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Neuropsychologia 46 (2008) 554–566

Alternative mode of presentation of Kanizsa figures sheds newlight on the chronometry of the mechanisms underlying the

perception of illusory figures�

M. Brodeur a,b, F. Lepore b, M. Lepage a, B.A. Bacon b,c, B. Jemel d, J.B. Debruille a,∗a Douglas Mental Health University Institute, McGill University, Canada

b Centre de Recherche en Neuropsychologie et Cognition, Universite de Montreal, Canadac Department of Psychology, Bishop’s University, Canada

d Research Laboratory in Neuroscience and Cognitive Electrophysiology, Hopital Riviere des Prairies, Canada

Received 27 February 2007; received in revised form 29 September 2007; accepted 3 October 2007Available online 10 October 2007

bstract

The mechanisms responsible for the perception of illusory modal figures are usually studied by presenting entire Kanizsa figures at stimulusnset. However, with this mode of presentation, the brain activity generated by the inducers (the ‘pacmen’) is difficult to differentiate from thectivity underlying the perception of the illusory figure. Therefore, in addition to this usual presentation mode, we used an alternative presentationode. Inducer disks remained permanently on the screen and the illusory figure was induced by just removing the notches from the disks. The

esults support the heuristic value of this alternative mode of presentation. The P1 deflection of the visual evoked potentials (VEPs) was foundo be greater for the illusory modal figure than for its control and for an amodal figure. This modulation is one of the earliest direct evidences

or a low-level processing of illusory forms in the human brain. Meanwhile, larger N1s were obtained for the control figures than for the illusorygures in the notch mode of presentation. While this new type of N1 modulation could shed some light on the stage of processing indexed by thiseflection, several propositions are put forward to account for the P1 and N1 variations found.

2007 Published by Elsevier Ltd.

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eywords: Kanizsa figure; Modal completion; Amodal completion; P1; N1; Ev

. Introduction

Modal completion (Kanizsa, 1969) involves the elabora-ion of illusory contours that circumscribe a surface definedy the connection of collinear inducers. The modally com-leted surface is also characterized by an increased brightnessnd an impression of foreground segmentation, giving it theppearance of a real, well-defined object. Modal figures are

ufficiently object-like to induce aftereffects (Smith & Over,975, 1979), stereoscopic depth perception (Gillam, Blackburn,

Nakayama, 1999) and, to a lesser degree, geometric illusions

� The study was carried out at the Douglas Mental Health University Institute,cGill University.∗ Corresponding author at: Human Neurocognitive Science Laboratory, Dou-las Mental Health University Institute, 6875 Boulevard, LaSalle, Verdun,uebec H4H 1R3, Canada. Tel.: +1 514 761 6131x3405; fax: +1 514 888 4099.

E-mail address: bruno.debruille@douglas.mcgill.ca (J.B. Debruille).

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028-3932/$ – see front matter © 2007 Published by Elsevier Ltd.oi:10.1016/j.neuropsychologia.2007.10.001

potentials

Li & Guo, 1995; Meyer & Garges, 1979; Walker & Shank, 1987;estheimer & Wehrhahn, 1997). Neural mechanisms involved

n modal completion are generally investigated by recordingrain activity while repeatedly flashing “pacmen”-like induc-rs that delineate the corners of an illusory triangle or squareSeghier & Vuilleumier, 2006). Although this is the traditionalay of presenting these types of figures, it might not be opti-al for investigating the underlying brain mechanisms for two

easons. First, the inducers themselves contain visual informa-ion that is much more salient than the illusory contours. As aesult, the neural response triggered by their sudden appearanceould overwhelm the more subtle response related to the forma-ion of the illusory contours. Second, the fact that the “notches”re flashed in temporal synchrony with the inducers arguably

avours the association rather than the dissociation of inducersnd notches. The dissociation is necessary in order to identify theotches as the key components of a (illusory) form that is inde-endent of the inducers. To circumvent these two problems, only

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he notches may be removed from disks displayed permanentlyn the screen. In doing so, the ratio of illusory contours over theeal inducing contours is maximized. Using this “notch” mode ofresentation, Davis and Driver (1994) observed superior visualearch performances compared to the performances reportedn other studies where stimuli were presented in the classic

ode (Gegenfurtner, Brown, & Rieger, 1997; Grabowecky &reisman, 1989; Gurnsey, Poirier, & Gascon, 1996).

In the present study, we investigated the impact of the modef presentation of illusory figures (classic versus notch mode)n two early brain potentials of the visual evoked potentialsVEPs): the P1 and the N1. The P1 is a positive deflection thateaks at around 100 ms. To this day, there is no consistent directvidence for a P1 effect specific to illusory contours despitehe fact that this deflection is highly sensitive to the amount ofhysical features, including contours (Jeffreys, 1996). One likelyossibility for this lack of effect is that the P1 signal evoked byllusory contours is overwhelmed by the larger P1 signal evokedy the highly contrasted inducers. We hypothesized that if theres indeed a P1 effect induced by illusory contours, it will beevealed by the notch mode of presentation, since this mode doesot give rise to most of the signal evoked by the inducers and,n turn, increases the ratio of illusory contours signal relative tootal signal evoked at onset. Such results would provide directupport for the idea that illusory contours are formed early inow-level visual areas, as suggested by monkey studies showinghat V2 cells respond maximally to illusory contours as soon asround 100 ms (Lee & Nguyen, 2001).

