

ct th

at it

is n

ot a

lway

s po

ssib

le to

identifya word un-

ambi

guou

sly u

ntil

one

has h

eard

the

wor

d af

ter i

t. Co

nsid

er. f

or e

xam

ple.

the

wor

d "t

ar,"

If w

e ar

e lis

teni

ng to

an

utte

ranc

e an

d ha

ve g

otte

n ju

stto the

Irl

in "

The

man

saw

the

tar b

ox. " though "

tar" will tend to be

the

pref

erre

d hy

poth

esis

at t

his

poin

t. w

e do

not

hav

e en

ough

info

rmat

ion

to sa

y un

equi

voca

lly th

at th

e w

ord

" tar

" will

not

turn

out

to b

e " t

arge

t,-I

arba

k,-

I-Iar

b,",-

I-I,rb

,k.-

I-Iar

baks

-Ft

G. 3

1. S

iale

of I

he T

race

al s

ever

al p

oint

s in

pro

cess

ing

"tar b

ox" and "

guita

r box

,

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

muc

h m

ore

likel

y th

at o

nly

one

wor

d ca

ndid

ate

will

rem

ain,

Inde

ed, w

ithlo

nger

wor

ds it

is o

ften

poss

ible

to h

ave

enou

gh in

form

atio

n to

iden

tify

the

wor

d un

ambi

guou

sly w

ell b

efor

e th

e en

d of

the

wor

d. A

n ill

ustra

tion

of th

is s

ituat

ion

is p

rovi

ded

by a

simulation using the utterance "

guita

rbo

x" Ig

tarb

aks/,

By

the

time

the

Irl h

as re

giste

red,

"gu

itar " is clearly

dom

inan

t at t

he w

ord

leve

l. an

d ca

n be

unai11biguously identified without

furth

er a

do.

Rece

ntly

, an

expe

rimen

t by

Gro

sjean

(198

5) h

as d

emon

strat

ed th

ese

sam

e ef

fect

s em

piric

ally

, Gro

sjean

pre

sent

ed su

bjec

ts w

ith lo

ng o

r sho

rtw

ords

follo

wed

by

ase

cond

w()r

d an

d m

easu

red

how

muc

h of

the

wor

dan

d its

succ

esso

r the

subj

ect n

eede

d to

hea

r to

iden

tify

the

targ

et. W

ithlo

nger

wor

ds, s

ubje

cts c

ould

usu

ally

gue

ss th

e w

ord

corre

ctly

wel

l bef

ore

the

end

of th

e w

ord,

; and

by

the

end

of th

e w

ord

they

wer

e qu

ite s

ure

ofth

e w

ord'

s ide

ntity

, With

mon

osyl

labi

c w

ords

, on

the

othe

r han

d, m

any

of th

e w

ords

cou

ld n

ot b

e id

entif

ied

corr~

ctly

unt

il w

ell i

nto

the

next

wor

d, O

n th

e av

erag

e, s

ubje

cts

wer

e no

t sur

e of

the

wor

d's

iden

tity

until

abou

t the

end

of t

he n

ext w

ord,

or t

he b

egin

ning

of t

he o

ne a

fter,

AsGrosjean (1985) points out, a m

~or r

easo

n fo

r thi

s is s

impl

y th

at th

esp

oken

inpu

t ofte

n do

es n

ot u

niqu

ely

spec

ify th

e id

entit

y of

a s

hort

wor

d.In

suc

h ca

ses,

the

perc

eptu

al s

yste

m is

ofte

n fo

rced

to p

roce

ss th

e sh

ort

wor

d, a

nd it

s suc

cess

or, a

t the

sam

e tim

e.RecoRllizilll: the words

ill

a short selltellce,

One

last

exam

ple

ofTR

ACE

II's

perfo

rman

ce in

seg

men

ting

wor

ds is

illu

strat

ed in

Fig

, 32,

The

figur

e sh

ows

the

stat

e of

the

lrace

at s

ever

al p

oint

s du

ring

the

pro-

cessing of the stream ISiS"

t"ba

ks/.

By th

e en

d, th

e w

ords

of t

he p

hras

eSh

e sh

ut a

box

, " w

hich

fils

the

inpu

t per

fect

ly w

ith n

o ov

erla

p, d

omi-

nate

all

othe

rs. .

This

exam

ple

illus

trate

s how

far i

t is s

omet

imes

pos

sible

to g

o in

pars

ing

a st

ream

of p

hone

mes

into

wor

ds, w

ithou

t eve

n co

nsid

erin

g sy

n-ta

clic

and

sem

antic

'(on

stra

ints

, or s

tress

, syl

labi

ficat

ion,

and

junc

ture

cues

to w

ord

iden

tific

atio

n. T

he e

xam

ple

also

illu

strat

es th

e di

fficu

lty th

em

odel

has

in p

erce

ivin

g sh

ort,

unstr

esse

d w

ords

like

". T

his

is, o

fco

urse

, jus

t an

extre

me

vers

ion

of th

e di

fficu

lty th

e m

odel

has

in p

ro-

cess

ing

mol

1osy

llabi

c w

ords

like

"ta

r," a

nd is

con

siste

nt w

ith G

rosje

anda

ta o

n th

e di

fficu

lty su

bjec

ts ha

ve w

ith id

entif

ying

shor

t wor

ds. I

n fa

ct,

Gro

sjean

and

Gee

(198

4) re

port

pilo

t dat

a in

dica

ting

that

thes

e di

fficu

lties

are

even

mor

e se

vere

with

func

tion

wor

ds li

ke "

" and "

of. " It should

be n

oted

that

TRA

CE m

akes

no

spec

ial d

istin

ctio

n be

twee

n co

nten

t and

func

tion

wor

ds, p

er se

, and

nei

ther

do

Gro

sjean

and

Gee

, How

ever

, fun

c-tio

n w

ords

are

usu

ally

uns

tress

ed a

nd c

onsid

erab

ly sh

orte

r tha

n co

nten

tw

ords

. Thu

s, it

is no

t nec

essa

ry to

poi

nt to

any

spec

ial m

echa

nism

s for

clos

ed v

ersu

s ope

n cl

ass m

orph

emes

to a

ccou

nt fo

r Gro

sjean

and

Gee

resu

lts.

m:n

IDJ

IH&

fnJiJ

EEJJ

5 I

s -

p u

s u

II i

S \I

-SIS

-l-ba

ks-

SiS-

t-bak

s-Si

S-l-b

aks-

tE:D

1~~

J3l~

Iru

s -

~ , 1t

-lt:: ~~

- 1- b

b.,;

s I S

s

I Silk

S I

pug S

SIS"

'I-ba

ks-

SIS-

l-bak

s-Si

S-l-b

llks-

FIG

. 32.

The

sta

te o

f the

Tra

ce a

t sev

erat

poi

nts

durin

g th

e pr

oces

sing

of t

he s

tream

ISis

Tbak

sl ("

She

shut

8 b

ox

,.

Sum

mar

y of

Wor

d Id

emiji

catio

n Si

mul

atio

nsW

hile

pho

nem

e id

entif

icat

ion

has

been

stu

died

for m

any

year

s, d

ata

from on-lin

e stu

dies

of w

ord

reco

gniti

on is

just

begi

nnin

g to

acc

umul

ate;

Ther

e is

an o

lder

lite

ratu

re o

n ac

cura

cy o

f wor

d id

entif

icat

ion

in n

oise

.bu

t it h

as o

nly

been

qui

te re

cent

ly th

at u

sefu

l tec

hniq

ues h

ave

been

de-

velo

ped

for s

tudy

ing

wor

d re

cogn

ition

in re

al ti

me.

Wha

t evi

denc

e th

ere

is. th

ough

indi

cate

s the

com

plex

ity o

f the

wor

d

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

iden

tific

atio

n pr

oces

s, W

hile

the

wor

d id

entif

icat

ion

mec

hani

sm is

sen-

sitiv

e to

eac

h ne

w in

com

ing

phon

eme

as it

arri

ves,

it is nevertheless

robu

st en

ough

to re

cove

r fro

m u

nder

spec

ifica

tion

or d

istor

tion

of w

ord

begi

nnin

gs. A

nd it

app

ears

h..

be c

apab

le o

f som

e sim

ulta

neou

s pro

cess

ing

of su

cces

sive

wor

ds in

the

inpu

t stre

am, T

RACE

app

ears

to c

aptu

re th

ese

aspe

cts o

f the

tim

e co

urse

of w

ord

reco

gniti

on. I

n th

ese

resp

ects,

it im

-pr

oves

upo

n th

e CO

HO

RT m

odel

, the

onl

y pr

evio

usly

ext

ant m

odel

that

prov

ides

an

expl

icit

acco

unt o

f the

on-

line

proc

ess

of w

ord

reco

gniti

on,

And

the

mec

hani

sms i

t use

s . to

acco

mpl

ish th

is ar

e th

e sa

me

ones

that

itus

ed fo

r the

sim

ulat

ions

of t

he p

roce

ss o

f pho

nem

e id

entif

icat

ion

de-

scribed in the preceding section,

lene

e,th

e m

odel

exh

ibits

imm

edia

te s

ensi

tivity

to in

form

atio

n fa

vorin

gon

e w

ord

inte

rpre

tatio

n ov

er a

noth

er. I

t sho

ws a

n in

itial

pre

fere

nce

for

shor

ter w

ords

rela

tive

to lo

nger

wor

ds, b

ut e

vent

ually

a se

quen

ce o

fph

onem

es th

at m

atch

es a

long

wor

d pe

rfect

ly w

ill b

e id

entif

ied

as th

atw

ord,

ove

rturn

ing

the

initi

al p

refe

renc

e fo

r the

shor

t-wor

d in

terp

reta

tion.

