induction of kanizsa contours requires awareness of the ... · 7/14/2016 · 86 a kanizsa...
TRANSCRIPT
1
InductionofKanizsacontoursrequiresawarenessoftheinducingcontext1
2
TheodoraBanica1&DietrichSamuelSchwarzkopf1,2*3
4
1.ExperimentalPsychology,UniversityCollegeLondon,26BedfordWay,London,UK5
2.InstituteofCognitiveNeuroscience,UniversityCollegeLondon,17QueenSquare,6
London,UK7
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*Towhomallcorrespondenceshouldbedirected:[email protected]
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Abstract11
12
Itremainsunknowntowhatextentthehumanvisualsysteminterpretsinformation13
about complex scenes without conscious analysis. Here we used visual masking14
techniquestoassesswhetherillusorycontours(Kanizsashapes)areperceivedwhen15
the inducing context creating this illusion does not reach awareness. In the first16
experiment we tested perception directly by having participants discriminate the17
orientationofan illusorycontour. In thesecondexperiment,weexploitedthe fact18
that the presence of an illusory contour enhances performance on a spatial19
localization task. Moreover, in the latter experiment we also used a different20
maskingmethodtoruleouttheeffectofstimulusduration.Ourresultssuggestthat21
participants do not perceive illusory contours when they are unaware of the22
inducingcontext.This isconsistentwiththeoriesofamultistage,recurrentprocess23
ofperceptualintegration.Ourfindingsthuschallengesomereports,includingthose24
from neurophysiological experiments in anaesthetized animals. Furthermore, we25
discusstheimportancetotestthepresenceofthephenomenalperceptdirectlywith26
appropriatemethods.27
28
Keywords29
30
Illusorycontours,Kanizsa,modalcompletion,awareness,masking31
32
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33Introduction34
35
What role does conscious processing of the environment fulfill and how much36
processingoccurs in theabsenceofawareness? It is self-evident thatmuchof the37
internal bodily functions and the learned motor behaviors, such as walking or38
driving,operatemostlywithoutawareness.Butforprocessingthroughtheclassical39
senses,likevision,therehavebeenwidelydiscrepantfindingsonhowmuchstimulus40
processing can occur and how it affects decision-making when the subject is41
unaware of the stimulus. Moreover, the approach to be used when studying42
unconsciousstimulusprocessinghasalsobeensubjectofcontroversy[1].43
44
Several experiments suggest that the effect of contextual stimuliwithin a target,45
such as the percept of visual illusions or adaptation effects, persists even when46
participantsareunawareofthepresentedcontextualinformation[2–6].Theuseof47
continuous flash suppression (CFS), in which a dynamic, high-contrast stimulus is48
presentedtooneeyetosuppressthestimulusintheothereyefromawareness,has49
become a popular way to probe unconscious stimulus processing [7]. Using this50
procedure ithasbeenclaimed that theperceptionofphysical facial attributes [8],51
the complex analysis of naturalistic scenes [9], and even linguistic processing and52
arithmetic canbeperformedwithout awareness [10]. However, several of these53
findings have recently failed to be replicated and were challenged on theoretical54
grounds[11–13].55
56
Neuroimaging experiments showed that while both simple and more complex57
stimuli haveaneural signature in the visual cortexundermasking conditions [14–58
16], the encodingof unconscious stimuli appears tobequalitatively different.Not59
only is theoverall response tounconsciousvisual stimuliweaker [15]butcoupling60
betweendifferentstagesinthevisualprocessinghierarchyisalsoreduced[17]and61
the information content differs [16,18,19]. In particular, the response to these62
stimuli ismore variable [20], and also localized inmore posterior regions than to63
conscious stimuli [18]. . One reason for this variability could be that only simple64
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positionalorgeometric information isprocessed in theabsenceofawareness,but65
that more complex abstraction and perceptual integration requires consciousness66
Underthishypothesis,theneuralencodingofstimuliisnoisybecauselocalstimulus67
interactionsarepreservedbutabstractandthereforecoherentrepresentationsare68
disrupted. Specifically, we used shape stimuli that were either defined by the69
positionortheorientationofsimpleimageelements.Wedemonstratedthatwhen70
suchstimuliwererenderedinvisibleusingfastcounter-phaseflicker(at120Hz)they71
couldspeedupperformanceonashapediscriminationtaskofconsciousstimuli[21].72
Critically,thisprimingeffectwasonlypresentforstimulisharingthesamepositions,73
or – if oriented elements were used for priming – the positions along the path74
implied by the elements. We observed no priming by invisible primes if the test75
stimuluswas smaller than the prime stimulus. This suggests that invisible priming76
operatedlocally,possiblyinretinotopicspaceinearlyvisualcortex,butnoabstract77
integration of individual elements into the concept of a shape occurred without78
awareness.79
80
We further tested whether two brightness illusions manifest when the inducing81
context is rendered invisible by means of CFS [22]. We found that masking the82
context (a smooth gradient in luminance) had little impact on simultaneous83
brightness contrast of two targets that were unmasked. In a stark dissociation,84
participantscouldnotdiscriminatetheorientationofanillusorycontour,definedby85
a Kanizsa triangle, when the inducing context (the ‘Pacman’ shapes whose open86
segments define the corners of the triangle)weremasked selectively by CFS. This87
could indicate that thegenerationof the illusorycontourperceptoccursata later88
stageofvisualprocessingthansimultaneousbrightnesscontrast,eitherintermsof89
the visual hierarchy or in the latency of processing. These findings were also90
consistentwithpreviousreports that illusorycontoursarenotperceivedwhenthe91
inducersaresuppressedfromawarenessduringbinocularrivalry[23].92
93
These findings challenge some previous reports, using psychophysical tests in94
healthy volunteers [24] and even neurophysiological experiments in anesthetized95
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animals[25]thatsuggestedthatillusorycontourscouldbeformedintheabsenceof96
awareness.However,noneofthesepreviousstudiesspecificallytestedwhetherthe97
experimental participants actually perceived an illusory contour. Similarly,98
neuropsychological studies in neglect patients [26–29] suggested that illusory99
contourprocessingoccurswhenpartof the inducingcontext isplaced in theblind100
hemifield. However, this also does not conclusively support the assertion that101
contoursareformedintheabsenceofanycontextualawareness.102
103
However, two issuesburden the interpretationofpreviousexperiments that show104
no evidence of illusory contours when the inducers are masked. First, there is105
evidence that illusory contours are processed by binocular neurons in early visual106
cortex [30–33]. It does in fact seem unsurprising that the mechanisms inducing107
illusory contours at least partially overlap with those for segmenting surfaces in108
depth[34].Whenretinaldisparityimpliesthatinducersareatdifferentdepthsfrom109
thebackground,thevisualsystemnotonlyproducestheperceptofillusorycontours110
butthesurfaceboundedbyillusorycontoursisalsoperceivedinstereoscopicdepth111
[31]. Thuswhen binocular processing is disrupted or overwhelmed by a dichoptic112
maskorbinocularrivalry,theillusorycontourperceptisalsobroken.Anotherrecent113
studyfurthercomplicatedthissituationbyfindingthatKanizsashapesbrokethrough114
CFSfasterthancontrolstimuli[35].Leavingasideconceptualissueswiththetime-to-115
emergenceparadigm, it is imperative that thedependenceof illusory contourson116
awarenessmustbeconfirmedusingmaskingmethodsotherthanCFS.117
118
A second confoundwith theseprevious studies [22,23] is that even if participants119
perceivedan illusorycontour, thisperceptwas far less salient than real luminance120
contours, as it may have been obscured by the dominant masking stimulus. The121
additionofasimplecontrolconditioncouldremedythisproblem:Oneexperimental122
stimulusshouldbearealcontourdefinedbyasubtleluminancecontrastthatmimics123
the illusory contour percept as closely as possible. If participants can detect and124
discriminatethisstimulusbutareunabletodosofortheillusorycontourcondition,125
thisindicatesthattheillusoryperceptisindeeddisruptedspecifically,ratherthanthe126
moregeneralabilitytodetectsubtlestimuli.127
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128
In thepresentpaper,wecarriedout twoexperiments toaddress theseconfounds129
andanswerthequestionofwhetherKanizsacontoursareformedwheninducersare130
not consciously perceived. In experiment 1, we used a similar design as in our131
previousstudy[22].Here,participantswereaskedtodiscriminatetheorientationof132
a Kanizsa triangle. However, instead of CFS we employed a temporal masking133
methodtorendertheinducersinvisible.Moreover,weincludedacontrolcondition134
in which a real, luminance-defined contour was present. Because this masking135
methodreliesonverybriefstimulusdurations, inexperiment2wepresented long136
(500ms)stimulirenderedinvisiblebymeansoffastcounter-phaseflicker[3,21].This137
iscriticalbecausetheformationofillusorycontoursarisecomparablyslowly[36–38]138
andthusmaybedisruptedbyafasttemporalmaskingtechnique..Inadditiontothis,139
previous research demonstrated that the presence of illusory contours boosts140
participants’abilitytodiscriminatethepositionofatinytarget[23,39],providinga141
specifictestofwhethertheparticipant infactperceivesan illusorycontourornot.142
Wethereforemeasuredtheabilityofagroupofparticipants,whowerewelltrained143
