the effect of the geometric form.pdf

7/27/2019 THE EFFECT OF THE GEOMETRIC FORM.pdf

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THE EFFECTOF THE GEOMETRICFORMAND MEANING

OFTHE STIMULUS ON THE CONFIGURATION OF THE

VISUALEVOKED RESPONSE

by

SHERRILL JEANSWIFT PURVES

B.Sc, McGill University,1967

M.D. UniversityofB r i t i s h Columbia,1971

A THESISSUBMITTED INPARTIAL FULFILMENTOF

THEREQUIREMENTS FOR THE DEGREE OF

DOCTOROF PHILOSOPHY

i n

THEDEPARTMENT OF PHYSIOLOGY

Weaccept this thesisascxsnforining

totherequired standard

THE UNIVERSITYOFBRITISHCOLUMBIA

May,1976


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In pre sen t in g th is th es is in p ar t ia l fu l f i l me nt of the requirements for

an advanced degree at the U n i v e r s i t y of B r i t i s h Colum bia, I agree that

the Li br ar y sh a ll make it fr ee ly a va il a bl e for r efere nce and study.

I fu rt he r agree tha t perm iss io n for ext ens ive copying of thi s t he s i s

f o r s c h o l a r l y purp oses may be gr an te d by the Head of my Department or

by hi s re pr es en ta t i ve s . I t is understood that copying or pu bl ic at io n

of t hi s th es is for f i n a n c i a l gai n sh al l not be allo wed withou t my

w r i t t e n p e r m i s s i on .

Depa rtment

The U n i v e r s i t y of Br i t ish Columbia

2075 Wesbrook Place

Vancouver, Canada

V6T 1W5


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ABSTRACT

Inthevisual system i t has been established that certain physical

parameters of thestimulusincluding intensity, focus,and sizeof

theelements of apatternhavesignificant effectson the configuration

ofthe evoked response as recorded from theoccipital region. While

claimshavebeenmadei n a fewpublications,tothisdate thequestion

ofwhether anypartof the evoked response i saffectedby higher

perceptualprocessesratherthan onesmoredirectly related tostimulus

inputhas not been resolved.

Evoked responses tofourgeometric shapes (a square,c i r c l e , e l and

omega)wererecorded frommultiple scalp locationsunder two experimental

conditions; i n the f i r s tthe shapeswerepresented atrandom intervals

and inrandomorder with nomeaningassigned tothem. In the second

theywerepresented i n a fixedrhythmic sequence, and two of the shapes

occasionally failed to appear intheir usualtime interval i n the

sequence. The subject"wasrequiredtorapidly signal such omissions by

a buttonpress. The cerebral electrical activityduring this time

intervalof the expected, but omittedstimuluswas recorded and

separatelyaveraged, and these responseswere called emittedpotentia ls.

Thereweresignificant,differencesbetweenthe evoked responses (in the

occipital region) to the square and e l shapes andbetweenthose to the

c i r c l e andomegashapes. These differencesweredemonstrated by three

measurementtechniques: performance of thediscriminant functions


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computed bySWDAi ncla ssi fyi ng single t r i a l responses, aratio

s t a t i s t i c calledXasdescribedbyJohn,Herrington&Sutton (1967),

and amplitude differences i n the N 2 and P 2 components.

Theemitted potentials recorded fromthevertex, containedasmall

and variable early negativecomponent and anobvious late positive

component similarto the oneseeni n theresponses to thepresent square

ande lduringthesecondexperimental condition. Manipulationof the

waveformsfromthesingle t r i a l s priortoaveraging showed that this

negativecomponent was nottime-lockedto the peak.

Theevokedpotential differences foundwere believed to be due to

twoclasses ofvariables; thephysical char acteristi csof thestimulus

(the contoursin thecentral1.5 of thevisual field)and task-related

changesi n the meaning of thestimulus; andthese affected earlierand

laterpartsof the waveform respectively.


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TABLE OFCONTENTS:

Page

t

Abstract i i

TableofContents . i v

L i s tofFigures v i i

L i s tofTables v i i i

L i s tofAbbreviations i x

Acknowledgement x

I. Introduction 1

II. Review of theLiterature 5

2.1 Therelationshipofscalp-recordedevokedresponsesto theunderlying brai na c t i v i t y 5

2.2 PatternVER's i n thestudyofvision 7

2.3 Evokedresponsesi n thestudyofpsychologicalvariablesand meaning 14

2.4 Suitniaryof theliterature review 20

III. Statementof theproblemf orinvestigation 22

IV. Methods 24

4.1 Dataacquisitionmethods 24

4.2 Dataanalysismethods 33

V. Results 38

5.1 Effectofdifferent experimental conditions

ontheevokedresponse 38


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5.2 Effectofdifferentstimulus shapes

ontheevoked responses 42

5.2.1 ResultsofVisu al analysis 43

5.2.2 ResultsofStepwise discriminantanalysis 50

5.2.3 ResultsusingtheXdescript or

computations 60

5.2.4 Summary of theeffecto f the

different stimulus shapeson theevoked responses 64

5.3 Emitted responses 67

5.3.1 Averaged emitted responses 67

5.3.2 Averaged "shifted"emitted responses 72

5.3.3 Control studies: motor potentials,

EOGcontributions,andtopography . . . 76

5.3.4 Summary offindingsfor omitted

stimuli 79

VI. Discussion 81

6.1 Studiesofevoked responsesandstimulusmeaning 81

6.2 Techniques fortheanalysisofevokedresponses 83

6.2.1 Visu al analysi s 85

6.2.2 Xvalues 85

6.2.3 Stepwisediscriminantanalysis 86

6.3 Topographicaldistributionof theevoked

responses 89

6.3.1 Locationofshape effects 89

6.3.2 Distributionofresponsecomponents.. 90


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v i

6.4 Intershape differences related to physical

propertiesof the stimuli 93

6.5 Effectsof stimulusmeaning 99

6.5.1 Present stimuli 99

6.5.2 Omitted stimuli 101

VII Summary& Conclusions 105

Bibliography 110

AppendixA. Computerhardware and data

acquisitionprogram (VER03) 118

AppendixB. Stepwise Discriminant Analysis 120

AppendixC. Termsfrom experimental psychology .. 124


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vii.

LISTOF FIGURES

Figure Page

1. Electrodeplacements 25

2. Dataacquisitionsystem 27

3. Stimulusshapes 29

4. Experimentalparadigms 30

5. Effectofexperimental conditions forcircleand omega . 39

6. Effectofexperimental conditions forsquareande l.... 41

7. Effectofstimulusshapeon theevokedresponses 44

8. Intersubjectvariability; occipitalVER toe l for

13 subjects 45

9. Effectofstimulus size 52

10. Distributionoflatency pointschosenby the4-groupdiscriminant functions 57

11. Distributionoflatency pointschosenby the2-groupdiscriminant functions 62

12. DistributionofXvalues 65

13. Averagedemitted potentialsat C infivesubjects 68

14. Averagedemitted potentialsatmultiple recording sites

inonesubject 69

15. "Shifted"averagedemitted potentialsat C zinfivesubjects 73

16. "Shifted"averagedemitted potentialsatmultiplerecordingsitesin twosubjects 74

17. Responsestoomitted stimuliwithEOGcontrol 77

18. Responsestoomitted stimuliinfrontal regions withabsentbutton press 78

19. Flowchartof VER03program(AppendixA) 119


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v i i i

LISTOFTABLES:

TABLE

I. Latencies of thecomponentso f occipital

evokedresponses to the fourshapes 48

II. Amplitudesof thecomponentsof occipital

evokedresponses to the fourshapes 50

III. Performanceof the "four-shape" discrinunant

functions 54

IV. Percentagesof t r i a l s incorrectly classified

separated in to pa ir s ofshapesconfused witheachother 56

V. Performanceof the "two-shape"discriminant

functions.One subject, five pairs ofshapes 59

VI. Performanceof the "two-shape" discrinunantfunctions. Fiv e subjects, two pai rs of

shapes 61

VII. Mean A values foreachsubject 63

VIII. Amplitude and latency of P^ forresponses

to omitted and present stimuli 70

IX. Amplitude and latency of N 2 forresponses

to omitted and present stimuli 71


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LISTOF ABBREVIATIONS

EEG Electroencephalogram

EOG Electro-oculogram

EP Evoked potential

ER Evoked response

SWDA Stepwise Discriminant Analys

VER VisualEvokedResponse


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X

The author gratefullyacknowledgesthe assistance inthis research

workprovided by anumberof people, especially

Dr.MortLow whoseadvice,encouragement,and t i r e l e s s personal

involvement i n the projectwereinvaluable fo r me

MichaelBakerfor hisinterest and a b i l i t y i n the design and writ ing

ofthecomputerprogramsf or thePDP-11system

JaniceGallowayand RichardFergusonfortheir technicalassistance

with the experimental apparatus and the subjects

JimMcEwenand JohnDoylefromtheDept.of E l e c t r i c a l Engineering

for their helpful instructioni n the use of the UBC Computing

Centre F a c i l i t i e s

Dr. Michael SchulzerfromtheDept.ofMathematicsfor advice on

s t a t i s t i c a l testing and

my committeemembersi n theDept.of Physiology; Drs.Tony Pearson,

HughMcLennanand PeterVaughanfortheir consideration, suggestions

and criticismso f the pro jec t.

Financial support was provided by the Medical ResearchCouncil of

Canadathrough an MRC Fellowship to the author fromSeptember 1972

u n t i lDecember1975 and MRC GrantMT-3313.


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Chapter I

INTRODUCTION

To attempt tounderstandtheenormouscomplexityof neural transactions

within thehumanbra in i s a formidable challenge to invest igato rs i n a l l

of the disciplines of neu rol ogi cal research. One of themethodsof

directly studying the neural processes i n the brai n, i s to record the

electrical activity occurring inresponse tosensory stimu lat ion . Two

classes of electrical activity can be recorded; theminutely localized

potentials of singlenerve c e l l s and themoredi ff us e and slowly changing

potential fieldsdetectable i n the v i c i n i t y of lar ge populations of

neurons. The terms"evokedpot ent ial s" and "evokedresponses" are

coimonlyusedtodenotethe sig nal s of the latter class of cerebral

e l e c t r i c a l a c t i v i t y that are recorded followin g stim ulation of sensory

receptors or afferentpathways.