Thes

e as

pect

s of

the

mod

el, a

re c

onsi

sten

t with

hum

an d

ata

from

gat

ing

expe

rimen

ts.7,

Tho

ugh

the

mod

el is

hea

vily

influ

ence

d by

wor

d be

ginn

ings

, it c

anre

cove

r fro

m u

nder

spec

ifica

tion

or d

isto

rtion

of a

wor

d's

begi

nnin

g.S,

The

mod

el c

an u

se it

s kno

wle

dge

of th

e le

xico

n to

par

se se

quen

ces

of p

hone

mes

into

wor

ds, a

nd to

esta

blish

whe

re o

ne w

ord

ends

and

the

next

one

beg

ins

whe

n cu

es to

wor

d bo

unda

ries

are

lack

ing.

9, L

ike

hum

an su

bjec

ts, th

e m

odel

som

etim

es c

anno

t ide

ntify

a w

ord

until

it h

as h

eard

par

t of t

he n

ext w

ord,

Also

like

hum

an su

bjec

ts. it

can

bette

r det

erm

ine

whe

re a

wor

d w

ill b

egin

whe

n it

is pr

eced

ed b

y a

wor

dra

ther

than

a n

onw

ord,

10. T

he m

odel

doe

s not

dem

and

a pa

rse

of a

pho

nem

e se

quen

ce th

atin

clud

es e

ach

phon

eme

in o

ne a

nd o

nly

one

wor

d, T

his a

llow

s it t

o co

pegr

acef

ully

with

elis

ion

. of p

hone

m~s

at w

ord

boun

darie

s. It

will

ofte

npe

rmit

seve

ral a

ltern

ativ

e pa

rses

to re

mai

n av

aila

ble

lor h

ighe

r lev

el in

-flu

ence

s to

choo

se a

mon

g,

In a

dditi

on to

thes

e ch

arac

teris

tics o

bser

ved

in th

e pr

esen

t pap

er, o

ursim

ulat

ions

with

TRA

CE I

show

seve

ral f

urth

er c

orre

spon

denc

es b

e-tw

een

the

mod

el a

nd h

uman

spee

ch. p

erce

ptio

n, M

ost i

mpo

rtant

of t

hese

is th

e fa

ct th

at th

e m

odel

is a

ble

to u

se a

ctiv

atio

ns o

f pho

nem

e un

its in

one

part

of th

e Tr

ace

to a

djus

t the

con

nect

ion

stren

gths

det

erm

inin

g w

hich

feat

ures

will

act

ivat

e w

hich

pho

nem

es in

adj

acen

t par

ts of

the

Trac

e. In

this WaY, the m

odel

can

adj

ust a

s hum

an su

bjec

ts do

to co

artic

uh.it

ory

influ

ence

s on

the

acou

stic

prop

ertie

s of p

hone

mes

(Fow

ler.

1984

; Man

n&

Rep

p, 19

80).

Ther

e is

, of c

ours

e. m

ore

data

on

som

e of

thes

e po

ints

than

oth

ers.

IIw

ill b

e ve

ry in

tere

sting

to se

e ho

w w

ell r

RACE

will

hol

d up

aga

inst

the

data

as f

urth

er e

mpi

rical

stud

ies a

re c

arrie

d ou

l.

ENER

AL

DIS

CUSS

ION

Sum

ma,

.y o

f TRA

CE' ~

' Suc

cess

esIn

this

artic

le, w

e ha

ve se

en th

at T

RACE

can

acc

ount

for a

num

ber o

fdi

ffere

nt a

spec

ts of

hum

an sp

eech

per

cept

ion.

We

begi

n by

list

ing

the

maj

or c

orre

spon

denc

es b

etw

een

TRA

CE a

nd w

hat w

e kn

ow a

bout

the

hum

an sp

eech

und

ersta

ndin

g pr

oces

s.

I, TR

ACE

, lik

e hu

man

s, us

es in

form

atio

n fro

m o

verla

ppin

g po

rtion

sof

the

spee

ch w

ave

to id

entif

y su

cces

sive

phon

emes

,2,

The

mod

el s

how

s a

tend

ency

tow

ard

cate

goric

al p

erce

ptio

n of

pho

-ne

mes

, as

do h

uman

sub

ject

s. T

he m

odel

' s te

nden

cy to

war

d ca

tego

rical

perc

eptio

n is

affe

cted

by

man

y of

the

sam

e pa

ram

eter

s whi

ch af

fect

the

degr

ee o

f cat

egor

ical

per

cept

ion

show

n by

hum

an su

bjec

ts; in

par

ticul

ar.

the

exte

nt to

whi

ch p

erce

ptio

n w

ill be

cat

egor

ical

incr

ease

s w

ith ti

me

betw

een

stim

uli t

hat m

ust b

e co

mpa

red,

3,

The

mod

el c

ombi

nes

feat

ure

info

rmat

ion

from

a n

umbe

r of d

ifter

ent

dim

ensio

ns, a

nd e

xhib

its c

ue tr

ade-

offs

in p

hone

me

iden

tific

atio

n. T

hese

char

acte

ristic

s of h

uman

spee

ch p

erce

ptio

n ha

ve b

een

dem

onstr

ated

in a

very

hirg

e nu

mbe

r of s

tudi

es,

4, T

he m

odel

aug

men

ts in

form

atio

n fro

m th

e sp

eech

stre

am w

ith fc

ed-

back

from

the

lexi

cal l

evel

in re

achi

ng d

ecisi

ons a

bout

the

iden

tity

ofph

onem

es, T

hese

lexi

cal i

ntlu

ence

s on

pho

nem

e id

entif

icat

ion

occu

r in

cond

ition

s si

mila

r to

thos

e in

whi

ch le

xica

l effe

cts

have

bee

n re

porte

d,bu

t do

not o

ccur

in c

ondi

tions

in w

hich

thes

e ef

fect

s hav

e no

t bee

n ob

-

~~. .

5, L

ike

hum

an su

bjec

ts th

e m

odel

exh

ibits

app

aren

t pho

nota

ctic

rule

effe

cts o

n ph

onem

e id

entif

icat

ion.

thou

gh it

has

no

expl

icit

repr

esen

tatio

nof

the

phon

otac

tic ru

les,

The

tend

ency

to p

refe

r pho

nota

ctic

ally

regu

lar

inte

rpre

tatio

ns o

f am

bigu

ous p

hone

mes

can

be

over

ridde

n by

par

ticul

arle

xica

l ite

ms,

just

as

it ca

n in

the

hum

an p

erce

iver

,6. In processing un

ambi

guou

s ph

onem

e se

quen

ces

prec

eded

by

si-

Som

e of

Ihe

Rea

solU

' fo,

. Ihe

Succ

es~'

es o

f TR

ACE

To w

hat d

oes t

he T

RACE

mod

el o

we

its su

cces

~ in

sim

ulat

ing

hum

ansp

eech

per

cept

ion'

! Som

e of

TRA

CE's

succ

esse

s sim

ply

depe

nd o

n its

abili

ty to

mak

e us

e of

the

info

rmat

ion

as it

com

es il

. For

exa

mpl

e. it

fails

to sh

ow c

onte

xt e

ffect

s onl

y w

hen

a re

spon

se m

ust b

e m

ade.

or c

an b

em

ade

with

hig

h ac

cura

cy. b

efor

e co

ntex

tual

info

rmat

ion

is a

vaila

ble.

