-
@Pergamon
VisionRes.,
Vol.36,No.17,pp.2721–2727,1996CopyrightCl1996ElsevierScienceLtd.All
rightsreserved
PII: S0042-6989(%)00004-1 Printedin
GreatBritain0042-6989/96$15.00+ 0.00
Temporal and Spatial Frequency Tuning of theFlicker Motion
AftereffectPETER J. BEX,*~$ FRANS A. J. VERSTRATEN,$ ISABELLE
MARESCHAL*
Received 18 September1995; in revisedform 10 December 1995
The motion aftereffect (MAE) was used to study the temporal and
spatial frequency selectivity ofthe visual system at
supra-threshold contrasts. Observers adapted to drifting sine-wave
gratings ofa range of spatial and temporal frequencies. The
magnitude of the MAE induced by the adaptationwas measured with
counterphasing test gratings of a variety of spatial and temporal
frequencies.Independently of the spatial or temporal frequency of
the adapting grating, the largest MAE wasfound with slowly
counterphasing test gratings (at approximately 0.125-0.25 Hz). The
largestMAEs were also found when the test grating was of similar
spatial frequency to that of the adaptinggrating, even at very low
spatial frequencies (0.125 c/deg). These data suggest that MAEs
aredominated by a single, low-pass temporal frequency mechanism and
by a series of band-pass spatialfrequency mechanisms. The band-pass
spatial frequency tuning even at low spatial frequenciessuggests
that the “lowest adaptable channel” concept [Cameron et al. (1992).
Vision Research, 32,561-568) may be an artifact of disadvantaged
low spatial frequencies using static test patterns.Copyright @ 1996
Elsevier Science Ltd.
Adaptation Motionaftereffects Temporaltuning Spatialtuning
INTRODUCTION
It has been suggested that the early stages of humanvisual
processing involve analysis by a parallel set of“channels” that may
be defined in terms of their spatialand temporal tuning
characteristics [see Graham (1989)for an overview]. Spatial
frequency selectivity is wellestablished and has been
demonstratedboth physiologi-cally (e.g. Enroth-Cugell & Robson,
1966) and psycho-physically (e.g. Campbell & Robson, 1968). The
resultsindicate that spatialprocessing involvesa range of band-pass
spatial frequency-selectivefilters each with a bandwidth of
approximately one octave (e.g., Maffei &Fiorentini, 1973).
Temporal frequency selectivity hasreceived less attention, but
existing data suggest thatthere may be two or three temporal
channels, one low-pass and one or two band-pass filters (Mandler
&Makous, 1984;Hess& Plant, 1985;see also Fredericksen&
Hess, 1996a,b).Physiologicaldata from the macaque(Foster et al.,
1985) show that neurons in VI and V2 are
*McGill Vision Research, Department of Ophthalmology,
McGillUniversity, 687 Pine Avenue West (H4 14), Montreal,
Quebec,H3A IA1, Canada.
~Present address: Center for Visual Science, Universityof
Rochester,274 Meliora Hall, Rochester, NY 14627,U.S.A.
~Vision Sciences Laboratory, Department of Psychology,
HarvardUniversity, 33 Kirldand St, Cambridge,MA 02138,U.S.A.
$To whom all em-respondenceshouldbe addressed at present
address[Fax +1 716271 3043;[email protected]].
2721
broadly tuned for temporal frequency and may be eitherlow-pass
or band-pass.
Motion aftereffects and channel theories
One interesting demonstration of the existence ofspatial
frequency tuned mechanismsin the human visualsystem has been
established using a well documentedvisual illusion known as the
Waterfall Illusion or themotionaftereffect [MAE:see Wade (1994)for
a selectiveoverview]. When a stationary image is examined
afterprolonged viewing of a moving image, the stationaryimage
appearsto move in the oppositedirectionto that ofthe inducing
image: the MAE. MAEs are particularlyinteresting because they can
be used for selectiveadaptation [see Sekuler & Pantle (1967)
for the rationaleof this procedure].