atpsychophysicaltasks,todiscriminatethepositionofadottargetforKanizsaand144
controlstimulipresentedwithorwithoutmasking.145
146
MaterialsandMethods147
148
Both experiments were carried out at the UCL Department of Experimental149
Psychology.ProceduresadheredtotheDeclarationofHelsinki.Ethicalapproval for150
thisstudywasobtainedfromtheUCLResearchEthicsCommitteeandallparticipants151
gavewritten,informedconsent.152
153
Participantsinexperiments1wererecruitedamongtheUCLstudentpopulation.In154
experiment2werecruitedparticipantswhowerefamiliarwithpsychophysicaltasks.155
AllParticipantshadnormalorcorrected-to-normalvisualacuity.156
157
Allexperimentswereconductedinadark,sound-attenuatedroom.Fortheduration158
of the experimental sessions, participantswere asked to stabilize their head on a159
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chinrestlocatedatafixeddistanceof48cmfromthestimuluspresentationscreen160
wherestimuliwerepresentedtothembinocularly.161
162
Stimuli were generated by a computer and presented on a 22-inch Samsung163
SM2233RZLCDmonitorataresolutionof1680x1050pixels.Screenrefreshratein164
experiment1wassetto60Hz.Inexperiment2itwassetto120Hz.Theexperiment165
was controlled and behavioral responses were recorded using MATLAB (The166
Mathworks,Inc.)andPsychtoolbox3[40]usingastandardkeyboard.167
168
Experiment1169
170
The experiment comprised two tasks: the first, henceforth called ‘Kanizsa’ task,171
investigatedwhetherparticipantscanperceiveillusorycontourswithoutawareness172
oftheirinducers.Thesecondtask,the‘Visibility’task,assessedtheeffectivenessof173
themaskingtechniquedirectly.174
175
Studydesign176
177
Like our earlier experiment using continuous flash suppression, this experiment178
aimed to measure the perception of illusory contours in a direct manner. We179
implementeda2x3designwithvisibility(invisible,visible)andtypeofstimulus(real,180
illusory,control)aswithinsubjectfactors.181
182
Everyparticipant completed twodifferent tasks. Each task comprised25blocksof183
trials,whereoneblockconsistedof24 trials in theKanizsa taskand8 trials in the184
visibilitytask.Acrossonetask,eachconditionappeared100times.Thevisibilitytask185
onlycomprisedthevisibleandinvisiblecontrolconditions.186
187
Participantsmade behavioral responses in a forced-choice design by button press188
(leftor rightarrow)onastandardcomputerkeyboard.Eachtrial requiredeithera189
leftora right responseandeachresponsetypeappearedtwiceperblock foreach190
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condition. Conditions were selected pseudo-randomly for every trial but were191
counterbalancedovereachblock.192
193
Participants194
195
Seventeen (13 female; age range: 18-29, mean age: 23.8±2.5) normal, healthy196
participantstookpartintheexperiment.Anadditionaltwoparticipantsweretested197
buttheyfailedtodiscriminatetherealluminancecontourundermaskingconditions198
and were therefore excluded from further analysis (see more details below). All199
participantswereunawareoftheexperimentalhypothesis.200
201
Stimuli202
203
Stimuliwerecreatedbyplacingfourdiscs(inducerelements,diameter=2.2°) inthe204
configurationofasquare(width=4.3°).Theconfigurationofthesediscswascentered205
onfixation.Asbothstandardedgetypeandanumberofline-endinducingelements206
havecomparableefficacyintheclaritywithwhichillusorycontoursareperceivedby207
participants[41,42],herethediscsweredefinedbypartialconcentriccircles.Eachof208
the four lines forming the circles had a width of approximately 0.07° with a209
luminanceof0.6cd/m2.Thepositioningofthegapwithinthecirclesgaverisetothe210
percept of a Kanizsa triangle (Figure 1A). Thus, a number of line-end inducing211
elementsgaverisetoperceptionofillusorycontours[43].212
213
Wealso includedareal luminancecondition.Here, thestimulididnotcontainany214
discsbutinsteadtherewasatriangledefinedbyarealbutsubtleluminancecontrast215
at theexact locationwhere theKanizsa trianglewouldbeperceived in the illusory216
condition(Figure1B).Wereasonedthatifparticipantswereunabletodiscriminate217
theorientationofthissubtleluminanceedgewhentheinducersweremasked,this218
implied that theywould be unable to detect any illusory contour that could have219
formedwithoutawarenessoftheinducers.Therefore,weremovedtwoparticipants220
whosediscriminationperformanceforthisconditionwasatchancelevelsfromany221
furtheranalyses.222
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223
Control stimuli did not form any triangle and were created by altering the224
orientations of the inducers by a systematic rotation of 180º (Figure 1C). This225
conditionwassomewhatunnecessarybecausethedependentvariable,accuracyfor226
discriminating theorientationof the triangle,wasorthogonal to theconditionand227
for thesecontrol stimuli therewasnostrictly correctanswer in thisdiscrimination228
task. The “correct” responses in this condition were dummy coded so that the229
stimulus-responsemappingmatchedthatfortheequivalentillusorytrianglestimuli230
beforerotatingtheinducers.Weincludedthisconditionascatchtrials–participants231
should be guessing here because there was no triangle to discriminate. Thus it232
provided information on whether participants could indeed perceive the triangle233
contoursorwhethertheyadoptedastrategyofmatchingtheinducerorientationto234
feedback.235
236
Thebackgroundhadaluminanceof76.6cd/m2andrealtrianglesweredefinedbya237
subtly greater luminance of 86.9 cd/m2. The mask was only used in the invisible238
conditions and was designed as a square configuration of four black discs (0.6239
cd/m2), each containing four white concentric circles (230 cd/m2). It was wide240
enoughtocoverall fourPacmen in the illusoryandcontrolconditions (Figure1D).241
Themaskingtechniqueconsistedofasequenceofthreeframeswhichwasrepeated242
threeconsecutivetimes:Atfirstthemaskappearedonthescreen,followedbythe243
appearanceofthestimulusandafinalblankscreen.Inthevisibleconditions,ablank244
greyscreenreplacedthemaskframe.245
246
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247Figure1. Illustrationofthestimuli.A)(i)Kanizsatrianglepointingright(ii)Kanizsatriangle248
pointingleft.B)(i)Realtrianglepointingright(ii)Realtrianglepointingleft.C)Controlstimuli249
werecreatedbyasystematic180°rotationof the individualPacmen.D) Illustrationof the250
maskingprocedureintheinvisibleconditions:Amask-stimulus-blankscreenframesequence251
wasrepeatedthreetimes.Inthevisibleconditionsablankscreenreplacedthemask.252253
254
255
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Procedure256
257
Kanizsa task:At first, participants were instructed that they would see a triangle258
appearonthescreen.Theywereaskedtojudgewhetheritshypotenusewastilted259
clockwiseorcounter-clockwisefromverticalbypressingthecorrespondingresponse260
key.We explained this task to them as a decisionwhether the right angle of the261
trianglewaspointingtotheleftortotherightbuttheyweretoldexplicitlytojudge262
thecontoursofthetriangle,inparticularthelonghypotenuseextendingthroughthe263
centerofthestimulusdisplay.Becausewewishedtokeepthecontrastbetweenreal264
triangleandthebackgroundaslowaspossible,wetrainedparticipantsonthevisible265
andinvisiblerealtriangleconditionuntiltheywereabletodetectthemcorrectly.266
267
Participantswereinstructedtofixateasmallblackdot(0.2°wide)thatwaspresent268
inthecenterofthescreenthroughouttheexperiment.Oneachtrial,thefixationdot269
was displayed alone for 500ms. Thiswas followed by a sequence of three frames270
thatdefinedwhethertheconditionwasavisibleoraninvisibleone.Intheinvisible271
condition a 300msmask followed the fixation period. Subsequently, the stimulus272
appeared on the screen for one frame of approx. 16.7 milliseconds (ms),273
immediatelyfollowedbyablankscreenthatwasshownforoneframeaswell.This274
mask-stimulus-blanksequencewasrepeatedthreetimesbeforeasecondandfinal275
post-stimulus blank screen was presented until participants gave their response.276
Figure2Ashowsthegeneralparadigmfortheexperimentalprocedure.277
278
Visibility task:We further tested whether participants indeed did not consciously279
perceiveanycontextualinformationofthestimuliintheinvisiblecondition.Forthis280
purpose, the visibility taskassessed theeffectivenessof themasking techniqueby281
measuring whether participants could consciously discriminate the inducer282
elements.Participantswereaskedtojudgewhetherarightanglewaspresentedin283
the left or right bottom inducer. The timing of the trial sequence in this taskwas284
identical to the Kanizsa task, with one exception: only control stimuli were used285
(Figure2B).286
287
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288Figure2. Illustrationof thetrialsequence inexperiment1.A)Kanizsatask:Eachtrialwas289composedoffiveframes:fixationdot,mask,stimulus,blank,andpost-stimulusblank.After290
theparticipant’sresponse, thefixationpointprovidedfeedbackfor100ms(green:correct;291
red:incorrect).Thedurationofeachframeisshownonthetime-line.Notethatinthevisible292
conditions,ablankscreenreplacedthemask.B)Visibilitytask:Trialsproceededinthesame293
way as in the Kanizsa task except that only control stimuli were being shown and294
participants judged whether the right-angle gap was in the bottom left or right inducer295
(indicatedbythegreencircle,whichwasnotpresentintheactualstimuli).296
297
298
299
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Dataanalysis300
301
Performance in both tasks was defined in terms of mean proportion of accurate302
responsesineachcondition.Initially,weconductedbinomialtestsattheindividual303
level to quantify how many participants performed significantly above chance. A304
condition for any participant’s data to be considered in the group analysis of this305