The other early deflection of interest with regards to illu-ory contours is the N1, a negative deflection that peaks around60–170 ms. Its amplitude is systematically larger in responseo illusory figures than to control figures (Brodeur, Lepore,

Debruille, 2006; Herrmann & Bosch, 2001; Herrmann,ecklinger, & Pfeifer, 1999; Korshunova, 1999; Kruggel,errmann, Wiggins, & von Cramon, 2001; Murray et al., 2002;roverbio & Zani, 2002; Sugawara & Morotomi, 1991). Thexact nature of the processes reflected by this N1 effect remainso be determined precisely but it appears likely that these pro-esses involve global object processing. Another N1 effect thateserves attention is the greater N1 evoked by modal figureselative to the N1 evoked by amodal figures (Brodeur et al.,006). The amodal figure is seen as a whole form even thought is occluded. Moreover, it is not defined by illusory contoursMichotte, Thines & Crabbe, 1964). It is hypothesized that the1 difference between modal and amodal figures is due to aeaker global object processing of the amodal figure that is due

o greater difficulty at dissociating the notches from the induc-rs. We therefore predict that presenting the figures in the notchode should minimize these difficulties and substantially reduce

r eliminate N1 differences between modal and amodal figures.

. Methods

.1. Subjects

Twenty-one right-handed subjects (16 females; ages ranging from 19 to 32)igned an informed consent form approved by the Research Ethics Board ofhe Douglas Mental Health University Institute. All subjects had a normal or

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logia 46 (2008) 554–566 555

orrected-to-normal vision and at least a college level of education. They werecreened to ensure that they and their first-degree relatives were free of neuro-ogical or psychiatric disorders. They received a compensation of 20 Canadianollars for their participation.

.2. Stimuli

The stimuli were all composed of four black pacmen inducers outlined by arey circle. In two stimuli, the right-angle notches of the inducers delineated theorners of an illusory square. In the modal square (see the rightmost stimulus inhe sequence depicted in Fig. 1), the segments of the grey circles passing insidehe area of the square were removed. In the amodal square (see the secondtimulus in the sequence depicted in Fig. 1), the grey circles were intact andherefore enclosed the notches inside the inducers. Once the figure is amodallyompleted, perception switches to that of a square seen through four holes locatedver its corners (Ringach & Shapley, 1996). This amodal square however doesot evoke illusory contours. In both the modal and amodal stimuli, the greyircles had a diameter of 3.4 cm (3◦15′ of visual angle) and the square had sides,ncluding the illusory contours, of 4.4 cm (4◦12′ of visual angle). The supportatio, that is the ratio between the length of the real inducing contours relative tohe length of the global figure (Shipley & Kellman, 1992), was therefore 0.77.

ith such a high ratio, illusory contours are usually easily perceived (SeghierVuilleumier, 2006). The two control stimuli were identical to the modal and

modal stimuli, except that their notches were oriented outward (see the firstnd third stimuli in the sequences depicted in Fig. 1).

.3. Procedure

The experiment included two blocks presented in an order that was counter-alanced across subjects. In the classic block (first sequence depicted in Fig. 1),he whole stimulus appeared at the onset and disappeared at the offset, leav-ng the screen totally blank during the interstimulus intervals. In the “notch”lock (second sequence depicted in Fig. 1), the computer screen continuouslyisplayed four complete black disks during the interstimulus intervals. At thenset, the appearance of the stimulus was achieved by having a quarter of eachisk (the notches) disappear. The removal of the notches transformed the disksnto pacmen inducers. At the offset, the notches were made to reappear. In thislock, the removal of the notches and the presentation of the inducers were tem-orally dissociated since the former appeared transiently while the latter alwaysemained on the screen.

Stimuli were presented on a computer screen set with a resolution of40 × 480 pixels and a refresh rate of 75 Hz. In random order, each of the fourypes of stimuli was presented 60 times, for a duration of 600 ms, in each of thewo experimental conditions. The intertrial interval was fixed at five seconds.he subjects’ task was to report the presence or the absence of a square byressing one of two keys on the computer keyboard with their right index finger.odal and the amodal squares were identified as deserving the square response

espite their incompleteness. Subjects were also instructed to fixate the center ofhe screen and not to blink or move their eyes during the stimulus presentation.

.4. Data acquisition

Subjects were fitted with an elastic cap of 32 electrodes positioned accordingo the modified expanded 10–20 system (Electrode Nomenclature Committee,991). Twenty-eight electrodes, distributed all over the scalp (see Fig. 3), weresed to record the electroencephalogram (EEG). The reference electrode waslaced on the right earlobe. Those used to monitor ocular movements werelaced above and below the dominant eye and at the outer canthus of each eye.mpedances were kept below 5 k� for all electrodes. The half-amplitude cut-offsf the bandpass were set at 0.01 and 100 Hz. A 60 Hz electronic notch filter waslso used. Signals were amplified 20,000 times by Contact Precision amplifiersnd digitized at a sampling rate of 256 Hz. Response codes and reaction timesere also recorded.

.5. Data processing and measures

EEG epochs started 200 ms before and ended 800 ms after stimulus onset.he epochs of trials with incorrect responses and those contaminated by ocular

556 M. Brodeur et al. / Neuropsychologia 46 (2008) 554–566

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ig. 1. Modes of presentation. In the classic block, the whole figure appears atnly the notches appear at onset and then disappear; the black disks that serveequence is the modal control. It is followed by the amodal square, the amodal

rtefacts, excessive myogram, amplifier saturation or analog to digital clippingere rejected after a visual inspection of the data. The remaining epochs were

orted by conditions and averaged accordingly. Overall, an average of 54.4pochs was used to obtain the VEP traces in the classic block and 54.1 epochsn the notch block. Voltage was measured relative to a baseline defined by theverage amplitude of the 200 ms signal preceding the stimulus onset.