Spatial frequency tuning. The contribution of spatialfrequency
selectivemechanismsto motion detection hasbeen establishedby
comparing MAE characteristics forgratingsof
differingspatialfrequencies.In thisparadigm,observersview a
driftinggrating(the adaptingpattern)ofa given spatialfrequency,then
the magnitudeof the MAEis measured for static gratings (the test
pattern) ofdiffering spatial frequencies. Using this
procedure,several researchers have demonstrated that the
strongestMAE is elicitedwhen the test and adaptinggratingsare
ofsimilar spatial frequency (Over et al., 1973;Cameron etal.,
1992). However, Cameron et al. showed that whenthe adapting grating
is lower in spatial frequency thanabout0.5 cldeg, this
relationshipbreaks down and a peak
.—
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2722 P. J. BEX et al.
MAE is always found at around 0.5 cldeg. This relation-ship
suggeststhat motion detection involvesmechanismsnarrowly tuned for
spatial frequency (at least for spatialfrequencies above 0.5
c/deg).
Temporal frequency tuning. Support for the existenceof temporal
frequency-selective mechanisms usingMAEs is more scarce. Most
research has examined thetemporal parameters which result in the
largest MAEs(e.g., Pantle, 1974; Wright & Johnston, 1985).
Specifi-cally, Pantle (1974) studied whether a constant velocityor
temporal frequency resulted in maximal MAEs for arange of spatial
frequencies. The optimal parameterswere a productof
spatialfrequencyand adaptingvelocity(i.e., temporal frequency), not
velocity per se. Noexperiments have been reported which describe
therelationshipbetween the temporal frequency of adaptingpatterns
and the temporal modulation frequency of thetest pattern.This
relationshipis exploredin experiment1.
In the research described above, the magnitude ofMAEs was
measured using static test patterns. However,a recent distinction
has emerged between MAEs whichare measured using static test
patterns and thosemeasured with dynamic “flickering” test patterns
(e.g.,Hiris & Blake, 1992).For example, in general no MAE
isinduced by non-Fourier motion stimuli if tested withstatic
patterns (Anstis, 1980; Derrington & Badcock,1985; Nishida et
al., 1994), but a flickering test patternreliably reveals a MAE
(McCarthy, 1993; Ledgeway,1994; Nishida et al., 1994). These
differences in thepsychophysicaldata have led to the
speculativesugges-tion that the two different types of MAE
originate atdifferentsites along the path of
visualmotionprocessing.Nishida & Sato (1995) suggested V1 as a
possiblecandidatefor the staticMAE and area MT or MST for
theflicker MAE (see also Ashida & Osaka, 1995).
Using flickeringtest patterns, Ashida & Osaka (1995)found
that the optimal temporal frequencyfor inducingaMAE is found to be
partiallyvelocity tuned,not temporalfrequency tuned, as is the case
for static MAE. Theseresults support the proposal of two different
sites ofMAE. Furthermore, Ashida & Osaka (1994) confirmedthat
the magnitudeof static MAEswas greatest if test andmatch
gratingswere of similar spatial frequency,but thisrelationship was
not found if the test grating wasflickering. In this case no
spatial frequency selectivitywas observed. This finding appears to
contradict aprevious finding in which spatial frequency tuning
wasshown using flicker MAE (von Gri.inau& Dub6, 1992).The
different results were attributed to experimentaldifferences
(Ashida & Osaka, 1994).