experimentwasthattheirperformancetothe invisiblerealstimulusconditionwas306
significantlyabovechance(0.5).Thisisbecausethefeaturesnecessarytodistinguish307
the contours of the real triangle were not masked and if a participant could not308
perform the task for this particular stimulus, any test of theperceptionof illusory309
contourswouldberedundant.310
311
One-samplet-testswerecarriedoutatthegrouplevelforeachconditionindividually312
to assess whether the participants’ level of performance was significantly above313
chance (>0.5). In addition to traditional frequentist statistical tests, we also314
quantified the evidence for or against the hypothesis that participants could315
discriminate the stimuli by calculating a Bayes Factor using a default Cauchy prior316
with scaling factor 0.707 for the alternative hypothesis [44]. For conditions with317
performancenear chance levels, thisenabledus toalsoquantifyhowstrongly the318
evidencesupportedthenullhypothesisthatparticipantswereactuallyguessing.319
320
321
Experiment2322
323
Thisexperimentonlycomprisedonetaskthattestedtheperceptofillusorycontours324
by measuring participants’ threshold on a spatial localization task. Such a325
manipulationhasbeenusedsuccessfullyinpreviousexperiments[23,39].Whileitis326
an indirect test, thecontouraidsperformanceonthespatial localization task.This327
provides independent evidence about whether participants perceived any illusory328
contours and therefore helps to address confounds with measuring the percept329
directly as we did in experiment 1. Pilot experiments using a task in which we330
directlymeasuredtheillusorypercept(thesametaskasexperiment1butwiththe331
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long-lasting masking technique employed in experiment 2) could encourage332
participants to pay close attention to the masked context and thus reduce the333
effectiveness of the masking procedure. This would mean that participants use334
residualawarenessoftheinducerstoperformthetaskinsteadofactuallymakinga335
perceptualjudgmentoftheillusorycontour.336
337
Moreover, because the masking procedure in experiment 1 used very brief338
presentationsoftheinvisibleKanizsastimuli(16.7ms)whiletheseweremuchlonger339
(300ms)forthevisibleones,inexperiment2weusedadifferentmaskingprocedure:340
stimuli were defined by sinusoidal gratings which reversed contrast polarity at341
120Hz.Thismethodcaneffectivelyrenderstimuliinvisibleforprolongedperiodsso342
thatparticipantsonlyperceiveagreyscreen.Previousresearchsuggeststhatstimuli343
masked in thiswayareprocessed inearlyvisual cortex [3].Moreover,weshowed344
thatstimulirenderedinvisiblebythismethodcouldinducelocalprimingeffectsona345
shapediscriminationtask[21].346
347
Participants348
349
Five normal, healthy participants (3 female; age range: 24-37; mean age: 30.2)350
completedthisexperiment,includingoneoftheauthors(DSS).Allparticipantswere351
experienced with psychophysical experiments. While the results from a larger352
subject basemight generalizemore to abroadpopulation,we reasoned thatonly353
precise measurements of participants’ visual performance would afford354
interpretabledata.Unlikeinthefirstexperiment,inexperiment2wemeasuredhow355
positiondiscriminationthresholdschangedbetweenexperimentalconditions.Naïve,356
untrained participants from the general population would be more likely to357
contributenoisydataandthismightobscurepotentialsubtleperceptionof illusory358
contoursinthemaskedcondition.359
360
Stimuli361
362
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WegeneratedaKanizsashapebypresentingfourGaborpatches(sinusoidalgratings363
with wavelength 0.33° visual angle convolved with a Gaussian with standard364
deviation0.8°at30%contrast)inthelocationsofthefourcornersofasquarewitha365
side-lengthof8.2°.Weturnedthesepatches intoPacmenbysettingaright-angled366
regionofeachpatchtozerocontrast(uniformbackgroundgrey).Thisstimulusthus367
describedanillusorysquare(Figure3A).Inthecontrolconditions,thePacmenwere368
rotated by 180° so that the corners faced outward breaking the illusory percept369
(Figure3B).Intherealluminancecontrol,wedidnotpresentanyGaborpatchesbut370
insteadasquareregionwithasubtleluminancecontrast(54cd/m2)relativetothe371
background (Figure 3C). The orientation and phase of each Gabor patch was372
randomizedineachtrial.Finally,wecreatedamaskstimulusbyoverlaying24Gabor373
patches in the four locations. These patches were presented at 5% contrast and374
covered the full range of orientations in equal steps of 15° but their phaseswere375
randomized.This resulted inapatchypatternwithoutanyobviousorientationcue376
(Figure3D).Themaskpatternwasalsogeneratedanewineverytrial.377
378
Thesestimuliwerepresentedinthecenterofthescreen.Thetargetstimuluswasa379
tiny dark grey dot (diameter: ~0.1°) which could appear somewhere along the380
horizontalmeridian,eitherneartherightortheleftverticalboundaryofthesquare.381
382
The square stimuli were either invisible or visible. In the invisible condition, the383
gratings reversed contrast polarity at 120Hz. In the visible conditions and the real384
luminance control, every even-numbered framewas uniform grey (except for the385
fixationdot).386
387
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388Figure3.Stimuli(A-D)andtrialsequence(E)inexperiment2.Duringthestimulusinterval389
eitheran illusoryKanizsasquare(A),acontrolstimuluswithrotated inducers(B),orareal390luminance square without any inducers (C) was presented. In visible trials every odd-391
numberedvideoframeat120Hzcontainedthestimuluswhileeveryeven-numberedframe392
contained only a blank screen with the blue fixation dot. In invisible trials, both frames393
contained the Gabor patches for A and B but their contrast polarity alternated between394
frames. For real luminance stimuli the even-numbered frames always contained a blank395
screen. D) A mask stimulus preceded and followed each stimulus interval. E) Each trial396
started with 500ms of fixation, followed by a 100ms mask, the 500ms stimulus interval,397
another100msmask,and finallyablankscreenwithouta fixationdot that remaineduntil398
participantsgavetheirresponse.Theirtaskwastolocatethesmalltargetdotinthestimulus399
displayanddecidewhether itappearedto the leftor therightof theverticalboundaryof400
thesquare(here,thecorrectresponseisright).401
402
403
Procedure404
405
Figure3EdepictsthesequenceofaninvisibleKanizsatrialinexperiment2.Eachtrial406
startedwith500msofablankgreyscreen thatonlycontainedablack fixationdot407
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16
(diameter0.26°).Thiswasfollowedbya100mspresentationofthemaskandthen408
500ms of the stimulus, after which another 100ms mask interval was presented.409
Then the screen turned grey and the fixation dot was removed, indicating that410
participantscouldgivetheirbehavioralresponse.411
412
Duringthestimulusintervalthefixationdotwasblueinsteadofblacktodenotethat413
thiswas the task-relevant interval. As described above, the stimuli in this interval414
either reversed in contrast polarity at 120Hz (invisible Kanizsa and control415
conditions)or the frameswere interleavedwithblank frames (visibleKanizsa, real416
luminance stimuli and control). During the stimulus presentation the small dark417
targetdotalsoappearedatsomelocationalongthehorizontalmeridian.Itsposition418
was randomized to be either near the left or the right vertical boundary of the419
squareregion.Thedotcouldeitherappearinsideoroutsidethesquareregion.420
421
Thedependentvariableinthisexperimentwasthedistancebetweenthetargetdot422
and the boundary of the square region and it was controlled by a 2-down, 1-up423
staircase procedure that converged on the threshold distance in each of the five424
experimental conditions at which performance was approximately 70.7% correct.425
That is, after every consecutive two correct trials the distancewould decrease by426
onepixel (~0.03°)whileafterevery incorrect trial it increasedbyonepixel.All five427
staircasesstartedatadistanceof15pixels(0.49°).Theminimumandmaximumthat428
theycouldreachwereonepixeland25pixels(0.82°),respectively.429
430
BeforetheactualexperimentweshowedparticipantsstaticexamplesoftheKanizsa431
andthecontrolstimuli.Participantswereinstructedtojudgewhetherthetargetdot432
was left or right of the (imaginary) vertical boundary of the square region.While433
showingthemstillimagesofthestimuli,wespecificallyexplainedtothemthatthis434
boundarywasdefinedby theexactcenterof theGaborpatches (i.e. thecornerof435
thePacman’smouth)andthatthiswasidenticalinboththeKanizsaandthecontrol436
conditions.Wefurtherinformedthemthattherewouldbeathirdconditioninwhich437
they should only see a subtly lighter grey square against the background but no438
Gaborpatches.Inordertobecomeacquaintedwiththetask,theythenperformed1-439
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17
2 blocks of the experimentwith only the visible Kanizsa, the control and the real440
luminance condition. Finally, the actual experimentwould commence. Participants441
wereinformedthatthetaskwaslargelyalwaysthesameasthefamiliarizationrun,442
although during this experiment there would be many more trials in which they443
eitheronlyseealightgreysquarebutnoGaborpatchesorthattheymightevenonly444
seeagreyblankscreen.Exceptfortheauthor,allparticipantswereunawareofthe445
experimentalhypothesis.Indebriefingnoneoftheparticipantsreportedseeingany446
Gaborpatchesduringtheinvisiblecondition.447
448
Participantsperformedtwoconsecutiverunscomprising500trialseach.Theseruns449
were further subdivided into 25 blocks. In every block each of the five conditions450
appeared4timesinapseudo-randomly interleavedorder.Blockswereinitiatedby451