A stimulus i n i t i a t e s asequenceof physiologicalevents,whichare the

substrates fo r i t s perception, as well as processes leading to an overt

behavioral response. Therefore, analys is of the e l e c t r i c a l activity

occurringbetween stimulus and response should provide clues to the

natureand anatomicalloca tion ofthese physiologicalevents.

Since the late1950's, there has beena largeamountof data published

aboutthe eff ec t of awidevar iet y of stimulus parameters on the evoked

responsesrecorded fromhumansubjects. The stimulus parameters

investigatedhave included bothphysical characteristicssuchas intensity,

1


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2.

durationandsize,andpsychological-contextualonessuchasexpectation,

task-relevance andstimulusmeaning. Withrespectto thelat ter,

conflicting reportshaveappearedi n theliterature. Inspiteofthe ir

importance forthedevelopmentoftheoriesofinformation processing,

somewidely publ iciz ed findingson theeffectofstimulusmeaningon the

evokedpote ntia l configurationhavebeen poorly substantiated.

Theuse ofpatterned visual stimulitoe l i c i t evokedresponsesi s arecent

developmenti nthis f i e l d ofstudy, becauseof therelativetechni cal

complexity of thestimulus presentationapparatus (ascomparedto the

simple stroboscopic flash thatwasusedpreviouslytostimulatethevis ual

system). Withpatterned stimulitheexperimentercanprovokeandexamine

the physiologicaleventsinvolvingmorecomplexaspectsofvisu al

perceptionandrela ted cogn itive processes thanwasformerly possible

using onlytheflas h.

Anotherrecentadvancei n thestudyofcogn itiv e processes rel ate dto

processingofsensory information, appearsi n somerecent reportsof the

recordingof aconsistentwaveform fromthescalpi n theintervalwhen a

subjectexpectsbuti snotac tu al ly presented withastimulus. The

responsesrecorded i n theintervalwhen nostimulushasbeen presented

are called emitted potentials,todistinguishthemfromtheevoked

potentials e l i c i t e d bystimuli thatareact ua ll y presented. Thestudyof

these emitted potentialwaveformsmayprovide important information

concerning bra in processes without inte rferen cefromtheactivity directly

relatedto thestimulus input.


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3.

In recordingsmadefrom thescalp, the evoked response i s ofmuchsmaller

amplitude (l-20yv) than the ongoingelectroencephalographic (EEG)

activitywhich i s 10 - 100 yv i n amplitude. In order to isolate and

studythe activity relatedto the stimulus, repeated responses are averaged

startingat the time of stimuluspresentation. The event-related activity

which, i spresumedt o be relatively constant in the repeated t r i a l si s

enhancedand the background non-event related EEG i scancelledby this

method. Thenumberof t r i a l sper average hasvariedwidely indiffer ent

reports,ranging from 10 to2000ormorerepetitionsof the stimulus.

The variance of these t r i a l s included i n the average i srarelyever recorded.

The resulting averaged evoked responses can beexamined. Thepeaksare

labelledand their latencyand amplitude can bemeasured. Whensimple

stimulusvariablessuch as luminance arealtered,theeffect i susual ly

only seen i n one o f thepeaksorcomponentsof the evoked response and i s

f a i r l yeasy to observe andmeasure. Howeverwhenchangesaremadei n

complexstimulusvariablessuch asmeaning,morethan onecomponentin

the waveformmay be alteredand i tbecomesquite d i f f i c u l t toquantify

differencesi n thewaveformsthatcan be relatedto stimulusvariable s.

Ifthe evoked EEG a c t i v i t y i sdigitizedand thentreatedas aseriesof

discreetobservations,somequite differentmethods,other than averaging

can be used to approach thisproblem. One suchmethodi scall ed

discriminant analysis; i t i s a technique based onmultivariate statistical

theories thatdetermines thevariables (or time points)thatbest account

for thedifferencesbetweentwo ormoresetsof observations (waveforms).


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4.

It requires verycomplexcomputationsand mustbe ca rr ie d out on a large

computer. (AprogramcalledBMD:07MStepwiseDiscriminant Analysis from

theUCLAseries i s availabl e inmostcomputingprogram libraries).

Inorder to investigate the natureand anatomical loca tion of the

(cortical) events that underlie v is ua l perception, a studywas designed

usingcomplexpatterned vi su al sti mu li (geometricshapes)todemonstrate

what,i f any, differences i n the evokedpot ent ial configuration could be

attributed to differences in stimulus meaningor to differences i n the

physical properties of the stimuli. The experimentaldesign also included

aparadigm to record the emittedpotential a c t i v i t y thatappeared during

the intervalwhenthe subject expected but did not see one of the geometric

shapes. A l l of the recordedwaveformsweredigitized and stored as single

t r i a l s so that a variety of techniques, includingaveragingand discriminant

analysis could be usedtomeasureany differencesbetweenthem.


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5.

Chapter II

REVIEWOF THE LITERATURE

2.1 The rela tionship of scalp recordedevokedresponsesto theunderlying brain activity.

A f a i r l y comprehensivereview of the experimental evidenceand current

theories isprovided by Kiloh,McComas&Osselton (1972)for the origin

of spontaneous a c t i v i t y and byRegan (1972)for the o ri gi n of both

spontaneousand evokedEEG a c t i v i t y . The availa bledataabout the origins

of scalp-recorded e l e c t r i c a l potentials are fragmentaryand do not yet

precisely define the sources of eitherspontaneousEEG a c t i v i t y or evoked

potentials.

Early reports suggested that the 0-60 Hz. "slow" a c t i v i t y in the EEG

recorded at the c o r t i c a l surface i s the re su lt of asummationof action

potentials of c o r t i c a l c e l l s . Otherslatershowedconclusively that neuronal

action potentialswerenot e ssential and suggested that thewavesof the

surface EEG resultfroma summationof the exc itatory or inhib ito ry

postsynaptic potentialsdeveloped by the somaand larg er dendrites of the

c o r t i c a lpyramidal c e l l s (Li &Jasper,1953). This suggestion received

further supportfrom studies including intracellular recordings of cortical

neurons (Jasper & Stefanis, 1965; Creut zfeldt,Watanabe& Lux, 1966) that

showeda close timerelationshipbetweenre pet it ive postsynaptic potent ials

andmacropotentials recordedon the surface of the ove rly ing cortex.

Insomemorerecent workE l u l (1968)demonstratedthe existence of slow,

spontaneousmembranepotential shifts i n c o r t i c a lneurons,in thesame

frequencyrangeas the EEG. On the basis of theseobservationshe suggested


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6.

that the EEG may r e f l e c tthese slowerpotentials ratherthanbeing a

temporalsummationof fas tera c t i v i t y , as the e a r l i e r studies of post

synaptic dend ritic pot ential s had implied.

The re la ti onsh ip of the EEG and evokedpotentiala c t i v i t y recorded from

the scalp to the e l e c t r i c a l a c t i v i t y recorded fromthe c o r t i c a l surface

has beeninvestigated by anumberofworkers. Geisl er & Gerstein (1961)

showedbothexperimentally (inmonkey)and i n theo reti cal formulations that

amajor attenuation of e l e c t r i c a lpotentialamplitudeoccurs in the highly

conductivelayers of the CSF and dura but not at the skull or scalp.

Cooper,Winter,Crow&Walter (1965)in studies onhumanpatientswith

simultaneousrecording from the scalp and implanted c o r t i c a l electrodes,

foundthat only "widely synchronized" cort ical activi tywas not obscured

at the scalp and that theamountof attenuationbetweencortex and scalp

dependeduponthe size of the area of cortex involved in the activity.

In a study of the EEG a c t i v i t y followingsensory (flash) stimulation,

Heath& Galbraith (1966),using c o r t i c a l and scalp electrodes i n the primary

visual area showedthat an earlycomponent (65msec.)was evident at the

samelatency i nboth c o r t i c a l and scalp recx)rdings; but for later

(100 to 300msec.)peakstherewerelatency differenc esbetweenthe response

componentsrecorded at the two electrodes. In explanation of thesewaveform

differences, theysuggested that a c t i v i t y generated i nnon-primary cortex

spreadto the o c c i p i t a l scalp lead but not to the c o r t i c a l lead. Hence

they concluded forVER'sto flash, that the earlycomponentsin the response

recorded fromthe s calp electrode directly overtheprimaryvisual cortex

are generatedonly in the immediately underlying brain area, while the


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7.

laterresponsecomponentsrepresentsummationof a c t i v i t y frommore

extensive (including both primary and non-primarycortex as faraway

as thetemporal area)brain regions.

Vaughan (1966)reported similar findings i n a patient explored with cortical

electrodesduring an intracranial surgicalprocedure. However,in studying

the distributionofevokedresponseto flash i n the c o r t i c a l regions

adjacent to the calcarinefissure, he found theresponsedifferedmarkedly

as the recording electrode was movedanteriorly fromthe occipital pole.

He concluded thatmostof the scalp VER usually recorded in the occipital

regions origin ated i n the occipitalpole of the cortex (foveal representation)

and not inmoreanter ior portions of the striate cortex. This findingi s

consistentwith other authors' observations (Devoe,Ripps&vaughan, 1968;

Harter, 1970) that the VER pri ma ri lyreflects foveal stimulation.

2.2 PatternVER'si n the study of vision.

The responserecorded i n theoccipital region e l i c i t e d by spatially

structured stimulus fields i s quitedifferent fromthe one evokedby a flash.

Thismajordif feren ce was f i r s t described bySphelmann(1965)who showed

thata patterned stimulusevokesaresponseofmuchgreater amplitude

and containingpeaksof differentlatency and polarity than a flash of

comparative l i g h t intensity.