In the present experiments,we were interested in thespatial and
temporal frequency tuning of flickerMAE. Inthe same way that
spatial frequency tuning of the (static)MAE reveals spatial
frequency-selectivemotion detec-tion mechanisms, we hypothesized
that any temporalfrequency selectivity of motion detection
mechanismswould be exhibited by temporal frequency tuning of
theflickering MAE. We measured the magnitude of MAEselicited after
adaptation to drifting sine gratings whose
Adaptation Period:(drift towards centre)
Test Period:~ (counterphase) ~
FIGURE1.Schematicdiagramillustratingthe geometryof the
display.Sinusoidalgratingwere presentedin
twohorizontalregions(7.5by 7.5deg), separated by a horizontal strip
of 1 deg width with a centralfixation point. The sine gratings were
all 50% peak contrast, theremainderof the displaywas at mean
luminance(32 cd/m2).Duringa20 sec adaptationperiod, the sine
gratings drifted towards the fixationpoint to aid steady fixation.
During the test phase, the gratings weresinusoidallycounterphasedin
each windowuntil the observer reported
the end of the MAE.
spatial and temporal frequencies were manipulated.Following
Ashida & Osaka (1994), the magnitude ofthe MAE was estimated by
recording the duration of theMAE. The test grating in each case was
a sine grating ofthe same spatial frequency and peak contrast as
theadapting grating, but whose contrast was
counterphasedsinusoidally at between 0.125 to 16 Hz. A
comparisonconditionwas recorded using stationarygratings
(OHz).Using this protocol, we found no evidence for narrowtemporal
frequency tuning of the flickering MAE. Forany spatial and temporal
adapting frequencies, the peakflickerMAE was found at low
counterphasefrequencies.
In a second experiment, we measured the spatialfrequency tuning
of flicker MAE for low spatialfrequency adaptinggratings (0.125–2
c/deg). The resultsshowedclear spatialfrequencytuningat all
spatialscales,in good agreementwith von Griinau& Dub6 (1992)
andsuggest that the absence of such tuning reported byAshida &
Osaka (1994) may be related to their stimulusparameters. The
spatial frequency tuning at low spatialfrequencies shows that the
lowest adaptable channel(Cameron et al., 1992) revealed using
static MAE doesnot exist using flicker MAE.
EXPERIMENT1: TEMPORALFREQUENCYTUNINGOF FLICKERMAE
Methods
Apparatus and stimuli. Stimuliwere generated using aVSG 2/1
graphicscard (CambridgeResearch Systems)ina host PC microcomputer
(DELL 333D) and werepresented on a Nanao Flexscan 6500 monitor with
P4
-
TEMPORALAND SPATIALFREQUENCYTUNINGOF THE
FLICKERMOTIONAFTEREFFECT 2723
r
\‘B: 4 e/deg0 .125.25 .5 1 2 4 8
r & A 16 Hz0 8 I+z~\
❑ 4 HzA 2 Hz● 1 Hz
7\N ■ o.5”-iiz.
\N
Countetphaae Frequeney (Hz)
FIGURE2. Magnitudeof MAEas a functionof the temporalfrequencyof
the counterphasingtest grating. The data for the two observers
areshown in separate columns and the data for the three
adaptingspatialfrequencies (as well as test spatial frequencies)
are shown in separaterows. The temporal frequency of the
adaptinggrating is shown in thecaption and the temporalfrequencyof
the adaptinggrating is showninthe legend. The spatial frequency of
the test grating was the same asthat of the adaptinggrating in each
case. The temporalfrequencyof thetest grating is shownon thex-axis
with semi-logcoordinates(to permitthe inclusionof the OHz data,
where the test grating was static). Theduration of the MAE is shown
on the y-axis. Each data point is the
mean of at least four observations.Error bars show f S.E.
phosphor and with a frame rate of 118 Hz. The meanluminanceof
the displaywas 32 cd/m2.The luminanceofthe display was linearized
using an ISR attenuator(Pelli& Zhang, 1991)and calibratedusing
a UDT Photometer.The image was 16 deg horizontally(512 pixels) by
13.4deg vertically (428 pixels) and was viewed from adistance of
118 cm. Subjects viewed the screenbinocularly in a dim room. The
spatial layout of thedisplay is shown schematicallyin Fig. 1. There
were twosquare windows on the screen, each subtending7.5 degby 7.5
deg. The windows were separated horizontallybya 1 deg strip of mean
luminance, in the center of whichwas a prominent fixation point.