button-press.Duringblockbreaksamessageonthescreenremindedtheparticipant452
ofthetaskinstructionsandofthenumberofblockstheyhadalreadycompleted.453
454
Dataanalysis455
456
Wedeterminedthethresholddistanceineachofthefiveexperimentalconditionsby457
calculating the mean distance across the final 15 reversals in the staircase458
procedure. Then we conducted a group level analysis in which we compared the459
average thresholds for conditions across participants. For thiswe used a two-way460
repeated-measures ANOVA with the factors visibility (visible vs invisible) and461
stimulustype(illusoryvscontrolstimulus).Astherewasonlyoneconditionwitha462
real luminance contour, the thresholds for this conditionwerenot included in the463
ANOVA. However, we used paired t-tests to compare results for the illusory and464
controlstimulidirectlytotherealluminancecondition.465
466
467
Results468
469
Experiment1470
471
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18
Inthisexperiment,17participantsjudgedtheorientation(leftvsright)ofatriangle472
thatwaseitherdefinedbyanillusorycontourorasubtlerealluminanceedge.Ina473
thirdcontrolconditionthePacmenstimuliinducingillusorycontourswererotatedso474
thatnotriangleshapecouldbeperceived.475
476
KanizsaTask477
478
Accuracyacrossparticipantsfordiscriminatingtheorientationofthetriangleforthe479
six conditions is displayed in Figure 4. We tested whether discrimination480
performanceineachconditionwasabovechanceatthegrouplevel.481
482
Inthevisiblecondition,themeanproportionofcorrectresponsesforthegroupwas483
closetoceilingandclearlybetterthanchancebothforthereal(M=0.97,t(16)=83.5,484
p<0.001,BF10>9.8*1018)andtheillusorycondition(M=0.99,t(16)=120.8,p<0.0001,485
BF10>2.4*1021).Inthecontrolcondition,however,thegroupperformedsignificantly486
below chance level (M=0.30, t(16)=-3.87, p=0.001, BF10=29.3). There was487
considerablevariabilityinperformanceforthisconditionrangingfrom0to0.74.488
489
In the invisible condition, only performance on the real triangle condition was490
significantly above chance level (M=0.71, t(16)=6.4, p<0.001, BF10=2608.7).491
Performance on both the illusory triangle condition (M=0.51, t(16)=1, p=0.332,492
BF10=0.385)andthecontrolcondition(M=0.50,t(16)=0.2,p=0.859,BF10=0.253)were493
at chance level. Importantly, performance on the real triangle condition was also494
significantly greater than for either the illusory contour (t(16)=6.1, p<0.0001,495
BF10=1475.4) or the control condition (t(16)=5.6,p<0.0001,BF10=688). In contrast,496
performance for the illusory contour did not differ from the control condition497
(t(16)=0.7,p=0.525,BF10=0.3).498
499
VisibilityTask500
501
In a supplementary control taskwe askedparticipants tomake a decisiondirectly502
basedontheinducersbyreportingwhichoneofthetwobottominducerscontained503
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19
the rightangle.Thisassessedany residualperceptionof the inducer shapesunder504
masking conditions. In the visible condition, the mean proportion of correct505
responsesacrossparticipantswasagainclosetoceilingandfarabovechance level506
(M=0.96, t(16)=22.2, p<0.001, BF10>2.9*1010). In contrast, participants’ mean507
performance in the invisibleconditiondidnotsignificantlydiffer fromchance level508
(M=0.48, t(16)=-1.3, p=0.227, BF10=0.489), suggesting that participants could509
generallynotperceivethe inducersundermaskingconditions.Note,however,that510
therewasonlyanecdotalsupportforthenullhypothesis(BF10>1/3).511
512
The first experiment thus suggested that if participants were unaware of the513
inducersbecausetheyhadbeenmasked,theywereunabletojudgetheorientation514
oftheKanizsatriangle.Thissupportstheinterpretationthatillusorycontoursarenot515
formedundertheseconditions.516
517
518Figure4.Resultsofexperiment1.Accuracyfordiscriminatingtheorientationofatriangle519
stimulus in visible or invisible (masked) trials. Each dot represents the performance of an520
individualparticipantineachoftheconditions.Thelargesymbolsanderrorbarsdenotethe521
mean ±1 standard error for each condition. Black: real luminance contour. Red: illusory522
(Kanizsa)contour.Blue:controlstimuli.523
524
Experiment2525
526
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20
Inexperiment2wechangedtheapproachinanumberofways.First,insteadofthe527
brief stimulus presentations used in the first experiment we used counter-phase528
flicker to render stimuli invisible for prolonged periods. Moreover, we used an529
indirect measure of illusory contour processing: we took advantage of previous530
reports that the presence of an illusory contour stimulus enhanced participants’531
thresholds at discriminating the position of a small dot [23,39]. Here we tested532
whetherthisalsooccurredwhentheinducersgeneratingtheillusorycontourwere533
invisible. Because this task was more challenging than those in previous534
experiments, and to rule out that our previous results might have been due to535
insufficient practice or familiarity with psychophysical experiments, in this536
experimentweonlytestedasmallgroupofwell-trainedpsychophysicsparticipants.537
538
Figure 5 plots position discrimination thresholds in the different conditions for all539
individualparticipantsandthegroupaverages.Overall,thresholdsmeasuredwhilea540
Kanizsa stimulus was presentedwere significantly lower than thosemeasured for541
control stimuli (F(1,4)=34.6, p=0.004). There was also a non-significant trend of542
lowerthresholdsduringvisiblethaninvisibletrials(F(1,4)=7.5,p=0.052).Importantly,543
therewasasignificantinteractionbetweenvisibilityandstimulustype(F(1,4)=15.9,544
p=0.016). In the visible condition thresholds measured while an illusory Kanizsa545
contour was present were significantly lower (M=0.08°, t(4)=-5.27, p=0.006,546
BF10=9.95) compared to the control condition without a contour (M=0.21°). In547
contrast, intheinvisibleconditionthedifferenceinthresholdsfor illusorycontours548
(M=0.20°)wasnotsignificantlydifferentfromthatforthecontrolstimulus(M=0.21°,549
t(4)=-0.16,p=0.882,BF10=0.4).550
551
Thresholdsmeasuredfortherealluminancecontourwereofasimilarmagnitudeas552
those measured for the visible illusory contour (M=0.10°, t(4)=-0.92, p=0.409,553
BF10=0.55).Incontrast,thresholdsfortherealluminancecontourweresignificantly554
lower than for invisible Kanizsa stimuli (t(4)=3.84,p=0.019,BF10=4.53). Thresholds555
forthereal luminancecontourwerealsosignificantly lowerthanforeithercontrol556
stimulus(visible:t(4)=4.4,p=0.012,BF10=6.19;invisible:t(4)=5.3,p=0.006,BF10=9.9).557
558
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21
559Figure 5. Results of experiment 2. Spatial discrimination thresholds per condition (real560
luminancevsillusoryvscontrol)onadotlocalizationtaskwhileparticipantswerepresented561
with visible (blue squares), invisible (orange diamonds), or real luminance stimuli (black562
circles). A-E) Plots for the five individual participants. F) Thresholds averaged across563
participants±1standarderrorofthemean.564
565
566
Discussion567
568
In two psychophysical experiments we tested whether the visual system forms569
illusory (Kanizsa) contours in the absence of awareness of the inducing context.570
Takentogether,withpreviousexperimentsusingdichopticstimulation[22,23]allthe571
findings suggest that illusory contours are not perceived when the inducers are572
masked.573
574
Inourfirstexperiment,wedirectlymeasuredillusorycontourperceptionbyasking575
participantstodiscriminatetheorientationofaKanizsatriangle.Thisprocedurewas576
akin to our earlier experiments [22] that showed no evidence either that illusory577
contours are formed when inducers are suppressed from awareness. In those578
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22
experimentsweemployedCFSinwhichadynamic,high-contrastmaskpresentedto579
one eye suppresses awareness of a stimulus viewed by the fellow eye. Similarly,580
previous studies showed that illusory contours are not formedwhen the inducers581
are suppressed from awareness during binocular rivalry [23]. However, because582
previousreports indicatethat illusorycontoursareprocessedbybinocularneurons583
[30–33]theuseofdichopticstimulimaysimplydisrupttheirprocessing.584
585
Thereforehereweusednormalbinocularviewingconditionsandadifferentmethod586
to render the inducers invisible. Inaddition,we includeda real luminancecontour587
conditionasabaselinecheck.Thishelpedtoruleoutanothertrivialexplanationfor588
ourearlierfindings:Thepresenceofthebrightmaskscouldpossiblyhaveobscured589
thedetectionofthesubtleillusoryboundary.Ourrealluminancecontourwasnever590
masked because the masks only overlapped with the corners of the triangle;591
therefore,ifparticipantswereabletodiscriminatethesubtlerealluminancecontour592
theyshouldalsobeabletodosoifanillusorycontourwereindeedformed.Weonly593
included participants for whom discrimination of this real luminance contour was594
significantly above chance levels. Nonetheless, participants were unable to595
discriminatetheorientationoftheillusorycontourwheninducersweremasked.The596
phenomenologicalexperienceof thereal luminanceandthe illusorycontour isnot597
perfectly identical (and itobviously isnot for thevisible conditions).However, the598
real luminance conditions were very faint while illusory contours tend to be599
subjectivelyquitesalient.Thusitseemsunlikelythatparticipantsshouldbeableto600
detectonlytherealluminancecontourbutnottheillusorycontour.601
602
Experiment1alsocontainedcontrolconditionsinwhichtheindividualinducershad603
been rotated by 180° and thus no illusory triangle should be formed. Thesewere604
essentiallycatchtrialsbecauseparticipantsshouldhavebeenguessing,astherewas605
noactualtriangletodiscriminate.Thiswasclearlythecaseforcontrolstimuliwhen606