Amplitudes and latencies of the P1 and N1 were measured. They werextracted by first defining the peak of these deflections for each subject acrossonditions. The P1 and N1 were respectively identified as the greatest posi-ivity and the greatest negativity reached within specific time-windows basedn the grand-average VEP waveforms. P1 measures were set between 80 and24 ms and N1 measures were set between 124 and 188 ms. These measuresere extracted over the electrodes located at the center of the topographic dis-

ribution of the P1 and N1 deflections as revealed by the scalp potential mapsllustrated in Fig. 3. As usually reported (Korshunova, 1999; Murray et al., 2002;roverbio & Zani, 2002), the two deflections were maximal over the posterioregions, and most particularly over O1/O2, T5/T6, and P3/P4. Analyses wereherefore restricted to these electrodes. A secondary analysis was conducted overhe P3 within a time-window set between 300 and 450 ms. This analysis aimed ateeing how the illusory contours and the completion influenced discriminability.

.6. Statistical data analyses

Amplitude and latency measures of the P1, N1 and P3 amplitude wereubmitted separately to a multivariate analysis (MANOVA) with five within-ubject factors: block (classic/notch), modality (modal/amodal), figure typesquare/control), laterality (left/right), and electrode (O/T/P). All significance

evels were two-tailed, with a significance criterion of 0.05. The between blockomparison of the P1 and N1 latency, amplitude and distribution was not intendedt testing the hypotheses but only at examining how the mode of presentationodulates P1 and N1. Within-block post hoc analyses were conducted in the

ig. 2. Reaction times. In the classic block (left panel), reaction times were 766 ms±118) for the amodal square, and 776 ms (±122) for the amodal control. In the noquare, 668 ms (±103) for the modal control, 703 ms (±103) for the amodal square,

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and then disappears, leaving a blank screen between trials. In the notch block,ducers remain on the screen between trials. The first figure presented in eachl, and the modal square.

ase of a significant interaction between the block and the modality and/or thegure type factor.

The MANOVA used for analyzing mean reaction times (RTs) had the block,he modality, and the figure type as factors. This analysis was made not onlyith the correct responses of all subjects but also with a subgroup including the1 fastest subjects. This subgroup analysis was found to be necessary becausehe average RTs were abnormally high for this simple discrimination task.

.7. Dipole analyses

In order to determine the approximate locations of the regions responsibleor P1 and N1 scalp distributions, we used the brain electrical source analysisBESA) algorithm software, version 5.18 (Scherg & Berg, 1991). This methodllows the generation of spatiotemporal dipoles that may be responsible for thecalp distribution of the VEP components of interest P1 and N1. The dipole mod-ling was performed on average re-referenced VEP data. The BESA analysesere first conducted on the average of the VEPs of all subjects for all conditionsith a model of symmetrical dipole. The models that provided the best fit for the1 in a time-window between 97 and 109 ms and for the N1 in a time-windowetween 160 and 179 ms were then applied to the corresponding deflections inach condition in order to test to what extent it accounted for the deflection mod-lations. The location solution was restricted to the grey matter, within a searchange of 3 mm, in the Talairach atlas coordinates (Talairach & Tournoux, 1988).

. Results

(±125) for the modal square, 732 ms (±116) for the modal control, 779 mstch block (right panel), reaction times were of 709 ms (±107) for the modal

and 691 ms (±119) for the amodal control.

.1. Behavioural data

Subjects performed well, with error rates inferior to 1%. RTsnd standard deviations are presented in Fig. 2 (see the legend

M. Brodeur et al. / Neuropsychologia 46 (2008) 554–566 557

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or details). The only within-block differences were shorter RTso modal control relative to the other figures in both blocks (clas-ic block: F(3,60) = 7.00, p = 0.004; notch block: F(3,60) = 8.31,= 0.005). Between-block comparisons showed that RTs to allgures were shorter in the notch block than in the classic blockF(1,20) = 8.38, p = 0.009). The amodal square and its controlarticularly benefited from the notch mode of presentation, withespective reductions of 76 and 85 ms of their RTs relative tohe classic block. These reductions were significantly more pro-ounced than those observed with the modal square (57 ms) andts control (64 ms) (F(1,20) = 26.2, p < 0.001).

Analyses conducted on the subgroup of the 11 fastestubjects arrived to a comparable pattern of results. RT tohe modal square (classic block: 598 ± 86 ms; notch block:75 ± 112 ms), amodal square (classic block: 609 ± 72 ms;otch block: 569 ± 105 ms) and amodal control (classic block:08 ± 88 ms; notch block: 575 ± 115 ms) were comparable buthey were all longer than RT to the modal control (classic block:62 ± 72 ms; notch block: 551 ± 109 ms). These figure differ-nces reached significance in the classic block (F(3,30) = 8.98,= 0.002) but only approached significance in the notch block

F(3,60) = 2.07, p = 0.126).