The type of pattern presentationusedto e l i c i t the "patternvisualevoked

response" i s important. The stimulus may be abriefpresentation of a

patternon a blank background; i t may be achangeof patternfrom one

formto another (called a "patternreversal" i n the literature) or i t may


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8.

be illuminationof adarkpatternby aflash. Thetheoreticaladvantage

ofapattern reve rsaltypeofstimulationi sthat therei s nochangei n

the t o t a l luminancereaching thesubject's eye, onlyachangei n

distributionofpattern structure withinthevisual f i e l d . Consequently

someworkers (Halliday&Michael, 1970)havesuggestedthattheresponse

to pattern re ver sal contains only "pattern specific"components,and not

any toluminancechangeasdoestheflashVER.

However,thepattern rever salmethodi susableonly for certaintypesof

symmetricalpatterns suchascheckerboardsorgratings. I t hasalsobeen

shownbyEstevez&Sprekreijse (1974)thatthepotentialgeneratedby

pattern reversal includescomponentsfound bothi nresponsesto a pattern

appearanceand apattern disappearance (analgousto an "on" and"off "

response), andhencei snot asuncomplicatedasoriginally suggestedby

Halliday.

The thirdmethodofpresenting thepatterned stimulus (byillurninatinga

darkpatternwithastroboscopic flash) requires le ss elaborate instrument

ationand hasbeenused i nmany studies includingtheoriginalone of

Sphelmann (1965). The VERproducedbythis typeofstimulation consists

ofasummationof theresponsestoboththelargeluminancechangeproduced

bytheflashand to thepatterned stimulus.

For a l lof theabovestimulationtechniquesthestimuliaresuff iciently

widelyspacedi ntimethatthenervous systemhasbeenregardedasreturning

tothe samei n i t i a l statebetweensuccessive stimuli,andhencethese


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9.

responsesaresometimescalled "transient"responsestodistinguishthem

from another typeofevoked pote ntia l described i n theliterature calle d

a "steady state"evoked response. (SeeRegan(1975)fora complete

descriptionofthisphenomenon). Theterm "steadystate"i semployedto

indicate thatthestimuliarepresentedatsucharapid rate (e.g.10 - 60 Hz.)

thatthec o r t i c a lresponsestoindi vidual st imuli overlap. Thesteady state

responsei sthen separated fromthenon-event relatedbackground EEGbecause

i t i scomposedofcomponents (or harmonics) whosefrequencies areexactlythe

same as ormultiplesof thestimulus frequency. AFourier analysisi s usedto

performtherequired computations and theamplitudeandphaseof thefrequency

componentscan bedisplayedandmeasured.

The techniquehas theadvantage ofspeed becausethestimulican bepresented

somuch morerapidlythan theycan beforthestudyoftransient evoked

responses. Regan(1974)hassuggested thatthepropertiesof thecomponents

ofthesteadystateresponseseemtodependonwhetherthestimulation

frequency i s(a)near10 Hz., (b) 13 - 25 Hz., or (c) 45 - 60 Hz. and has

foundthatsomediseasesinvolvingthevisualsystemmayaffectthese

frequencybandsdifferently. Thesteady statetechniquehasalsobeenused

by Campbell &Kulikowski(1972)i nstudiesof therelationshipof a subject's

a b i l i t yto see asinewavegrating patternof lowcontrastto thesizeof the

evoked response e l i c i t e d by the samegrating pattern. Theyseparately tested

thesubject's thresholdbyhavinghimreportwhen thegrati ng pattern

disappearedas thecontrastwasdecreased,andthenbyrecordingtheevoked-

responseto arapid pattern rev ersa lof thegratingatdiffe rent levelsof

contrast. Theyfoundgood agreementbetweenthethreshold determinedby the


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10.

subject's verbal report and the contrast level atwhichthe evoked response

could no longer be detected. The steady state patternevokedresponse

thusprovides an objective indi cator of visual function and perception.

Transient and steadystateevokedresponsesgivecomplementary insights into

brain function. Van Hof (1960)and Tweel& Verduynlunel (1965)showedthat

i t i s not possib le to pred ict the parametersof the steadystateresponses

from the characteristics of the transientresponses. This i s not surp risi ng

i nviewof the very different stimulation techniques, but consequently

results obtained with one methodare not directlycomparablewith those

obtained with the other.

The remainderof the literature on patternVER'sto be reviewed i sconcerned

with the transienttypeofresponse, as i s the experimental work described

i n thisthes is.

The individualpeaksof the t ransi ent pattern VER have been studied using

topographicalmappingtechniques in an attempttodetermine the area of

cortex inwhichthesecomponentsoriginate. Jeffreys(1971)and Je ffre ys &

Axford (1972a& b) foundthat the f i r s t two peaksof the VER occurring

65 - 80msec,and 90 - 110msec,af te r presentation of acheckerboard pattern

(they called thesepeaks "C I" and "C I I" respec tively)weregre atly

influenced by the retinal lo ca tion of the stimulus. The polarity of these

componentsreversedwhentheupperrather thanthe lower f i e l d of vision

was stimulated, and theamplitude of C IIchangeddependingupon the

distance of the recording electrode from the calcarinefissure inboth the


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11.

horizontaland vertical planes. Basedon these and other observations of

responsesto right and l e f t half f i e l d stimu latio n, Je ff re ys & Axford

concluded th at the two peakshad spatially separate sources; the striate

cortex was the source of C I and the ext ra -s tri at e cortex on the outer

surface of the occipital lobes was the source of C I I . Ha ll id ay & Michael

(1970)described a sing lecomponentat 100 msec,i n the response to a

reversingcheckerboard stimulus that alsoshowedpolarity reversaldepending

on the part of the visual f i e l d that was stimulated. Theysuggested that

thiscomponentoriginated i nextra-striatecortex i n the occipital pole

aboveandbelowthe calcar inefissure. A similarcomponentin the responses

to a reversi ng pattern of red lightswas described by Purves& Low (1975)

using separate stimul ati on ofupperand lowervisual fields. The polarities

and latenc ies of thecomponentsdescribed by a l l of these groupsaresomewhat

differentprobably due i n part to differences i n the luminanceof the stimulus

and i n the techniques of stimulus presenta tion. The lack of standardized

stimulation and recording techniquesmakesthe find ings of the different

investigators d i f f i c u l t tocompare.

Datafromintracellular recording in the visual cortex in cats andmonkeys

(Hubel& Weisel, 1965 &1968); Bishop, Coombs&Henry,1971) haveshown

thatmanyc e l l s i nthis brain region aremuchmoresen si ti ve to contoured

thanto unstructured light stLmuli. Suchworkhas r esul ted i n important

advancesi n the understanding of the c e l l u l a r physiology of the occipital

lobe and of the fu nctio nal substrates of vision. Datafrom the study of

VER'srecorded i nawakehumansubjects (see theabove observation,

localizing the origin of these pattern specific responsecomponents to


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12.

striateandextra-striatecortex)inturn,haveprovided supportive

evidenceof theexistenceofthese patternsensitivemechanismsinthi s

regionandtheymayproveto be animportant toolfor further investigation

of visual perceptual processes inman.

In additionto thestudiesconcernedwiththetopographicalmappingof the

responsecomponents,therehave beenmanyothers thathavedemonstrated

the effectof avarietyofpatterned stimulusparameterson theevoked

c o r t i c a l response.

Clynes&Kohn(1967)reportedone of theearliest studies using stimulus

patterns other thantherelatively simpleones (checkerboards andstripes)

thatwereusedbySphelmann (1965). Theystudiedthespatial distribution

oftheevokedpotentialsto alargenumberofvisu al stimuliincluding

complexpatternsandcoloursandfound that therewere "spatially independent

components"thatwerepeculiartoeachtypeofstimulus foragiven individual.

Theyalso noted that whilethelatenciesofthesecomponentswere very

constant, their amplitudes werequite variab le,a finding replicatedi n

manylater studies. Thechanging polaritiesandamplitudes of theresponse

componentstodiffe rent stimuliwhichthey reporteddosuggestthepossib ility

ofchangingsourcesandsinksofcurrenti n thec o r t i c a l a c t i v i t yevokedby

the different stimuli. Howevertheir ratheridiosyncratic electrode

placements (inarosette patternaroundtheoccipitalregions),theiruse

ofbi pola r recording deriv ations,andtheir largenumberofstimulus types

makesi t impossibletocomparetheir resultstothoseof anylat er

investigators,or to make anydefiniteinterpretationsofthemeaningof

their results.


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13.

Rietveld, Tordoir,Hagenouw,Lubbers & Spoor (1967)and Harter &White

(1968)presentedmoredetailed studiesof the responses to a checkerboard

patternandshowedthatthe amplitude of the response from100 to200msec,

post-stimuluswas sensitive to thesizeo f the checks i n the pattern.

Theyshowedthati ncontrastto theflashevoked responses i n which the

luminance of the stimulus i s an important determinant of thelatencyof

thecomponents, (Creutzfeld &Kuhnt,1967; Tepas, Guterus & Klingman, 1974),

thelatencyof thecomponentsi n thepatternresponse did notchangeover a

widerange of luminances. Theysuggestedthat i twas the presence of the

intersecting contrastbordersthatwas essential for the patterned type of

evoked c o r t i c a lresponse. How sharplythe stimuluspatternwas focused was

alsofound tohavea significant effecton the VER amplitude i n these

studies. Manysubsequent reports (Arden & Lewis,1973; Harter & White,

1970; John, 1974) havedemonstrated thatthe amplitude of the VER to

patterns consistingofdifferent sizedchecks i s auseful tool i n the

determinationof therefractive error i nc l i n i c a l ophthalmology. The

patient'sresponse i s oflargestamplitude f or thesmallest sizeof checks

on which he can focus clearly.