inducers were invisible. However, for visible control stimuli discrimination607
performancevariedwidelyfrom0-0.74proportioncorrectresponsesandwaseven608
significantly below chance level. The “correct” responses in this condition were609
dummy coded so that the stimulus-response mapping matched that for the610
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23
equivalent illusory triangle stimuli before rotating the inducers. Therefore one611
inducerinthetoprowhadaright-angledcutoutthatpointedinwards(Figure1)and612
this would be consistentwith the presence of a triangle oriented in the opposite613
direction (even though no actual triangle should be perceived). Some participants614
may have adopted this stimulus responsemappingwhile others did not. Itwould615
thereforehavebeenbetter tohaveall inducerspointoutwards in thiscontroland616
randomize their locations.However, our actual results serendipitously support the617
fact that masking was effective in this experiment: The fact that participants618
performedatchancelevelwhenthecontrolstimuliweremaskedrulesoutthatthey619
hadanyresidualawarenessoftheinducers.TheresultsofourexplicitVisibilitytask620
afterthemainexperimentalsocorroboratedthisconclusion.621
622
Theresultsofexperiment1contradictpreviousclaimsthata“ghostlytriangle”could623
beperceivedwhen the inducersofaKanizsa triangleweremasked [24].However,624
follow-upexperiments failed to replicate theseexperimentsbut instead suggested625
that conscious processing of the inducers precedes awareness of the illusory626
contoursandthattheseearlierresultswereinfactduetoresidualawarenessofthe627
inducers[45].Anothercriticalissuewiththeseexperimentsisthattheperceptionof628
illusorycontoursisbasedonaYesorNojudgmentofwhetheratriangleshapewas629
present. Such a task could theoretically be performed based on any residual630
awarenessof thecorners in the inducers.Our task requiredanexplicitorientation631
discriminationofthe illusorycontour. Ifparticipantshadbeenabletoperformthis632
task in spiteofbeingunable toperceive the inducers, thiswouldhavebeenmore633
conclusive evidence that illusory contours are indeed formed when inducers are634
masked.635
636
Naturally,ourdesignthatsplitthemaintaskfromatestofawarenessdidnotallow637
ustomeasuretheawarenessoneachactual trialof themainexperiment–onlya638
dual-taskdesign inwhichvisibility isprobeddirectly inthemainexperimentwould639
permitthis.However,adual-taskdesigncannotclassifytheawarenessofindividual640
trials perfectly as the participant’s judgment of their own awareness is subject to641
variability. Dual-tasks also entail a division of attentional resources across the642
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24
different task components. It could be argued that the Visibility task was more643
difficult than theKanizsa taskbecause the former required the judgmentofonea644
small,peripheralfeatureofthestimulus.Thismaycomplicatetheinterpretationof645
chanceperformanceonthistask.646
647
Experiment1alsousedverybriefstimuluspresentationsandpowerful,high-contrast648
maskstorenderstimuliinvisible.Ratherthantheabsenceofawareness,thereason649
why invisible contours were not formed could be the brevity of the stimuli or650
because themask fundamentally disrupted stimulus processing. This is certainly a651
possibility because previous research indicated that the processing of illusory652
contoursoccursrelativelyslowly[36–38].Therefore, inoursecondexperiment,we653
used counter-phase flicker tomask the inducers instead of themaskingmethods654
employed in the earlier experiments. This allowed us to present stimuli for655
prolonged periods.While this is also a temporalmasking procedure, the stimulus656
energyisconstantduringtheentirepresentationbecauseonlythepolaritychanges657
betweenframes.Furthermore,weexploitedthefactthatthepresenceofanillusory658
contourenhancesperformanceonadotlocalizationtaskbecauseitprovidesavisual659
aid fordetermining itsspatial location [23,39].Weconfirmedthisadvantagewhen660
inducerswerenotmaskedandperformancewascomparabletowhenwepresented661
realluminancecontoursonly.However,wheninducerswererenderedinvisiblethis662
advantageforillusorycontoursdisappeared.Thistaskmayactuallybeanevenmore663
appropriatetestofthe inductionof illusorycontoursthantestingdiscriminationof664
the contour itself as in experiment 1. For that experiment, the interpretation is665
unproblematicbecausewefoundnoevidenceofdiscriminationwheninducerswere666
masked.However,ifwehadfoundabovechanceperformanceformaskedstimuli,it667
wouldhavebeenimpossibletoconcludethatthiswasnotduetodiscriminationof668
theinducers.Pilotexperimentsforexperiment2suggestedthatwiththelong-lasting669
counter-phaseflickermasking,participantsmightinoccasionaltrialshavehadsome670
residual awareness of the corners in the inducers. Thus they might still have671
performed above chance at a shape/orientation discrimination on the purported672
illusorycontoureventhoughtheydidnotinfactperceiveanyillusorycontour.This673
problemalsoplaguesmanypreviousexperimentsthatdirectlytestedthepresence674
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25
ofanillusoryshape[36,37].Therefore,onlyataskthatexploitsthepresenceofthe675
illusory contour to modulate performance on an orthogonal task, like the dot676
localization taskweused, canprovideconclusiveevidence thatan illusorycontour677
wasinfactperceived.Analternativepossibilitycouldbeataskthatreliesonafine678
discriminationofafeatureoftheillusorycontoursuchasitscurvature[38]buteven679
such discriminations may be confounded (with?) discrimination of the inducers680
[46,47].681
682
InalltestsofperformanceagainstchancelevelsweusedBayesianhypothesistests683
that can quantify the strength of evidence for the null hypothesis indicating that684
participantswereguessing[44].Noneofthesetestsrevealedstrongevidenceforthe685
null hypothesis as typical Bayes Factors fell between 0.3-0.5. To establish more686
compellingsupportforthenullhypothesisinthosecasesmuchlargersampleswould687
be required. Crucially, the Bayes Factor indicates by how much the observed688
evidenceshouldupdateone’spriorbeliefinthenulloralternativehypothesis.Even689
if theevidence is relativelyweak,anyBayesFactorbelow1 isevidence in favorof690
thenullandnotforthealternativehypothesis.Unlessonestartswithapriorbelief691
thatpeopleareclearlyabletodiscriminatemaskedstimuli,eventhesemodestBayes692
Factorssuggestthatparticipantswereprobablyguessing.693
694
Several previous studies used stimulusmanipulations that seek to disentangle the695
factors associated with Kanizsa-type illusory contours [39,48,49]. Rounding the696
cornersofthePacmaninducersresultsinanotablereductionintheillusorycontour697
percept and abolishes the concordant improvement on a spatial localization task.698
However, stimuli like this nonetheless activated higher extrastriate cortex to a699
similar degree as Kanizsa stimuli [39]. This is consistentwith the theory that later700
stages of visual processing, presumablymediated by higher visual areas, segment701
surfacesandassignboundariestoobjects.Thesesegmentationsarethenfedbackto702
earlyvisualcortextogeneratesignalsthatareinterpretedasillusorycontours[48].703
Practical support for this idea comes from transcranial magnetic stimulation704
experiments that disrupted neural processing either in object-sensitive lateral705
occipital(LO)cortexorinearlyvisualareasV1andV2[50].Critically,thedisruption706
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26
of LOcortexonlyabolished the illusory contourperceptearlyafter stimulusonset707
while disruption of early visual areas only did so at a later stage – presumably708
affectingfeed-backsignalsratherthantheearlyfeed-forwardresponse.709
710
Additional stimulusprocessing that is unrelated to the actual formationof illusory711
contourscouldalsoexplainpreviousreportsthatKanizsastimuliarefastertobreak712
through CFS masking than control stimuli [35]. The collinearity of edges and the713
thereby inferred surfacemay be processed evenwhile the stimulus is suppressed714
from awareness – this may in turn produce an attentional signal that causes the715
stimulus to break suppression. Recent experiments that tested a range of visual716
controlstimuliunderCFSsuggestthatlow-levelpropertiesofthestimulusdetermine717
thetimeittakestobreaksuppression[51].Suchstimulus-dependenteffectsarealso718
plausible because attentional processing can occur without awareness of the719
stimulus [52–54]. However, this does not prove that any percept of an illusory720
contourwasactuallyformed.Thisprocessmayalsoexplainwhycrowdinginterferes721
with discrimination of the inducer orientation but not with illusory contour722
formation[55].723
724
Whilewe did notmanipulate the presence of illusory contours in thisway in our725
experiments, we nonetheless controlled this factor by including real luminance726
contour conditions. In experiment 2 the presence of a contour should afford an727
improvement on an orthogonal spatial localization task. Such an improvement in728
localizationthresholdsonlyoccurredfortherealluminancecontourorwhenKanizsa729
inducerswerevisible.Improvementslikethisarenotobservedforstimulithatmatch730
theglobalcharacteristicsbutwhichdonotproduceillusorycontours[39].Therefore731
our results from this experiment strongly support the conclusion that illusory732
contoursweresimplynotformedwheninducersweremasked.733
734
Ourresultsalsoagreewithpreviousfindingsthatonlylocalprocessingoccursinthe735
absenceof awarenessbut thatmore complexanalysisof scenegeometry requires736
conscious processing [21]. This would also explain why the nature of stimulus737
representationsinhighervisualareasdiffersdependingonawareness.738
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27