Ther

e ar

e se

vera

l oth

er re

ason

s for

TRA

CE's

succ

ess,

One

, we

thin

k.is

the

use

of c

ontin

uous

act

ivat

ion

and

com

petit

ion

proc

esse

s in

plac

e of

/'"

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

disc

rete

dec

isiv

e pr

oces

ses

such

as

segm

enta

tion

and

labe

ling,

Act

ivat

ion

and

com

petit

ion

are

mat

ters

of d

egre

e an

d pr

otec

t TRA

CEfr~

m c

ata-

strop

hic

com

mitm

ent i

n m

argi

nal c

ases

. and

they

pro

vide

a n

atur

iif m

eans

for c

ombi

ning

man

y ~i

ffere

nt so

urce

s: of

info

rmat

ion,

Of c

ours

e. th

is fe

a-tu

re o

f the

mod

el is

shar

ed w

ith se

vera

l oth

er m

odel

s (e,

g.. M

orto

n. 19

69;

Ode

n &

Mas

saro

. 197

8). t

houg

h on

ly N

usba

um a

nd S

iow

iacz

ek (1

982)

have

pre

viou

sly in

corp

orat

ed th

ese

kind

s of a

ssum

ptio

ns in

a m

odel

of.

the

time

cour

se o

f wor

d re

cogn

ition

.Pa

rt of

the

succ

ess o

f TRA

CE is

spec

ifica

lly d

ue to

the

use

of c

om-

petit

ive

inhi

bito

ry in

tera

ctio

ns in

stead

of b

otto

m-u

p (o

r top

-dow

n) in

hi-

bitio

n. C

ompe

titio

n al

low

s the

mod

el to

sel

ect t

he b

est i

nter

pret

atio

nav

aila

ble.

sell

ling

for a

n im

perfe

ct o

ne w

hen

no b

elle

r one

is a

vaila

ble.

but o

verri

ding

poo

r one

s whe

n a '

goo

d on

e is

at h

and,

The

se a

nd o

ther

virtu

es o

f com

petit

ive

inhi

bitio

n ha

ve b

een

note

d be

fore

(e,g

.. Fe

ldm

an& Ballard. 1982: G

ro~s

berg

. 197

3; L

evin

. 197

6: R

atlif

f. 19

65; v

on B

ekes

y.1967) in other co

ntex

ts. T

heir

usef

ulne

ss h

ere

atte

sts to

the

gene

ral u

tility

of th

e co

mpe

titiv

e in

hibi

tion

mec

hani

sm.

The

elim

inat

ion

of b

etw

een-

leve

l inh

ibiti

on fr

om th

e in

tera

ctiv

e ac

ti-va

tion

mec

hani

sm p

uts

us in

a v

ery

nice

pos

ition

with

resp

ect t

o on

ege

nera

l crit

ique

of i

nter

activ

e-ac

tivat

ion

mod

els.

It is

ofte

n sa

id th

at a

c-tiv

atio

n m

odel

s are

~oo

unc

onst

rain

ed a

nd to

o fle

xibl

e to

be

anyt

hing

mor

e th

an a

lang

uage

for c

onve

nien

tly d

escr

ibin

g in

form

atio

n pr

oces

sing,

We

are

now

in a

pos

ition

to su

gges

t tha

t a re

stric

ted

vers

ion

of th

e fra

me-

wor

k is

not

onl

y su

ffici

ent b

ut s

uper

ior.

Inte

ract

ive-

activ

atio

n m

odel

sco

uld

expl

oit b

oth

exci

tato

ry a

nd in

hibi

tory

con

nect

ions

bot

h be

twee

nan

d w

ithin

leve

ls, b

u~ in

the

orig

inal

inte

ract

ive-

activ

atio

n m

odel

of l

ette

rpe

rcep

tion.

onl

y in

hibi

tory

inte

ract

ions

wer

e al

low

ed w

ithin

a le

vel.

Inm

ore

rece

nt v

ersi

ons

of th

e vi

sual

mod

el (M

cCle

lland

. 1985, 1986),

and

in T

RACE

, we

have

gon

e ev

en fu

rther

, allo

win

g on

ly e

xcita

tory

conn

ectio

ns b

etw

een

leve

ls an

d on

ly in

hibi

tory

con

nect

ions

with

in le

vels.

From

our

exp

erie

nce,

i t a

ppea

rs th

at m

odel

s w

hich

adh

ere

to th

ese

con-

strai

nts w

ork

as w

ell a

s or b

ette

r tha

n m

embe

rs o

f the

mor

e ge

nera

l cla

ssth

at d

o no

t. W

e ha

sten

to a

dd th

at w

e ha

ve n

o pr

oof t

hat t

his

is tr

ue, W

eha

ve, h

owev

er. n

o re

ason

to fe

el th

at w

e co

uld

impr

ove

the

perfo

rman

ceof

our

mod

el b

y al

low

ing

eith

er b

etw

een-

leve

l inh

ibito

ry in

tera

ctio

ns o

rw

ithin

- leve

l exc

itatio

n,

Oth

er a

spec

ts of

the

succ

esse

s of T

RACE

dep

end

on it

s use

offe

edba

ckfro

m h

ighe

r to

low

er re

vels,

Fee

dbac

k pl

ays a

cen

tral r

ole

in th

e ac

coun

tsof

cat

egor

ical

per

cep t

ion.

lexi

cal e

ffect

s on

phon

eme

iden

tific

atio

n, a

ndph

onot

actie

rule" e

ffect

s,

We

do n

ot c

laim

that

any

of t

hese

phe

nom

ena,

take

n in

divi

dual

ly, r

e-qu

ire th

e as

sum

ptio

n ; o

f a fe

edba

ck m

echa

nism

, For

exa

mpl

e, c

onsid

erth

e ph

enom

enon

of t

ateg

oric

al p

erce

ptio

n. W

e us

e fe

edba

ck fr

om th

e

phon

eme

to th

e fe

atur

e le

vel t

o dr

ive

feat

ure

patte

rns c

lose

r to

the

pro-

totype of the ph

onem

e th

ey m

ost s

trong

ly a

ctiv

ate,

Thi

!! m

echa

nism

.co

uple

d w

ith th

e co

mpe

titio

n m

echa

nism

at t

he p

hone

me

leve

l, ac

coun

tsfo

r bet

ter d

iscrim

inat

ion

betw

een

than

with

in c

ateg

orie

s, H

owev

er, w

eco

uld

acco

unt f

or c

ateg

oric

al p

erce

ptio

n by

sugg

estin

g th

at su

bjec

ts do

not h

ave

acce

ss to

the

acou

stic

leve

l at a

ll, b

ut o

nly

to th

e re

sults

of t

heph

onem

e id

entif

icat

ion

proc

ess.

Sim

ilarly

, lex

ical

effe

cts o

n ph

onem

eid

entif

icat

ion

can

be a

ccou

nted

for b

y as

sum

ing

that

subj

ects

(som

etim

es)

read

out

from

the

wor

d le

vel a

nd in

fer t

he id

entit

y of

pho

nem

es fr

om th

ele

xica

l cod

e (M

arsl

en-Wilson, 1980: Marslen-

Wils

on &

Wel

sh, 1

978;

Mor

ton.

1979), In the case of "ph

onot

actic

rule

" effects, other interpre-

tatio

ns a

re o

f cou

rse

avai

labl

e as

wel

l, O

ne c

ould

. for

exa

mpl

e, s

impl

ysu

ppos

e th

at su

bjec

ts us

e kn

owle

dge

of th

e ph

onot

actic

con

strai

nts.

per-

haps

cap

ture

d in

uni

ts sta

ndin

g fo

r leg

al p

hone

me

pairs

. and

that

it is

the

outp

ut o

f suc

h un

its th

at a

ccou

nts

for t

he in

fluen

ce o

f pho

nota

ctic

reg-

ularity on phoneme identification.

We

know

of n

o sin

gle

conv

inci

ng e

mpi

rical

reas

on to

pre

fer f

eedb

ack

acco

unts

to o

ther

pos

sibili

ties,

How

ever

, we

have

two

theo

retic

al re

ason

sfo

r pre

ferri

ng to

reta

in to

p-do

wn

as w

ell a

s bot

tom

-up

inte

ract

ions

in o

urac

tivat

ion

mod

els,

One

reas

on h

as to

do

with

the simplicity of the re-

sulti

ng d

ecisi

on m

echa

nism

s, Fe

edba

ck a

llow

s hig

herle

vel c

onsid

erat

ions

to in

fluen

ce th

e ou

tcom

e of

pro

cess

ing

at lo

wer

leve

ls in

just

the

sam

ew

ay th

at lo

wer

leve

l con

sider

atio

ns in

fluen

ce th

e ou

tcom

e of

pro

cess

ing

at h

ighe

r lev

els,

The

influ

ence

s of

lexi

cal a

nd o

ther

con

stra

ints

on

pho-

nem

e id

entif

icat

ion

need

not

be

push

ed o

ut o

f the

theo

ry o

f spe

ech

per-

cept

ion

itsel

f int

o de

cisio

n pr

oces

ses,

but a

re in

tegr

ated

dire

ctly

into

the

perc

eptu

al p

roce

ss in

a u

nifie

d w

ay, G

iven

top-

dow

n as

wel

l as

botto

m-

up p

roce

ssin

g, th

e de

cisio

n m

echa

nism

s req

uire

d fo

r gen

erat

ing'

ove

rtre

spon

ses t

hat r

eflectlexical and other contextual influences are g

reat

lysim

plifi

ed; n

o sp

ecia

l pro

visio

n ne

eds t

o be

m.,d

e fo

r com

bini

ng le

xica

lan

d ph

onet

ic o

utpu

ts in

the

deci

sion

mec

hani

sm,

A se

cond

reas

on fo

r ret

aini

ng fe

edba

ck c

omes

up

whe

n w

e co

nsid

erth

e pr

oble

m o

f lea

rnin

g. A

lthou

gh w

e ha

ve n

ot d

iscus

sed

how

lear

ning

mig

ht o

ccur

in T

RACE

, we

have

ass

umed

that

the

mec

hani

sms o

f spe

ech

perc

eptio

n ar

e ac

quire

d th

roug

h m

odifi

catio

n of

con

nect

ion

stren

gths

.V

ery

roug

hly,

in m

any

lear

ning

sche

mes

, con

nect

ions

bet

wee

n un

its a

rest

reng

then

ed w

hen

two

units

tend

to b

e ac

tivat

ed s

imul

tane

ousl

y, a

t the

expe

nse

of c

onne

ctio

ns b

etw

een

units

that

tend

not

to b

e ac

tivat

ed a

t the

sam

e tim

e (c

f, G

ross

berg

, 197

8; R

osen

blat

t. 19

62: R

umel

hart

& Z

ipse

r,19

85).