The remainder of thedisplay was blank and at the mean
luminance.
Adapting and test stimuli were vertical sinusoidalgratings of
50% Michelson contrast, which werepresented in the square windows.
The adapting gratingsdrifted towards the fixationpoint. The test
gratingsweresinusoidally counterphase flickering. The spatial
fre-
quencyof the adaptinggratingswas either 1,2 or 4 cldeg.The
temporal frequency of the adapting gratings wasvaried between 0.125
and 16 Hz, in steps of one octave.The spatialfrequencyof the test
patternwas also 1,2 or 4c/deg and in experiment 1 was always the
same spatialfrequency as the adaptinggrating which had preceded
it.The test gratings were counterphased at a temporalfrequency
between 0.125 and 16 Hz, in steps of oneoctave. An additional
conditionwas measured in whichthe test gratingwas static (OHz
counterphasefrequency).The startingphase of all gratingswas
randomizedbeforeeach presentation.
Procedure
The subjectwas instructed to maintain steady fixationduring
adaptation and testing and initiated each trial bypressing a mouse
button. This was followed by a 20 secadaptationperiod duringwhich
the adaptingsine gratingwas presented.The adaptinggrating was
always driftingtowards the center of the screen to facilitate
steadyfixation. The adaptation period was immediately fol-lowed by
a brief tone and the test period. During the testperiod, the
counterphasingtest grating was presented inboth windows.The
subjectwas requiredto press a mousebutton when the MAE had
finished. If the subjectexperiencedno MAE, the durationwas recorded
as zerosec. Subjectspracticed the task many timesbefore formaldata
collection. The direction of the MAE was alwaysseen in the opposite
direction to that of the adaptinggrating (in this case it always
appeared to move awayfrom the fixationpoint)and it was not
necessaryto recordthe perceived direction of MAE. Several studies
(e.g.Georgeson & Harris, 1978) have found that counter-phasing
gratings viewed parafoveally appear to driftaway from the center
even withoutadaptation:the foveo-fugal drift effect (FFDE).
However, the FFDE wouldresult in motion away from fixation which
neverterminated whereas the MAE measured in the presentstudy
reliably halted. Furthermore, on some trials,observers reported
that they experienced no MAE evenafter adaptation, again
inconsistentwith the intrusion ofFFDE into our results.Subjectshad
normal or eorrected-to-normalvision.
Each trial was followed by a inter-trial recoveryintervalof not
less than 1 min. The whole procedurewasrepeated for each of the
combinations of spatial andtemporal frequencies measured. The
presentation se-quence for the various spatial and temporal
frequencieswas randomizedand the datawere collectedover a periodof
severalweeks. The mean and standard deviation of atleast four
estimatesof MAE duration for each conditionwere recorded.
Results
Estimates of the MAE duration as a function of testtemporal
frequency are shown for the two observers inFig. 2. The top panels
represent the results where theadaptingand test
spatialfrequencieswere 1c/deg.For themiddlepanels the
spatialfrequencieswere 2 c/deg and in
-
2724 P. J. BEX et al.
201 T
0 PB: 2 dsglsec
O 2 tidsq❑ 1 c/d~A 0.5 e/decI● 0.25 c/dw
%
■ 0.12s Clde
IM : 1Hz
.1 1 10.1 1Test spatial Fraquency (c/deg)
FIGURE3. Magnitudeof MAE as a function of the spatial
frequencyof the counterphasingtest grating. The data for the two
observers areshown in separate columns and the data for the
separate temporalfrequencies are shown in separate rows. The
spatial frequency of theadaptinggrating is shown in the caption. In
the top row, the temporalfrequency of the adapting grating was 1 Hz
(its speed varied), in thebottom row, the speed of the adapting
grating was 2 degjsec (itstemporal frequency varied). The spatial
frequency of the adaptinggrating is shownon thex-axiswith
semi-logcoordinates.The temporalfrequencyof the test gratingwas
0.25Hz in each case. The durationofthe MAEis shownon they-axis.
