7/31/2019 Untangling Spatial From Temporal Illusions 2002-3516

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TRENDSi n Neurosciences Vol.25 No.6 June 2002 293News& Comment

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Letters

Untan gling spatial

from tem poral

i l lusions

The flash -lag effect (FLE ) is a

phenomenon in which a flash a ligned

with a moving object appear s t o spatially

lag behind the st imulus. In their r ecent

review of the F LE, Krekelberg and La ppe

conclude t ha t th e influence of differen tial

lat encies on th e per ception of position of

moving objects is u nden iable[1]. They

refer to th e hypothesis tha t differences in

physiological lat encies m ight t ra nslate

directly into perceptua l time differen ces.

This idea ha s enjoyed recent popularity a s

a proposed explanation for the

flash -lag illusion [24]. However,this is inconsistent with evidence that

timing judgement s between flashes an d

moving objects a re ver y accur at e [5].

Recent r esearch su ggests tha t t he

FLE, a s opposed to being a tem pora l

illusion, is a consequ ence of spat ial

inter polation [57].

The a ppeal t o differences in

physiological lat ency to explain th e FLE

ru ns t he r isk of oversimplificat ion, given

the m any shortcomings tha t hypothesis

must contend with. First, th e different ial

laten cy (DL) model predicts th at t heoutcome of a ra cebetween de novo

movement a nd a flash can be changed,

by giving the flash a hea d sta rt ; however,

this hypothesis has been tested an d

disproved [6]. Second, the DL model

predicts t hat the onset t ime of a flashed

an d m oving object will be m isperceived;

this hypothesis has also been tested an d

disproved [8]. Third, th e DL model runs

into difficulties in th e flash -initiated

para digm, in which t he flash and moving

object appear s imult an eously: th e DL

model predicts th at th e moving object

will suffer th e same delay as the flash, as

it suddenly appear s from nowhere.

However, the cont inuous a nd flash-

initiat ed conditions yield the sam e

psychophysical result [6]. Fourt h,

skepticism about th e DL explana tion of

the F LE is war ran ted by the conspicuous

absen ce of physiological s upport . In fact,

as Krek elberg and Lappe acknowledge,

th e available physiology in t he m edial

temporal area (MT) speaks against t he

DL model. Thus, th e differential lat ency

hypothesis lacks a sufficient ba se of

support , and a ny direct conn ection

between t he timing of neur al signals and

th e timing of perception is cast into

doubt by a critical an alysis of the extan t

data .

Much of th e confusion in th e storm y

world of flash-lag liter at ur e can be

att ributed to a single assumption that h as

not been critically assess ed: the

assumption tha t a measured spatial

differen ce can be directly tran slated in to a

temporal difference. Having assu med

such tra nslation, almost all reports on t he

FLE m easu re a perceived spat ial offset

(e.g. 1 of visual an gle), but r eport a

put at ively corr esponding time difference

(e.g. 70 ms).

Our alter na tive view, if corr ect, is fat al

to this ass um ption: even while a movingobject has a rea l-world time

corr esponding to ea ch position, it could be

th at th e pairin g is no longer veridical, or

even r etained, in the r epresentat ion of the

stimulus in t he ner vous system. Given th e

distributed pr ocessing in th e visual

system , a logical possibility is th at

position inform at ion is not persisten tly

represent ed, but instea d is only computed

when needed. In other words, when an

observer is ask ed wher e a m oving object

was at a part icular m oment, a special (and

possibly ra re) comput at ion is thenperform ed. Asm ear of spatial positions

must be evaluat ed (deblurred) into a

single, una mbiguous ans wer. The resu lt of

th is compu ta tion can be non-veridical

tha t is, a sm ear of spatial activity across

cortex can be evaluated at some

inter mediat e position. In this view, time

can be stam ped with high fidelity, but th e

position a ssociated with t hat time is th e

result of a deblurring process tha t

interpolates over a smear of recent

positions [57]. The observer is only able

to report a per ceived position after th is

compu ta tion is complete [9]. This

framework nat ura lly explains other

illusions, su ch as th e Fr hlich effect, in

which a moving object t hat appears

suddenly is not seen in its true st art ing

position, but instead some distance into

the t rajectory [10]. This suggests tha t t he

FLE is an other incar nat ion of the

Fr hlich effect, one in which th e spat ial

landmark ta kes on a temporal stamp as a

resu lt of being flash ed inst ead of being

sta tic [6].

Spat ial int erpolation over occupied

positions of the m oving object offers an

explanation for the FLE t hat h as man y

advanta ges. First, it is consistent with

other illusions (e.g. th e Fr hlich effect).

Second, it is consist ent with su bjects

ability to accurat ely judge tempora l

relationships between flashed an d

moving objects [5]. Thir d, it does n ot

embed the assum ption tha t a measur ed

spatial judgement tra nslates directly

into a temp oral illusion. Fourth , it

accounts n at ur ally for the roun ding of

the curve seen in Whitney and

Mura kam is r eversal of the moving

object, for wh ich th ey were forced to

appeal to an additional m echanism

(neural delay variability or a separat e

spat iotempora l avera ging filter) [3].

Thus, in contr ast to Krekelberg andLappes sta tement that latency

differences un denia blyinfluence

perception, a spat ial explanat ion could

prove more par simonious.

David M. Eagleman

Terrence J. Sejnow ski

The Salk Institu te for Biol ogical Studies,

10010 N. Torrey Pines Road, La Jolla,

CA 92037, USA.

e-mail: eagleman@salk.edu

References

1 Krekelberg, B. and Lappe, M. (2001) Neuronallatencies a nd the position of moving objects.

Trends Neurosci. 24, 335339

2 Purushothaman, G. et al. (1998) Moving ahea d

thr ough different ial visual lat ency.N at ur e 396,

424

3 Whitney, D. and Murak ami, I. (1998) Latency

difference, not spat ial extra polation.N at .

N eur osci. 1, 656657

4 Baldo, M.V. and Klein, S.A. (1995) Extra polation

or attent ion shift?N atur e 378, 565566

5 Eagleman , D.M. and Sejnowski, T.J. (2000)

Latency difference versus postdiction: Response

to Patel et al.S cience 290, 1051

6 Eagleman , D.M. and Sejnowski, T.J. (2000)

Motion int egration an d postdiction in visua l

awareness. S cience287, 203620387 Eagleman , D.M. and Sejnowski, T.J. (2000) The

position of moving objects: Respons e to

Krekelberg et al.S cience 289, 1107

8 Eagleman, D.M. and Sejnowski, T.J. (2001)

The flash la g illusion is a consequence of

adapt ive spatial filtering. S oc. N eurosci. Abstr.

165.29

9 Eagleman, D.M. (2001) Visual illusions

an d n eur obiology.N at . Rev. N eurosci. 2,

920926

10 Frhlich, F.W. (1923) Uber die Messun g

der Empfindungszeit [Measur ing the

time of sensation].Z . S in nesp hys iol . 54 ,

5878