induction of kanizsa contours requires awareness · 4/27/2016 · 88 a kanizsa triangle, when the...

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InductionofKanizsacontoursrequiresawareness1

TheodoraBanica1&DietrichSamuelSchwarzkopf1,2*231.ExperimentalPsychology,UniversityCollegeLondon,26BedfordWay,London,UK42.InstituteofCognitiveNeuroscience,UniversityCollegeLondon,17QueenSquare,5London,UK67*Towhomallcorrespondenceshouldbedirected:s.schwarzkopf@ucl.ac.uk89Abstract1011Itremainsunknowntowhatextentthehumanvisualsysteminterpretsinformation12about complex scenes without conscious analysis. Here we used visual masking13techniquestoassesswhetherillusorycontours(Kanizsashapes)areperceivedwhen14theinducingcontextcreatingthisillusiondoesnotreachawareness.Inexperiment115wetestedperceptiondirectlybyhavingparticipantsdiscriminatetheorientationof16an illusory contour. In experiment 2, we probed perception indirectly by using17Kanizsa shapes toprimeperformanceonavisual search task. Inexperiment3,we18exploitedthefactthatthepresenceofanillusorycontourenhancesperformanceon19a spatial localization task. Moreover, in experiment 3 we also used a different20maskingmethodtoruleouttheeffectofstimulusduration.Ourresultssuggestthat21participants do not perceive illusory contours when they are unaware of the22inducing context.While perceptual grouping and surface segmentationmay occur23without awareness of the stimuli, the induction of the phenomenal percept of an24illusory shape does not. This is consistentwith theories of amultistage, recurrent25process of perceptual integration. Our findings thus challenge some reports,26including those fromneurophysiologicalexperiments inanaesthetizedanimals.We27discusstheimportancetotestthepresenceofthephenomenalperceptdirectlywith28appropriatemethods.2930Keywords3132Illusorycontours,modalcompletion,awareness,masking333435 36

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Introduction3738What role does conscious processing of the environment fulfill and how much39processingoccurs in theabsenceofawareness? It is self-evident thatmuchof the40internal bodily functions and controlling learnedmotor behaviors, like walking or41driving, operate mostly without awareness. But for processing by the classical42senses,likevision,therehavebeenwidelydiscrepantfindingsonhowmuchstimulus43processingcanoccurandhowmuch itaffectsdecision-makingwhenthesubject is44unawareofthestimulus.Theapproachwithwhichunconsciousstimulusprocessing45isstudiedhasalsobeensubjecttocontroversy[1].4647Several experiments suggest that the effect of a contextual stimulus on a target,48suchasinthetilt illusionoradaptationeffects,persistsevenwhenparticipantsare49unawareofthepresentedcontextual information[2–5]. Inrecentyears,theuseof50continuous flash suppression (CFS), in which a dynamic, high-contrast stimulus is51presentedtooneeyetosuppressthestimulusintheothereyefromawareness,has52become a popular way to probe unconscious stimulus processing [6]. Using this53procedure ithasbeenclaimed that theperceptionofphysical facial attributes [7],54the complex analysis of naturalistic scenes [8], and even linguistic processing and55arithmeticcanbeperformedwithoutawareness[9].5657Neuroimaging experiments showed that while both simple and more complex58stimuli haveaneural signature in the visual cortexundermasking conditions [10–5912], the encodingof unconscious stimuli appears tobequalitatively different.Not60only is theoverall response tounconsciousvisual stimuliweaker [11]butcoupling61betweendifferentstagesinthevisualprocessinghierarchyisalsoreduced[13]and62the information content differs [12,14,15]. In particular, the response to these63stimuli is more variable, and also localized in more posterior regions than to64consciousstimuli[14].6566These findings corroborate behavioral experiments suggesting that unconscious67stimulus representations aremore variable [16]. Further, it is consistent with our68proposalthatonlysimplepositionalorgeometric informationmaybeprocessed in69the absence of awareness, but that more complex abstraction and perceptual70integrationrequiresconsciousness[17].Specifically,weusedshapestimulithatwere71either defined by the position or the orientation of simple image elements. We72demonstrated that when such stimuli were rendered invisible using fast counter-73phaseflicker(at120Hz)theycouldspeedupperformanceonashapediscrimination74taskof conscious stimuli.Critically, thisprimingeffectwasonlypresent for stimuli75sharing the samepositions,or– if orientedelementswereused forpriming– the76positions along the path implied by the elements. We observed no priming by77



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invisible primes if the test stimulus was smaller than the prime stimulus. This78suggeststhatinvisibleprimingoperatedlocally,possiblyinretinotopicspaceinearly79visualcortex,butnoabstractintegrationofindividualelementsintotheconceptofa80shapeoccurredwithoutawareness.81

82We further tested whether two brightness illusions manifest when the inducing83context is rendered invisible by means of CFS [18]. We found that masking the84context (a smooth gradient in luminance) had little impact on simultaneous85brightness contrast of two targets that were unmasked. In a stark dissociation,86participantscouldnotdiscriminatetheorientationofanillusorycontour,definedby87a Kanizsa triangle, when the inducing context (the ‘Pacman’ shapes whose open88segments define the corners of the triangle)weremasked selectively by CFS. This89could indicate that thegenerationof the illusorycontourperceptoccursata later90stageofvisualprocessingthansimultaneousbrightnesscontrast,eitherintermsof91the visual hierarchy or in the latency of processing. These findings were also92consistentwithpreviousreports that illusorycontoursarenotperceivedwhenthe93inducersaresuppressedfromawarenessduringbinocularrivalry[19].9495These findings challenge some previous reports, using psychophysical tests in96healthy volunteers [20] and even neurophysiological experiments in anesthetized97animals[21]thatsuggestedthatillusorycontourscouldbeformedintheabsenceof98awareness.However,noneofthesepreviousstudiesspecificallytestedwhetherthe99phenomenological percept was indeed formed in these conditions. Similarly,100neuropsychological studies in neglect patients [22–25] suggested that illusory101contourprocessingoccurswhenpartof the inducingcontext isplaced in theblind102hemifield.However, thisdoesnotconclusively support theassertion thatcontours103are formed in the absence of any contextual awareness. The absence of illusory104contoursunderdichopticmasking [18,19] thereforehighlights thatexperimentson105Kanizsa stimuli must use appropriate methods and should be interpreted with106caution.107108However, two issues confound the interpretation of these findings. First, there is109evidence that illusory contours are processed by binocular neurons in early visual110cortex [26–29]. It does in fact seem unsurprising that the mechanisms inducing111illusory contours at least partially overlap with those for segmenting surfaces in112depth[30].Whenretinaldisparityimpliesthatinducersareatdifferentdepthsfrom113thebackground,thevisualsystemnotonlyproducestheperceptofillusorycontours114butthesurfaceboundedbyillusorycontoursisalsoperceivedinstereoscopicdepth115[27]. Thuswhen binocular processing is disrupted or overwhelmed by a dichoptic116maskorbinocularrivalry,theillusorycontourperceptisalsobroken.Anotherrecent117



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studyfurthercomplicatedthissituationbyfindingthatKanizsashapesbrokethrough118CFSfasterthancontrolstimuli[31].Leavingasideconceptualissueswiththetime-to-119emergenceparadigm, it is imperative that thedependenceof illusory contourson120awarenessmustbeconfirmedusingmaskingmethodsotherthanCFS.121122A second confoundwith theseprevious studies [18,19] is that even if participants123perceivedan illusorycontour, thisperceptwas far less salient than real luminance124contours, as it may have been obscured by the dominant masking stimulus. The125additionofasimplecontrolconditioncouldremedythisproblem:Oneexperimental126stimulusshouldbearealcontourdefinedbyasubtleluminancecontrastthatmimics127the illusory contour percept as closely as possible. If participants can detect and128discriminatethisstimulusbutareunabletodosofortheillusorycontourcondition,129thisindicatesthattheillusoryperceptisindeeddisruptedspecifically,ratherthanthe130moregeneralabilitytodetectsubtlestimuli.131132Herewecarriedoutthreeexperimentstoaddressthesequestions.Inexperiment1,133we used a similar design as in our previous study [18] by asking participants to134discriminate the orientation of a Kanizsa triangle. However, instead of CFS we135employeda temporalmaskingmethod to render the inducers invisible.Moreover,136we included a control condition in which a real, luminance-defined contour was137present.Previousexperimentsusedillusorycontourstocaptureattentioninavisual138searchtask[32]orforpriming[33,34].Inexperiment2,wethereforefurthertested139if Kanizsa triangles rendered invisible using a similar masking procedure could be140usedasprimesinavisualsearchtaskonasubsequent,visiblestimulusarray.Finally,141because both these masking methods rely on very brief stimulus durations, in142experiment3wepresentedlong(500ms)stimulirenderedinvisiblebymeansoffast143counter-phase flicker [3,17]. This is critical because the formation of illusory144contours arise comparably slowly [35–37] and thus may be disrupted by the fast145temporal masking techniques in our first two experiments. Previous research146demonstrated that the presenceof illusory contours boosts participants’ ability to147discriminatethepositionofatinytarget[19,38]provingamoreobjectivemeasure148of whether the participant in fact perceives an illusory contour. We therefore149measured the ability of a group of participants, who were well trained at150psychophysical tasks, to discriminate the position of a dot target for Kanizsa and151controlstimulipresentedwithorwithoutmasking.152153MaterialsandMethods154155All three experiments were carried out at the UCL Department of Experimental156Psychology.ProceduresadheredtotheDeclarationofHelsinki.Ethicalapproval for157