Each data point is the mean of at least
four observations.Error bars show ~ 1 S.E.
the bottompanels theywere 4 c/deg. In all cases, it can beseen
that the longest MAE was found when the testgrating was
counterphasingat a low temporal frequency.Three combinations of
adapting spatial and temporalfrequency resulted in no or negligible
MAE (1 c/degadapting temporal frequency 16 Hz, 4 c/deg
adaptingtemporal frequency 1 or 0.5 Hz).
The results of the temporal frequency tuning of theflicker MAE
clearly show that the most pronouncedMAEs are found when the test
grating is slowlycounterphasing.This effect is found for both
observersand for all conditions measured. The effects of
theadapting frequency are not as clear. The data show weakband-pass
tuning of the MAE, but there is no clearsupport that the MAE is
tuned to either the temporalfrequency or the speed of the adapting
grating. Togetherwith different observations of temporal frequency
anddrift speed determinantsof MAE (Pantle, 1974;Ashida&Osaka,
1995),these data suggestthat both may contributeto MAE magnitude in
the same way that both contributeto the perceived speed of moving
images (Smith &Edgar, 1994).
EXPERIMENT2: SPATIALFREQUENCYTUNING OFFLICKER MAE
In experiment2, the spatial frequencytuning of flickerMAE was
measured. The procedurewas the same as for
experiment 1 except that the spatial frequency of
theadaptinggratingswas between 0.125 and 2 c/deg in stepsof one
octave. We included these low spatial frequenciesbecause Cameron et
al. (1992) presented evidence thatthe spatial frequency tuning
breaks down for spatialfrequencies below 0.5 c/deg. Although the
results ofAshida & Osaka (1994) argue against spatial
frequencyselectivity for flicker test stimuli, to our
knowledgenobody has reported whether the flicker MAE at lowspatial
scales is spatial frequency tuned. Moreover, thedifferencein
resultsbetweenAshida & Osaka (1994)andthose reported by von
Griinau and Dub6 (1992: experi-ment 4) requires investigation.The
latter found spatialfrequencyselectivitywith flickeringtest
stimuli,whereasAshida & Osaka (1994) did not. Differing
techniqueshave been suggestedas the sourceof the discrepanciesinthe
results (Ashida & Osaka, 1994).
The temporal frequency of the adapting patterns waseither 1 Hz
or was varied with spatialfrequencysuch thatdrift speed was 2
deg/sec. For each adapting spatialfrequency, MAEs were measured
using five test spatialfrequencies, one of the same spatial
frequency, two ofhigherand two of lower spatialfrequency,in stepsof
oneoctave. This range was employed except at the lowestspatial
frequenciesmeasured,which would have resultedin too few
visiblecycles of the grating. In these cases thelowest frequency
measuredwas 0.125 c/deg. The resultsof experiment1 showedthat a
maximalMAE occurswitha counterphase frequency of around 0.125-0.25
Hz,independently of spatial frequency. Test gratings
werecounterphased at a temporal frequency of 0.25 Hzbecause this
was near or at the peak of the temporalfrequencytuningcurve.The
startingphase of all gratingswas randomizedbefore each
presentation.
Results
Estimates of the MAE duration as a function of testspatial
frequencyare shown for the two observersin Fig.3. In the top
panels, the temporal frequency of theadapting grating was 1 Hz,
which means that the speedvaried. In the bottom panels the speed of
the adaptinggratingwas 2 deg/see,so temporalfrequencyvaried.
Thespatial frequency of the adapting grating is shown alongthe
x-axis.