739
However, our findings do not accordwith a number of other studies that suggest740
that illusory contours are processed unconsciously. Experiments on a patientwith741
extinctionduetoaparietallesionsuggestthatperceptionofaKanizsashapeoccurs742
evenwhensomeinducersareplacedintopartsofthevisualfieldwherethepatient’s743
conscious perception is impaired [26–29]. These findings indicate that visual744
processingoperatesatthesurface-orobject-basedlevel.Segmentingandgrouping745
the local features of an image into a coherent, global shape may only require746
awareness of some component features but it then spreads to the whole object.747
However,thisisnotsufficientevidencetodemonstratethattheseprocesseswould748
occurwhentheparticipantisunawareofallcomponents.Moreimportantly,italso749
does not demonstrate that the percept of illusory contours was actually formed750
under these conditions but only that some processing of the features producing751
illusorycontoursundernormalviewingconditionsstilloccurred.752
753
Another study used functional magnetic resonance imaging (fMRI) and an754
inattentional blindness paradigm to study illusory contours processing [49].755
Participantswerepresentedwith a sequenceof images, someofwhich contained756
Kanizsashapeswhilstotherswerevarious typesofcontrol stimuli.Simultaneously,757
participantswereengagedinademandingattentionaltaskatfixation.Asub-group758
of participants subsequently reportednot to have seen any Kanizsa stimuli. These759
participants nonetheless showed stronger fMRI responses to Kanizsa than control760
stimuli.Multivariateclassificationmethodsfurtherdemonstratedthattheactivation761
patterns produced by unseen Kanizsa stimuli were more reliable than those762
produced by control stimuli. The authors suggested that the neural signature of763
illusory contours differs from that of other, carefully matched stimuli and thus764
arguedthatillusorycontoursareprocessedevenwithoutawarenessofthestimuli.765
766
This finding is interestingbecause it tests the consequencesof awarenesswithout767
any experimentalmanipulation of the stimuli. The distinction of what is or is not768
processed without awareness only depends on the contents of the participant’s769
consciousness.However,thisalsomakesitdifficulttointerprettheseresults.First,it770
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28
isunclearwhetherparticipantsapparentlyoblivious to thepresenceof the stimuli771
really did not perceive any illusory shapes. Due to the design of the experiment,772
awarenesscouldonlybeassessedafterthemainfMRIexperimentratherthanona773
trial-by-trialbasis(butseeaboveourdiscussionwhysuchtrial-by-trialjudgmentsof774
awarenessarecomplicated).Onlyparticipantswhoreportedhavingseentheactual775
Kanizsastimulusduringtheexperimentwereclassifiedashavinghadawarenessof776
the stimuli. However, many of the candidate stimuli were similar Kanizsa shapes.777
Thereforeitispossiblethatparticipantshadsomeawarenessofthestimuli,evenif778
animpreciseone.779
780
Second,while thecontrol stimuli in themain fMRIexperimentwerevery carefully781
matchedtoruleouttheinfluenceofglobalcharacteristicsthiscanbydefinitiononly782
beanapproximation: if conditionswereperfectlymatched, the stimuluswouldbe783
identical and thus an illusory contour would be perceived. It is possible that the784
conditions resulting in an illusory contourpercept are alsoparticularly effective in785
producing discriminable activation patterns in visual cortex. For instance, the786
contrast energy along the mouths of the Pacmen inducers differed considerably787
between theKanizsaand theglobal control stimuli. Thus, surface segmentationor788
collinearinteractionsmayhavealsodifferedbetweentheseconditions.789
790
Alltheseissuesagainhighlightanimportantpoint:Theonlywayonecantrulyinfer791
that illusory contours are formed is by using a measure that is specific to the792
presence of an illusory contour. This is indeed what we did in our previous CFS793
experiment [22], our present experiment 1, or what was done in several other794
previous experiments [23,34,50,55], by asking participants to directly report a795
featureof the contour.Anotherusefulmanipulation is the spatial localization task796
used by other studies [23,39] as well as our experiment 2. This is because the797
performanceenhancementonlyoccursinthepresenceofanactualcontourhelping798
theparticipanttolocalizethetarget.Manypreviousstudiessuggestingthatillusory799
contoursareprocessedwithoutawarenessoftheinducersareconfrontedwiththis800
problem. In the appendix we included another experiment in which we tested801
whether Kanizsa shapes masked from awareness could nevertheless provide an802
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29
attentionalcue forasubsequentvisualsearchtask.Even ifa robustprimingeffect803
were found in such an experiment, this design simply cannot rule out alternative804
explanations. Itonly tests theconsequencesof the stimuli thatmay in factnotbe805
specifictoillusorycontours.806
807
In the same vein, neurophysiological and neuroimaging experiments have shown808
that illusory contours are encoded by neurons even in the early visual cortex809
[25,33,56–67].Theoverwhelmingmajorityoftheseexperimentswereconductedon810
awake participants. However, some neurophysiological studies reported neuronal811
tuning to illusory contours even in anesthetized animals [25,58,67]. Such findings812
seem to superficially contradict our conclusion that awareness of the inducing813
stimuli is necessary for the formation of illusory contours. However, these814
experiments are in fact a perfect illustration of the importance to distinguish815
between perceptual experience and correlated processing. Because animals are816
anesthetizedintheseexperimentsandthusbydefinitionunawareofthestimuli,itis817
impossible to determine whether illusory contours were formed. The neural818
correlates of these stimuli could be related to the contextual processing of the819
inducingstimuli,suchasthediscontinuitiesdetectedby“end-stoppedcells”orthe820
detection of collinearity in the image. Such processing may indeed occur in the821
absenceofawarenessasissupportedbyourfindingofcollinearpriming[21]andthe822
inductionofcontextualvisualillusionsundermaskingconditions[2,3]orinattention823
[6]. Furthermore, it is quite likely that such stimulus processing is a necessary824
prerequisite for the formation of illusory contours. However, they do not825
conclusivelyprovethatillusorycontoursareformedunderanesthesia.826
827
Arelatedissueisthatallofthesestudiesuseillusorycontoursinducedbyabutting828
lines (offset gratings) rather than Kanizsa shapes. While the two share a similar829
phenomenology, contours inducedbyabutting linesarearguably simpler andmay830
bebasedmostlyonlocalprocessingwhileKanizsashapesarelikelytoinvolvemore831
complexinferencesofsurfacedepthandboundaryownership.Ourexperimentsdid832
notexplicitly test illusory contoursgeneratedwithabutting linesand thereforedo833
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30
not speak to the question whether such simpler illusory contours are in fact834
perceivedwhentheinducersaremasked.835
836
We conclude that there is little evidence that Kanizsa-type illusory contours are837
processedwhenparticipantsarenotawareoftheinducingcontext.Thisappearsto838
be the case for a range of different methods to render the inducers invisible.839
However,alloftheseexperimentsemployedphysicalstimulusmanipulations,while840
it is likelythatanexperimentaldesignthatallowstheuseoftheparticipant’sown841
report to determine awareness on a trial-by-trial basis whilst also testing directly842
whether an illusory contour was in fact formed can answer this question843
conclusively.844
845
846
Acknowledgements847
848
This researchwas supportedby an ERC StartingGrant (310829) toDSS.We thank849
PieterMoors,JacobJolij,andDavidShanksaswellastwoanonymousreviewersfor850
commentsonapreviousmanuscriptandJonasLarssonforinspiringthisstudyinthe851
firstplace.852
853
854
855
DataandMaterials856
857
Allstimuluscodeanddataforthisstudyareavailablefordownloadat:858
https://dx.doi.org/10.6084/m9.figshare.2009952.859
860
861
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1064
1065
Appendix1066
1067
Primingexperiment1068
1069
Previousexperimentsused illusorycontourstocaptureattention inavisualsearch1070
task [68]or forpriming [69,70]. Inourprimingexperiment,we therefore tested if1071
Kanizsatrianglesrenderedinvisibleusingasimilarmaskingprocedurecouldbeused1072
as primes in a visual search task on a subsequent, visible stimulus array. Like1073
experiment 1, this experiment comprised two consecutive tasks. The first, again1074
called ‘Kanizsa’ task,wasan indirect test that investigatedwhetherprimingwitha1075
Kanizsa triangle in the location of the target improves detection of the target. A1076
secondtaskconsistedofa‘Visibility’taskthatdirectlytestedtheeffectivenessofthe1077
maskingtechnique.1078
1079
Studydesign1080
1081
Inthecontextofthisexperiment,primingreferstotheattentionalcueprovidedby1082
theprimeshapeaboutthelocationofthesubsequentlypresentedtarget inoneof1083
.CC-BY-NC 4.0 International licenseavailable under anot certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made
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37
fourpossiblelocationsthatinturnaffordsaboostinbehavioralperformanceonan1084
orthogonaldiscrimination task. In theprimedconditions the locationof the target1085
was always identical with the location of the prime, whereas in the unprimed1086
conditionstheprimeshapewasreplacedbyacontrolstimulus,whichlookedexactly1087
likethedistractors intheotherthree locations.Therefore, theunprimedcondition1088