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thisstudywasobtainedfromtheUCLResearchEthicsCommitteeandallparticipants158gavewritten,informedconsent.159160Participants in experiments 1 and 2 were recruited among the UCL student161population. In experiment 3 we recruited participants who were familiar with162psychophysical tasks. All Participants had normal or corrected-to-normal visual163acuity.164165Allexperimentswereconductedinadark,sound-attenuatedroom.Fortheduration166of the experimental sessions, participantswere asked to stabilize their head on a167chinrestlocatedatafixeddistanceof48cmfromthestimuluspresentationscreen168wherestimuliwerepresentedtothembinocularly.169170Stimuli were generated by a computer and presented on a 22-inch Samsung171SM2233RZLCDmonitorataresolutionof1680x1050pixels.Screenrefreshratein172experiments 1 and 2 was set to 60Hz. In experiment 3 it was set to 120Hz. The173experimentwascontrolledandbehavioralresponseswererecordedusingMATLAB174(TheMathworks,Inc.)andPsychtoolbox3[39]usingastandardkeyboard.175176Experiment1177178The experiment comprised two tasks: the first, henceforth called ‘Kanizsa’ task,179investigatedwhetherparticipantscanperceiveillusorycontourswithoutawareness180oftheirinducers.Thesecondtask,the‘Visibility’task,assessedtheeffectivenessof181themaskingtechniquedirectly.182183Studydesign184185Like our earlier experiment using continuous flash suppression, this experiment186aimed to measure the perception of illusory contours in a direct manner. We187implementeda2x3designwithvisibility(invisible,visible)andtypeofstimulus(real,188illusory,control)aswithinsubjectfactors.189190Everyparticipant completed twodifferent tasks. Each task comprised25blocksof191trials,whereoneblockconsistedof24 trials in theKanizsa taskand8 trials in the192visibilitytask.Acrossonetask,eachconditionappeared100times.Thevisibilitytask193onlycomprisedthevisibleandinvisiblecontrolconditions.194195Participantsmade behavioral responses in a forced-choice design by button press196(leftor rightarrow)onastandardcomputerkeyboard.Eachtrial requiredeithera197leftora right responseandeachresponsetypeappearedtwiceperblock foreach198



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condition. Conditions were selected pseudo-randomly for every trial but were199counterbalancedovereachblock.200201Participants202203Seventeen (13 female; age range: 18-29, mean age: 23.8±2.5) normal, healthy204participantstookpartintheexperiment.Anadditionaltwoparticipantsweretested205buttheyfailedtodiscriminatetherealluminancecontourundermaskingconditions206and were therefore excluded from further analysis (see more details below). All207participantswereunawareoftheexperimentalhypothesis.208209Stimuli210211Stimuliwerecreatedbyplacingfourdiscs(inducerelements,diameter=2.2°) inthe212configurationofasquare(width=4.3°).Theconfigurationofthesediscswascentered213onfixationasbothstandardedgetypeandanumberofline-endinducingelements214havecomparableefficacyintheclaritywithwhichillusorycontoursareperceivedby215participants[40,41].Here,thediscsweredefinedbypartialconcentriccircles.Each216of the four lines forming the circles had a width of approximately 0.07° with a217luminance of 0.6 cd/m2. The positioning of the gap in the circles gave rise to the218percept of a Kanizsa triangle (Figure 1A). Thus, a number of line-end inducing219elementsgaverisetoperceptionofillusorycontours[42].220221Wealso includedareal luminancecondition.Here, thestimulididnotcontainany222discsbutinsteadtherewasatriangledefinedbyarealbutsubtleluminancecontrast223at theexact locationwhere theKanizsa trianglewouldbeperceived in the illusory224condition(Figure1B).Wereasonedthatifparticipantswereunabletodiscriminate225theorientationofthissubtleluminanceedgewhentheinducersweremasked,this226implied that theywould be unable to detect any illusory contour that could have227formedwithoutawarenessoftheinducers.Therefore,weremovedtwoparticipants228whosediscriminationperformanceforthisconditionwasatchancelevelsfromany229furtheranalyses.230231Control stimuli did not form any triangle and were created by altering the232orientations of the inducers by a systematic rotation of 180º (Figure 1C). These233stimuliwereusefulinthattheyprovidedinformationonwhetherparticipantscould234indeed perceive the triangle contours or whether they adopted a strategy of235matchingtheinducerorientationtofeedback.236237Thebackgroundhadaluminanceof76.6cd/m2andrealtrianglesweredefinedbya238subtly greater luminance of 86.9 cd/m2. The mask was only used in the invisible239



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conditions and was designed as a square configuration of four black discs (0.6240cd/m2), each containing four white concentric circles (230 cd/m2). It was wide241enoughtocoverall fourPacmen in the illusoryandcontrolconditions (Figure1D).242Themaskingtechniqueconsistedofasequenceofthreeframeswhichwasrepeated243threeconsecutivetimes:Atfirstthemaskappearedonthescreen,followedbythe244appearanceofthestimulusandafinalblankscreen.Inthevisibleconditions,ablank245greyscreenreplacedthemaskframe.246247

248Figure1. Illustrationofthestimuli.A)(i)Kanizsatrianglepointingright(ii)Kanizsatriangle249pointingleft.B)(i)Realtrianglepointingright(ii)Realtrianglepointingleft.C)Controlstimuli250



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werecreatedbyasystematic180°rotationof the individualPacmen.D) Illustrationof the251maskingprocedureintheinvisibleconditions:Amask-stimulus-blankscreenframesequence252wasrepeatedthreetimes.Inthevisibleconditionsablankscreenreplacedthemask.253254Procedure255256Kanizsa task:At first, participants were instructed that they would see a triangle257appearonthescreen.Theywereaskedtojudgewhetheritshypotenusewastilted258clockwiseorcounter-clockwisefromverticalbypressingthecorrespondingresponse259key.We explained this task to them as a decisionwhether the right angle of the260trianglewaspointingtotheleftortotherightbuttheyweretoldexplicitlytojudge261thecontoursofthetriangle,inparticularthelonghypotenuseextendingthroughthe262centerofthestimulusdisplay.Becausewewishedtokeepthecontrastbetweenreal263triangleandthebackgroundaslowaspossible,wetrainedparticipantsonthevisible264andinvisiblerealtriangleconditionuntiltheywereabletodetectthemcorrectly.265266Participantswereinstructedtofixateasmallblackdot(0.2°wide)thatwaspresent267inthecenterofthescreenthroughouttheexperiment.Oneachtrial,thefixationdot268was displayed alone for 500ms. Thiswas followed by a sequence of three frames269thatdefinedwhethertheconditionwasavisibleoraninvisibleone.Intheinvisible270condition a 300msmask followed the fixation period. Subsequently, the stimulus271appeared on the screen for one frame of approx. 16.7 milliseconds (ms),272immediatelyfollowedbyablankscreenthatwasshownforoneframeaswell.This273mask-stimulus-blanksequencewasrepeatedthreetimesbeforeasecondandfinal274post-stimulus blank screen was presented until participants gave their response.275Figure2Ashowsthegeneralparadigmfortheexperimentalprocedure.276277Visibility task:We further tested whether participants indeed did not consciously278perceiveanycontextualinformationofthestimuliintheinvisiblecondition.Forthis279purpose, the visibility taskassessed theeffectivenessof themasking techniqueby280measuring whether participants could consciously discriminate the inducer281elements.Participantswereaskedtojudgewhetherarightanglewaspresentedin282the left or right bottom inducer. The timing of the trial sequence in this taskwas283identical to the Kanizsa task, with one exception: only control stimuli were used284(Figure2B).285



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286Figure2. Illustrationof thetrialsequence inexperiment1.A)Kanizsatask:Eachtrialwas287composedoffiveframes:fixationdot,mask,stimulus,blank,andpost-stimulusblank.After288theparticipant’sresponse, thefixationpointprovidedfeedbackfor100ms(green:correct;289red:incorrect).Thedurationofeachframeisshownonthetime-line.Notethatinthevisible290conditions,ablankscreenreplacedthemask.B)Visibilitytask:Trialsproceededinthesame291way as in the Kanizsa task except that only control stimuli were being shown and292participants judged whether the right-angle gap was in the bottom left or right inducer293(indicatedbythegreencircle,whichwasnotpresentintheactualstimuli).294295296Dataanalysis297298Performance in both tasks was defined in terms of mean proportion of accurate299responsesineachcondition.Initially,weconductedbinomialtestsattheindividual300