The results are unambiguous: in all cases, it can beseen that
the longest MAE is found when the adaptinggratingand test grating
are of the same spatial frequency.The data showclear evidencefor
spatialfrequencytuningeven at the lowest spatial
frequenciesmeasured.
GENERALDISCUSSION
In this paper we have investigated both spatial andtemporal
frequency tuning of the flicker motion after-effect. We found no
evidence for band-pass temporalfrequency tuning of the flicker MAE,
but clear evidencefor band-pass spatial frequency tuning was
revealed bymeasuring MAE duration. The data from experiment 1show
that maximum MAEs were found using
flickeringtestpatterns,counterphasingat low
temporalfrequencies.
—.
-
TEMPORALAND SPATIALFREQUENCYTUNINGOFTHEFLICKERMOTIONAFTEREFFEm
2725
The tuning was independent of the spatial or temporalfrequency
of the adapting grating.
The low-passtemporal tuning of flickerMAE suggeststhat
flickerMAE may be dominatedby a single low-passtemporal mechanism.
The low-pass mechanism must bebroadly tuned because test patterns
of high temporalfrequencycan produce robustMAEs, but the peak
tuningof the MAE is always at a low counterphase
temporalfrequency.It shouldbe emphasizedthat the resultsdo
notpreclude the existence of additional temporal mechan-isms which
are band-pass and tuned to higher temporalfrequencies. Instead, the
results suggest the contributionof such mechanismsto flickerMAE
maybe substantiallyless than that of a single, low-pass temporal
mechanism.
A comparison of the MAE for the static and counter-phasing test
patterns shows no clear distinctionbetweenthe two. Instead, there
is a steady transition of MAEmagnitudefrom high
temporalfrequencycounterphasinggratings to gratings counterphasing
at zero Hz (static).This is supported by the subjective impressions
of theMAEs which were approximately the same under thevarious
conditions, although of differing duration. Thedata provide no
evidence to suggest that the two types ofMAE may be mediated by
separate mechanisms. Thisdoes not imply that there are no such
mechanisms. Forexample, it is known that the MAE direction
oforthogonallydirected transparentmotion (see Verstratenet al.,
1994a) can change drastically depending onwhether the test pattern
is dynamic or static (Verstratenet al., 1994b). Moreover,
differences found in recoveryfrom adaptation with static and
dynamic stimuli favor atsvo mechanism interpretation (Verstraten et
al., 1996).Also, Culham & Cavanagh (1994) have shown that
afterattentive trackingof a radial grating, a MAE is perceivedfor a
counterphasing test grating, not for a stationarygrating. MAE
studies using inter-ocular transfer techni-ques (IOT) also
showgreatdifferencesbetween staticanddynamic test patterns
(Raymond, 1993; Nishida et al.,1994; Steiner et al., 1994). In sum,
there is plenty ofevidence for different gain controls along the
path ofmotion processing. However, the temporal
tuningcharacteristicswe report here do not justify the conclu-sion
as drawn by Ashida & Osaka (1994).
In experiment 2, we tested a range of spatialfrequencies for
spatial frequency tuning of flickerMAE. The results show that the
maximum MAE wasfound using flickering test patterns of similar
spatialfrequency to that of the adaptingpattern. This result is
ingood agreement with von Grunau & Dub6 (1992),notwithstanding
the fact that they used a differenttechnique. However, the results
are not consistent withthose of Ashida & Osaka (1994), even
though they alsoused MAE duration as the dependent variable.
Thisinconsistencycontributesto a growing body of evidenceshowing
that MAE varies with a numberof experimentalparameters, including
the stimulus geometry and themethod of MAE magnitude estimation
(Wade, 1994).One key parameter which may contribute to
thesedifferences is the use of sub-optimaltemporalconditions
for the flicker frequency of the test pattern. In
ourexperiments, we used the optimal temporal frequencydetermined in
experiment 1, which should reveal tuningdifferencesmore
clearly.