Anumberof other stimulus parametershavebeenshownto be reflectedi n

theVER configurationbesides focus, check size and luminance as indicated

above. The overall sizeo f the stimulus for both unstructured(Rietveld,

Tordoir& Duyff,1965) and structured (Harter,1971; Rietveldet a l . , 1967)

f i e l d shasbeenfound to be important with amarkeddifferencebetween

fovealand extra-fovealstimulation. The colourof the stimulus can also


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14.

influence the response (Kinney,McKay,Mensch& Luria, 1972; Clynes &

Kohn,1967; Shipley, Jones & Fry, 1965) as can i t s or ie nt at io n

(Kulikowski, 1974; Yoshida,Iwahara &Nagamura,1975) althoughthere i s

somedispute aboutthe l a t t e r (Kakigi,Miyazaki &Mori,1972). Responses

have beendemonstratedboth to on- and of f- se t of diff use (White &Easton,

1966) and patterned l i g h t (Harter, 1971). The specific findings inthese

reports that are relevant to the presentstudy w i l l be considered i nmore

detail i n the Discussion Section.

2.3 Evokedresponsesin the study of psychological variables and meaning.

A very largenumberof publications i n the evokedpotentialliteraturehave

originated i n laboratories of physiologic al or experimentalpsychology.

A review of the relevantworksof nece ssity includes termsand phrases,

the precise def ini tio ns ofwhichare outside the scopeof th is thes is.

InappendixC a discussion of the terms "psychological variables" and

"stimulus meaning"has been included toprovidesomebackground for the

readerunfamiliarwiththese concepts, and to c l a r i f y th ei r use i n th is text.

The concept of stimulus meaningi s also considered i n relat ionship to the

experimental findings of th is di ss er ta ti on i n section 6.5 of the

Discussion chapter.

A greatmanyreports of the effec ts of a va riet y of psychological variables

on the evoked potential configuration are reviewedby Regan (1972) and

Beck (1975). These studies can be divided into two categories, those that

are concernedwith the entireevokedresponse configuration and those that


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15.

are primarily concerned with a late positive componentof theevoked response

(also called P^ or"P3QQ"becausei t appears as a prominent positive peakat approximately 300msec,after the stimulus).

Chapman&Bragdon (1964)i n one of theearlieststudies of theeffect of

psychologicalmeaningon theevokedresponse not specifically concerned with

the component,comparedevokedresponses to task relevant (blank) and

irrelevant (patterned) stimuli and found that the responses to task relevant

stimuliwerelarger and differentthan those to the irrelevant sti muli.

Howeverthey f a i l e d to consider that therewould have beenadifferencebetween

the responses to theirblank and patternedstimulianywaywhethertheywere

taskrelevant or not. L i f s h i f t z (1966)and Begleiter, Gross &Kissin (1971)

i n similar studies claimed that the VER to pleasant,repulsive and neutral

categorieso fvisual stimuliweredifferent for each category, and that the

effectdisappearedwhenthe stimulus was blurredso as to be unrecognizable.

John, Herrington & Sutton (1967)i n a widely citedstudy onmeaning found

thatthe VER was different fordifferent geometric formsof equal area and

similar fo r versions of thesamegeometric form of unequal area. They

concluded thatthis evidence suggested thewaveform of theevoked response

reflects atleast i n part the symbolicmeaningof the stimulus. Austt,

Buno&Vanzulli (1971)i n a similar study claimed th at both the occipital

visual evoked response and the visualevokedresponse recorded at the

vertex arerelated to the processing of sensory information. The occipital

response was saidto bemorespecificforvisu al stimuli than the one from

the vertex and was modified by changesi nvisualperception, significance

ofthe stimulus and theprogramthe subject was performing.


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16.

Buchsbaum& Fedio (1969) i n a study of the visualevokedresponse to

differentpatterns, includingsomewords,also found differences i n the

VER waveformsthatwererelated tomeaning. Becauseof their relevance to

the findings of the present study these l a t t e r three studiesw i l l be

considered in furtherdetail i n the Discussion section,

Begleiter& Platz (1969) showedthat the responseevokedby a particular

visualstimulus could be modified by c l a s s i c a l conditioning. A negative

peakat 155

msec,to 160

msec,latency was enhancedwhenthe stimulus was

conditionedby asso ciation with a loud trainof clicks and thisenhancement

was reversiblewith extinc tion and reconditioning. In threemorerecent

reports (Begleiter, Porjesz, Yerre &Kissin, 1973; Porjesz & Begl eiter,

1975; Begleiter & Porjesz, 1975) of the effect of the subject'sexpectancy

or perception of a stimulus, these authors haveshownthat the amplitude of

the vertex VER to a flashofmediumintensitywas higher i f the subject

expected orthought he saw a brightflash and significantly smaller i f he

expected orthoughthe saw a dim flash. Thesefindings are reminiscent of

John, Shimokocki&Bartlett's (1969) and Ruchkin&John's, (1966) findings

of "neural read-out frommemoryduring generalizat ion" i n cats . These

investigatorstrai ned cats to discriminatebetweentwo stimulus flicker

frequencies. Theyfound that theevoked potential i n various regions of

the brain to a stimulus of an intermediate flicker frequency closely

resembledthe responseto the appropriate signalfor the behavior which

was displayed on that t r i a l ; the configuration of theresponseevoked by

the intermediate stimulus was differentdependingonwhetherthe catmade

the behavioralresponseassociated with the high or low frequency flicker.


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17.

The authors concluded thatthis const itu ted evidence fo r relea se of a

neural process representing previous experience and provided somefurt her

support fo r the s t a t i s t i c a l theory of lea rning andmemorysuggested by

John (1967).

Investigatorswhoseworkhas centred on the experimental manipulation of the

latepositive (P^)componentofevokedpotentialshavesuggested a large

numberof p ossi ble psychologi cal variab les that may relate to thispeak.

It hasbeenvariablyproposedthat the "P^" i s a physi ol og ic al sig n of

"deliveryo f task-relevan t information" (Donchin&Cohen,1967),of the

"resolutionof prior uncertainty" (Sutton, Tueting, Zubin &John,1967) of

"cognitiveevalu ation of stimulus significance" (Ritter &Vaughan,1969), of

"a decision regarding the stimulus" (Rohbaugh,Donchin &ErUcsen,1974), of a

"reactivechangein the state of arousal" (Karlin, 1970),and of "stimulus

salience" (Jenness, 1972). Someauthors (Karlin, 1970, Naatanen, 1969)

haveargued a non-cognitive explanation f or a l l of these fin din gs, suggesting

thatthe dif fer enc e i n theresponsesto the relevan t or attended stimuli i n

a l l of the experiments can be explained by a "prior-state" hypothesis,

meaningthat a l l of the f ind ing s could be explained on the basis of achange

i n the subject's sta te of arousal or expectancy and not by any specific

parameters of the stimulus i t s e l f . However,Picto n &Hillyard (1974) and

Ford, Roth, Dirks & Kopell (1973)i nmorerecentworkstudying evoked

responsecorrel ates of selective attentionhaveshownthat this i s not true .

A l lhavedemonstrated two separate systemsoperating during selective

attention (for auditory stimuli); one ind ica ted by increased amplitude


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18.

of wavesand invo lvingselectionof a particular input channel and

thesecondindicated by themorewidespread complexand involving

complexinformation processing or decisionmakingbased on information

provided by the stimulus. Harter &Salmon (1972),i n an earlier study,

had also found a consistent "differencewaveform"with an earlier negative

and later positivepeakbetweenresponsesto task relevant and task irrelevant

patterned visual stimuli (although the non-specific"priorstate" of the

subjectwas not as carefully controlled i nthisworkas i t was i n the later

studiesthatusedauditory stimuli).

I t has alsobeenshownthat the componenti s not e l i c i t e d by a l l attended

and task relevantstimulior emitted stimuli (seebelow)that require

decision,but only by those forwhichcertain requirements of detection

confidence and prior subject uncertaintyhave beenmet (Squires,Hillyard &

Lindsay, 1973; Sutton, Braren, Zubin &John,1965; Tueting, Sutton &

Zubin, 1971). In a recent report on the P^ componentduring an auditory

detectiontask withcuedobservation intervals Squires, Squires & Hillyard

(1975a& b)havefurtherc l a r i f i e d the effect of these two majorfacto rs of

decision confidence and expectancy. They suggested that the P^ (becauseof

i t s topographyand sensitivityto the probabilityof the stimulus occurrence

orabsence)i s thesamep hys iol ogi cal bra in event,whetheri t occurs i n a

threshold detection task, omitted stimulusparadigm (Picton,Hillyard &

Galambos,1973) or a single-double stimulus discrimLnation task (Sutton

et a l . ,1967).

A few authorshavereported that a c o r t i c a l responsecan be e l i c i t e d by

omission of an expected stimulus; theseresponsesto stimuli that are


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19.

expected but do not occurhave beencalled "emitted potentials" by

Weinberg,Walter &Crow(1970)and this termhasbeenadoptedbymostother

investigatorsof thisphenomenon. Sutton, Ruchkin& Tueting (1974) reviewed

mostof the relevant studies of the componentof the emitted potential

and the experimental paradigmsusedfor i t s production. Squires etal.(1975b)

providedsomefurther experimental evidence that c l a r i f i e ssomeof the

questions on theappearanceof the P^ i n threshold detection tasks raised

i nthe presentation of Sutton et a l .(1974) and they alsodemonstratedthat

the essential c r i t e r i a fo r a stimulusabsenceto e l i c i t a P^waveare that

i t i sclearly recognizable and relatively improbable.

Therehasbeensomecontroversyaboutthe s p e c i f i c i t y of the emitted response.

Sutton et a l .(1974)stated that there was no evidence available indicating

thatthe P^componentof the emitted potential i sspecific for the modality

or intensity of the stimulus. Ritter,Simson&Vaughan (1974)howeveri n

another paperfromthesamesymposiumd id providesomeexperimental evidence

thatthe negativecomponente l i c i t e d by the missing stimulus was different

forthe two modalities theyused (visualand auditory) but they did not

investigate s p e c i f i c i t ywithin a modality. Otherauthors who havedescribed

emitted responses have been investigating effectsof stimulus timing

(Klinke,Fruhstorfer & Fin kenzell er, 1968) selective attenti on (Picton et a l . ,

1973) or the subject's expectancy i n a guessing paradigm (Weinberget a l . ,

1970) andnoneof these experiments provides any evidence on the specificity

of the response to the modality ormeaningof the omitted stimulus.