In s

uch

sche

mes

. how

ever

. the

re is

a s

erio

us p

robl

em if

act

ivat

ion

is e

ntire

ly b

otto

m-u

p: fo

r in

that

cas

e. o

nce

a pa

rticu

lar u

nit h

as b

een

tune

d" to

resp

ond

to a

par

ticul

ar p

atte

rn. i

t is d

iffic

ult t

o re

tune

it: i

tfir

es w

hen

its "

expe

cted

" pat

tern

is p

rese

nted

. and

whe

n it

fires

. its

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

tend

ency

to re

spon

d to

that

pat

tern

onl

y in

crea

ses.

Feed

back

pro

vide

s aw

ay to

bre

ak th

is vi

ciou

s cyc

le, I

f hig

her l

evel

s in

sist

that

a p

artic

ular

phon

eme

is pr

esen

t. th

en th

e un

it fo

r tha

t pho

nem

e ca

n be

com

e ac

tivat

edev

en if

the

botto

l11-

up in

put w

ould

nor

mal

ly a

ctiv

ate

som

e ot

her p

hone

me

inste

ad; t

hen

the

lear

ning

mec

hani

sm c

an "

retu

ne" the detector for the

phon

eme

so th

at it

will

nee

d to

dep

end

less

on

the

top-

dow

n in

put t

hene

xt ti

me

arou

nd,

In general. the use offeedback appears to place more

of

the

inte

lligen

cere

quire

d fo

r per

cept

ion

and

perc

eptu

al le

arni

ng in

to th

e ac

tual

per

cept

ual

mec

hani

sm it

self.

and

to m

ake

the

mec

hani

sms w

hich

exh

ibit

this

inte

l-lig

ence

exp

licit.

As f

orm

ulat

ed h

ere.

thes

e m

echa

nism

s ar

e in

cred

ibly

sim

ple;

yet

they

app

ear t

o bu

y qu

ite a

lot w

hich

ofte

n ge

ts p

ushe

d in

toun

spec

ified

"de

cisio

n" and "postperceptual guessing" p

roce

sses

(e.

Forster. 1976),

Fina

lly. t

h~ .s

ucce

ss

of

TRA

CE a

lso d

epen

ds u

pon

its ar

chite

ctur

e,ra

ther

than

the

fund

amen

tal c

ompu

tatio

nal p

rinci

ples

' of a

ctiv

atio

n an

dcompetition. or the decision to include feedback, By

arc

hite

ctur

e, w

emean the organization

of

the Trace structure into layers consisting

of

units

corre

spon

ding

to it

ems o

ccur

ring

at p

artic

ular

tim

es w

ithin

the

utte

ranc

e,A

s we

note

d in

the

intro

ducL

ion.

this

arch

itect

ure

is on

e w

e de

cide

d up

ononly after several other kinds

of

arch

itect

ure

had

faile

d,Th

ere

are

thre

e pr

inci

ple

posi

tive

cons

eque

nces

of

th

e TR

ACE

arc

hi-

tect

ure.

Firs

t, it

keep

s stra

ight

wha

t occ

urre

d w

hen

in th

e sp

eech

stre

am,

Com

petit

ion

occu

rs o

nly

betw

een

units

com

petin

g to

repr

esen

t the

sam

eportion of the input stream, Multiple copies of the same phoneme and

wor

d un

its c

an b

e ac

tive

at th

e sa

me

time

with

out p

rodu

cing

con

fusi

on,

Furth

erm

ore.

the

arch

itect

ure

perm

its th

e sa

me

com

petit

ion

mec

hani

smthat chooses among alternative word interpretations

of

a si

ngle

-wor

d ut

-te

ranc

e to

segm

ent l

onge

r utte

ranc

es in

to w

ords

, No

sepa

rate

con

trol

structure. resetting the mechanism at the beginning

of

each

new

wor

d, is

requ

ired,

Se

cond

, the

arc

hite

ctur

e pe

rmits

bot

h fo

rwar

d an

d ba

ckw

ard

inte

rac-

tions

. Bac

kwar

d in

tera

ctio

ns a

re a

bsol

utel

y es

sent

ial i

f the

mod

el is

toaccount for the fact that the identity

of

a ph

onem

e (o

r a w

ord;

War

ren

&Sh

erm

an, 1

974) can be influenced by w

hat c

omes

afte

r it a

s w

ell a

s w

hat

comes beforeiL Some kind

of

reco

rd

of

the

past

is ne

cess

ary

to c

aptu

rethese kinds

of

influences. as well as to provide a clear picture

of

the

sour

ces

of

the more conventional effects

of preceding context, and the

Trace construct lays this out in a way that is both comprehensible' a

ndef

ficie

nLTh

ird, t

he T

race

stru

ctur

e pr

ovid

es a

n ex

plic

it m

echa

nism

whi

ch in

-st

antia

tes

the

idea

that

ther

e m

ay b

e no

dis

tinct

ion

betw

een

the

mec

ha-

nism

s whi

ch c

arry

out

per

cept

ual p

roce

ssin

g an

d th

ose

whi

ch p

rovi

de a

wor

king

mem

ory

for t

he re

sults

of t

he p

erce

ptua

l pro

cess

. At o

ne a

ndth

e sa

me

time.

the

Trac

e is

a p

erce

ptua

l pro

cess

ing

syst

em a

nd a

mem

ory

syst

em, A

s a

resu

lt. th

e m

odel

aut

omat

ical

ly a

ccou

nts f

orih

e fa

ct th

atco

here

nt m

emor

y tra

ces

pers

ist l

onge

r tha

n in

cohe

rent

one

s. T

he c

o-he

rent

one

s re

sona

te th

roug

h in

tera

ctiv

e (th

at is

, bot

tom

-up

and

top-

dow

n) a

ctiv

atio

n. w

hile

inco

here

nt o

nes f

ail t

o es

tabl

ish a

reso

nanc

e an

dth

eref

ore

die

away

mor

e ra

pidl

y,Se

vera

l of

th

ese

aspe

cts o

f TRA

CE o

verla

p w

ith a

ssum

ptio

ns m

ade

inot

her m

odel

s, as

men

tione

d in

pre

viou

s sec

tions

; con

tinui

ty b

etw

een

wor

king

mem

ory

and

the

perc

eptu

al p

roce

ssin

g str

uctu

res h

as b

een

sug-

geste

d by

a n

umbe

r of o

ther

aut

hors

(e.g

,. Co

nrad

,19

62),

and

the

notio

nth

at w

orki

ng m

emor

y is

a d

ynam

ic p

roce

ssin

g st

ruct

ure

rath

er th

an a

pass

ive

data

stru

ctur

e ha

s pre

viou

sly b

een

advo

cate

d by

Cro

wde

r (19

78,

1981) and Grossberg (1978), Indeed, G

ross

berg

has

not

ed th

at re

sona

ting

activ

atio

n/co

mpe

titio

n pr

oces

ses

can

both

enh

ance

a p

erce

ptua

l rep

re-

sent

atio

n an

d in

crea

se th

e re

tent

ion

of a

repr

esen

tatio

n; h

is a

naly

sis

ofin

tera

ctiv

e-ac

tivat

ion

proc

esse

s in

perc

eptio

n an

d ap

emor

y ca

ptur

es th

eco

ntin

uity

of p

erce

ptio

n an

d m

emor

y as

wel

l as

man

y ot

her d

esira

ble

prop

ertie

s of

in

tera

ctiv

e-ac

tivat

ion

mec

hani

sms,

Som

e D

efic

ienc

ies

of T

RACE

. Alth

ough

TRA

CE h

as h

ad a

num

ber o

f im

porta

nt su

cces

ses,

it a

lso h

asa number of equally important deficiencies. A number

of these deficien-

cies

rela

te to

sim

plify

ing

assu

mpt

ions

of t

he si

mul

atio

n m

odel

. It i

s im

-portant to be clear that such deficiencies are not in

trins

ic to

the

basic

stru

ctur

e of

the

mod

el b

ut to

the

sim

plifi

catio

ns w

e ha

ve im

pose

d up

onit

to in

crea

se o

ur a

bilit

y to

und

ersta

nd it

s bas

ic p

rope

rties

. Cer

tain

de-

ficiencies-such as the assumption th

at a

ll ph

onem

es a

re th

e sa

me

leng

th, t

hat a

ll fe

atur

es a

re e

qual

ly sa

lient

and

use

ful,

and

over

lap

an e

qual

amount from one phoneme to another-are no

t pre

sent

in T

RAC

E I.