.2. VEP data

Fig. 3 compares the topographic scalp distribution of the mostrominent early VEP peaks, P1 and N1, elicited in both thelassic and notch blocks for each figure type condition. Both P1

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deflections in each block. A different scale was used for the P1 evoked in eachthe notch block. The N1 evoked in the two blocks is illustrated using the same

nd N1 voltage peaks were mainly located over posterior scalpegions, at occipital and occipito-temporal electrodes, with alear right lateralization of the P1 peak, particularly in the notchlock. The P1, and to a lesser extent the N1, peaked earlier in thelassic than in the notch block. Block effects on P1 and N1 peakmplitude and latency measures were tested separately usingANOVAs with block, figure type, electrode and laterality asithin-subject factors.

.2.1. Modulation of the P1 and N1 by the mode ofresentation

P1 resulting from the average of all subjects peaked at00 ms (±8) in the classic block and significantly later, at05 ms (±3), in the notch block (F(1,20) = 5.55, p = 0.029).he P1s evoked in the notch block were, nevertheless, nots well-defined as those evoked in the classic block. To ver-fy that the 5 ms shift between blocks did not result from thisack of definition, a comparison of latencies was conductedut only for the modal square, because as it can be seen inig. 4, the modal square was the only one evoking clear P1 inoth blocks. Results remained unchanged with peaks at 99 ms±2) in the classic block and 105 ms (±2) in the notch block.his 6 ms difference was significant (F(1,20) = 5.83, p = 0.025).omparisons of P1 amplitude measures revealed additional

ifferences of interest. First, the averaged P1 evoked in thelassic block (3.5 ± 1.0 �V) was larger than the P1 evoked inhe notch block (1.8 ± 0.7 �V) (F(1,20) = 39.3, p < 0.001). Sec-nd, the P1 recorded over right-sided electrodes (3.0 ± 1.1 �V)

558 M. Brodeur et al. / Neuropsychologia 46 (2008) 554–566

Fig. 4. Grand averaged VEPs (n = 21) elicited by the four stimuli in the classic and in the notch block. The full waveforms are presented on the sides. Enlargedportions of the waveforms that include only the P1 and the N1 are placed in the center of the figure. Modal figures are represented by blue lines and amodal figures,b rove tT

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as significantly greater than the P1 recorded over left-sidedlectrodes (2.2 ± 1.2 �V) (F(1,20) = 7.40, p = 0.013) but this lat-rality effect did not interacted with the block factor. Finally,he distribution of the P1 amplitude was different from onelock to the other (F(2,19) = 22.21, p < 0.001). In the classiclock, P1 was greater over occipital electrodes (4.4 �V as com-ared to 3.6 and 2.4 �V for the temporal and parietal electrodes,espectively), while in the notch block, amplitudes did not varyuch between electrodes (1.9, 1.8, and 1.6 �V for the temporal,

arietal and occipital electrodes, respectively).Like the P1, The N1 peaked later in the notch

lock (167 ± 4 ms) than in the classic block (162 ± 3 ms)F(1,20) = 7.20, p = 0.014) but in both blocks, it peaked later

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ver the temporal electrodes (168 ± 3 ms) than over thearietal (163 ± 3 ms) and occipital (164 ± 2 ms) electrodesF(1,20) = 7.42, p = 0.004). Still like the P1, the N1 amplitudeas modulated by the mode of presentation (F(1,20) = 14.8,= 0.001) but it showed no laterality effect. It was also observed

hat the N1 was distributed differently according to the modef presentation (F(1,20) = 10.7, p = 0.001). The maximal ampli-ude was reached over the temporal electrodes (−8.9 ± 1.4 �Vs compared to −7.9 ± 1.5 and −4.4 ± 0.9 �V for the occipi-

al and parietal electrodes, respectively) in the classic block andver the occipital electrodes (−11.1 ± 2.0 �V as compared to10.1 ± 2.1 and −7.0 ± 1.1 �V for the temporal and parietal

lectrodes, respectively) in the notch block.

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.2.2. P1 effectsThe block × figure type × modality interaction revealed that

he mode of presentation induced a marginally significantodulation of the amplitude of the P1 differences exist-

ng between figures (F(1,20) = 4.74, p = 0.042). The figureype and modality factor did not interact with the electrodend laterality factors except for a block × modality × lateralitynteraction (F(1,20) = 5.00, p = 0.037). Further analyses wereherefore conducted to verify whether modality interactedith the laterality in one of the two blocks. Interac-

ion in the classic block was not significant but it almosteached significance (F(1,20) = 4.05, p = 0.058) in the notchlock.

Four within-block analyses per block were conducted toocate the source of the block × figure type × modality inter-ction. Each analysis included a figure factor with two levelshat depended on the analysis (modal square/modal control;modal square/amodal control; modal square/amodal square;nd modal control/amodal control) and an electrode factorith 6 levels. The laterality factor was removed from the

nalyses since it did not interact with the figure type andodality factors. These analyses conducted on the classic

lock data led to no main effect of figure or figure × electrodenteraction. Conversely, analyses performed on the notchlock data showed that P1 to the modal square was signif-cantly greater than P1 to the modal control (F(1,20) = 7.89,

ig. 5. Source analysis of the electric brain activity by a spatiotemporal dipoles modight panel, were based on time-windows of 97–107 ms and 160–179 ms. Time-windowith the latency change specific to each block.

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logia 46 (2008) 554–566 559

= 0.011) and P1 to the amodal square (F(1,20) = 7.21,= 0.014). No other significant difference or interaction was

ound.