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20.

24 Summaryof the literature review.

It hasbeenestablished that sensory evokedresponsesrecorded from the

scalp r e f l e c t den dri tic post-synaptic pote nti ala c t i v i t y i n the underlying

c o r t i c a l c e l l s . I t has alsobeenshownthat only widely synchronized

cortical activityappearsi n scal p recordingsbecauseof the att enuation

by the intervening layers ofmeninges,cerebrospinal f l u i d and skull.

The EEGwaveformrecorded after presentation of a sensory stimulus consists

(after theon-going,non-eventre lat ed EEG a c t i v i t y hasbeenextracted) of

a ser ies of well definedpeaksorcomponents. Experimental evidence

suggeststhat at leastsomeof thesecomponentsoriginate i n different

c o r t i c a l regions. I t has al sobeenshownthat these differentcomponents

are se nsi ti ve tochangesi n a var ie ty of stimulus parameters.

The use of patterned visual stimuli toproduceevokedresponsesi s a

relatively recentdevelopmentthat allows the study ofmuchmorecomplex

aspects of stimulusmeaningthandid the oldermethodof stroboscopic

flashpresentation. Componentsof the pattern VER have beenshown to be

sensitive to focus, to thenumberof lines per uni t area (contour density)

and to a les ser extent t o the luminanceof the presented stimulus. Steady

stateevokedresponses (related i nprinciple to transientevoked responses,

but e l i c i t e d by a different stimulationtechnique) to grating patterns of

varying contrast levels and spatial frequencieshave beenshownto begood

indicatorsof the subject's thresh old fo r these patterns asdetermined by

separate psychophysical threshold testing.


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21.

Thereis also a largebodyof literature concernedlesswith the physical

parametersof the stimulus andmorewith the ef fe ct of psychological

variables on theevokedresponseconfiguration; bothwithnon-specific

variablessuchas attention, and withmorespecific cognitive variables such

as stimulus meaningor decision-makingbasedon information provided by a

stimulus. I t has beenshown,forexample,that a late positivecomponent

of the responsei n any modality i s related to this decisionmaking process,

but that thewaveappearsi n theevokedresponseonly i f the subject i s

certainhe detects the "stimulus" and i f the stimulus occurs relatively

infrequently during the paradigm.

I t has alsobeenclaimed that theevokedresponseconfiguration i s affecte d

by the symbolicmeaning of the stimulus. However,the few published studies

purporting todemonstratesucha relationshiphave failed to define clearly

either the experimentalchangesmadein stimulus meaningor the effects

theyproduce on the configuration of theevokedresponse.

Therehave been a few reports of the recording of c o r t i c a l responses (at

the scalp) e l i c i t e d by s ti mu li that areexpectedby the subject but do not

occur. Thesehavebeencalled emitted potentials since they are not evoked

by an external stimulus butpresumably r e f l e c tsomebrainprocessrelated

to therememberedstimulus. I t has beensuggestedthat this typeofresponse

recorded at the scalpmightprovide a usef ulmethod of examiriing brain

processes rela te d to stimulus rteaningwithoutcmtaminationby activity

that isdirectlyrelated to the sensory input. To thisdatehowever,

neither the techniques for obtaining emitted pot ent ial s, nor the configuration

and topographyof thesewaveformshave beenwelldocumented in the

literature.


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22.

Chapter I I I

STATEMENT OF THE PROBLEM FORINVESTIGATION

The questionofwhetheranypartof theresponse evokedbypatterned stimuli

reflectshigher perceptualratherthan simple sensory processi ss t i l l

unresolved. In tworecent, comprehensive reviewsof thef i e l d (Ciganek,1975;

Regan,1972),theclaimsofJohneta l . (1967)weresingledout ascr ucial

becauseoftheir significance forthestudyofevoked response correlates

of higher processes. Both reviewers urged thatthesefindings require

independent validationanddevelopment. Tothisdate therehasbeenno

reportofsuch studies.

Thereforeaseriesofexperimentswasdesignedtostudytheeffectof the

geometric formandmeaningof thestimuluson theconfigurationof the

visual evoked response. More s p e c i f i c a l l y i twas proposed:

(1) Toestablishwhethertherearereliableorcons istent

differencesbetweenevoked responsestogeometricshape

stimuliand todeterminethespatial (scalp distribution)

of anydiffere nces.

(2) Todemonstrate theeffecton theevoked responseof

assigningameaning(ascuesi n atask)to thediff erent

shapes.

(3) Torecordthe"emittedpotentials" occurringwhen asubject

expectsbutdoesnotactuallyseethese shapes,andthus

determine i f i t i spossiblet orecordEEGcorrelatesof

brainprocessesrelatedto themeaningof astimulusi n

the absenceof thestimulus i t s e l f .


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23.

( 4 ) Toapplyavarietyofanalytical techniquesto theevoked

potentialsobtainedtoexploremeansofquantifyingany

differencebetween thecomplexwaveformsrecordedi n the

differentexperimentalparadigms.

I n a l lof theexperimentalwork,data collectionandanalysiswere performed

byad i g i t a lcomputerwherepossible. Singlet r i a l samplesweresavedso

that'a variableandsmallnumber oft r i a l s couldbeusedi n theaverages

andsothat alternate analysis techniques suchas astepwise discriminant

analysis,manipulationofsingle t r i a l s priortoaveragingand computation

of theratio s t a t i s t i c calledtheAdescriptor (Johneta l . , 1967)could

becarriedout. (Seesection6.2 for adetailed discussionofthese

analyticaltechniquesandAppendixB for adescriptionofstepwise

discrimiiiant analysis.

Becauseinaveragesofsmallnumbers o ft r i a l s ,anya c t i v i t yo frelatively

highvoltage presenteveni nonlyone t r i a l affectstheaveragedwaveform

to adisproportionate extent,anautomatic artefact rejectionroutinewas

includedi n thedataacquisitionprogram (seeMethods).


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ChapterIV

METHODS

4.1 DataAcquisitionMethods.

Forthef i r s t part (paradigm1)oftheexperimentt hir tee n subjectswere

used. Ten ofthesewerepaidwomenstudent volunteers betweentheages

of19 and28. Theother threeweremembersoftheLaboratory St af f. A l l

ofthesubjectsweretestedatleastonceandsomeweretested repeatedly

but alwaysondifferentdays separatedby atleastaweek. At o t a lof

26runsofparadigm1wereusedfortheanalysi s.

Fiveofthethirt een subjects, includingtwo ofthestaffmembers,were

usedforthesecondpartoftheexperiments (paradigm2),i nwhichfrom

threetotenseparate recording sessions foreachsubjectwerecarriedout.

The subjectssatalonei nashieldedroomwithlowintensityoverhead

lighting (.5ft-candles)andwatchedatelevision screen1meterdista nt

forthepresentationofthestimulus.

The electrodeswereGrassgold discs applied with Grasselectrode paste

accordingtotheInternational10-20system (Jasper,1958)forelectrode

placement. (SeeFig.1fordiagramatic representation). Fora l lof

paradigm1thestandard locationsof 0 2, 0^ P 4, P , C 4, C 3and C zwereused.

Inparadigm2theselocationswerealsousedbut in someexperiments

0 , P , C , F and twolocati ons designated CP 0and CP.locatedhalfway

Z Z Z Z j 4betweenCj and P^ and C^ and P respectivelywereused. A l l recordingwas

referredtothecontralateralearor tothel e f tear i n thecaseof

midline locations.


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25.

Fig. 1 Diagramaticrepresentationof thepositionof the

electrodes on thescalp.


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26.

The EEG was recorded on aBeckmanDynograph TypeR with a time constant

settingof .3 sec andhigh frequencycut-off f i l t e r a t 30 Hz (aset ting

of 3 on thepoweramplifier f i l t e r ) givingan effectivesystem band width

of 0.5 -30Hz (3dB points).

FromtheDynographthe EEG signalson seven channelswereled to A-D

convertersof the computer (adetailed descript ionof the computer system

used i n these experiments i sgiveni n Appendix A) and sampled every 2msec,

for a 512msec,epoch, beginning 56msec,before thestimuluspresentation.

Single t r i a l s (256pointsper trial) werestoredon thediscof the PDP 11

computer system and a l ldata was transferredto Dectapes forpermanent

storageat the end of each paradigm.

The experimental set-up i sshowni n the diagram i nFig. 2. The experiments

were a l l controlled i nrealtime by a program calledVER03which ran on the

PDP 11 system. The flowchartof the program i sgiveni n Appendix A with

someexplanation. An automaticartefact reject ionroutinewas included.

This routinechecked each t r i a l sample to determine i f i t exceededsome

presetamplitude limits, and i f i t di d the t r i a lwas not saved. The program

thencontinued the experimentu n t i l therequirednumberof t r i a l s had been

collected i n each group to be sampled. (Thenumberwas 30 t r i a l sof each

shape i n paradigm 1, and 10 t r i a l so f each shape in paradigm 2). I t was

found fromobservingthe paperwriteouto f the EEG a c t i v i t y that this

technique effectively excluded any t r i a l swheretherewas eye blink

or subjectmovement. Insomeexperiments the EGG (electro-oculogram) was

recordedon one o f the seven channels to check f or any event-related slow

eyemovementswhich might nothavebeen rejectedby the system.


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D a t a A c q u i s i t i o n S y s t e m .

Fig. 2 Experimental se t up.


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The stimuliwere the fourdifferentgeometricshapesshownin Fi g. 3.

They included two familiarshapes, (one with corners and one without),

a square, and a c i r c l e , and two unfamilia r and le sseasilynamedshapes

calledhere the e l and theomega. Theywerea l lshownaswhiteoutli nes

on a blackbackground. A l l had approximately equal length of contrasting

border. Theywerepresented as slidesby aKodakCarousel pr ojecto r.