Obv

ious

ly a

fully

real

istic

mod

el w

ould

take

acc

ount

of s

uch

dine

renc

es.

Oth

er fa

ctor

s th

at s

houl

d be

inco

rpor

ated

in a

mor

e co

mpl

ete

mod

el in

-clude some provision for effects

of

wor

ll fre

quen

cy, a

nd so

me

mec

ha-

nism

s for

exp

loiti

ng a

vaila

ble

cues

to w

ord

boun

darie

s,Another deficiency

of

the

mod

el is

that

the

deci

sion

mec

hani

sms h

ave

not b

een

fully

eno

ugh

elab

orat

ed. F

or e

xam

ple,

as

it st

ands

the

mod

eldo

es n

ot p

rovi

de a

mec

hani

sm fo

r ,de

cidi

ng w

hen

a no

nwor

d ha

s bee

npr

esen

ted.

Nor

hav

e w

e sp

ecifi

ed h

ow d

ecis

ion

proc

esse

s w

ould

act

ually

use

the

info

rmat

ion

avai

labl

e at

the

wor

d le

vel t

o lo

cate

wor

d-in

itial

pho

-ne

mes

, A re

late

d pr

oble

m is

the

lack

of a

n ex

plic

it pr

ovisi

on fo

r vari-

abili

ty in

the

activ

atio

n an

d/or

read

out p

roce

sses

. Inc

orpo

ratin

g va

ri-ab

ility

dire

ctly

into

a si

mul

atio

n m

odel

wou

ld g

reat

ly in

crea

se th

e co

m-

plex

ity

of the simulation process, but would also increase the model's

TRAC

E M

OD

EL

abili

ty to

cap

ture

the

deta

iled

prop

ertie

s of

reac

tion

time

dist

ribut

ions

and

erro

rs (R

atcl

iff. 1

978)

.So far we have considered de

ficie

ncie

s whi

ch w

e w

ould

attr

ibut

e to

sim

plify

ing

assu

mpt

ions

ado

pted

to k

eep

TRAC

E as

sim

ple

and

trans

-pa

rent

in it

s beh

avio

r as p

ossib

le. H

owev

er. t

here

are

som

e pr

oble

ms t

hat

are

intri

nsic

to th

e ba

sic

stru

ctur

e of

the

mod

el,

One

fund

amen

tal d

efic

ienc

y of

TRA

CE is

that

fact

that

it requires

mas

sive

dupl

icat

ion

of u

nits

and

conn

ectio

ns. c

opyi

ng o

ver a

nd o

ver

agai

n th

e co

nnec

tion

patte

rns

thaI

' det

erm

ine

whi

ch fe

atur

es a

ctiv

ate

whi

ch p

hone

mes

and

whi

ch p

hone

mes

act

ivat

e w

hich

wor

ds. A

s w

~ al

-re

ady

note

d. le

arni

ng in

act

ivat

ion

mod

els (

e,g" Ackley, Hinton. &

Sejn

owsk

i. 19

85: G

ross

berg

. 197

6: R

umel

hart

& Z

ipse

r, 1985) usually

invo

lves

the

retu

ning

of c

onne

ctio

ns b

etw

een

units

dep

endi

ng o

n th

eir

sim

ulta

neou

s ac

tivat

ion,

Giv

en T

RAC

E's

arch

itect

ure.

suc

h le

arni

ngw

ould

not

gen

eral

ize

from

one

par

t of t

he T

race

to a

noth

er a

nd so

wou

ldno

t be

acce

ssib

le fo

r inp

uts a

risin

g at

diff

eren

t loc

atio

ns in

the

Trac

e, A

seco

nd p

robl

em is

that

the

mod

el. a

s is

. is

inse

nsiti

ve to

var

iatio

n in

glo

bal

para

met

ers,

such

as s

peak

ing

rate

, spe

aker

cha

ract

erist

ics a

nd a

ccen

t,an

d am

bien

t aco

ustic

cha

ract

erist

ics,

A th

ird d

efic

ienc

y is

that

it fa

ils to

acco

unt f

or th

e fa

ct th

at o

ne p

rese

ntat

ion

of a

wor

d ha

s an

effe

ct o

n th

epe

rcep

tion

of it

a v

ery

shor

t tim

e la

ter (

Nus

baum

& S

iow

iacz

ek, 1

982)

,These two pr

esen

tatio

ns. i

n th

e cu

rrent

ver

sion

of th

e m

odel

. sim

ply

exci

te s

epar

ate

toke

ns fo

r the

sam

e w

ord

in d

iffer

ent p

arts

of t

he T

race

,A

ll th

ese

defic

ienc

ies r

efle

ct th

e fa

ct, t

hat t

he T

RAC

E co

nsis

ts o

f ala

rge

set o

f ind

epen

dent

toke

ns o

f eac

h fe

atur

e. p

hone

me,

and

wor

d un

it,W

hat a

ppea

rs to

be

calle

d fo

r ins

tead

is a

mod

el in

whi

ch th

ere

is a

sin

gle

store

d re

pres

enta

tion

of e

ach

phon

eme

and

each

wor

d in

som

e ce

ntra

lrt;

pres

enta

tiona

l stru

ctur

e, If

this

struc

ture

is a

cces

sed

ever

y tim

e th

ew

ord

is p

rese

nted

, the

n w

e co

uld

acco

unt f

or re

petit

ion

prim

ing

effe

cts,

Like

wise

. if t

here

wer

e a

singl

e ce

ntra

l stru

ctur

e. le

arni

ng c

ould

occ

ur in

just

one

set

of u

nits

, 'as could dynamic returning offeature-

phon

eme

and

phon

eme-

wor

d co

nnec

tions

to ta

ke a

ccou

nt o

f cha

nges

in g

loba

l par

am~

eter

s or

spe

aker

cha

ract

eris

tics,

How

ever

. it r

emai

ns n

eces

sary

to k

eep

stra

ight

the

rela

tive

,tem

pora

llo

catio

n of

diff

eren

t fea

ture

. pho

nem

e. a

nd w

ord

activ

atio

ns. T

hus

it w

illnot do to simply abandon the Trace in favor of a single set of units

cons

istin

g of

just

one

copy

of e

ach

phon

eme

and

one

copy

of e

ach

wor

d,It

seem

s tha

t we

need

to h

ave

thin

gs b

oth

way

s: w

e ne

ed a

cen

tral

repr

esen

tatio

n th

at p

lays

a ro

le in

pro

cess

ing

ever

y ph

onem

e an

d ev

ery

wor

d an

d th

iit is

subj

ect t

o le

arni

ng, r

etun

ing.

and

prim

ing,

We

also

nee

dto

kee

p a

dyna

mic

trac

e of

the

unfo

ldin

g re

pres

enta

tion

of th

e sp

eech

stre

am. s

o th

at w

e ca

n co

ntin

ue to

acc

omm

odat

e bo

th le

ft an

d rig

ht c

on"

text

ual e

ffect

s.

MC

CLE

LLAN

D A

ND

ELM

AN

We

are

curre

ntly

beg

inni

ng to

dev

elop

a m

odel

that

has

thes

e pr

oper

-tie

s, ba

sed

on a

sche

me

for u

sing

a ce

ntra

l net

wor

k of

uni

ts to

tune

the

conn

ectio

ns b

etw

een

the

units

in th

e Tr

ace

in th

e co

urse

of p

roce

ssin

g.th

ereb

y ef

fect

ivel

y pr

ogra

min

g it

"on

the

fly,"

Sim

ilar i

deas

hav

e al

read

ybe

en a

pplie

d to

visu

al w

ord

reco

gniti

on (M

cCle

lland

, 198

5, 1

986)

. Our

hope

is th

at a

new

ver

sifJn

of th

e m

odel

bas

ed o

n th

ese

idea

s will

pre

serv

eth

e po

sitiv

e fe

atur

es o

f TRA

CE I

and

TRA

CE II

. whi

le o

verc

omin

g th

eir

principle deficiencies,

Som

e G

ener

al Is

sues

in S

peec

h an

d La

ngua

ge P

erce

ptio

nTh

ere

are

a nu

mbe

r of g

en~r

al is

sues

in sp

eech

and

lang

uage

' per

cep-

tion,

Fou

r que

stion

s in

parti

cula

r app

ear t

o lie

clo

se to

the

hear

t of o

urco

ncep

tion

of w

hat s

peec

h pe

rcep

tion

is al

l abo

ut. F

irst.

wha

t are

the

basic

uni

ts in

spee

ch p

erce

ptio

n? S

econ

d. w

hat i

s the

per

cept

. and

whi

chas

pect

s of t

he p

roce

ssin

g of

spok

en la

ngua

ge sh

ould

be

calle

d pe

rcep

tual

?Th

ird. w

hat i

s the

repr

esen

tatio

n of

ling

uisti

c ru

les?