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level to quantify how many participants performed significantly above chance. A301condition for any participant’s data to be considered in the group analysis of this302experimentwasthattheirperformancetothe invisiblerealstimulusconditionwas303significantlyabovechance(0.5).Thisisbecausethefeaturesnecessarytodistinguish304the contours of the real triangle were not masked and if a participant could not305perform the task for this particular stimulus, any test of theperceptionof illusory306contourswouldberedundant.307308One-samplet-testswerecarriedoutatthegrouplevelforeachconditionindividually309to assess whether the participants’ level of performance was significantly above310chance (>0.5). In addition to traditional frequentist statistical tests, we also311quantified the evidence for or against the hypothesis that participants could312discriminate the stimuli by calculating a Bayes Factor using a default Cauchy prior313with scaling factor 0.707 for the alternative hypothesis [43]. For conditions with314performancenear chance levels, thisenabledus toalsoquantifyhowstrongly the315evidencesupportedthenullhypothesisthatparticipantswereactuallyguessing.316317318Experiment2319320This experiment comprised two consecutive tasks as well. The first, again called321‘Kanizsa’task,wasanindirecttestthatinvestigatedwhetherprimingwithaKanizsa322triangleinthelocationofthetargetimprovesdetectionofthetarget.Asecondtask323consisted of a ‘Visibility’ task that directly tested the effectiveness of themasking324technique.325326327Studydesign328329Inthecontextofthisexperiment,primingreferstotheattentionalcueprovidedby330theprimeshapeaboutthelocationofthesubsequentlypresentedtarget inoneof331fourpossiblelocationsthatinturnaffordsaboostinbehavioralperformanceonan332orthogonaldiscrimination task. In theprimedconditions the locationof the target333was always identical with the location of the prime, whereas in the unprimed334conditionstheprimeshapewasreplacedbyacontrolstimulus,whichlookedexactly335likethedistractors intheotherthree locations.Therefore, theunprimedcondition336provided a baseline that allowed the assessment of whether the triangle prime337boostedtheparticipants’performancebycapturingtheirattention.338339We tested priming with a 2x2x2 design with attentional priming (primed vs.340unprimed), the type of priming triangle (real vs. illusory), and prime visibility341



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(invisible vs. visible) as within subject factors. There were two different prime342shapes:arealtriangledefinedbyasubtleluminancecontrastoranillusory(Kanizsa)343one.Thevisibilityofthecuewasmanipulatedbyvaryingpresentationtimesofthe344priming array that followed the mask. A short presentation time of 16.7ms was345selected for the invisible conditions to render the prime invisible. In the visible346conditions,however,theprimingarraywaspresentedfor300ms,whichleftenough347timefortheprimetoenterawareness.348349Everyparticipant completed twodifferent tasks. Each task comprised20blocksof350trials,whereoneblockconsistedof32trials.Acrossonetask,theunprimedvisible351and invisible conditions appeared for a total of 160 times each,whereas the four352primed conditions (real vs illusory and visible vs invisible)occurred80 timeseach.353Thus,halftheblockconsistedofunprimedtrialsandtheotherhalfofprimedones.354355Behavioral responseswere given on a forced-choice discrimination task by button356press (left or right arrow) on a standard computer keyboard. Each trial required357either a left or a right response (see experimental procedure) and each response358type appeared twice per block for each condition, except for the unprimed ones359whereitappearedfourtimesperblock.Conditionswereselectedpseudo-randomly360foreverytrialbutwerecounterbalancedwithineachblock.361362Participants363364Seventeen (13 female, age range: 21-32 years,mean age 24.7±2.5 years) normal,365healthyparticipantstookpartintheexperiment,includingthetwoauthors.Allother366participantswereunawareoftheexperimentalhypothesis.367368Stimuli369370Inhalfthetrials,participantswerecuedwithoneoffourpossiblecues.Wecreated371shapestimulibyplacing fourblackdiscs (inducerelements,diameter=1.96°) in the372configurationof a square (width=2.8°). Inorder toproduce the illusory shapeof a373rightisoscelestrianglesittingontopofthesquare,awedgewasremovedfromeach374inducer element (Pacman).Wedges started at the center of their respective discs375andextendedtotheiredges.Onewedgemeasured90°andwasalwayspositioned376eitherinthebottomleftorbottomrightinducer.Thetwowedgesthatformedthe377apexes of the triangle measured 45°. Finally, the wedge of the fourth inducer378element measured 90°. This Pacman did not form part of the triangle and was379positionedopposite the hypotenuse of the right isosceles triangle. It always faced380the outside of the disc configuration.When the inducers were oriented with the381



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gaps forming the three corners of a triangle, an illusory light greyKanizsa triangle382wasvisibleontopoftheblackdiscs(Figure3A,iandii).383384Real triangleswere identical to thecorresponding illusoryshapesexcept that their385contour was defined by a luminance contrast with a black line connecting the386cornersofthetriangle(Figure3A,iiiandiv).387388ThecontrolstimuliwerecreatedbyalteringtheorientationsofthethreePacmenby389a systematic rotation of 180º. This modification broke the link between these390inducers and thus no illusory triangle was perceived (Figure 3B). Control stimuli391couldalsobehorizontalmirrorimagesoftheillustratedexamples.392393Stimuli in thesearcharraywereright isosceles triangleswithacontourdefinedby394luminance (Figure 3C, i and ii). Target stimuli were triangles with the right angle395pointingdown,eithertotheleftortotheright,while indistractorstherightangle396pointedup (Figure3C, iii and iv).Participantswereasked todetect the targetand397respondviabuttonpresswhether its rightanglewaspointing to the leftor to the398right.399400Themask consisted of a square of black lines connecting four discs, one at each401corner. Itwaswideenoughtocoverall fourPacmen,aswellasthelocationofthe402trianglewithinthem(Figure3D,i).Inthe‘Visibility’task,therewasnosearcharray.403Instead,thestimulusarrayfollowingtheprimewasasquarecontainingacrossthat404gavenoclueaboutthelocationandorientationofthetargettriangle(Figure3D,ii).405406The luminance of the background was 156 cd/m2. The luminance of inducer407elements, thecontourof the real triangleand theblackof themaskwas2cd/m2.408Finally,thegreyofthemaskinthefirsttaskwas58.2cd/m2.409410Notethatalthoughthereweredifferencesbetweenstimuli,the individual inducers411remained the same and there was no change in the local image properties (i.e.412square configuration of the four Pacmen and their position among the other 3413configurations).Thisenabledanassessmentofthedifferenteffectsofprimingwith414theKanizsa shape, as compared to the corresponding real triangleandungrouped415Pacmenordistractorsintheunprimedconditions.416417



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418419Figure3.Illustrationofthestimuliinexperiment2.A)Thefourprimeswereformedoffour420inducerelements,ofwhichthreecreatedeitherarealtriangleortheillusoryimpressionofa421triangle. In half the trials, participants were cued with one of the four possible cues: (i)422Kanizsa triangle pointing right, (ii) Kanizsa triangle pointing left, (iii) Real triangle pointing423right, (iv) Real triangle pointing left. B) Examples of control stimuli used during cue424presentation. Control stimuliwere created by a systematic 180° rotation of the individual425Pacmen, so that all gaps within the black discs pointed to the outside of the formation.426Controlstimulicouldalsobethehorizontalmirrorimageofthepresentedexamples.C)The427twopossible targets in the search taskwere right isosceles triangleswith their rightangle428pointingdown,eithertotheright(i)ortotheleft(ii).Distractorstimuliinthesearcharray429alwayspointedup,butvariedbetweenpointingleftandright.D)(i)Themaskusedtorender430theprime invisible. (ii) Theuninformativepost-stimulusmask replacing the targetarray in431the‘Visibility’task.432433Procedure434435Kanizsatask,indirecttestassessingpriming:436Atfirst,participantswereinstructedthattheywouldseeanarrayoffourtrianglesof437which only one, the target, had a right angle pointing down and were asked to438decidewhether this anglewas pointing to the left or to the right by pressing the439respectiveresponsekeys.440



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441The20experimentalblockswereprecededbyasmanypracticeblocksasneededin442order to fully familiarize the participants with the task requirements. Participants443could initiateanewblockbypressinganybuttonon thekeyboardand, ifneeded,444takesmallbreaksbetweenblocks.445446Participantswereinstructedtofixateasmallblackdot(0.2°wide)thatwaspresent447inthecenterofthescreenthroughouttheexperiment.Oneachtrial,thefixationdot448wasdisplayedalonefor500ms.Subsequently,amaskarraywasdisplayedfor100ms449andwasimmediatelyfollowedbyoneoftheprimingarrays(seeconditionsinstudy450design),whichwaspresentedforeither16.7msor300ms.Thisarraycouldcontain451oneof the twopossibleprimeshapes (realor illusory triangle)ora simplecontrol452stimulusamongtheotherthreecontrolstimuli.Immediatelyafterthis,atargetarray453containingone targetamong threedistractorswasdisplayedand remainedon the454screenuntiltheparticipant’sresponse.455456The responsewas self-paced;however,participantswere instructed to respondas457quicklyaspossible.Responsesweremadewiththeindex(forleft)andthemiddleor458ring finger (for right), depending on how participants felt comfortable. For every459response, the fixationdotprovided feedback (100ms)bychanging itscolor (green:460correct; red: incorrect). Then, the next trial started without any delay. Figure 4A461showsthegeneralparadigmfortheexperimentalprocedure.462463Visibilitytask,directtestassessingprimediscrimination:464Thesecondtaskassessedtheeffectivenessofthemaskbymeasuringtheconscious465discriminabilityoftheprimesinadirectmanner.Forthispurpose,participantswere466askedtomakethesamedecisionasintheprevioustask,butthistimewithrespect467to theprime.Thus, insteadofdiscriminatingwhether the rightangleof the target468waspointingtotheleftortotheright,participantsdiscriminatedthisaspectofthe469primeshape.470471Procedure,stimuliandtimingofthetrialsequenceinthistaskwerekeptidenticalto472thepreviousindirecttest,withasingleexception:Insteadofatargetarray,anarray473comprisingnewidenticalmaskshapescoveredthefourstimulilocations(Figure4B).474