More interesting,perhaps, is the comparison betweenthe results
of experiment 2 and those of Cameron et al.(1992). Using static
test patterns, these researcherspresented evidence for a “lowest
adaptable channel” of0.5 cldeg. This hypothesiswas based on the
observationthat the peak MAE for a 0.25 c/deg grating was foundwhen
testedwith a static0.5 c/deg grating(Fig. 2, p. 516).This is
apparentlynot the case for dynamic “flicker” testpatterns, even at
lower spatial frequencies than thosemeasured by Cameron et al. Part
of this difference mayarise from the particular method of MAE
magnitudeestimation. Cameron et al. used a tracking procedure,where
a subject manually matched the speed anddirection of the MAE over a
fixed period, using apotentiometer.These data were later combined
to give amean velocity estimation. Using this procedure,
theseauthors found “no measurable MAE at spatial frequen-cies lower
than 0.25 c/deg” (p. 561).Usingour methodofMAE
durationestimation,we found robustMAEs at verylow spatial
frequencies (we measured as low as 0.125c/deg), even using static
test gratings. We have alsoverified that MAEs can be measured using
the durationmethod with the same stimulus geometry used byCameron
et al. (two horizontal adapting fields, oneabove and one below
fixation).This shows that stimulusgeometryalone cannotaccountfor
these differences.It istempting to conclude that it might not be
possible torecord MAEs using the tracking methodology for
lowspatial frequency gratings. Consequently, with thisprocedure it
is not possible to record a MAE for lowspatial frequencies or to
measure any spatial frequencytuning.
This finding is redolent of early observationsthat thelowest
spatial frequency channelwas originallybelievedto be around 1–3
c/deg (Blakemore & Campbell, 1969;Campbellet al.,
1981).However, when larger field sizesand higher temporal
frequencies were used, separatelyadaptableand
maskablemechanismswere found to existdown to 0.2 c/deg (Stromeyer
& Julesz, 1972;Tolhurst,1973; Kranda & Kulikowski, 1976;
Stromeyer et al.,1982). This suggests that an absence of
temporalmodulation,combinedwith a MAE magnitudeestimationtechnique
which does not detect MAEs for very lowspatial frequencies,may have
contributedto the differentresults of Cameron et al. (1992) and our
experiment 2.
Our results and the suggestionsby Ashida & Osaka(1994)may
appearto complicatethe understandingof thespatial and temporal
tuning of MAEs. Ashida and Osakasuggest that since spatial
frequency selectivity is aproperty of mechanisms (channels) at a
relatively earlystage of the visual system, the absence of
spatialfrequency tuning is evidence for higher level
MAEs.However,the narrow spatialfrequencytuningreported inthis paper
and by von Griinau & Dub6 (1992) make thisargument disputable.
We show that selecting optimal
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2726 P. J. BEX et al.
conditionsfor maximizingMAEs can avoid confoundingvariables and
reveal the tuning characteristics of themotion mechanisms in human
vision.
CONCLUSION
We used the flicker MAE to study the temporal andspatial
frequency tuning of the visual system at supra-threshold contrasts.
The results show that the magnitudeof the flicker MAE is dependent
on the temporalfrequency of the counterphasing test grating, such
thatlowest temporal modulation frequenciesgive the largestMAEs. The
relationshipis independentof the spatial andtemporalfrequencyof the
adaptinggrating.This suggeststhat the flicker MAE is dominated by a
single, low-passtemporal mechanism. The data show that the
magnitudeof flickerMAE is also dependenton the spatialfrequencyof
the test grating, such that the largest MAE is foundwhen the
adapting and test patterns are of similar spatialfrequency, even at
very low spatial frequencies. Thisrelationship suggests that the
flicker MAE involves aseries of band-pass spatial
frequency-selectivemechan-isms. The differences between our data
and someprevious data may be based on the use of
sub-optimalconditions, resulting in weak or non-measurableMAEs,the
tuning of which are consequently difficult todetermine.
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