Fou

rth. i

s the

re a

ny-

thin

g un

ique

or s

peci

al a

bout

spe

ech

perc

eptio

n? W

e co

nclu

de th

is a

rticl

eby

con

sider

ing

each

issu

e fro

m th

e pe

rspe

ctiv

e w

e ha

ve d

evel

oped

thro

ugh

the

cour

se o

f our

~xp

lora

tions

of T

RACE

.What is the perceptual "nit?

Thro

ugho

ut th

is ar

ticle

. we

have

con

sid-

ered

thre

e le

vels

of p

roce

ssin

g-fe

atur

e, p

hone

me,

and

wor

d. A

t eac

hle

vel,

indi

vidu

al p

roce

ssin

g un

its st

and

for h

ypot

hese

s abo

ut th

e fe

atur

es,

phon

emes

. and

wor

ds th

at m

ight

be

pres

ent a

t diff

eren

t poi

nts i

n th

e in

put

strea

m, I

t is w

orth

not

ing

that

mos

t asp

ects

of th

e m

odel

's pe

rform

ance

, are

inde

pend

ent o

f the

spec

ific

assu

mpt

ions

that

we

have

mad

e ab

out t

heun

its, o

r eve

n th

e le

vels.

Thu

s. if

we

repl

aced

the

phon

eme

leve

l with

demisyllables (Fujimura & Lovins, 1978) or

pho

nem

e tri

ples

(Wic

kelg

ren.

1969

), ve

ry li

ttle

of th

e be

havi

or o

f the

mod

el w

ould

cha

nge.

The

se u

nits

can

capt

ure

som

e of

the

coar

ticul

ator

y in

fluen

ces o

n ph

onem

e id

~ntit

y.an

d th

ey w

ould

redu

ce s

ome

of th

e w

ord-

boun

dary

am

bigu

ities

face

d by

the

curre

nt v

ersio

n of

the

mod

el, b

ut n

eith

er co

articulatory influences

nor w

ord

boun

dary

am

bigu

ities

wou

ld d

isap

pear

alto

geth

er (s

ee E

lman

& M

cCle

lland

. in

pres

s, fo

r fur

ther

dis

cuss

ion)

, In

fact

, int

erac

tive

activ

atio

n m

odel

s lik

e TR

ACE

can

be

form

ulat

edin

whi

ch e

ach

perc

eptu

al o

bjec

t is

repr

esen

ted.

not

by

a si

ngle

uni

t. bu

tby

a p

atte

rn o

f act

ivat

ion

over

a c

olle

ctio

n of

uni

ts, F

or e

xam

ple.

the

phon

eme

units

in e

ach

time

slice

of T

RAC

E m

ight

be

repl

aced

by

adi

ffere

nt se

t of u

nits

whi

ch d

id n

ot h

ave

a on

e-to

-one

cor

resp

onde

nce

toph

onem

es. A

pho

nem

e w

ould

be

repr

esen

ted

by a

par

ticul

ar p

atte

rn o

fac

tivat

ion

over

the

set o

f uni

ts (e

ach

repr

esen

ting,

per

haps

. to

som

e co

n-ju

nctio

n of

low

er le

vel f

eatu

res)

rath

er th

an b

y a

singl

e un

it in

the

set.

Ther

e ar

e so

me

com

puta

tiona

l adv

anta

ges o

f dist

ribut

ed re

pres

enta

tion

com

pare

d to

our

"on

e un

it on

e co

ncep

t" assumption (Hinton, Mc-

""'

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

Cle

lland

, & R

umel

hart,

in p

ress

), bu

t it i

s ve

ry d

iffic

ult t

o fin

d pr

inci

pled

way

s of d

istin

guish

ing

betw

een

loca

l and

dist

ribut

ed re

pres

enta

tiona

lsc

hem

es e

mpi

rical

ly, I

ndee

d, in

cer

tain

cas

es th

ere

is an

exa

ct m

appi

ngan

d, in

gen

eral

, it i

s pos

sible

to a

ppro

xim

ate

mos

t asp

ects

of th

e be

havi

orof

a lo

cal s

chem

e w

ith a

dist

ribut

ed o

ne a

nd v

ice

vers

a (S

mol

ensk

y,1986). In light of this, our u

se o

f loc

al a

s op

pose

d to

dis

tribu

ted

repr

e-se

ntat

ions

is n

ot p

erha

ps a

s si

gnifi

cant

as

it m

ight

app

ear a

t firs

t gla

nce.

Wha

t is e

ssen

tial i

s the

info

rmat

ion

that

the

repr

esen

tatio

n ca

ptur

es,

rath

er th

an w

heth

er it

doe

s so

via

dis

tribu

ted

or lo

cal r

epre

sent

atio

n, T

heus

e of

loca

l rep

rese

ntat

ior..

., w

ith e

ach

unit

(at t

he p

hone

me

and

wor

dle

vels

, any

way

) rep

rese

ntin

g a

mut

ually

exc

lusi

ve a

ltern

ativ

e m

akes

itm

uch

easie

r to

rela

te th

e sta

tes o

f the

pro

cess

ing

syste

m to

ove

rt re

spon

seca

tego

ries b

ut is

not

oth

erw

ise a

fund

amen

tal f

eatu

re o

f the

stru

ctur

e of

the

mod

el.

What is the percept?

At a

num

ber o

f poi

nts i

n th

is ar

ticle

, we

have

allu

ded

to w

ays i

n w

hich

our

con

cept

ion

of p

erce

ptio

n di

ffers

from

the

usag

e of

oth

er a

utho

rs, S

uch

conc

epts

as p

erce

ptio

n ar

e in

here

ntly

tied

to th

eory

, and

onl

y de

rive

thei

r mea

ning

with

resp

ect t

o pa

rticu

lar t

heo-

retic

al c

onst

ruct

s, W

here

doe

s th

e TR

ACE

mod

el p

lace

us,

then

, with

resp

ect t

o th

e qu

estio

n, w

hat i

s spe

ech

perc

eptio

n?Fo

r one

thin

g, T

RAC

E bl

urs

the

dist

inct

ion

betw

een

perc

eptio

n an

dother aspects of cognitive processing, T

here

is re

ally

no

clea

r way

inTR

ACE

to sa

y w

here

per

cept

ual p

roce

ssin

g en

ds a

nd c

once

ptua

l pro

-ce

sses

or m

emor

y be

gin,

How

ever

, fol

low

ing

Mar

rs (1

982)

def

initi

on o

fvi

sual

per

cept

.ion,

we

coul

d sa

y th

at s

peec

h pe

rcep

tion

is th

e pr

oces

s of

form

ing

repr

esen

tatio

ns o

f the

stim

ulus

-the

spea

kers

utte

ranc

e-at

seve

ral l

evel

s of d

escr

iptio

n, T

RACE

pro

vide

s suc

h a

set o

f rep

rese

nta-

tions

, as w

ell a

s pro

cess

es to

con

struc

t the

m. O

n th

is vi

ew, t

hen,

the

Trac

e is

the

perc

ept,

and

inte

ract

ive

activ

atio

n is

the

proc

ess o

f per

cep-

tion, A

spec

ts of

this

defin

ition

are

app

ealin

g. F

or e

xam

ple,

on

this

view

, the

perc

ept i

s a v

ery

rich

obje

ct, o

ne th

at re

fers

bot

h to

abs

tract

, con

cept

ual

entit

ies l

ike

wor

ds a

nd p

erha

ps a

t hig

her l

evel

s eve

n m

eani

ngs,

as

wel

las

to m

ore

conc

rete

ent

ities

like

aco

ustic

sign

als a

nd fe

atur

es, P

erce

ptio

nis

not r

estri

cted

to o

ne o

r a su

bset

of l

evel

s, as

it is

in c

erta

in m

odel

s(e

.g.,

Mar

slen

-Wils

on, 1

980;

Mor

ton,

1979

).O

n th

e ot

her h

and,

the

defin

ition

seem

s ove

rly li

bera

l, fo

r the

re is

evid

ence

sug

gest

ing

that

per

cept

ual e

xper

ienc

e an

d ac

cess

to th

e re

sults

of perceptual processing for the purposes of overt responding ,m

ay n

otbe

com

plet

ely

unco

nstn

tined

, A n

umbe

r of e

xper

imen

ts, b

oth

in sp

eech

(e.g

., Fo

ss &

Sw

inne

y, 1

973;

McN

eil &

Lin

dig,

1973) and reading (D

rew

-no

wsk

i & H

ealy

, 1977; Healy, 1976) suggest that under certain conditions

low

er le

vels

of p

roce

ssin

g ar

e in

acce

ssib

le, o

r are

' at b

est a

cces

sed

only

with

ext

ra ti

me

or e

ffort.