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475Figure4. Illustrationofanexample trial sequence inexperiment2.A)Kanizsatask:Each476trial was composed of four frames: fixation period, mask, cueing array and target array.477After the participant’s response, the fixation point provided feedback for 100ms (green:478correct;red:incorrect).Thedurationofeachframeisshownonthetime-line.Theduration479of the cueingarraywasvariedbetweenconditions:16.7ms in the invisible conditionsand480300ms in the visibleones.Note that in theunprimed conditions, the cueingarraydidnot481containanyprimeshapeandwascomposedexclusivelyofcontrolstimuli.B)Visibilitytask:482Trialprocedurewas identical to theone in theKanizsataskexcept for the fourth frame in483whichthesearcharraywasreplacedbyanarrayoffouridenticalmaskshapes.484485486487488



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Dataanalysis489490Response times (inmilliseconds)andperformanceaccuracy (inproportioncorrect)491weremeasuredforeachofthesixconditions.Toanalyzeprimingeffects,response492timeswerecalculatedoncorrecttrialsonly.493494FortheKanizsatask,accuracyofresponseswaslessinformative,asthetargetarray495was present until the participant’s response. We reasoned that priming in the496indirect Kanizsa taskwouldbe reflectedby fastermean response times toprimed497than unprimed conditions. Therefore the priming effect was calculated by498subtractingresponsetimesunderprimingfromthosewithoutpriming.499500Primingeffectsfromallparticipantsforallconditionswereenteredintoatwo-way501repeated-measures analysis of variance (ANOVA) with prime visibility (visible vs502invisible) and prime stimulus (real vs illusory triangle) aswithin subject factors. In503addition to this, further comparisons between experimental conditions (visible or504invisible/real or illusory) and control conditions (unprimed) were assessed with505paired t-tests. We also determined the significance of the priming effect by506comparingitagainstzerowithone-samplet-tests.507508In the visibility task, the variableof interestwas theaccuracyof responses as this509allowed us to test whether participants could discriminate the orientation of the510primes. One-sample t-testswerecarriedoutat thegroup level foreachcondition511individually in order to assesswhether the participants’ level of performancewas512significantly above chance (>0.5) and paired-tests were used to compare513performanceindifferentexperimentalconditions.Asinexperiment1,forallt-tests514wealsocalculatedthedefaultBayesFactorinadditiontofrequentiststatistics.515516Experiment3517518Thisexperimentonlycomprisedonetaskthatindirectlytestedtheperceptofillusory519contoursbymeasuringparticipants’ thresholdonaspatial localization task.Sucha520manipulationhasbeenusedsuccessfullyinpreviousexperiments[19,38].Whileitis521an indirect test, thecontouraidsperformanceonthespatial localization task.This522provides independent evidence about whether participants perceived any illusory523contours and therefore helps to address confounds with measuring the percept524directlyaswedidinexperiment1.Pilotexperimentsusingataskdirectlymeasuring525theillusorypercept(thesametaskasexperiment1butwiththelong-lastingmasking526technique employed in experiment 3) could encourage participants to pay close527attention to themaskedcontextand thus reduce theeffectivenessof themasking528procedure.Thiswouldmeanthatparticipantsuseresidualawarenessoftheinducers529



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toperformthetaskinsteadofactuallymakingaperceptualjudgmentoftheillusory530contour.531532Moreover,because themaskingprocedure inexperiments1and2usedverybrief533presentationsoftheinvisibleKanizsastimuli(16.7ms)whiletheseweremuchlonger534(300ms)forthevisibleones,inexperiment3weusedadifferentmaskingprocedure:535stimuli were defined by sinusoidal gratings which reversed contrast polarity at536120Hz.Thismethodcaneffectivelyrenderstimuliinvisibleforprolongedperiodsso537thatparticipantsonlyperceiveagreyscreen.Previousresearchsuggeststhatstimuli538masked in thiswayareprocessed inearlyvisual cortex [3].Moreover,weshowed539thatstimulirenderedinvisiblebythismethodcouldinducelocalprimingeffectsona540shapediscriminationtask[17].541542Participants543544Five normal, healthy participants (3 female; age range: 24-37; mean age: 30.2)545completedthisexperiment,includingoneoftheauthors(DSS).Allparticipantswere546experienced with psychophysical experiments. While the results from a larger547subject basemight generalizemore to abroadpopulation,we reasoned thatonly548precise measurements of participants’ visual performance would afford549interpretable data. Unlike in the first two experiments, in experiment 3 we550measured how position discrimination thresholds changed between experimental551conditions. Naïve, untrained participants from the general population would be552more likely to contribute noisy data and this might obscure potential subtle553perceptionofillusorycontoursinthemaskedcondition.554555Stimuli556557WegeneratedaKanizsashapebypresentingfourGaborpatches(sinusoidalgratings558with wavelength 0.33° visual angle convolved with a Gaussian with standard559deviation0.8°at30%contrast)inthelocationsofthefourcornersofasquarewitha560side-lengthof8.2°.Weturnedthesepatches intoPacmenbysettingaright-angled561regionofeachpatchtozerocontrast(uniformbackgroundgrey).Thisstimulusthus562describedanillusorysquare(Figure5A).Inthecontrolconditions,thePacmenwere563rotated by 180° so that the corners faced outward breaking the illusory percept564(Figure5B).Intherealluminancecontrol,wedidnotpresentanyGaborpatchesbut565insteadasquareregionwithasubtleluminancecontrast(54cd/m2)relativetothe566background (Figure 5C). The orientation and phase of each Gabor patch was567randomizedineachtrial.Finally,wecreatedamaskstimulusbyoverlaying24Gabor568patches in the four locations. These patches were presented at 5% contrast and569covered the full range of orientations in equal steps of 15° but their phaseswere570



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randomized.This resulted inapatchypatternwithoutanyobviousorientationcue571(Figure5D).Themaskpatternwasalsogeneratedanewineverytrial.572573Thesestimuliwerepresentedinthecenterofthescreen.Thetargetstimuluswasa574tiny dark grey dot (diameter: ~0.1°) which could appear somewhere along the575horizontalmeridian,eitherneartherightortheleftverticalboundaryofthesquare.576577The square stimuli were either invisible or visible. In the invisible condition, the578gratings reversed contrast polarity at 120Hz. In the visible conditions and the real579luminance control, every even-numbered framewas uniform grey (except for the580fixationdot).581582

583Figure5.Stimuli(A-D)andtrialsequence(E)inexperiment3.Duringthestimulusinterval584eitheran illusoryKanizsasquare(A),acontrolstimuluswithrotated inducers(B),orareal585luminance square without any inducers (C) was presented. In visible trials every odd-586numberedvideoframeat120Hzcontainedthestimuluswhileeveryeven-numberedframe587contained only a blank screen with the blue fixation dot. In invisible trials, both frames588contained the Gabor patches for A and B but their contrast polarity alternated between589frames. For real luminance stimuli the even-numbered frames always contained a blank590screen. D. A mask stimulus preceded and followed each stimulus interval. E. Each trial591started with 500ms of fixation, followed by a 100ms mask, the 500ms stimulus interval,592another100msmask,and finallyablankscreenwithouta fixationdot that remaineduntil593participants made their response. Their task was to locate the small target dot in the594