The imagesfromthe projectorweremonitoredby a televisioncamera and

were transmitted to the T.V. monitor placed one meteri n front of the

subject. An electromagnetic shutter (Gerbrant) on the lens of the

projector controlled the brief 20msec,presentation of the stimulus.

Advancementof the carousel and the shutteropeningwerecont rol led by

thecomputer.

The size of the shapeson the T.V. screen was approximately 6 cm. by 6 cm.;

therefore the shapessubtendedan angle of 3.6 degrees at the centre of

the subject's visual f i e l d . The luminancemeasuredat the screen was

10 ft-candle s fo r thebackgroundand approximately25 ft -candles for a l l

four stimuli. The subject was instructed to fixate visually on a point on

the centre of the screen throughout the recording period.

Forparadigm1 (see Figure 4 for a diagramatic representation), the four

stimuliwerepresented i n arandomsequenceat from 3-5 sec. in te rv al s

u n t i l 30 t r i a l s ofeachshapewerecol lec ted . The number30 was chosen

becausei t was the lowestnumberper averagethatproducedconsistent

evokedpotentialwaveformsand was the highestnumberof si ngle trials

i twouldbe p ossibl e to store in thecomputermemoryduring a sin gleexperiment.


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29.

Fig.3 StiraulijsShapes 1.circle 2.square 3.omega 4.el


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Stimulus presentation

20 msec

fc==l-*H

56

msec*


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31.

The subjectswereinstructedt owatchthepresentationof thediffer ent

shapesand totrytostay alertandattentive. Total time f or paradigm1,

includingone or twobrief interruptions,wasfrom10 to 15minutes. The

ongoingEEG wasmonitoredon thepaper writeoutby theexperimenterand

when theAlpha rhythmbegantodominatetherecordor anyTheta activity

appeared,theexperimentwasinterruptedand thesubjectaskedtorefocus

her attention afterabrief rest.

Inparadigm2 (seeFigure4foradiagramaticrepresentation), five shapes,

a c i r c l e , e l ,cross, squareand omegawerepresentedi n the sameordera t

regular intervalsof 1.4sec. Inapproximatelyonethirdof thesequences

eitherthesquareor thee lwasomitted (theshutterdidnotopen). The

experiment continuedu n t i lthe EEGduringtenomissionsofeach shapewas

collectedandstored. Withnorejectionsdue toartefact this required

63 sequencesoffiveshapes,or 315stimuli, which tooklhminutes.

Ten responsestoeachof thefourshapes usedi nparadigm1 whenthey d id

appearwerealso saved.

The subjectwasinstructedtolearnthesequenceoffive shapes, thenwas

told that occasionallythesquareore l would f a i ltoappear. She wasgiven

two buttons (handheldthumb-switches) andinstructed thati fthesquare

f a i l e d toappearat theexpected time,shemustpresstheleft-handbutton

andwhen thee lwasemittedshemustpresstheright-hand button. Failure

torespondcorrectlywithin400msec,of when thestimulus shouldhave

appearedi n therhythmic sequence,orpressingthebuttonwhen thestimulus

did appear resultedi n awarning tone being sounded (controlledby the

computer).


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The rhythmicpresentation sequence was not broken by the warning tone.

Button presseswererecorded as squarewavepulses on one of the seven

datachannels sothatthe exacttimingof the responsecouldbecompared

to the EEG a c t i v i t y . I talsoprovided a way of counting thenumberof

correctresponses at the end of the experiment. The subjectswere allowed

to practise this f a i r l y d i f f i c u l t taskpriorto therecordingsession and

a l lbecamequiteproficienti nmakingthecorrectresponsewithin400msec,

in from70 - 100%of the t r i a l swith the missing stimulus, and i nmaking

veryfew responseswhenthe stimulus was present.

At the end of an experimentalsessionthe datawereaveraged, 30 t r i a l s per

average for paradigm 1,and 10 t r i a l s per average for paradigm 2and the

averageswereimmediatelywrittenout. A l l of the data (assingle trials)

was thentransferredto Dectapes f orpermanentstorage andcould be

accessedfrom there by aprogramcalledGASPf orfurther analysis.

Q

Calibrationof the dataacquisitionsystem was doneat the beginning of

each experiment by insertinga5 Hz. 50yv squarewavesignalfrom a Grass

calibrator intoone pairof inputs at the headboard. A l l seven recording

channelswerecalibratedusingthispairofelectrodes. The VERprogram

was then run and a writeout ofsomesingle t r i a l smade. The gainswere

adjustedon the EEG amplifiers sothatthe50 yv signal i n a single t r i a l

appeared with an amplitude of25mm. on theplotter. Sincea l l averaged

EEG signalson the f i n a l plotweretruemeans (ratherthansums)a50 yv

signalappeared with a25mm. amplitude i n the final plotwhetheri t was

the signalof a single t r i a l or the signal i n the averaged response.


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4.2 DataAnalysisMethods.

The differencesbetweenthe evokedpotentialwaveformse l i c i t e d by the

different shapeswereanalyzed by a va ri et y of techniques. They include

visual inspection foridentification andmeasurementof the prominent

peaksof thewaveformsand twomorecomplexquantitativeapproaches.

The simpler of the two i s the computationof a descriptors t a t i s t i c called X

that was described by Johnet a l .(1967). This s t a t i s t i c gives a

quantitativemeasureof how different two waveforms are and the formula

andmethodsusedtocomputei t are givenbelow.

The secondmethodof determining the dif fer enc esbetweenthe responsesi s

called discriminant anal ysis , a techniquewhichi sbasedon multi varia te

s t a t i s t i c a l theory. A desc riptio n of the technique and the assumptions

onwhichi t i sbasedi s given i nAppendixB. The discriminant ana lys is

performedon this experimental data was carried out by theBMD:07Mprogram

i n the UBC computingcenter. The input data fo r theprogramconsi sts of

two ormoregroupsof observations (inthis experimental workeach "group"

was ashapeand eachobservation was the EEG a c t i v i t y recorded during a

single presentation of thatshape). Eachobservation i n turn consists of

anumberof variableswhichi nthis studywerethe digitized values of the

EEG voltage at different timepoints.

The programproceedstochoosea subset of the var iab les (i .e . voltages

at different timepoints) and cal cul ate s a discriminant function using these

variables thatw i l l discriminatebetweenthe differentgroupsthatwere

entered in to theprogram. Thismathematical functio n can be evaluated fo r


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34.

anewobservation (asingleEEGsample)andi tw i l l provideapredict ion

ofwhich group (shape) thisnewobservationwasmost l i k e l ytohavecome

from. Themethodtheprogramusestosel ect the varia blesand tocompute

thisdiscrimin ant function i s described i nAppendixB. Certainparameters

for thisprocessmustbesetatthe timethe data i s enteredandthese are

also given in thatappendix. Anotherre late d, butmuchsimpler program

calledUBC:CLASSevaluates the discriminant function fo rnewdata.

In ordertosubmittheEEGdatatotheUCLABMD:07Mprogram (runonthe

UniversityIBM 370computer) the selecteddigitized t r i a l s (one channelat

a time)weretransferredfromDectapetoeitherpapertapeormagnetictape

on thePDP 11system. Duringthe transfe r the digitized t r i a l representation

was reduced from256pointsto 67pointsbyomitting the f i r s t26points

(whichrepresented the52msecs.ofthe pre-stimulus bas eli ne) ,then taking

the averageofeachofthe67setsof 3poin ts t hat followedandomitting

the l a s t29points. Thedata fromonechannelat atimewerethentransferred

to f i l e sonthe Uni versi tyIBM 370system. Each67point single t r i a l

representation includeda 5character label indicatingwhichshapei tcame

from. The BMD:07Mprogramwasrun for ei th er2 or 4groups (each group

consistingof 30t r i a l s e l i c i t e d by adifferent shape). On a fewruns

60 t r i a l s per groupwere included but the separationofthegroupsbythe

programdi d not appearto beimprovedbythis increase in thenumberof

observations per group. Thenumberofvaria ble s per observationwas 67 as

indicatedin the previous paragraphandtheprogramselected6 or 7 ofthese

(i.e.6 or 7latency points)asthe b asisofthe discriminant function.


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35.

P i l o t studiesshowedthat allowing theprogram to run throughmorethan

6 or 7steps (i. e. tochoosemorevariablesthanthis) did not improve

the discrimination and so the programwas instructed to stopatthisnumber

of steps. This discriminant function producedby each run of the program

was stored i n aseparate f i l e and thenamesof the variab les i t selected

wererecorded separately (e.g. the 5th, 21st, 35th,42nd, 51st and 60th

variable)..

For test ing one of thesefunctions, datafroma l l of the experiments for

thesamesubject fo r thesamechannel that had not beenused fo r input to

theBMD:07Mprogramthatdetermined the function,wereput i n a sing le f i l e .

The programUBC:Classwas run on this f i l e with the discrhntlnant function

and aformat statement indicatingwhichwere the six or seven variables

(latency points) to be selected fromthe responsefor thi s partic ular

function as it s input. The r es ul ts of th is testing wereexpressed as

percentagesof correc tlyc l a s s i f i e d t r i a l s and i n the case of the 4group

choices, thewrongly c l a s s i f i e d t r i a l swerealsogrouped as towhichshape

theyweremistakenfor and thesepercentageswere tabulated.

Tocomputethe X rati os as described by Johnet al .(1967)averages of

10 t r i a l swereprepared on thePDP-11systemwiththeGASPprogram. These

weretransfer red bymagnetictapeto the IBM 370 wherea Fortr an program

computed the X values for the 4averagesgiven i tfrom the following formula:

= ( dl , 3 + d2,4 } / ( dl , 2 + d3,4 )

whered.. i s the absolute value of the rootmeansquare (r.m.s.) difference

between waveformsi and j . /"r.m.s.= x. /nwherex. i s the i sample


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36.

of thewaveformwith the valueof thebaselineascomputedfromthef i r s t

3points(18msec.)of theresponsesubtracted fromi t V Thesubscript s

1and 2denotethe tworeplicatedresponsesto oneshapeand 3 and 4

denotethereplicatedresponsesto thesecond shape.