.2.3. N1 effectsThe differences in N1 amplitude existing between the figures

ere influenced by the mode of presentation, as indicated bysignificant block × figure type × modality × laterality interac-

ion (F(1,20) = 5.55, p = 0.029). The four within-block analysesonducted in the classic block revealed greater N1 to the modalquare as compared to its control (F(1,20) = 14.8, p = 0.001) ando the amodal square (F(1,20) = 61.1, p < 0.001). These differ-nces were not asymmetrically distributed over the scalp ando other figure comparison reached significance. In the notchlock, figure differences diverged considerably from those notedn the classic block, with the N1 evoked by the modal squareow being the smallest. Its amplitude was significantly smallerhan that of the N1 evoked by the modal control (F(1,20) = 33.5,< 0.001) and the amodal square (F(1,20) = 17.5, p < 0.001).modal square also evoked a smaller N1 than its control

F(1,20) = 6.49, p = 0.019) but the difference between con-rols was not significant. Differences between figures in the

el. The model for the P1 and the N1, respectively illustrated on the left and thes of the deflections to which these models were fit were adjusted in accordance

otch block were more important on the right hemiscalpF(1,20) = 7.48, p = 0.013) mostly because N1s evoked byhe modal and amodal square were smaller over the rightemiscalp.

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.2.4. Dipole source modelingFig. 5 shows the modeled dipole sources for the P1 and

1 evoked components using the BESA algorithm. Dipole fit-ings were first performed on the grand-averaged VEP data ofhe two modality presentation blocks and the four figure types.fter several iterations from different start-up locations, a sta-le solution with a pair of symmetrical dipoles was reached forhe average P1. These symmetrical dipoles accounted for morehan 98% of the scalp recorded P1 (residual variance = 1.6%).heir approximate coordinates were x: −33, y: −71, z: −15.ccordingly, the dipoles were situated within the extrastriate

ortex, and more precisely over the fusiform gyrus, in Broad-ann area 19. The dipoles’ locations in the BESA sphere are

hown in Fig. 5 along with their location in anatomical brain atlaspace (the MNI averaged brain). This projection is intended toive only an approximation of the anatomical region of compo-ent origin. The symmetrical dipole solution, fixed in locationut unconstrained in orientation, was then fitted to the grand-verage P1 evoked by each figure type condition over the intervalf 94–109 ms for the classic block and over the interval of06–117 ms for the notch block. Residual variances obtained forndividual conditions after applying this model are described inig. 5. They were all very low, ranging from 1.6% to 3.1% in

he classic block and from 3.7% to 5.9% in the notch block.In order to model the dipole sources for the N1, we used a

imilar strategy as for the P1. This resulted in a stable symmetri-al dipole solution that reached an even better fit with a residualariance of 0.8%. Again, the symmetrical dipoles were locatedithin the extrastriate cortex (x: −27, y: −78, z: −10), near the

usiform gyrus (see Fig. 5). The fit of this solution to the N1voked in each condition was also very satisfactory, with resid-al variance between 0.8% and 1.4% in the classic block androm 1.1% to 2.3% in the notch block.

.2.5. P3 effectsThere was a significant block × electrode × figure

ype × modality interaction (F(2,19) = 5.52, p = 0.013).he post hoc analyses made in the classic block to search

or the source of this interaction revealed that the P3 to theodal square was significantly greater than the P3 evoked

y the amodal square (F(1,20) = 7.77, p = 0.011) and theodal control (F(1,20) = 32.2, p < 0.001) which in turn were

ignificantly larger than the P3 evoked by the amodal controlF(1,20) = 9.59, p = 0.006 and F(1,20) = 4.59, p = 0.045 on P3nd P4, respectively). In the notch block, the modal square stillvoked a larger P3 (F(1,20) = 20.1, p < 0.001 when comparedo the amodal square and F(1,20) = 42.0, p < 0.001 whenompared to the modal control). However, there was no moreifference between the two amodal figures and the P3 was nowarger for the amodal control as compared to the modal controlF(1,20) = 10.7, p = 0.004).

. Discussion

The classic mode of presentation led to a replication of theattern of results existing in the literature (Brodeur et al., 2006;errmann & Bosch, 2001; Herrmann et al., 1999; Korshunova,

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logia 46 (2008) 554–566

999; Kruggel et al., 2001; Murray et al., 2002; Proverbio &ani, 2002; Sugawara & Morotomi, 1991). The modal squarevoked P1 that was identical to those evoked by the amodalquare and by the control figures but it elicited N1 that wasignificantly larger. The results were therefore not affected byhe relatively long stimulus presentations and by the long inter-timulus interval that extended beyond that used in most of thetudies investigating the VEPs to illusory figures. VEPs wereuite different in the notch mode of presentation as the modalquare evoked larger P1 and smaller N1 than the figures withoutllusory contours. These findings shed a new light on the inter-retation of VEPs to illusory figures. They will be examinedeparately after a discussion of the reaction time data.

.1. Behavioural results

Reaction times (RTs) were surprisingly long for a simpleiscrimination task. Usually, RTs recorded in classic paradigmssing imaging techniques such as VEPs and functional magneticesonance imagery (fMRI) to this type of stimuli in this kind ofask are comprised between 450 and 650 ms (Proverbio & Zani,002; Ritzl et al., 2003). A likely cause for these extended RTslonger than 700 ms in the classic block) is the slow pace of thegure presentation. Subjects may have delayed their responsesince they knew they had ample time between figure appear-nces. Additionally, the fact that they were asked not to blinkuring the figure appearance might have led some of them toait for the offset of the figure before responding. In any case,

he extension of RT seems not to have influenced the behaviouralifference between conditions since the fastest subjects, who hadeaction times closer to those usually reported in the literature,resented the same pattern of figure differences as the wholeroup of subjects.