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stimulus display and respond whether it appeared to the left or the right of the vertical595boundaryofthesquare(here,thecorrectresponseisright).596597Procedure598599Figure5EdepictsthesequenceofaninvisibleKanizsatrialinexperiment3.Eachtrial600started with 500ms of a blank grey screen in which only a black fixation dot601(diameter 0.26°) was present. This was followed by a 100ms presentation of the602maskandthen500msofthestimulus,afterwhichanother100msmaskintervalwas603presented.Thenthescreenturnedgreyandthefixationdotwasremoved,indicating604thatparticipantscouldmaketheirbehavioralresponse.605606Duringthestimulusintervalthefixationdotwasblueinsteadofblacktodenotethat607thiswas the task-relevant interval. As described above, the stimuli in this interval608either reversed in contrast polarity at 120Hz (invisible Kanizsa and control609conditions)or the frameswere interleavedwithblank frames (visibleKanizsa, real610luminance stimuli and control). During the stimulus presentation the small dark611targetdotalsoappearedatsomelocationalongthehorizontalmeridian.Itsposition612was randomized to be either near the left or the right vertical boundary of the613squareregion.Thedotcouldeitherappearinsideoroutsidethesquareregion.614615Thedependentvariableinthisexperimentwasthedistancebetweenthetargetdot616and the boundary of the square region and it was controlled by a 2-down, 1-up617staircase procedure that converged on the threshold distance in each of the five618experimental conditions at which performance was approximately 70.7% correct.619That is, after every consecutive two correct trials the distancewould decrease by620onepixel (~0.03°)whileafterevery incorrect trial it increasedbyonepixel.All five621staircasesstartedatadistanceof15pixels(0.49°).Theminimumandmaximumthat622theycouldreachwereonepixeland25pixels(0.82°),respectively.623624BeforetheactualexperimentweshowedparticipantsstaticexamplesoftheKanizsa625andthecontrolstimuli.Participantswereinstructedtojudgewhetherthetargetdot626was left or right of the (imaginary) vertical boundary of the square region.While627showingthemstillimagesofthestimuli,wespecificallyexplainedtothemthatthis628boundarywasdefinedby theexactcenterof theGaborpatches (i.e. thecornerof629thePacman’smouth)andthatthiswasidenticalinboththeKanizsaandthecontrol630conditions.Wefurtherinformedthemthattherewouldbeathirdconditioninwhich631they should only see a subtly lighter grey square against the background but no632Gaborpatches.Inordertobecomeacquaintedwiththetask,theythenperformed1-6332 blocks of the experimentwith only the visible Kanizsa, the control and the real634luminance condition. Finally, the actual experimentwould commence. Participants635



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wereinformedthatthetaskwaslargelyalwaysthesameasthefamiliarizationrun,636although during this experiment there would bemanymore trials in which they637wouldeitheronlyseea lightgreysquarebutnoGaborpatchesor that theymight638even only see a grey blank screen. Except for the author, all participants were639unaware of the experimental hypothesis. In debriefing none of the participants640reportedseeinganyGaborpatchesduringtheinvisiblecondition.641642Participantsperformedtwoconsecutiverunscomprising500trialseach.Theseruns643were further subdivided into 25 blocks. In every block each of the five conditions644appeared4timesinapseudo-randomly interleavedorder.Blockswereinitiatedby645button-press.Duringblockbreaksamessageonthescreenremindedtheparticipant646ofthetaskinstructionsandofthenumberofblockstheyhadalreadycompleted.647648Dataanalysis649650Wedeterminedthethresholddistanceineachofthefiveexperimentalconditionsby651calculating the mean distance across the final 15 reversals in the staircase652procedure. Then we conducted a group level analysis in which we compared the653average thresholds for conditions across participants. For thiswe used a two-way654repeated-measures ANOVA with the factors visibility (visible vs invisible) and655stimulustype(illusoryvscontrolstimulus).Astherewasonlyoneconditionwitha656real luminance contour, the thresholds for this conditionwerenot included in the657ANOVA. However, we used paired t-tests to compare results for the illusory and658controlstimulidirectlytotherealluminancecondition.659660Results661662Experiment1663664Inthisexperiment,17participantsjudgedtheorientation(leftvsright)ofatriangle665thatwaseitherdefinedbyanillusorycontourorasubtlerealluminanceedge.Ina666thirdcontrolconditionthePacmenstimuliinducingillusorycontourswererotatedso667thatnotriangleshapecouldbeperceived.668669KanizsaTask670671Accuracyacrossparticipantsfordiscriminatingtheorientationofthetriangleforthe672six conditions is displayed in Figure 6. We tested whether discrimination673performanceineachconditionwasabovechanceatthegrouplevel.674675



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Inthevisiblecondition,themeanproportionofcorrectresponsesforthegroupwas676closetoceilingandclearlybetterthanchancebothforthereal(M=0.97,t(16)=83.5,677p<0.001,BF10>9.8*1018)andtheillusorycondition(M=0.99,t(16)=120.8,p<0.0001,678BF10>2.4*1021).Inthecontrolcondition,however,thegroupperformedsignificantly679below chance level (M=0.30, t(16)=-3.87, p=0.001, BF10=29.3). There was680considerablevariabilityinperformanceforthisconditionrangingfrom0to0.74.681682In the invisible condition, only performance on the real triangle condition was683significantly above chance level (M=0.71, t(16)=6.4, p<0.001, BF10=2608.7).684Performance on both the illusory triangle condition (M=0.51, t(16)=1, p=0.332,685BF10=0.385)andthecontrolcondition(M=0.50,t(16)=0.2,p=0.859,BF10=0.253)were686at chance level. Importantly, performance on the real triangle condition was also687significantly greater than for either the illusory contour (t(16)=6.1, p<0.0001,688BF10=1475.4) or the control condition (t(16)=5.6,p<0.0001,BF10=688). In contrast,689performance for the illusory contour did not differ from the control condition690(t(16)=0.7,p=0.525,BF10=0.3).691692VisibilityTask693694Ina supplementarycontrol taskweaskedparticipants tomakeadecisiondirectly695basedontheinducersbyreportingwhichoneofthetwobottominducerscontained696the rightangle.Thisassessedany residualperceptionof the inducer shapesunder697masking conditions. In the visible condition, the mean proportion of correct698responsesacrossparticipantswasagainclosetoceilingandfarabovechance level699(M=0.96, t(16)=22.2, p<0.001, BF10>2.9*1010). In contrast, participants’ mean700performance in the invisibleconditiondidnotsignificantlydiffer fromchance level701(M=0.48, t(16)=-1.3, p=0.227, BF10=0.489), suggesting that participants could702generallynotperceivethe inducersundermaskingconditions.Note,however,that703therewasonlyanecdotalsupportforthenullhypothesis(BF10>1/3).704705The first experiment thus suggested that if participants were unaware of the706inducersbecausetheyhadbeenmasked,theywereunabletojudgetheorientation707oftheKanizsatriangle.Thissupportstheinterpretationthatillusorycontoursarenot708formedundertheseconditions.709



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710Figure6.Resultsofexperiment1.Accuracyfordiscriminatingtheorientationofatriangle711stimulus in visible or invisible (masked) trials. Each dot represents the performance of an712individualparticipantineachoftheconditions.Thelargesymbolsanderrorbarsdenotethe713mean ±1 standard error for each condition. Black: real luminance contour. Red: illusory714(Kanizsa)contour.Blue:controlstimuli.715716Experiment2717718InthesecondexperimentweaskedwhethermaskedKanizsashapescouldaffordan719attentionalprimingeffect.WetestedwhetheraKanizsastimuluspresentedaspart720ofanarrayof four stimuli couldprimeparticipants to the locationofa target ina721visual search array presented subsequently. The prime could either be a Kanizsa722triangleoratriangledefinedbyarealluminancecontour.Intheunprimedcondition723allfourstimuliintheprimingarraywerecontrolstimuliwithoutatriangleshape.724725KanizsaTask726727Overallparticipantsmadeveryfewerrorsindeterminingwhetherthetargetpointed728to the left or to the right. Indeed, themean accuracy in all conditionswas above7290.94.Therefore,foreachexperimentalconditionwequantifiedtheeffectofpriming730by the difference in response time with priming subtracted from that without731priming(Figure7A).732



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733734Figure7.Resultsofexperiment2.Primingeffect(responsetimeforunprimedtrialsminus735primedtrials)ontheKanizsatask(A)andaccuracyfordiscriminatingtheinducerlocationin736theVisibility task (B) for visible or invisible (masked) trials. Eachdot represents themean737performanceof an individual participant in eachof the conditions. The large symbols and738error bars denote the mean ±1 standard error for each condition. Black: real luminance739contour.Red: illusory (Kanizsa)contour. InB)Grey:unprimed trialsdummycoded for real740contourprime.Orange:unprimedtrialsdummycodedforillusorycontourprime.741742We conducted a two-way repeated-measures analysis of variance, with visibility743(visiblevsinvisible)andstimulus(illusoryvsrealtriangle)aswithinsubjectfactors,to744compare the priming effects (response time on unprimed minus primed trials)745between conditions. There was a significant main effect of visibility of the prime746(F(1,16)=60.06, p<0.001), and a significant main effect of stimulus type747(F(1,16)=13.82, p=0.002). The interaction between visibility and the stimulus type748wasnotsignificant(F(1,16=3.52,p=0.079).749750There was a clear difference in the strength of priming between the visible and751invisible condition. However, the critical test is whether the priming effect was752significant, that is, if itdiffered fromzero (i.e.apaired t-testbetweenprimedand753unprimed response times). Unsurprisingly, the very pronounced priming effect for754visibletrialswasextremelysignificantforbothconditions(real:M=336ms,t(16)=8.2,755p<0.001, BF10>4.1*104; illusory: M=275ms, t(16)=6.9, p<0.001, BF10>5.8*103).756However, there was also a much more modest but nonetheless very significant757primingeffectforbothstimulustypesintheinvisibleconditionwheninducerswere758masked (real: M=43ms, t(16)=3, p=0.01, BF10=5.6; illusory: M=36ms, t(16)=4.4,759p<0.001,BF10=80.8).760 761VisibilityTask762763As in experiment 1 we used a direct Visibility test to assess whether participants764mighthavehadresidualawarenessof theprimingstimuliundermasking.Thetask765