provided a baseline that allowed the assessment of whether the triangle prime1089
boostedtheparticipants’performancebycapturingtheirattention.1090
1091
We tested priming with a 2x2x2 design with attentional priming (primed vs.1092
unprimed), the type of priming triangle (real vs. illusory), and prime visibility1093
(invisible vs. visible) as within subject factors. There were two different prime1094
shapes:arealtriangledefinedbyastrongluminancecontrastoranillusory(Kanizsa)1095
one.Thevisibilityofthecuewasmanipulatedbyvaryingpresentationtimesofthe1096
priming array that followed the mask. A short presentation time of 16.7ms was1097
selected for the invisible conditions to render the prime invisible. In the visible1098
conditions,however,theprimingarraywaspresentedfor300ms,whichleftenough1099
timefortheprimetoenterawareness.1100
1101
Everyparticipant completed twodifferent tasks. Each task comprised20blocksof1102
trials,whereoneblockconsistedof32trials.Acrossonetask,theunprimedvisible1103
and invisible conditions appeared for a total of 160 times each,whereas the four1104
primed conditions (real vs illusory and visible vs invisible)occurred80 timeseach.1105
Thus,halftheblockconsistedofunprimedtrialsandtheotherhalfofprimedones.1106
1107
Behavioral responseswere given on a forced-choice discrimination task by button1108
press (left or right arrow) on a standard computer keyboard. Each trial required1109
either a left or a right response (see experimental procedure) and each response1110
type appeared twice per block for each condition, except for the unprimed ones1111
whereitappearedfourtimesperblock.Conditionswereselectedpseudo-randomly1112
foreverytrialbutwerecounterbalancedwithineachblock.1113
1114
Participants1115
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38
1116
Participants in this experimentwere recruitedamong theUCL studentpopulation.1117
Seventeen (13 female, age range: 21-32 years,mean age 24.7±2.5 years) normal,1118
healthyparticipantstookpartintheexperiment,includingthetwoauthors.Allother1119
participantswereunawareoftheexperimentalhypothesis.1120
1121
Stimuli1122
1123
Inhalfthetrials,participantswerecuedwithoneoffourpossiblecues.Wecreated1124
shapestimulibyplacing fourblackdiscs (inducerelements,diameter=1.96°) in the1125
configurationof a square (width=2.8°). Inorder toproduce the illusory shapeof a1126
rightisoscelestrianglesittingontopofthesquare,awedgewasremovedfromeach1127
inducer element (Pacman).Wedges started at the center of their respective discs1128
andextendedtotheiredges.Onewedgemeasured90°andwasalwayspositioned1129
eitherinthebottomleftorbottomrightinducer.Thetwowedgesthatformedthe1130
apexes of the triangle measured 45°. Finally, the wedge of the fourth inducer1131
element measured 90°. This Pacman did not form part of the triangle and was1132
positionedopposite the hypotenuse of the right isosceles triangle. It always faced1133
the outside of the disc configuration.When the inducers were oriented with the1134
gaps forming the three corners of a triangle, an illusory light greyKanizsa triangle1135
wasvisibleontopoftheblackdiscs(FigureS1A,iandii).1136
1137
Real triangleswere identical to thecorresponding illusoryshapesexcept that their1138
contour was defined by a luminance contrast with a black line connecting the1139
cornersofthetriangle(FigureS1A,iiiandiv).1140
1141
ThecontrolstimuliwerecreatedbyalteringtheorientationsofthethreePacmenby1142
a systematic rotation of 180º. This modification broke the link between these1143
inducers and thus no illusory triangle was perceived (Figure S1B). Control stimuli1144
couldalsobehorizontalmirrorimagesoftheillustratedexamples.1145
1146
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39
Stimuli in thesearcharraywereright isosceles triangleswithacontourdefinedby1147
luminance (Figure S1C, i and ii). Target stimuliwere triangleswith the right angle1148
pointingdown,eithertotheleftortotheright,while indistractorstherightangle1149
pointedup(FigureS1C, iiiandiv).Participantswereaskedtodetectthetargetand1150
respondviabuttonpresswhether its rightanglewaspointing to the leftor to the1151
right.1152
1153
Themask consisted of a square of black lines connecting four discs, one at each1154
corner. Itwaswideenoughtocoverall fourPacmen,aswellasthelocationofthe1155
trianglewithinthem(FigureS1D,i).Inthe‘Visibility’task,therewasnosearcharray.1156
Instead,thestimulusarrayfollowingtheprimewasasquarecontainingacrossthat1157
gavenoclueaboutthelocationandorientationofthetargettriangle(FigureS1D,ii).1158
1159
The luminance of the background was 156 cd/m2. The luminance of inducer1160
elements, thecontourof the real triangleand theblackof themaskwas2cd/m2.1161
Finally,thegreyofthemaskinthefirsttaskwas58.2cd/m2.1162
1163
Notethatalthoughthereweredifferencesbetweenstimuli,the individual inducers1164
remained the same and there was no change in the local image properties (i.e.1165
square configuration of the four Pacmen and their position among the other 31166
configurations).Thisenabledanassessmentofthedifferenteffectsofprimingwith1167
theKanizsa shape, as compared to the corresponding real triangleandungrouped1168
Pacmenordistractorsintheunprimedconditions.1169
1170
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40
1171Figure S1. Illustration of the stimuli in the priming experiment.A)The fourprimeswere1172
formedoffourinducerelements,ofwhichthreecreatedeitherarealtriangleortheillusory1173impression of a triangle. In half the trials, participants were cued with one of the four1174
possible cues: (i) Kanizsa triangle pointing right, (ii) Kanizsa triangle pointing left, (iii) Real1175
triangle pointing right, (iv)Real triangle pointing left. B) Examples of control stimuli used1176
duringcuepresentation.Controlstimuliwerecreatedbyasystematic180°rotationof the1177
individual Pacmen, so that all gaps within the black discs pointed to the outside of the1178
formation. Control stimuli could also be the horizontal mirror image of the presented1179
examples.C)Thetwopossibletargets inthesearchtaskwereright isoscelestriangleswith1180
theirrightanglepointingdown,eithertotheright(i)ortotheleft(ii).Distractorstimuli in1181
the search array alwayspointedup, but variedbetweenpointing left and right.D) (i) The1182
maskusedtorendertheprimeinvisible.(ii)Theuninformativepost-stimulusmaskreplacing1183
thetargetarrayinthe‘Visibility’task.1184
1185
1186
1187
1188
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41
Procedure1189
1190
Kanizsatask,indirecttestassessingpriming:1191
Atfirst,participantswereinstructedthattheywouldseeanarrayoffourtrianglesof1192
which only one, the target, had a right angle pointing down and were asked to1193
decidewhether this anglewas pointing to the left or to the right by pressing the1194
respectiveresponsekeys.1195
1196
The20experimentalblockswereprecededbyasmanypracticeblocksasneededin1197
order to fully familiarize the participants with the task requirements. Participants1198
could initiateanewblockbypressinganybuttonon thekeyboardand, ifneeded,1199
takesmallbreaksbetweenblocks.1200
1201
Participantswereinstructedtofixateasmallblackdot(0.2°wide)thatwaspresent1202
inthecenterofthescreenthroughouttheexperiment.Oneachtrial,thefixationdot1203
wasdisplayedalonefor500ms.Subsequently,amaskarraywasdisplayedfor100ms1204
andwasimmediatelyfollowedbyoneoftheprimingarrays(seeconditionsinstudy1205
design),whichwaspresentedforeither16.7msor300ms.Thisarraycouldcontain1206
oneof the twopossibleprimeshapes (realor illusory triangle)ora simplecontrol1207
stimulusamongtheotherthreecontrolstimuli.Immediatelyafterthis,atargetarray1208
containingone targetamong threedistractorswasdisplayedand remainedon the1209
screenuntiltheparticipant’sresponse.1210
1211
The responsewas self-paced;however,participantswere instructed to respondas1212
quicklyaspossible.Responsesweremadewiththeindex(forleft)andthemiddleor1213
ring finger (for right), depending on how participants felt comfortable. For every1214
response, the fixationdotprovided feedback (100ms)bychanging itscolor (green:1215
correct; red: incorrect). Then, the next trial startedwithout any delay. Figure S2A1216
showsthegeneralparadigmfortheexperimentalprocedure.1217
1218
Visibilitytask,directtestassessingprimediscrimination:1219
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42
Thesecondtaskassessedtheeffectivenessofthemaskbymeasuringtheconscious1220
discriminabilityoftheprimesinadirectmanner.Forthispurpose,participantswere1221
askedtomakethesamedecisionasintheprevioustask,butthistimewithrespect1222
to theprime.Thus, insteadofdiscriminatingwhether the rightangleof the target1223
waspointingtotheleftortotheright,participantsdiscriminatedthisaspectofthe1224
primeshape.1225
1226
Procedure,stimuliandtimingofthetrialsequenceinthistaskwerekeptidenticalto1227
thepreviousindirecttest,withasingleexception:Insteadofatargetarray,anarray1228
comprising new identical mask shapes covered the four stimuli locations (Figure1229
S2B).1230
1231
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43
1232FigureS2. Illustrationofanexampletrialsequenceintheprimingexperiment.A)Kanizsa1233task:Eachtrialwascomposedoffourframes:fixationperiod,mask,cueingarrayandtarget1234
array. After the participant’s response, the fixation point provided feedback for 100ms1235
(green:correct; red: incorrect).Thedurationofeach frame isshownonthetime-line.The1236
duration of the cueing array was varied between conditions: 16.7ms in the invisible1237
conditionsand300msinthevisibleones.Notethatintheunprimedconditions,thecueing1238
arraydidnotcontainanyprimeshapeandwascomposedexclusivelyofcontrolstimuli.B)1239
Visibility task: Trial procedurewas identical to the one in the Kanizsa task except for the1240