Three behavioural effects need to be highlighted. First, inhe classic block, responding to amodal figures took more timehan responding to modal figures. Since RT is known to varyccording to the levels of discriminability of incomplete stim-li (Boucart & Humphreys, 1992), it can be assumed that theresent RT effects were mostly caused by the greater difficultyf discriminating amodal squares from controls than of discrimi-ating modal squares from controls. This is most likely becauseerception of the notches, which is necessary for discrimina-ion, is more difficult on amodal inducers than on modal ones.ndeed, the notches do not delimit the global boundary in themodal inducers while they do so in the modal inducers. As aesult, notches are easier to detect in the modal figures since theyodify the global shape of the figure (Gegenfurtner et al., 1997;urnsey et al., 1996).The second interesting but surprising finding was the shorter

Ts to the modal control as compared to the RTs to the othergures in both modes of presentation. Generally, the compar-

son of the RTs to modal and control figures shows invertedattern, with shorter RTs to the modal figure (Proverbio & Zani,

002; Ritzl et al., 2003). This result might be consequent to thelow pace of the figure presentation. When presented with thequare, subject focused their resources on perceiving the illusionithin the square and not on extracting the basic elements that

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llow the simple discrimination required by the task. Althoughechanisms initiated for the induction of illusory contours might

ccur very early, we know that there are later cognitive modu-ations changing the saliency of the illusory contours (Bradley

Dumais, 1975; Coren, Porac, & Theodor, 1986; Pritchard &arm, 1983; Wallach & Slaughter, 1988). These modulationsere probably awaited by the subjects before they decided, whenresented with a modal square, for the presence or the absencef a square.

The third series of relevant behavioural results are the shorterTs observed in the notch block than in the classic block. Theseifferences cannot be caused specifically by an easier perceptionf the squares in the notch block since RTs to control figures werelso reduced. They are more likely to be due to an easier percep-ion of the notches themselves, due to their isolated occurrence.t is noteworthy that RTs to amodal figures were more reduced byhe notch condition than RTs to modal figures, therefore suggest-ng that the notch mode of presentation facilitated the perceptionf the notches in amodal inducers to a greater degree than thosen modal inducers. This probably occurred because the notch

ode of presentation reduced the deleterious effect producedy the arc-lines enclosing the notches of the amodal inducers.

Behavioural effects reflecting difference of discriminabilityan have an impact on VEPs but normally, they occur overatencies beyond the deflections focused upon in the presenttudy (Hagen, Gatherwright, Lopez, & Polich, 2006; Sawaki &atayama, 2007; Senkowski & Herrmann, 2002). For instance,enkowski and Herrmann (2002) reported that N2b (after the1) were larger when evoked in difficult discrimination tasks

s compared to easier discrimination tasks. No effects related toiscrimination difficulties were observed earlier than that deflec-ion. Additionally, as described below, the usual pattern of VEPifferences was replicated in the classic block, which suggestshat the causes for the difference in RTs were not much involvedn the VEP effects.

.2. P3 effect

The fact that the modal square evoked the largest P3 ampli-ude relative to the other stimuli is not surprising for severaleasons: modal square had the highest degree of significance forhe task; it was the most parsimonious stimulus configuration;nd it represented the configuration from which the other stim-lus configurations were derived (see Donchin & Coles, 1988;ohnson, 1984). These larger P3s may however appear incon-istent with the fact that the modal control was responded fasterut, as we highlighted above, reaction times to the modal squareould have been delayed by phenomenological considerations.

At posterior electrode sites, the amplitude of the P3 to a visualarget is known to be larger in conditions of easy discriminationhan in conditions of difficult discrimination (Comerchero &olich, 1999; Hagen et al., 2006). This is consistent with thereater P3 observed in the classic block for the modal square

nd the modal control than for the amodal square and the amodalontrol, respectively. However, in the notch block, the amplitude3s to the two amodal figures were not different, probably result-

ng from an increase of the P3 to the amodal control relative to

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logia 46 (2008) 554–566 561

he P3 to the other figures. This increase might be related to thereater facilitatory effect of the notch mode of presentation onhe discrimination of the amodal figures, a facilitation suggestedy the reaction time effects.

.3. P1 effects

In the notch block, the fact that the P1 evoked by the modalquare was greater than those evoked by the amodal square andhe modal control represents a strong and direct evidence for thexistence of very early mechanisms involved in the perception ofllusory figures. To our knowledge, this effect is the earliest elec-rophysiological correlate of the perception of illusory figures inumans. Although they do not necessarily reflect the same mech-nisms, the P1 effect observed herein parallels cell responses tollusory contours in the extrastriate cortex of monkeys. Indeed,he P1 effect starts around 70 ms and peak around 100 ms, likehe response of most V2 cells that are responsive to illusory con-ours in monkeys (Lee & Nguyen, 2001). Of great relevance islso the fact that the P1 evoked by the amodal square was signif-cantly smaller than the P1 evoked by the modal square, just likehe single-cell responses in area V2 of the monkey, which arelso smaller for amodal than for modal figures (von der Heydt