Analysisof theresponsestothe omittedstimuliwas madesomewhat

d i f f i c u l tbytherelatively smallnumberofthese responses available.

The squareore lwasomitted from i t spositioni n thesequence only

occasionally. Anexperiment consistingofpresentationof atlea st

320 stimuliprovidedonly20"emitted" responses, ten each forthesquare

and e l . In theaveragesofonlytent r i a l sthebackgroundEEG(noise)

levelwassometimeshighenoughto makei td i f f i c u l ttovisuallyseparate

the componentsofevent-related a c t i v i t y , especially i f thesewere

relatively smalli n amplitude. However,combining data frommorethanone

experimenttoprovidemoreresponses fo r averagingintroduced added

v a r i a b i l i t yto theresponses becauseofchangesi n thesubject's performance

level, stateofalertnessor i ntechnical elements suchasminor differences

i n electrodeplacement. Inviewoftheseconsiderations, forpresentat ion

of theresults, themeasurementsmade byvisual inspection fo r compiling

tablesVTI andVTIIweremade onaveragesof tent r i a l s from each single

experiment,butforthei l l u s t r a t i o n sof theresponses30t r i a l speraverage

wereusedsothattheevoked responsecomponentscouldbemore easily

visualized.

Aftertheconventional averageswereproducedandexamined,analternate

techniquesimilartotheoneusedbyRuchkin (1974)wast r i e d tocompensate

fortheeffectof avaryingestimateof thetimeofoccurrenceof the


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37.

omittedstimulus by thesubject. Its purpose was tohave a l lof the

peaks from thesingle t r i a l s coincidei n timeprior to averaging, so that

any activity thatwas time-locked to the P peaks would be enhanced.

The single t r i a l s at C were f i r s t filtered by a 0 - 6 Hz band-pass

d i g i t a l f i l t e r tomakethepeakof the slow positive activity clearerfo r

measurement. The filtered trial swerethenmovedto adisplayscreen.

Aclear positivewavecouldbe seen i n about 90% of the responsesexamined,

between240 and 450msec,afterthe stimulus shouldhaveoccurred.

(If nopeakcouldbe seen inthat interval the t r i a l was notincludedfo r

the "shifted" average). The latencyof themostpositivepeakwasmeasured

and thesizeo f the shift along the timeaxis (inmsec.)thatwas required

tomovethepeakof thelargest positivewaveto an arbitrarily chosen

centralpoint (350msec.)was calculated. As a simplecontrol, this

shiftingprocedure was also appliedtosomeresponses from single trials

elicited by the square and e lwhentheywerepresent. Thisi sfurth er

discussed i nsection5.3.2.

TheGASPprogram mentioned included routinesforretrieving the single

t r i a l s from any experiment from Dectapes, averaging anynumberof responses,

switchingaround the seven channels i f necessary i n caseswhere different

channelshadbeenused for a givenscalp location, displayingaverages or

single t r i a l son thedisplay screen and f orplottingany of these responses

beforeo raftermanipulation on aprinter-plotter. The program also had

the capacity fordigital filter ing of data through whatever bandpass was

required.


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38.

Chapter V

RESULTS

5.1 Effectof the DifferentExperimental Conditions on theEvokedResponse.

Before examiningthe effectof the stimulusshapeon the configuration of

theevokedresponse, i t i s necessary to assess the contribution of the

differentpresentation conditions of the two experimental paradigmsi n

order to decidewhetherthe datafromthe two could becombinedfor

subsequent analysis.

In the stimulussequenceofparadigm 2, the c i r c l e andomegawerealways

present and served only as cue stimuli. For the square and e l , on the

otherhand,since theappearanceof the appropriateshapemeantthat no

button press was to bemade (detected omissionsweresignalledby pressing

one of the two switches) and since there was someuncertainty as towhether

the square or e lwouldappeari n i t s time interval in any given stimulus

sequence, the presence of these stimulusshapeswas informational!/more

important for the subject.

In theevokedpotentialse l i c i t e d by the c i r c l e andomegathere was

essentiallyno differencebetweenthe responses recorded i n the two paradigms

at any of the five recordingsitescommonto both experiments. Figure 5

showsthese responses for one subject in the two paradigmsand i t can be

seenthat theevokedpotentialsfromparadigm1 and 2 are very similar.


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39.

100msec

Fig.5 Effectofexperimental conditionsfo r thec i r c l eand

omega. Paradigm1 (notask) responsesare shown bys o l i d linesandparadigm2 (task)responsesare shownby dotted lines. n=30trials/average. (SubjectSC

Exp. 18A,34,35)


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In theevokedpotentialse l i c i t e d by the squareand e lhowever, there

wereconsistent differenc esbetweenthe two paradigms, but only in the

componentslater than 220msec. Inparadigm 2, but not inparadigm 1,

abroadpositivecomponentwith amaximumamplitudeof 8 to 18 pv

(depending on the subject)between 220 and 320msec,post-stimulus was

seenin the responses e l i c i t e d by the squareand e lshapes. This diff ere nce

i s i l l u s t r a t e d i n Figure 6 for one of the subjects. Thi s lateposit ive

waveappearing i nparadigm 2 was la rg es t at C , smaller i namplitude but

definitely present i n the right and l e f t parietal regions, and small and

inconsistentlypresent i n the occipital regions. For a given subject this

positivewavewas of thesameamplitudeand config uratio n fo rboth the

squareand e l responses. Therewereno amplitudeasymmetriesbetween

and C^, CP^ a n a- CP^ (notshowni n Fig. 6) and P^ and P .

Insummary,the effect of the task imposed i nparadigm 2 on theevoked

responsesto the four present shapeswas as follows:

1. The effectwas to introduce abroadpositivewavei n theevoked

responses at C ,whichwas only varia bl y present and of smaller

2

amplitude i noccipital regions.

2. Thiswavewas only seeni n theresponsese l i c i t e d by the square

and e lshapes and not to the c i r c l e andcmegashapes.

3. I t was seenonly i ncomponents220msec,or later post-stimulus.


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41.

+100 msec

F i g . 6 E f f e c t o f e x p e r i m e n t a l c c n d i t i o n s fo r the square and e l .

Paradigm 1 (no task) respon ses ar e shown by d o t t e d l i n e sand paradigm 2 (task) respon ses a r e shown by s o l i d l i n e s .

n=30 t r i a l s / a v e r a g e . ( Su b je c t MP, Ex p. 7,53,54 )


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Inview of theabove findings, i nconsideringthe data foranalysisof

theeffectof thedifferentshapes on the evoked responses, the circle

andonega t r i a l s from paradigm 1 and 2weremixed. For the square and

e l shapes there was a differencebetweentheparadigms whichwas maximal

atC and startedonly 220msec,post-stimulus. Since i t was very small

in occipital regions theresultsof computerized analysisof the

differentresponses at the occipital locationsfor these two paradigms

werealsocombinedforconsiderationi n the sectionto follow.

5.2 Effectofdifferentstimulus shapes on the evoked responses.

Consistentdifferencesbetweenthe evoked responses to the four different

shapeswereobserved. In order to best define and quantify these

differences,three techniqueswereused. F i r s t ,the data from the

standard paradigm 1condition (no task) was examinedvisually, the

componentslabelled and amplitude and latencymeasurementsmadeas

describedbelow. Secondly, thedigitized single t r i a l s from occipital

channels of one experimentweresubmitted to theBMD:07MStepwise

DiscariminantAnalysisProgram; a function todiscriminatebetween shapes

foreach subject was computedand thentestedon largenumbersof single

t r i a l s from other experiments with this subject. Finally,the A

s t a t i s t i c as described by John et a l . (1967)was computedusing

averages of ten t r i a l s formostof the experiments.


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5.2.1 Resultsofvis ual analysis.

For v is ua l analysistheevoked responses fromthirteen subjects (who

had eachbeen run i nparadigm 1 atleastoncewitho c c i p i t a l , parietal,

central,andvertex electrodes) wereused. Someof thesubjectswere

testedup tofive timesi nthisparadigmbut theaveragenumber of

runspersubjectwas two.

Thirty t r i a l s foreach stimulus shapewereaveragedtoobtainthe

VER's forexamination. With subjects thatweretestedmorethan

oncetheresponseswerefoundto benearly identical fromone

experimenttoanother. A typicalset ofresponsesto thefour shapes

in seven channelsfor onesubject i s shown i nFig.7. Theresponses

showedsomevariationi nlatencyandconfigurationbetweensubjects,

butdidexhibit es sential lythesameseriesof components fora l l

subjects. Fig.8 shows theresponseat 0 to the e lshapefora l l

thirteen subjects.

Ina fewsubjectsmid (T^ or T ) orpo sterio r (Tj-or T )temporalor

frontal (F^,F^ or F^) electrodeswereused instead of therightand

l e f t centrals. Noclearresponsewasseenatmid-temporal or frontal

locations. Theresponseat theposteriortemporals appeared very

similartothat seenat 0, and 0.


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100 msec

Fig. 7 Evokedresponsese l i c i t e d by thefour di ff er en t stirnulusshapes at the 7electrode locations used i n paradigm 1.

n=30trials/ aver age. (Subject SC, Exp. 1)


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45.

LP

msec

Fig. 8 Intersubject v a r i a b i l i t y . Responserecordedfrom

^1 - A2 e l i c : i - t e d DY t b ee lshape for each of the 13 subjects.n=30trials /avera ge.


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For further desc ript ionof theresponsesthelabellingshowni n

Fig.7 wasperformed. The f i r s tmajorpositivepeakseenatabout

95 msec., i n most subjects,was calledPp Thiswas sometimes

precededby adefi ni te small negative peakcalledN . Following P^

the nextlarge negative peak (whichwas sometimesdoublei nsome

channels)was calledN,,, thebroaderpositivewavefollowingN 2 was

calledP 2, and alater positivepeak (wheni toccurred)was called

P . It wasnotedthattherelative amplitudesofthesecomponents

variedi n theresponsestodifferentshapes andamongsubjects,but

i twasusually possibletoidentifythem ina l lof therecorded channels

for a l l subjects.