On

this

evid

ence

, if p

erce

ptio

n is

to fo

rm re

p-re

sent

atio

ns, a

nd if

the

repr

esen

tatio

ns. a

re a

nyth

il18

like

thos

e po

stul

ated

in T

RACE

, the

n pe

rcep

tion

is qu

ite in

depe

nden

t of t

he e

xper

ienc

e of

the

perc

eive

r and

of a

cces

s to

the

perc

ept.

Put a

noth

er w

ay, w

e m

ay c

hoos

eto

def

ine

the

'Irac

e as

the

perc

ept,

but i

t is n

ot th

e pe

rcep

tual

"exp

eric

nce.

This

doe

s no

t see

m to

be

a ve

ry s

atis

fact

ory

stat

e of

affa

irs.

One

coh

eren

t 're

spon

se to

thes

e ar

gum

ents

wou

ld b

e to

say

that the

'Irac

e is

not

the

expe

rienc

e its

elf,

but t

hat s

ome

part

or p

arts

of i

t may

be the

obje

ct

of perceptual experience, It seems sensible

, for

exa

mpl

e,to

sup

pose

that

the

perc

ept i

tsel

f con

sist

s of

that

par

t of t

he T

race

und

ersc

rutin

y by

the

deci

sion

mec

hani

sms,

On

this

view

, it w

ould

not

be

in-

cohe

rent

to s

uppo

se th

at re

pres

enta

tions

mig

ht b

e fo

rmed

whi

ch w

ould

nevertheless be inaccessible either to experience or to overt re

spon

sepr

oces

ses.

It w

ould

be

a m

atte

r sep

arat

e fro

m th

e an

alys

is of

the

inte

r-ac

tive-

activ

atio

n pr

oces

s its

elf t

o sp

ecify

the

scop

e an

d co

nditi

ons o

fac

cess

to th

e Tr

ace.

In o

ur si

mul

atio

ns, w

e ha

ve a

ssum

ed th

at th

e de

ci-

sion

mec

hani

sm c

ould

be

dire

cted

with

equ

al fa

cilit

y to

all

leve

ls. b

ut th

ism

ay tu

rn o

ut to

be

an a

ssum

ptio

n th

at d

oes n

ot a

pply

in a

ll ca

ses.

HolY are rules represe~ted?

It is

com

mon

in th

eorie

s of l

angu

age

toas

sum

e w

ithou

t disc

ussio

n th

at li

ngui

stic

rule

s are

repr

esen

ted

"...'

SlIc

hin the mind of the perceiver, and that perception is guided primarily by

cons

ulta

tion

of s

uch

rule

s, H

owev

er, t

here

are

a nu

mbe

r of d

iftic

ultie

sas

soci

ated

with

this

view

, Firs

t, it

:doe

s not

exp

lain

how

exc

eptio

ns a

reha

ndle

d; it

wou

ld se

em th

at fo

r eve

ry e

xcep

tion,

ther

e w

ould

hav

e to

be

a sp

ecia

l rul

e th

at ta

kes p

rece

denc

e ov

er th

e m

ore general formulation.

Seco

nd, i

t doe

s no

t exp

lain

asp

ects

of r

ule

acqu

isiti

on b

y ch

ildre

n le

ai'n

ing

lang

uage

, par

ticul

arly

the

fact

that

" rul

es a

ppea

r to

be a

cqui

red,

at l

east

to a

larg

e ex

tent

, on

a w

ord

by w

ord

basis

; acq

uisit

ion

is m

arke

d by

agr

adua

l spr

ead

of th

e ru

le fr

om o

ne le

xica

l ite

m o

r set

of l

exic

al it

ems t

oot

hers

. Thi

rd, i

t doe

s not

exp

lain

how

rule

s com

e ,in

to e

xiste

nce

histo

ri-ca

lly; a

s with

acq

uisit

ion,

it a

ppea

rs th

at ru

les s

prea

d gr

adua

lly o

ver t

hele

xico

n, It

is d

iffic

ult t

o re

conc

ile se

vera

lofth

ese

findi

ngs

with

trad

ition

alru

le-b

ased

acc

ount

s of

lang

uage

kno

wle

dge

and

lang

uage

pro

cess

ing.

Mod

els l

ike

TRA

CE a

nd th

e in

tera

ctiv

e-ac

tivat

ion

mod

el o

f wor

d re

c-og

nitio

n ta

ke a

ver

y di

ffere

nt p

ersp

ectiv

e on

the

issue

of l

ingu

istic

rule

s,Th

ey a

re n

ot re

pres

ente

d as

suc

h, b

ut ra

ther

they

are

bui

lt in

to th

e pe

r-ce

ptua

l sys

tem

via

the

exci

tato

ry a

nd in

hibi

tory

con

nect

ions

nee

ded

for

proc

essin

g th

e pa

rticu

lar i

tem

s whi

ch e

mbo

dy th

ese

rule

s. Su

ch a

mec

h-an

ism a

ppea

rs to

avo

id th

e pr

oble

m o

f exc

eptio

ns w

ithou

t dift

icul

ty, a

ndto

hol

d ou

t the

hop

e of

acc

ount

ing

for t

he o

bser

vatio

n th

at ru

le a

cqui

sitio

nan

d ru

le c

hang

e ar

e st

rong

ly ti

ed to

par

ticul

ar it

ems

whi

ch e

mbo

dy th

eru

~.

Wha

t is

sped

t,! a

bollt

...'p

eech

? We dose by ra

ising

a q

ucsti

on th

at o

ften

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

l ".'I

AN

REFE

REN

CES

Bagl

ey, W

. C. (

I~), The apperception o

r the

:spo

ken

sent

ence

: A st

udy

in th

e ps

ycho

logy

of language,

Am

eric

an Jo

llrnc

II of

P,fy

dlol

oR.",

12

, 80-

130.

Ballard. D. H.. Hinton, G. E,. &: Sejnowski, T. J. (1983). l'arallel visual computation.

Na-

Illre (LmIJOII).

306.

21-2

6.Ba

rrow

, H. G

.. &:

Ten

enba

um. J

. M. (

1978

). In

A. R

. Han

son

&: E

. M. R

isem

an (E

ds.

Com

pllle

r ";,f

;C1/

l .V

Slem

s (p

p. 3

-26)

. New

Yor

k: A

cade

mic

Pre

ss.

von Bekesy, G. (1967).

Senmry ;nlrib;l;ml.

Prin

ceto

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v. P

ress

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nd, Z

. S.,

&: G

arne

s, S.

(198

0). M

isper

cept

ions

of l

1uen

t spe

ech.

In R

. Col

e (E

d,

),

Per-

cep;

;t1/l and pmJllcl;on of/11lenl

fpe'

e'dr

, Hills

dale

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aum

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vdw

loRk

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Rel';

ell'

74, 1-

15.

Cole

, R. A

. (19

73).

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ning

for m

ispro

nunc

iatio

ns: A

mea

sure

of w

hat w

e he

ar d

urin

gsp

eech

. Pe

'rupl

;on

& P

.fydl

ClpI

IY,fi

l".f,

13,

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-156

, , .

Col

e, R

, A.,

&: J

akim

ik, J

, (19

78),

Und

erst

andi

ng s

peec

h: H

ow w

ords

are

hea

rd. I

n G

.U

nder

woo

d (E

d.

),

Sll"

Ole

R;e,f

af;n

fimllC

/l;cJ

/I pm

ce,u;

IIR.

New

Yor

k: A

cade

mic

Pre

ss.

Col

e, R

. A..

&: J

akim

ik. J

. (19

80).

A m

odel

of s

peec

h pe

rcep

tion.

In R

. Col

e (E

d,

),

Per-

cepl;an and proJllcl;on offllle'nl speech.

Hill

sdal

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lbau

m.

Col

e, R

. A.,

&: R

udni

cky,

A. (

1983

). W

hats

new

in sp

eech

per

cept

ion?

The

rese

arch

and

idea

s of W

illia

m C

hand

ler B

agle

y. 1

874-

1946

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dwla

Ricc

II Re

',';el

\', 9

0, 9

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nrad

, R. (

1962

). A

n as

soci

atio

n be

twee

n m

emor

y er

rors

and

erro

rs d

ue to

aco

ustic

mas

king

ofsp

e~ch

.Nal

llre'(

LonJ

OII)

.I9

6,

1314

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5.