Peterhans, 1989; von der Heydt, Peterhans, & Baumgartner,984). Finally, according to the present dipole analysis, P1 gen-rators were most likely located in the extrastriate cortex, likeost cells responsive to illusory contours induced by Kanizsa-

ike inducers (Lee & Nguyen, 2001; Nieder, 2002). However,he present generators were located in higher-order areas than2, over the fusiform gyrus, where hemodynamic activations

o illusory figures have already been reported (Kruggel et al.,001; Mendola, Dale, Fischl, Liu, & Tootell, 1999). It must beoted, nevertheless, that the participation of fusiform gyrus inhe perception of illusory contours has not be tested directlyhrough single-cell recordings. Still, selective lesions of V4 in

onkeys, an area covering the fusiform area (Rottschy et al.,n press), has been shown to considerably impair the percep-ion of illusory contours (De Weerd, Desimone, & Ungerleider,996). The infero-temporal (IT) gyrus of monkey, which exhibitsunctions similar to those of the fusiform area in humans, islso likely to be involved in the perception of illusory figure.ts cells respond to illusory stimuli (Sary et al., 2007) and itselective lesion prevents the perception of illusory contoursHuxlin, Saunders, Marchionini, Pham, & Merigan, 2000). Oneay point out that the number of electrodes used to determine

he source was limited. Nevertheless, this number allowed thedentification of a brain area that globally well matched theocation of the P1 reported in studies that used subdural elec-rodes in humans (Arroyo, Lesser, Poon, Webber, & Gordon,997), combined visual evoked potentials with functional mag-etic resonance imaging (Di Russo, Martinez, Sereno, Pitzalis,

Hillyard, 2002), and used simple dipole models based onhe known organization of the human visual cortex (Jeffreys &

xford, 1972).As mentioned in the introduction, a significant increase in the

atio of illusory contours signal relative to total signal evokedt onset can explain why the P1 effect occurred in the notch

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ode of presentation but not in the classic mode. It must beecalled that because illusory forms are not as salient as realorms (Bradley, 1982; Westheimer & Wehrhahn, 1997), cellsoding for these contours are activated to a smaller extent thanhen presented with a real contour (von der Heydt & Peterhans,989; von der Heydt et al., 1984). Results obtained with brainmaging techniques in human, nevertheless, go against the ideaf a smaller response to illusory form. It is generally foundhat hemodynamic brain activities common to both illusory andeal figures are more widespread over the cortex with illusorygures (Ffytche & Zeki, 1996; Hirsch et al., 1995; Larssont al., 1999; Ritzl et al., 2003). However, because of the pooremporal resolution of the fMRI and positron emission tomog-aphy (PET) techniques, it cannot be determined whether thesextended activities to illusory figures do not result from strongop-down modulations which are known to start mostly later,hat is, at the time of the N1 (Halgren, Mendola, Chong, & Dale,003; Yoshino et al., 2006). A second reason for the apparentbsence of a P1 effect to illusory contours in the classic blocks that the latter cover a relatively small space in the stimulusisplay, and therefore activate a small number of cells. Seghiernd Vuilleumier (2006) estimated that, on average, illusory con-ours represent around 40% of the global contour of the illusorygure used in the literature. This proportion does not even take

nto account the circular contours of the inducers and the sur-ace covered by the inducers. A third reason why the P1 effect tollusory contours does not emerge with the classic mode of pre-entation could simply be that the effect indexing the perceptionf the illusory form occurs later, over another deflection. Thislaim is consistent with the fact that illusory figures are detectedaster, in an automatic fashion, when presented according to theotch mode of presentation than when presented according tohe classic mode (Davis & Driver, 1994, 1998; Gurnsey et al.,996). In such case, the harder dissociation between the notchesnd the inducers in the classic block would be responsible forhe delay in the activation to illusory figure processes.

The notch mode of presentation might have revealed the P1ffect to the illusory figure not only because it increased theatio of illusory contours signal over the figure signal but alsoecause it enhanced the visibility of the illusory figure. Theemporal dissociation between the transient occurrence of theotches and the sustained presentation of the inducers indeedacilitates the exclusive integration of the notches into one objectnd, at the same time, the temporal segregation of this objectrom the inducers (see Blake & Lee, 2005 for a review on theeneficial impact of the temporal cues on integration and seg-egation). Only few brain recording studies have used cues tonhance the perception of illusory forms (Bakin, Nakayama,

Gilbert, 2000; Lee & Nguyen, 2001; Seghier et al., 2000).hey all reported activities that were more extensive in very

ow-level visual areas than when no temporal cues where used.or instance, Lee and Nguyen (2001) presented their figuressing the notch mode of presentation and found cells respon-

ive to illusory contours in V2 but also in V1 area (although atater latencies than in V2), where cells are usually found to benresponsive to Kanizsa-like illusory contours (von der HeydtPeterhans, 1989; von der Heydt et al., 1984). In another study,

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logia 46 (2008) 554–566

akin et al. (2000) used disparity cues instead of temporal cueso enhance the depth segregation of modal and amodal bars.hey found that there were cells in V2 that became respon-ive to illusory contours only once they benefit from these cues.inally in humans, Seghier et al. (2000) reported activities in1 that were evoked by illusory figures moving from a set of

nducers to another set (and thus benefiting from the temporalue of motion), but could not report such activities in conditionshere illusory figures were static.One may point out that with the notch mode of presenta-

ion, because of the sustained presence of the inducers, spatialttention was more easily focused over the location where thequare appeared and that this lead to an enhancement of the P1mplitude. P1, but also N1, are indeed known to be increasedhen stimuli appear over an attended location (Heinze, Luck,angun, & Hillyard, 1990; Luck, Heinze, Mangun, & Hillyard,