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procedure was the same as in the Kanizsa task except that the search array was766replaced with a foil in which all four stimuli were identical and non-informative767about the correct answer. We quantified the accuracy (proportion correct) with768whichparticipantscoulddiscriminatetheorientationoftheprimingtriangle(Figure7697B).770771Unsurprisingly, in the visible condition performance for discriminating the772orientation of a real (M=0.88, t(16)=14.9, p<0.001, BF10=9.5*107) and an illusory773triangle (M=0.87, t(16)=14,p<0.001,BF10=4.0*107)was significantly above chance.774Conversely,performance for theunprimedconditions, that is, stimulusarrays that775contained no triangle shape, performance was consistently at chance (“real”:776M=0.52, t(16)=1.3, p=0.21, BF10=0.52; “illusory”: M=0.53, t(16)=1.3, p=0.219,777BF10=0.5;butnotethatthesetwoconditionsweredummycodedinthiscaseasthey778containedthesamestimuli).779780Critically, performance in the invisible condition showed that discrimination in the781primedwith an illusory triangle conditionwas also significantly above chance (M=7820.55,t(16)=3.24,p=0.005,BF10=9.4),anddiscriminationofa real triangleshoweda783similarresulteventhoughitdidnotreachsignificance(M=0.53,t(16)=1.9,p=0.075,784BF10=1.1).Performanceforthetwoinvisibletrianglesalsodidnotdiffersignificantly785(t(16)=-0.9,p=0.385,BF10=0.35).Again,asexpectedbothoftheunprimedconditions786were at chance level (“real”: M=0.51, t(16)=0.6, p=0.534, BF10=0.3; “illusory”:787M=0.51,t(16)=0.7,p=0.494,BF10=0.31).788789Takentogether,theresultsofthesecondexperimentcontradictedthoseofthefirst,790astheysuggestedthatKanizsatrianglesrendered invisiblebymaskingcouldafford791anattentionalprimingeffectinasubsequentvisualsearchtask.However,acontrol792experiment testingparticipants’ability todiscriminate theprimeshapeorientation793directlysuggestedthat thissmallprimingeffect for invisible trialscouldhavebeen794duetosomeresidualawarenessofthemaskedtrianglestimuli.795796Experiment3797798In experiment 3 we therefore changed the approach in a number of ways. First,799insteadofthebriefstimuluspresentationsusedinthefirsttwoexperimentsweused800counter-phase flicker to render stimuli invisible for prolonged periods. As in801experiment2weusedan indirectmeasureof illusorycontourprocessing:wetook802advantage of previous reports that the presence of an illusory contour stimulus803enhanced participants’ thresholds at discriminating the position of a small dot804[19,38]. Herewe testedwhether this also occurredwhen the inducers generating805the illusory contour were invisible. Because this task was more challenging than806



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thoseinpreviousexperiments,andtoruleoutthatourpreviousresultsmighthave807beendue to insufficientpracticeor familiaritywithpsychophysicalexperiments, in808this final experiment we only tested a small group of well-trained psychophysics809participants.810811Figure 8 plots position discrimination thresholds in the different conditions for all812individualparticipantsandthegroupaverages.Overall,thresholdsmeasuredwhilea813Kanizsa stimulus was presentedwere significantly lower than thosemeasured for814control stimuli (F(1,4)=34.6, p=0.004). There was also a non-significant trend of815lowerthresholdsduringvisiblethaninvisibletrials(F(1,4)=7.5,p=0.052).Importantly,816therewasasignificantinteractionbetweenvisibilityandstimulustype(F(1,4)=15.9,817p=0.016). In the visible condition thresholds measured while an illusory Kanizsa818contour was present were significantly lower (M=0.08°, t(4)=-5.27, p=0.006,819BF10=9.95) compared to the control condition without a contour (M=0.21°). In820contrast, intheinvisibleconditionthedifferenceinthresholdsfor illusorycontours821(M=0.20°)wasnotsignificantlydifferentfromthatforthecontrolstimulus(M=0.21°,822t(4)=-0.16,p=0.882,BF10=0.4).823824Thresholdsmeasuredfortherealluminancecontourwereofasimilarmagnitudeas825those measured for the visible illusory contour (M=0.10°, t(4)=-0.92, p=0.409,826BF10=0.55).Incontrast,thresholdsfortherealluminancecontourweresignificantly827lower than for invisible Kanizsa stimuli (t(4)=3.84,p=0.019,BF10=4.53). Thresholds828forthereal luminancecontourwerealsosignificantly lowerthanforeithercontrol829stimulus(visible:t(4)=4.4,p=0.012,BF10=6.19;invisible:t(4)=5.3,p=0.006,BF10=9.9).830831



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832Figure 8. Results of experiment 3. Spatial discrimination thresholds per condition (real833luminancevsillusoryvscontrol)onadotlocalizationtaskwhileparticipantswerepresented834with visible (blue squares), invisible (orange diamonds), or real luminance stimuli (black835circles). A-E. Plots for the five individual participants. F. Thresholds averaged across836participants±1standarderrorofthemean.837838Discussion839840In three psychophysical experiments we tested whether the visual system forms841illusory (Kanizsa) contours in the absence of awareness of the inducing context.842Noneoftheexperimentsshowedanyconclusiveevidencethatillusorycontoursare843perceivedwhentheinducersaremasked.844845Inthefirstexperiment,wedirectlymeasured illusorycontourperceptionbyasking846participantstodiscriminatetheorientationofaKanizsatriangle.Thisprocedurewas847akin to our earlier experiments [18] that showed no evidence either that illusory848contours are formed when inducers are suppressed from awareness. In those849experimentsweemployedCFSinwhichadynamic,high-contrastmaskpresentedto850one eye suppresses awareness of a stimulus viewed by the fellow eye. Similarly,851previous studies showed that illusory contours are not formedwhen the inducers852are suppressed from awareness during binocular rivalry [19]. However, because853previousreports indicatethat illusorycontoursareprocessedbybinocularneurons854theuseofdichopticstimulimaysimplydisrupttheirprocessing.855856



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Thereforehereweusednormalbinocularviewingconditionsandadifferentmethod857to render the inducers invisible. Inaddition,we includeda real luminancecontour858conditionasabaselinecheck.Thishelpedtoruleoutanothertrivialexplanationfor859ourearlierfindings:Thepresenceofthebrightmaskscouldpossiblyhaveobscured860thedetectionofthesubtleillusoryboundary.Ourrealluminancecontourwasnever861masked because the masks only overlapped with the corners of the triangle;862therefore,ifparticipantswereabletodiscriminatethesubtlerealluminancecontour863theyshouldalsobeabletodosoifanillusorycontourwereindeedformed.Weonly864included participants for whom discrimination of this real luminance contour was865significantly above chance levels. Nonetheless, participants were unable to866discriminatetheorientationoftheillusorycontourwheninducersweremasked.The867phenomenologicalexperienceof thereal luminanceandthe illusorycontour isnot868perfectly identical (and itobviously isnot for thevisible conditions).However, the869real luminance conditions were very faint while illusory contours tend to be870subjectivelyquitesalient.Thusitseemsunlikelythatparticipantsshouldbeableto871detectonlytherealluminancecontourbutnottheillusorycontour.872873Experiment1alsocontainedcontrolconditionsinwhichtheindividualinducershad874beenrotatedby180°andthusnoillusorytriangleshouldbeformed.Thiscondition875was somewhat unnecessary because the dependent variable, accuracy for876discriminating theorientationof the triangle,wasorthogonal to theconditionand877forthesecontrolstimulitherewasnostrictlycorrectanswerinthisdiscrimination878task.Weincludedthisconditionascatchtrials–participantsshouldbeguessinghere879becausetherewasnotriangletodiscriminate.Thiswasclearlythecaseforcontrol880stimuli when inducers were invisible. However, for visible control stimuli881discriminationperformancevariedwidelyfrom0-0.74proportioncorrectresponses882and was even significantly below chance level. The “correct” responses in this883conditionweredummycodedsothatthestimulus-responsemappingmatchedthat884for the equivalent illusory triangle stimuli before rotating the inducers. Therefore885oneinducerinthetoprowhadaright-angledcutoutthatpointedinwards(Figure1)886andthiswouldbeconsistentwiththepresenceofatriangleorientedintheopposite887direction (even though no actual triangle should be perceived). Some participants888may have adopted this stimulus responsemappingwhile others did not. Itwould889thereforehavebeenbetter tohaveall inducerspointoutwards in thiscontroland890randomize their locations.However, our actual results serendipitously support the891fact that masking was effective in this experiment: The fact that participants892performedatchancelevelwhenthecontrolstimuliweremaskedrulesoutthatthey893hadanyresidualawarenessoftheinducers.TheresultsofourexplicitVisibilitytask894afterthemainexperimentalsocorroboratedthisconclusion.895896