Cotton. Soo &: Grosjean, F. (1

984)

. The

gad

ng p

arad

igm

: A c

ompa

rison

of s

ucce

ssiv

e an

dindividual presentation formats,

Pe',,

'ep'

;an

f.\:c

llopl

l.\'f

ics,

35. 4

1-48

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row

der,

R, G

. (19

78).

Mec

hani

sms o

f aud

itory

bac

kwar

d m

aski

ng in

the

stim

ulus

suffi

xef

fect

. Ps

ycho

loRi

cal

Re"~

;e'I\

'85

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owde

r, R.

G. (

l98\

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e ro

le o

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mem

ory

in s

peec

h pe

rcep

tion

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disc

rimi-

nation. In T. Myer:s. J. Laver. &: J

. And

erso

n (E

ds.).

TIle ('O

RII;I

;,'e repre.fe

nlal

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Drewnowski. A.. &: Healy. A, (1977). Detection errors on

IIle

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Elman, J, L. (1983). Unpublished results.

Elm

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mod

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f spe

ech

per-

cept

ion.

In N

orm

an L

ass (

Ed.),

LanR

llaR

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nJfp

eed,

(p

p. 3

37-3

74).

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Elm

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cCle

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Expl

oitin

g th

e la

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wav

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J. S

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kell.

&: D

. H. K

latt

(Eds

./n

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an

d I'O

r;abi

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,fpe

ech

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esse

s. H

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aum

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man

. L. D

.. &:

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ser,

U, R

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80).

The

Hea

rsay

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h un

ders

tand

ing

syste

m: A

, tutorial. In W. A. Lea,

Tre'n

Js;n

spee

ch re

coRI

I;I;c

1/I

(pp.

361

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81).

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ewoo

d Cl

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NJ. Prentice-

Hal

l.Fe

ldm

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alla

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nnec

tioni

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s and

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CoR-

n;I;,'e Science', 6

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I. . (

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alke

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s.N

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Amst

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m: N

orth

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land

..Fo

ss, D

. J..

&: B

lank

. M. A

. (19

80).

Identifying the speech codes.

CoRn

;I;,'e

P,f

ydw

loR.

\12

, 1-31

. Fo

ss, D

, J,.

&: G

erns

bach

er. M

. A. (

1983

), C

rack

ing

the

dual

cod

e: T

owar

d a

unita

ry m

odel

of phoneme identification.

Jollr

nal o

f Ver

bal L

earn

;nR

and

Ver

bal B

ella

,';or

, 22

. 609

-63

3.

com

es u

p in

disc

ussio

ns o

f the

mec

hani

sms o

f spe

ech

perc

eptio

n. Is

spee

ch sp

ecia

l? If

so. i

n w

hat w

ays?

It h

as b

een

argu

ed th

at sp

eech

issp

ecia

l bec

ause

of t

he d

istin

ctiv

e ph

enom

enon

of c

ateg

oric

al p

erce

ptio

n;be

caus

e of

the

enco

dedn

ess o

f inf

orm

atio

n ab

out o

ne p

hone

me

in th

ose

porti

ons o

flhe

spee

ch st

ream

that

are

gen

eral

ly th

ough

t to

repr

esen

t oth

erph

onem

es; b

ecau

se th

e in

form

atio

n' in

the

spee

ch s

tream

that

indi

cate

sth

e pr

esen

ce o

f a p

artic

ular

pho

nem

e ap

pear

s not

to b

e in

varia

nt a

t any

obvi

ous p

hysic

al le

vel;

beca

use

Qf t

he la

ck o

f seg

men

t bou

ndar

ies.

and

for a

var

iety

of o

ther

reas

ons.

Ove

r the

last

seve

ral y

ears

, a n

umbe

r of e

mpi

rical

arg

umen

ts ha

ve b

een

put f

orw

ard

that

sugg

est t

hat p

erha

ps sp

eech

may

not

be

so sp

ecia

l, or

at le

ast,

not u

niqu

e, C

ue tr

ade-

offs

and

con

text

ual i

nfluences are. of

cour

se, p

rese

nt in

man

y ot

her d

omai

ns (M

edin

& B

arsa

lou,

in p

ress

).an

d a

larg

e nu

mbe

r of s

tudi

es h

ave

repo

rted

cate

goric

al p

erce

ptio

n in

othe

r mod

aliti

es (s

ee R

epp.

198

4. fo

r a d

iscus

sion)

, Com

puta

tiona

l wor

kon

pro

blem

s in

vis

ion

have

mad

e cl

ear t

hat i

nfor

mat

ion

that

mus

t be

extra

cted

from

vis

ual d

ispl

ays

is -o

ften

com

plex

ly e

ncod

ed w

ith o

ther

info

rmat

ion

(Bar

row

& T

enen

baum

; 197

8; M

arl'.

1982

), an

d th

e la

ck

clea

r bou

ndar

ies

bet;w

een

perc

ept m

il un

its in

visi

on is

not

orio

us (B

alla

rdet a!.. 1983: M

arr.

1982

), Th

us, t

he p

sych

olog

ical

phe

nom

ena

that

cha

r-ac

teriz

e hu

man

spe

ech

perc

eptio

n, a

nd th

e co

mpu

tatio

nal p

robl

ems

that

mus

t be

met

by

any

mec

hani

sm o

f spe

ech

perc

eptio

n, a

re n

ot, i

n ge

nera

l,un

ique

to s

peec

h. T

o' b

e su

re. t

he p

artic

ular

con

stel

latio

n of

pro

blem

s th

atm

ust b

e so

lved

in sp

eech

per

cept

ion

is di

ffere

nt th

an th

e co

nste

llatio

n of

prob

lem

s fa

ced

in a

ny o

ther

par

ticul

ar c

ase.

but

mos

t of t

he in

divi

dual

prob

lem

s th

emse

lves

do

appe

ar to

hav

e an

alog

s in

oth

er d

omai

ns.

We

ther

efor

e pr

efer

to v

iew

spee

ch a

s an

exce

llent

test

bed

for t

hede

velo

pmen

t of a

n un

ders

tand

ing

of m

echa

nism

s whi

ch m

ight

turn

out

to h

ave

cons

ider

ably

bro

ader

app

licat

ion,

Spe

ech

is sp

ecia

l to

us, s

ince

it so

rich

ly c

aptu

res t

he m

ultip

licity

of t

he so

urce

s of c

onstr

aint

whi

chm

ust b

e ex

ploi

ted

in p

erce

ptua

l pro

cess

ing,

and

bec

ause

it so

cle

arly

indi

cate

s the

pow

erfu

l inf

luen

ces o

f the

mec

hani

sms o

f per

cept

ion

on th

eco

nstru

cted

per

cept

ual r

epre

sent

atio

n. W

e se

e th

e TR

ACE

mod

el a

s an

exam

ple

of a

larg

e cl

ass o

f mas

sivel

y pa

ralle

l. in

tera

ctiv

e m

odel

s tha

tho

lds g

reat

pro

mise

to p

rovi

de a

dee

per u

nder

stand

ing

of th

e m

echa

nism

sge

nera

lly u

sed

in p

erce

ptio

n,

Ackley, D.. Hinton. Goo & Sejnowski, T. (l9

8.'i)

, Bol

lzm

ann

mac

hine

s: C

onst

rain

t sat

isfa

c-tion networks thai learn.

C"N

n;I;I

'e Sciellce, 9, '13- 147.

AlU

lcr~

on. 1

. 1\,

t 19771, Neural models with cognitive im

plic

atio

ns. I

n D

. LaB

erge

& S

. J.

Sam

uels

tEds

. ). 8

".fk p/"(/('

f('f;I

I re

"JillN

: Per

cepl

i"n C

/IIJ

mm

pref

rell. d

on.

Hm

sdal

e,N

J: E

rlhau

m.

TRAC

E M

OD

ELM

C C

LELL

AND

AN

D E

LMAN

Foss

, D. J

., &

Swin

ney.

D. A

. (19

73).

On

the

psyc

holo

gica

l rea

lity

of th

e ph

onem

e: P

er-

ception, identification, and consciousness.

Jollr

nal o

j Ver

bal L

earn

ing

tllld

Ver

bul

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l'ior

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, 24

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Fowler, C. A. 11984). Segmentation of coaniculated speech in perception.

Pert'

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, Unp

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Jollrmll oj Experi-

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sycl

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llllltl

ll Pe

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Per

fimna

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., Ei

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, (19

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iden

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atio

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Langllage and Speech,

5, 171-

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Fujim

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J. B

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Sylla

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as c

onca

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ts. In

A. B

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& J. B. Hooper

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i. H

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heidentification and, discrimination of synthetic speech sounds.

Rept

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h In

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it's.

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. F. 1

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para

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,'iel

l', 7

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understanding system. In W. A.

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