For a l l subjectstheamplitudeof the P 1 component was approximately

equali noccipitalandparietal recordings and slightly smaller than

thisi n thecentr al leads.

The N 2 component in six of thethirt een subjectswas largesti n the

occipital regions andeit her reversed p ol ar it ycompletelyorbecame

biphasic (i.e. partofi t reversed i npolarity) anter iorly. The

reversaltookplace ei the r jus t anter iorto orposteriorto the

parietal electrodes (asi tdoes i nFig.9). In theremainingseven

subjects thiscomponentbecameprogressively smaller anter iorto the

occipital locations rather thanreversingi npolarity,(asi tdoes i n

Fig.7).


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The ?2 conponent had abroadconfigurationand thelatencyof this

peak wasmorevariablebetween thedifferent recording sites than

was thelatencyof theothercomponents. Insevenofthirte en

subjects thiswavewas ofhighest amplitudei n theoccipitaland

parietal regions,and i n six ofthirteen i twas largesti n the

centralorvertex channels.

Becausethe components were most clearly definedi n theoccip ital

regions therightandl e f t occipital channelswereusedto measure

the latencyandamplitudeof thethreecomponents fora l lof the

subjects. Theaveragesoft h i r t y t r i a l swereusedforthese

measurements, althoughi nsomesubjects i twasnotedthattheaverage

of justtent r i a l s showed the componentsequally well.

Itwas notedthatthelatencyof thethreecomponents for a

particular subjectwas quite consistentfora l l fourshapes.

Table Ishowsthemeanlatenciesof thethreecomponents for the

thirteen subjects' averageresponsesto thefour different shapes.

Itcan beseenbyinspectiono f the magnitude of thestandard deviations

ofthemeanvalues i nthis Table that therewere no significant

differencesin thelatencyofthesethreecomponents between the

different shapes. Themeanlatencyfora l l fourshapesanda l l

thirteen subjects for P- was 95 msec, for N 2 was 148 msec, and for

P~ was 219 msec.


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48.

TABLEI:

Latenciesof thepeaksof thethree principalcomponentsofthe occipitalevoked responsesto thefourshapespresented

inparadigm1. Eachvalue (inmsec.) i s themeanand

standarddeviation for13subjects.

N.2

square 95- 17 145- 20 212- 25

e l 92 - 20 150- 21 225- 18

circle 101- 23 150- 26 219- 21

omega 93- 24 148- 25 221- 24

Mean of 4shapes 95 148 219


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49.

Therewerenoconsistent differencesi n thelatencyof thecomponents

fo r any of theshapesbetweenrightandl e f t occipital regions. I f any

small differenc ebetweenrightandl e f twasseenforagiven subject,

themean o f the two wastakenas thevalue used fo r preparing TableI.

The amplitudeofthese threecomponentswasmeasuredfromthe pre-

stimulusbase l i n e . Tocomparetheamplitudeof thea c t i v i t yat

0- and0^thesubjectsweredivid ed accordingtohandedness. There

wereeight right-handers andfour left-handers (datafromonesubject

wereomitted becausethe EEGfrom 0^wastechnically inadequate).

Ameanamplitudedifference foreachof thefourshapeswascomputed

for eachsubject. For theright-handed groupno meandiffere nces

between0- and 0 2amplitudesofgreaterthan1 yvwereseenforany

ofthecomponentsandthesewerenotevenconsistentlyi n the same

direction. For thefour left-handed subjects therewas aconsisten t

tendencyforN 2to bes l i g h t l y higher overtherighthemisphere(mean

differenceof 2 uv)for a l l fourof theshapes.

Themeanamplitudeofeachof thethreecomponentsrecorded at for

the fourshapesfor12 of thesubjectsi sshowninTableII(datafrom

one subjecthad to bediscardedbecauseoftechnicalproblems). The

amplitudesof the N 2componentof thesquarevs. e l ,c i r c l e vs.omega

and c i r c l e vs.e l and of the P 2componento f thesquarevs. e l ,

c i r c l e vs.omega andsquarevs.omegawerea l lfoundto be significantly

different usingtheScheffe's (1959)methodoftestingmult iple


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TABLE II :

Amplitudesof thethreeprincipalcomponentsof theresponses

at 02to thefourshapes presentedi nparadigm1. Each value

(inyv) i s the mean andstandarddeviationfor12 subjects.*

P

lN

2P

2

square 2.9 2.4 6.4 4.0 6.2 3.1

circle 3.4 -2.4 5.2 2.8 7.2 4.5

cmega 3.8 3.1 8.1 3.8 10.5 5.5

e l 4.8 3.4 10.4 4.8 9.5 5.0

ByScheffe's(1959)methodoftesting contrasts(asprovidedi n the UBCComputingCenterProgramMFAV),theamplitudesof thecomponentsof thefollowingpairsofshapesweresignificantly different (p= .01):forthe Ncomponentthesquareandel ,thec i r c l eandomega and thec i r c l eande l were a l ldifferent,andfo rthe P~componentthesquareandel ,thec i r c l eand omegaandthesquareand omegapairswere a l ldifferent. For

the P^component noneo f the6possiblepairsofshapesweresignificantly different.


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51.

contrasts (p = .01). Therewereno significant differences (p = .01)

in the amplitude of the component. I t i s notedthattheScheffe's

methodi s based on a two way analysisofvarianceand thus i t i s

essentially testingthedifferencebetweenthe shapes for each of

the12 subjects.

Fortwo subjects,paradigm 1 was repeated wi th the T.V. camera lens

adjustedsothatthe shapes appeared on the viewing screen at

approximatelyone half theirusual size. The evoked responses to

thesesmallerstimulus shapeswerenotdifferent (byvisual inspection)

from the ones evoked by the standardsized shapes. The responses to

thesmallandlargesquare, e l ,c i r c l e andomegafor one subjectare

showni nFig. 9.

5.2.2 Resultsof StepwiseDiscrirninant Analysis.

Inap i l o t study with theBMD:07Mprogram a sampling of data from a l l

five subjectsfrom a l l of therecording locations (occipitals, parietals

and centrals)was t r i e d to determine which ones would bebestfo r

ccnpletetesting. Using the U s t a t i s t i cof the program as an indicator

ofhow wellthe groupswereseparated(see,Appendix B) i t was foundthat

one of theoccipital locationsalways had the lowest U value ( i . e .

showedthebest separation) for anypairo f groups from agivenexperiment.


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52.

Fig. 9 Effectofstimulus size. Responses tostandard sized

stimuli (visualangleof 3.6) areshownassol id

lines, those to thehalf-sizedones asdotted lines,

(subjectML, Exp. 8,21) n=30 trials/average


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Therefore, fortestingtheabilityofdiscriminant analysistoclas sify

newdataforeachsubject, onlytheoccipital datawere completely

testedandarereported here.

TheworkwiththeBMD:07Mand thetestingof thefunctionsproducedby

i t (withtheprogramUBC:CLASS)wasdonei n twophases. In thef i r s t

phase,data collected fromfour subjectsi nparadigm1wereused. Single

t r i a l s froma l lfour stimulus shapesweresubmittedto theBMD:07Mprogram

at once,hencetheresulting discriminant functionswereintendedto

classifyanysingle t r i a l (orobservation)asbelongingto one of the

fourpossiblegroups (shapes). In thesecondphasethedata fromonly

twostimulus shapesat atimeweresubmittedto theprogram.

In TableIIItheresultsoftestingthediscriminant functions

classifying a l l fourshapesforthefour subjectsondata fromrightand

l e f t occipital locationsarepresented. Thepercentages giveni n the

Tablearetheproportionsof thetotalnumberoft r i a l sthefunction

correctly classified. Theclassificationsmade on thet r i a l s usedto

computethefunction (i.e.ap o s t e r i o r i c l a s s i f i c a t i o n )wereincluded

inthepercentages andtotalnumbersoft r i a l s giveni nthis Table.

120 t r i a l s ,30 ofeach shape,wereusedtocomputeeachfunction.

Itcan beseen fromtheTable thatthefunctionsclassifiedfrom35 to 44%

of the waveformsfromthesingle t r i a l s correctly, proportionswhichare

significantly greater thanthelevelof 25%expected witharandom

c l a s s i f i c a t i o n (z =3.58f or35%whichi sgreater thanzn 1 =2.57).


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TABLEII I:

Performanceof the "Fourshape"Discriminant Functions.

Percentageof t r i a l s correctly c l a s s i f i e d by theBMD:07M

StepwiseDiscriminant Analysis technique on data from right

and l e f t occipital locations. Totalnumberof trials

c l a s s i f i e d foreach subject i s given in parentheses. A l l

percentages given are significantly* greater than the 25%

level expected with arandom c l a s s i f i c a t i o n (p = .01).

For 35% z = 3.58 whichi s greater than z m = 2.57.

Subject 0 2 (right) 0-j (left)

MP (260) 41% 44%

SC (480) 37% 37%

VS (240) 39% 40%

ML (360) 35% 41%

meanof 4 subjects 38% 40%

* significancedeterminedby evaluating the test statisticz = p - p//pq/n-wherep i s the observed proportion,

p i s the expected proportion, q = 1 - p and n i s thesamplesize, z i s considered to be normally distributed.(Bahn, 1972).


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TABLEII I:

Performanceof the "Fourshape"Discriminant Functions.

Percentageof t r i a l s correctly c l a s s i f i e d by theBMD:07M

Stepwise Discriminant Analysis technique on data from right

and l e f t occipital locations. Totalnumberof trials

c l a s s i f i e d foreach subject i s given i n parentheses.

Subject 0 2(right) O-^left)

MP (260) 41% 44%

SC (480) 37% 37%

.VS (240) 39% 40%

ML (360) 35% 41%

meanof 4 subjects 38% 40%


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55.

To ascertainwhichof thefourshapesthefunctionswerebest ableto

identify,thepatternof theincorrect classificationswas studied

(i.e.whentheresponse fromoneshapewas c l a s s i f i e d

the effect of the geometric form.pdf

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