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Theresultsofexperiment1contradictpreviousclaimsthata“ghostlytriangle”could897beperceivedwhen the inducersofaKanizsa triangleweremasked [20].However,898follow-upexperiments failed to replicate theseexperimentsbut instead suggested899that conscious processing of the inducers precedes awareness of the illusory900contoursandthattheseearlierresultswereinfactduetoresidualawarenessofthe901inducers[44].Anothercriticalissuewiththeseexperimentsisthattheperceptionof902illusorycontoursisbasedonaYesorNojudgmentofwhetheratriangleshapewas903present. Such a task could theoretically be performed based on any residual904awarenessof thecorners in the inducers.Our task requiredanexplicitorientation905discriminationofthe illusorycontour. Ifparticipantshadbeenabletoperformthis906task in spiteofbeingunable toperceive the inducers, thiswouldhavebeenmore907conclusive evidence that illusory contours are indeed formed when inducers are908masked.909910In our second experiment,we used an indirect approach to inferwhether illusory911contourswereprocessedwithoutawarenessoftheinducers.Wemeasuredwhether912thepresentationof a Kanizsa triangleoronewith a real luminanceedgeafforded913participants with a response time advantage on a subsequent visual search task914comparedtothesituationwherenotrianglewaspresented.Previousresearchhas915shown thatKanizsa shapes can capture attention in thismannerwhenvisible [32]916and illusory contours can also prime performance on shape or orientation917discriminationtasks[33,34].Asexpected,therewasaverystrongprimingeffectfor918boththerealluminanceandtheillusorytrianglewhenprimingstimuliwerevisibleas919the two shapes produce a salient “pop-out” effect. Interestingly, even when the920primes were removed from awareness by forward masking, they nonetheless921producedaweakbutsignificant response timeadvantage foreitherstimulus type.922However,oursubsequentVisibilitytask,inwhichparticipantsjudgedtheorientation923oftheprimingtriangledirectly,suggestedthatmaskingmaynothavebeenentirely924effective because participants performed significantly above chance level even for925themaskedstimuli.Therefore,themostparsimoniousinterpretationofthepriming926results is that participants had some residual awareness of the primes and that927Kanizsacontourswereprobablynotprocessedwheninducersweremasked.928929InbothexperimentswealsoincludedanexplicitVisibilitytaskinwhichwemodified930the task to testwhether participants had any residual awareness of the inducers.931Whenconductingsuchtestsofawareness,itisofparamountimportancetoensure932that trials with masked and unmasked conditions are interspersed. When only933masked conditions are tested for awareness, it is possible that the participant’s934performanceisatchanceeventhoughthereisinfactresidualawarenessinthemain935taskduetowhathasbeenreferredtoas“primingofawareness” [45].Conversely,936the ability to correctly identify purportedly subliminal prime stimuli (as in our937



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experiment2)mayhavebeenenhancedbytheinclusionoftrialswithclearlyvisible938primes[46].939940Naturally,ourdesignthatsplitthemaintaskfromatestofawarenessdidnotallow941ustomeasureoftheawarenessoneachactualtrialofthemainexperiment–onlya942dual-taskdesign inwhichvisibility isprobeddirectly inthemainexperimentwould943permitthis.However,adual-taskdesigncannotclassifytheawarenessofindividual944trials perfectly as the participant’s judgment of their own awareness is subject to945variability. Dual-tasks also entail a division of attentional resources across the946different task components. In experiment 2, where the Visibility task involved a947simplechangeofthesearcharrayandtheinstructiontonowjudgedirectlybasedon948theprimingarray,theconditionsbetweentheKanizsaandtheVisibilitytaskswere949verycloselymatched.Inexperiment1,itcouldbearguedthattheVisibilitytaskwas950moredifficult than the Kanizsa task because the former required the judgment of951one a small, peripheral feature of the stimulus. This may complicate the952interpretationofchanceperformanceonthistask.953954Bothexperiments1and2usedverybriefstimuluspresentationsandpowerful,high-955contrastmaskstorenderstimuliinvisible.Ratherthantheabsenceofawareness,the956reasonwhyinvisiblecontourswerenotformedcouldbethebrevityofthestimulior957because themask fundamentally disrupted stimulus processing. This is certainly a958possibility because previous research indicated that the processing of illusory959contours occurs relatively slowly [35–37]. Therefore, in our third experiment, we960used counter-phase flicker tomask the inducers instead of themaskingmethods961employed in the earlier experiments. This allowed us to present stimuli for962prolonged periods.While this is also a temporalmasking procedure, the stimulus963energyisconstantduringtheentirepresentationbecauseonlythepolaritychanges964betweenframes.Furthermore,weexploitedthefactthatthepresenceofanillusory965contourenhancesperformanceonadotlocalizationtaskbecauseitprovidesavisual966aid fordetermining itsspatial location [19,38].Weconfirmedthisadvantagewhen967inducerswerenotmaskedandperformancewascomparabletowhenwepresented968realluminancecontoursonly.However,wheninducerswererenderedinvisiblethis969advantageforillusorycontoursdisappeared.Thistaskisamoreobjectivetestofthe970induction of the illusory contours than directly testing the contour itself as in971experiment 1. Pilot experiments had suggested thatwith the long-lasting counter-972phaseflickermasking,participantsmightinoccasionaltrialshavehadsomeresidual973awareness of the corners in the inducers. Thus they might still have performed974above chance at a shape/orientation discrimination on the purported illusory975contoureventhoughtheydidnotinfactperceiveanyillusorycontour.Thisproblem976also plagues many previous experiments that directly tested the presence of an977illusory shape [35,36].Only tasks that specifically relyona finediscriminationofa978



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featureoftheillusorycontoursuchasitscurvature[37]orthatexploitthepresence979oftheillusorycontourtomodulateperformanceonanorthogonaltask,likethedot980localization taskweused, canprovideconclusiveevidence thatan illusorycontour981wasinfactperceived.982983InalltestsofperformanceagainstchancelevelsweusedBayesianhypothesistests984thatcanquantify thestrengthofevidencefor thenullhypothesis thatparticipants985were guessing [43]. None of these tests revealed strong evidence for the null986hypothesis as typical Bayes Factors fell between 0.3-0.5. To establish more987compellingsupportforthenullhypothesisinthosecasesmuchlargersampleswould988be required. Crucially, the Bayes Factor indicates by how much the observed989evidenceshouldupdateone’spriorbeliefinthenulloralternativehypothesis.Even990if theevidence is relativelyweak,anyBayesFactorbelow1 isevidence in favorof991thenullandnotforthealternativehypothesis.Unlessonestartswithapriorbelief992thatpeopleareclearlyabletodiscriminatemaskedstimuli,eventhesemodestBayes993Factorssuggestthatparticipantswereprobablyguessing.994995Taken together, our present findings and previous experiments using dichoptic996stimulation[18,19]allindicatethatawarenessoftheinducersisrequiredtoforma997perceptofillusorycontours.Evenifweinterpretthesomewhatinconclusiveresults998of the second experiment as showing that masked triangles produced priming999effects,thisdoesnotprovidedirectevidencethatthisadvantageisactuallycaused1000bythepresenceofillusorycontours.AKanizsatrianglecanclearlyprovideasalient1001attentionalcuebutthismaybeduetosharedfeatures,suchasthecollinearityofthe1002edges in the inducers or the implication of a surface. This is also consistent with1003previous reports that priming of a shape discrimination task by subliminal stimuli1004dependsonthestrengthof thesalient region [47]andthatsalient regionsofsuch1005stimuliaredetectedefficiently regardlessofwhether theyareboundedby illusory1006contours[38,48].10071008Severalpreviousstudiesusedstimulusmanipulationsthatseektodisentanglethese1009factors[38,49,50].RoundingthecornersofthePacmaninducersresultsinanotable1010reductionintheillusorycontourperceptandabolishestheconcordantimprovement1011onaspatiallocalizationtask.However,stimulilikethisnonethelessactivatedhigher1012extrastriatecortextoasimilardegreeasKanizsastimuli[38].Thisisconsistentwith1013the theory that later stages of visual processing, presumably mediated by higher1014visual areas, segment surfaces and assign boundaries to objects. These1015segmentationsarethenfedbacktoearlyvisualcortextogeneratesignals thatare1016interpreted as illusory contours [49]. Practical support for this idea comes from1017transcranial magnetic stimulation experiments that disrupted neural processing1018eitherinobject-sensitivelateraloccipital(LO)cortexor inearlyvisualareasV1and1019