fourth frame in which the search array was replaced by an array of four identical mask1241
shapes.1242
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44
1243
1244
Dataanalysis1245
1246
Response times (inmilliseconds)andperformanceaccuracy (inproportioncorrect)1247
weremeasuredforeachofthesixconditions.Toanalyzeprimingeffects,response1248
timeswerecalculatedoncorrecttrialsonly.1249
1250
FortheKanizsatask,accuracyofresponseswaslessinformative,asthetargetarray1251
was present until the participant’s response. We reasoned that priming in the1252
indirect Kanizsa taskwouldbe reflectedby fastermean response times toprimed1253
than unprimed conditions. Therefore the priming effect was calculated by1254
subtracting response times under priming from thosewithout priming for correct1255
responses.1256
1257
Primingeffectsfromallparticipantsforallconditionswereenteredintoatwo-way1258
repeated-measures analysis of variance (ANOVA) with prime visibility (visible vs1259
invisible) and prime stimulus (real vs illusory triangle) aswithin subject factors. In1260
addition to this, further comparisons between experimental conditions (visible or1261
invisible/real or illusory) and control conditions (unprimed) were assessed with1262
paired t-tests. We also determined the significance of the priming effect by1263
comparingitagainstzerowithone-samplet-tests.1264
1265
In the visibility task, the variableof interestwas theaccuracyof responses as this1266
allowed us to test whether participants could discriminate the orientation of the1267
primes. One-sample t-testswerecarriedoutat thegroup level foreachcondition1268
individually in order to assesswhether the participants’ level of performancewas1269
significantly above chance (>0.5) and paired-tests were used to compare1270
performanceindifferentexperimentalconditions.Asinexperiment1,forallt-tests1271
wealsocalculatedthedefaultBayesFactorinadditiontofrequentiststatistics.1272
1273
1274
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45
Results1275
1276
IntheprimingexperimentweaskedwhethermaskedKanizsashapescouldaffordan1277
attentionalprimingeffect.WetestedwhetheraKanizsastimuluspresentedaspart1278
ofanarrayof four stimuli couldprimeparticipants to the locationofa target ina1279
visual search array presented subsequently. The prime could either be a Kanizsa1280
triangleoratriangledefinedbyarealluminancecontour.Intheunprimedcondition1281
allfourstimuliintheprimingarraywerecontrolstimuliwithoutatriangleshape.1282
1283
KanizsaTask1284
1285
Overallparticipantsmadeveryfewerrorsindeterminingwhetherthetargetpointed1286
to the left or to the right. Indeed, themean accuracy in all conditionswas above1287
0.94.Therefore,foreachexperimentalconditionwequantifiedtheeffectofpriming1288
by the difference in response time with priming subtracted from that without1289
primingoncorrectlyansweredtrials(FigureS3A).1290
1291
1292FigureS3.Resultsofexperiment2.Primingeffect(responsetimeforunprimedtrialsminus1293primedtrials)ontheKanizsatask(A)andaccuracyfordiscriminatingtheinducerlocationin1294
theVisibility task (B) for visible or invisible (masked) trials. Eachdot represents themean1295
performanceof an individual participant in eachof the conditions. The large symbols and1296
error bars denote the mean ±1 standard error for each condition. Black: real luminance1297
contour.Red: illusory (Kanizsa)contour. InB)Grey:unprimed trialsdummycoded for real1298
contourprime.Orange:unprimedtrialsdummycodedforillusorycontourprime.1299
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46
1300
We conducted a two-way repeated-measures analysis of variance, with visibility1301
(visiblevsinvisible)andstimulus(illusoryvsrealtriangle)aswithinsubjectfactors,to1302
compare the priming effects (response time on unprimed minus primed trials)1303
between conditions. There was a significant main effect of visibility of the prime1304
(F(1,16)=60.06, p<0.001), and a significant main effect of stimulus type1305
(F(1,16)=13.82, p=0.002). The interaction between visibility and the stimulus type1306
wasnotsignificant(F(1,16=3.52,p=0.079).1307
1308
There was a clear difference in the strength of priming between the visible and1309
invisible condition. However, the critical test is whether the priming effect was1310
significant, that is, if itdiffered fromzero (i.e.apaired t-testbetweenprimedand1311
unprimed response times). Unsurprisingly, the very pronounced priming effect for1312
visibletrialswasextremelysignificantforbothconditions(real:M=336ms,t(16)=8.2,1313
p<0.001,BF10>4.1*104; illusory:M=275ms,t(16)=6.9,p<0.001,BF10>5.8*103).There1314
was also amuchmoremodest but nonetheless very significant priming effect for1315
both stimulus types in the invisible condition when inducers were masked (real:1316
M=43ms, t(16)=3, p=0.01, BF10=5.6; illusory: M=36ms, t(16)=4.4, p<0.001,1317
BF10=80.8).1318
1319
VisibilityTask1320
1321
As in experiment 1 we used a direct Visibility test to assess whether participants1322
mighthavehadresidualawarenessof theprimingstimuliundermasking.Thetask1323
procedure was the same as in the Kanizsa task except that the search array was1324
replaced with a foil in which all four stimuli were identical and non-informative1325
about the correct answer. We quantified the accuracy (proportion correct) with1326
whichparticipantscoulddiscriminatetheorientationoftheprimingtriangle(Figure1327
S3B).1328
1329
Unsurprisingly, in the visible condition performance for discriminating the1330
orientation of a real (M=0.88, t(16)=14.9, p<0.001, BF10=9.5*107) and an illusory1331
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47
triangle (M=0.87, t(16)=14,p<0.001,BF10=4.0*107)was significantly above chance.1332
Conversely,performance for theunprimedconditions, that is, stimulusarrays that1333
contained no triangle shape, performance was consistently at chance (“real”:1334
M=0.52, t(16)=1.3, p=0.21, BF10=0.52; “illusory”: M=0.53, t(16)=1.3, p=0.219,1335
BF10=0.5;butnotethatthesetwoconditionsweredummycodedinthiscaseasthey1336
containedthesamestimuli).1337
1338
Critically, performance in the invisible condition showed that discrimination in the1339
primedwith an illusory triangle conditionwas also significantly above chance (M=1340
0.55,t(16)=3.24,p=0.005,BF10=9.4),anddiscriminationofa real triangleshoweda1341
similarresulteventhoughitdidnotreachsignificance(M=0.53,t(16)=1.9,p=0.075,1342
BF10=1.1).Performanceforthetwoinvisibletrianglesalsodidnotdiffersignificantly1343
(t(16)=-0.9,p=0.385,BF10=0.35).Again,asexpectedbothoftheunprimedconditions1344
were at chance level (“real”: M=0.51, t(16)=0.6, p=0.534, BF10=0.3; “illusory”:1345
M=0.51,t(16)=0.7,p=0.494,BF10=0.31).1346
1347
1348
Discussion1349
1350
Taken together, the results of the priming experiment contradict those of1351
experiment 1 and 2 , as they suggest that Kanizsa triangles rendered invisible by1352
masking could afford an attentional priming effect in a subsequent visual search1353
task.However,acontrolexperimenttestingparticipants’abilitytodiscriminatethe1354
primeshapeorientationdirectlysuggestedthatthissmallprimingeffectforinvisible1355
trials could have been due to some residual awareness of the masked triangle1356
stimuli.Whenconductingsuchtestsofawareness,itisofparamountimportanceto1357
ensure that trials with masked and unmasked conditions are interspersed. When1358
onlymaskedconditionsaretestedforawareness,itispossiblethattheparticipant’s1359
performanceisatchanceeventhoughthereisinfactresidualawarenessinthemain1360
taskduetowhathasbeenreferredtoas“primingofawareness” [71].Conversely,1361
the ability to correctly identify purportedly subliminal prime stimuli (as in our1362
primingexperiment)mayhavebeenenhancedbytheinclusionoftrialswithclearly1363
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48
visibleprimes[72].Eitherway,becausewecontrolledforthispossibilityourvisibility1364
test should have provided a robust test of awareness. Therefore, the most1365
parsimonious interpretation of the priming results is that participants had some1366
residual awareness of the primes and that Kanizsa contours were probably not1367
processed when inducers were masked. Using a different masking technique, for1368
examplethefastcounter-phaseflickerusedinexperiment2couldbemoreeffective1369
and provide a better test of whether unconscious Kanizsa stimuli can act as1370
attentionalcues.1371
1372
Critically,however,becauseprimingexperimentslikethisareonlyanindirecttestof1373
an effect that is not specific to illusory contour formation, this would not be1374
informative. Even if we interpret the somewhat inconclusive results of this1375
experiment as showing thatmasked triangles produced priming effects, this does1376
notprovidedirectevidencethatthisadvantageisactuallycausedbythepresenceof1377
illusorycontours.AKanizsatrianglecanclearlyprovideasalientattentionalcuebut1378
this may be due to shared features, such as the collinearity of the edges in the1379
inducersortheimplicationofasurface.Thisisalsoconsistentwithpreviousreports1380
that priming of a shape discrimination task by subliminal stimuli depends on the1381
strength of the salient region [73] and that salient regions of such stimuli are1382
detected efficiently regardless of whether they are bounded by illusory contours1383
[39,74].Findingaprimingeffectthusonlysuggeststhatattentionalcuingcanoccur1384
without awareness but it does not rule out that other factors produced this1385
attentionalcapture.Conversely,theabsenceofaprimingeffectwouldonlyconfirm1386
thatwhatever acts as attentional cue is disrupted. Therefore, theonlymeaningful1387
testthatillusorycontoursareindeedformedisviaameasurethatisspecifictothis1388
percept.1389
1390
1391
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