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V2 [51]. Critically, the disruption of LO cortex only abolished the illusory contour1020perceptearlyafterstimulusonsetwhiledisruptionofearlyvisualareasonlydidsoat1021a later stage – presumably affecting feed-back signals rather than the early feed-1022forwardresponse.10231024Possibly theattentionalprimingeffectweobserved inour secondexperimentwas1025thereforenotduetothe illusorycontoursbutcouldhavealsobeenobservedwith1026other salient shapes thatmatch the conditions of the Kanizsa triangle in all other1027ways except for forming the illusory contour percept. This confound could also1028explainpreviousreportsthatKanizsastimuliarefastertobreakthroughCFSmasking1029thancontrolstimuli[31].Thecollinearityofedgesandthetherebyinferredsurface1030maybeprocessedevenwhilethestimulusissuppressedfromawareness–thismay1031inturnproduceanattentionalsignalthatcausesthestimulustobreaksuppression.1032Recentexperimentsthat testedarangeofvisualcontrolstimuliunderCFSsuggest1033that low-level properties of the stimulus determine the time it takes to break1034suppression [52]. Such stimulus-dependent effects are also plausible because1035attentional processing can occur without awareness of the stimulus [53–55].1036However, thisdoesnotprove that anyperceptof an illusory contourwasactually1037formed.Thisprocessmayalsoexplainwhycrowding interfereswithdiscrimination1038oftheinducerorientationbutnotwithillusorycontourformation[56].10391040Whilewe did notmanipulate the presence of illusory contours in thisway in our1041experiments, we nonetheless controlled this factor by including real luminance1042contour conditions. In experiment 3 the presence of a contour should afford an1043improvement on an orthogonal spatial localization task. Such an improvement in1044localizationthresholdsonlyoccurredfortherealluminancecontourorwhenKanizsa1045inducerswerevisible.Improvementslikethisarenotobservedforstimulithatmatch1046theglobalcharacteristicsbutwhichdonotproduceillusorycontours[38].Therefore1047our results from this experiment strongly support the conclusion that illusory1048contoursweresimplynotformedwheninducersweremasked.10491050Ourfindingsalsoagreewithpreviousfindingsthatonlylocalprocessingoccursinthe1051absenceof awarenessbut thatmore complexanalysisof scenegeometry requires1052conscious processing [17]. This would also explain why the nature of stimulus1053representationsinhighervisualareasdiffersdependingonawareness.10541055However, our findings do not accordwith a number of other studies that suggest1056that illusory contours are processed unconsciously. Experiments on a patientwith1057extinctionduetoaparietallesionsuggestthatperceptionofaKanizsashapeoccurs1058evenwhensomeinducersareplacedintopartsofthevisualfieldwherethepatient’s1059conscious perception is impaired [22–25]. These findings indicate that visual1060



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processingoperatesatthesurface-orobject-basedlevel.Segmentingandgrouping1061the local features of an image into a coherent, global shape may only require1062awareness of some component features but it then spreads to the whole object.1063However,thisisnotsufficientevidencetodemonstratethattheseprocesseswould1064occurwhentheparticipantisunawareofallcomponents.Moreimportantly,italso1065does not demonstrate that the percept of illusory contours was actually formed1066under these conditions but only that some processing of the features producing1067illusorycontoursundernormalviewingconditionsstilloccurred.10681069Another study used functional magnetic resonance imaging (fMRI) and an1070inattentional blindness paradigm to study illusory contours processing [50].1071Participantswerepresentedwith a sequenceof images, someofwhich contained1072Kanizsashapeswhilstotherswerevarious typesofcontrol stimuli.Simultaneously,1073participantswereengagedinademandingattentionaltaskatfixation.Asub-group1074of participants subsequently reportednot to have seen any Kanizsa stimuli. These1075participants nonetheless showed stronger fMRI responses to Kanizsa than control1076stimuli.Multivariateclassificationmethodsfurtherdemonstratedthattheactivation1077patterns produced by unseen Kanizsa stimuli were more reliable than those1078produced by control stimuli. The authors suggested that the neural signature of1079illusory contours differs from that of other, carefully matched stimuli and thus1080arguedthatillusorycontoursareprocessedevenwithoutawarenessofthestimuli.10811082This finding is interestingbecause it tests the consequencesof awarenesswithout1083any experimentalmanipulation of the stimuli. The distinction of what is or is not1084processed without awareness only depends on the contents of the participant’s1085consciousness.However,thisalsomakesitdifficulttointerprettheseresults.First,it1086isunclearwhetherparticipantsapparentlyoblivious to thepresenceof the stimuli1087really did not perceive any illusory shapes. Due to the design of the experiment,1088awarenesscouldonlybeassessedafterthemainfMRIexperimentratherthanona1089trial-by-trialbasis(butseeaboveourdiscussionwhysuchtrial-by-trialjudgmentsof1090awarenessarecomplicated).Onlyparticipantswhoreportedhavingseentheactual1091Kanizsastimulusduringtheexperimentwereclassifiedashavinghadawarenessof1092the stimuli. However, many of the candidate stimuli were similar Kanizsa shapes.1093Thereforeitispossiblethatparticipantshadsomeawarenessofthestimuli,evenif1094animpreciseone.10951096Second,while thecontrol stimuli in themain fMRIexperimentwerevery carefully1097matchedtoruleouttheinfluenceofglobalcharacteristicsthiscanbydefinitiononly1098beanapproximation: if conditionswereperfectlymatched, the stimuluswouldbe1099identical and thus an illusory contour would be perceived. It is possible that the1100conditions resulting in an illusory contourpercept are alsoparticularly effective in1101



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producing discriminable activation patterns in visual cortex. For instance, the1102contrast energy along the mouths of the Pacmen inducers differed considerably1103between theKanizsaand theglobal control stimuli. Thus, surface segmentationor1104collinearinteractionsmayhavealsodifferedbetweentheseconditions.11051106All these issueshighlightan importantpoint:Theonlywayonecantruly inferthat1107illusory contoursare formed isbydirect report. This is indeedwhatwedid inour1108previous CFS experiment [18], our present experiment 1, or what was done in1109several other previous experiments [19,30,51,56]. Another useful manipulation is1110thespatiallocalizationtaskusedbyotherstudies[19,38]aswellasourexperiment11113.Thisisbecausetheperformanceenhancementonlyoccursinthepresenceofan1112actualcontourhelpingtheparticipantto localizethetarget.Manypreviousstudies1113suggesting that illusory contoursareprocessedwithoutawarenessof the inducers1114are confounded by this problem, including our experiment 2 (if one assumes that1115thoseresultsarenottriviallyexplainedbyresidualawareness).11161117Neurophysiological and neuroimaging experiments have shown that illusory1118contoursareencodedbyneuronsevenintheearlyvisualcortex[21,29,57–68].The1119overwhelmingmajorityoftheseexperimentswereconductedonawakeparticipants.1120However, some neurophysiological studies reported neuronal tuning to illusory1121contourseveninanesthetizedanimals[21,59,68].Suchfindingsseemtosuperficially1122contradictourconclusionthatawarenessoftheinducingstimuliisnecessaryforthe1123formationofillusorycontours.11241125However, theseexperiments are in fact aperfect illustrationof the importance to1126distinguish between perceptual experience and correlated processing. Because1127animalsareanesthetizedintheseexperimentsandthusbydefinitionunawareofthe1128stimuli, it is impossible to determinewhether illusory contourswere formed. The1129neural correlatesof these stimuli couldbe related to the contextual processingof1130the inducingstimuli,suchasthediscontinuitiesdetectedby“end-stoppedcells”or1131thedetectionofcollinearity intheimage.Suchprocessingmayindeedoccurinthe1132absenceofawarenessasissupportedbyourfindingofcollinearpriming[17]orthe1133induction of the tilt illusion undermasking conditions [2]. Furthermore, it is quite1134likelythatsuchstimulusprocessingisanecessaryprerequisitefortheformationof1135illusorycontours.However,theydonotconclusivelyprovethatillusorycontoursare1136formedunderanesthesia.11371138Arelatedissueisthatallofthesestudiesuseillusorycontoursinducedbyabutting1139lines (offset gratings) rather than Kanizsa shapes. While the two share a similar1140phenomenology, contours inducedbyabutting linesarearguably simpler andmay1141bebasedmostlyonlocalprocessingwhileKanizsashapesarelikelytoinvolvemore1142



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complexinferencesofsurfacedepthandboundaryownership.Ourexperimentsdid1143notexplicitly test illusory contoursgeneratedwithabutting linesand thereforedo1144not speak to the question whether such simpler illusory contours are in fact1145perceivedwhentheinducersaremasked.11461147Weconcludethatthereislittleevidencethatillusorycontoursareprocessedwhen1148participantsarenotawareoftheinducingcontext.Thisappearstobethecasefora1149range of differentmethods to render the inducers invisible. However, all of these1150experiments nonetheless employed physical stimulus manipulations. Only an1151experimentaldesignthatallowstheuseoftheparticipant’sownreporttodetermine1152awareness on a trial-by-trial basis whilst also testing directly whether an illusory1153contourwas in fact formed,eitherbehaviorallyor throughphysiologicalmeasures,1154cananswerthisquestionconclusively.Suchadesignwillhoweverrequiregreatcare1155toensurethatmisclassificationsofawaretrialsdoesnotskewtheresults.11561157Acknowledgements11581159This researchwas supportedby an ERC StartingGrant (310829) toDSS.We thank1160PieterMoors,JacobJolij,andDavidShanksaswellastwoanonymousreviewersfor1161commentsonapreviousmanuscriptandJonasLarssonforinspiringthisstudyinthe1162firstplace.11631164DataandMaterials11651166Allstimuluscodeanddataforthisstudyareavailablefordownloadat:1167https://dx.doi.org/10.6084/m9.figshare.2009952.11681169References117011711. NewellBR,ShanksDR.Unconsciousinfluencesondecisionmaking:acriticalreview.1172

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