990). This possibility does not contradict the fact that the P1ffect remains strongly linked to the perception of illusory con-ours since the attentional enhancement of P1 reflects a gainf the signal and not a different signal (Mangun & Hillyard,995). From this point of view, the notch mode of presentationllows for the discernment of a P1 effect because it amplifieshe signal evoked by the illusory square through the benefit ofn increased spatial attention. Nevertheless, this attentional ideaannot account for the lack of a greater P1 evoked by the amodalquare as compared to its control. Therefore, the larger P1 to theodal square is unlikely to be due to greater attention to the

ocation of the square.One interesting aspect of the results is that amodal figures

voked P1s that were more lateralized over the right hemiscalphereas modal figures, including both the square and the control,

voked bilateral P1s. This difference approached statistical sig-ificance (p = 0.058). Bilateral activations in response to modalgures are discrepant with studies reporting that illusory figuresvoked larger hemodynamic (Hirsch et al., 1995; Larsson et al.,999) and magnetoencephalographic (Halgren et al., 2003) sig-als in the right hemisphere as compared to the left hemisphere.imilarly, it has also been shown that repetitive transcranial mag-etic stimulation applied so as to induce a temporary inhibitionf cortical extrastriate activity of the right hemisphere impairedhe perception of illusory figures (Brighina et al., 2003). How-ver, as pointed out by Seghier and Vuilleumier (2006), such anding has so far failed to be systematically replicated in otherrain imaging studies (Mendola et al., 1999; Murray et al., 2002;roverbio & Zani, 2002). It can, nevertheless, be noted that in

he present study, the reference was located on the right ear-obe. This should have attenuated the amplitude of the signalecorded over the right-sided electrodes. However, the lateral-zation of the reference did not prevent the recording of larger1 over the right hemiscalp for amodal than for modal figures.his finding is consistent with results showing that callosotomyatients more easily discriminate amodal figures when they areresented in their left hemifield as compared to when they are

resented in their right hemifield (Corballis, Fendrich, Shapley,

Gazzaniga, 1999).The P1 generators, in addition to their participation to the

nduction of the illusory contours, might also play a role in the

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nduction of other aspects of modal completion. For instance,odal figures are known to be subjectively brighter than the

ackground (Brigner & Gallagher, 1974; Frisby & Clatworthy,975). Difference in brightness, like difference of contours, canodulate the amplitude of the P1 (Johannes, Munte, Heinze,Mangun, 1995; Kaskey, Salzman, Klorman, & Pass, 1980).

owever, the effect of brightness on the present P1 effect isebatable since it is not systematically the brightest figurehat evokes the greatest P1 (Kaskey et al., 1980). In addition,here is a contradictory report arguing that difference in stim-lus brightness intensities does not affect the P1 (Blenner &ingling, 1993). The P1 effect could alternatively be evoked by

he figure-ground segmentation of the modal form. Segmenta-ion is closely associated to the formation of the illusory contourss it comes along with the impression that the form is placed inhe foreground (Coren, 1972; Rock & Anson, 1979). Howevereffreys (1996) showed that the P1s evoked by patterns present-ng monocular or stereoscopic depth information are not greaterhan the P1 evoked by patterns presenting no such information.he VEP effects to segmentation can start as soon as 100 ms but

t generally peaks within the time latencies of the N1 deflectionJeffreys, 1996).

To conclude, the present P1 findings support brain imag-ng (Ffytche & Zeki, 1996; Kruggel et al., 2001; Larsson etl., 1999) and psychophysical studies (Corballis et al., 1999;resp & Bonnet, 1991, 1995; Pillow & Rubin, 2002; Rubin,akayama, & Shapley, 1997) suggesting that illusory contoursay occur very early in the human visual system, probably at the

ame stage where real contours are processed. It must howevere noted that the P1 effect to illusory contours does not rule outhe later participation of cognitive variables in the perception ofllusory contours (Gregory, 1972; Rock & Anson, 1979). Manyate cognitive processes, such as the visual interpretation of thegures, are known to have a significant impact on the saliencyf the illusory figure, but this influence probably follows an ini-ial low-level elaboration of the illusory contours (Bradley &umais, 1975; Coren et al., 1986; Pritchard & Warm, 1983;allach & Slaughter, 1988).

.4. N1 effects

The greater N1 evoked by the modal square in the classiclock replicated what is generally observed when these fig-res are presented in the classic way (Brodeur et al., 2006;errmann & Bosch, 2001; Herrmann et al., 1999; Korshunova,999; Kruggel et al., 2001; Murray et al., 2002; Proverbio &ani, 2002; Sugawara & Morotomi, 1991). As usual, these N1ffects were maximal over T5/T6 and they originated from theateral occipital cortex (LOC). However, there was no asymme-ry in their scalp distribution. Growing evidence suggests thathis N1 modulation reflects object processing, that is, the pro-essing of what can be identified as an object within a limitedpace (Murray et al., 2002; Stanley & Rubin, 2003; Yoshino et

l., 2006). The N1 might also be modulated by the depth process-ng of the modal and amodal figures (Jeffreys, 1996) althoughhere is no evidence showing that this modulation should occuro a lesser degree for the backward segmentation of the amodal

afico

logia 46 (2008) 554–566 563

gure as suggested by its smaller N1. In fact, completion oforms behind occluders is also known to elicit more negativeotentials around the N1 (Caputo, Romani, Callieco, Gaspari,

Cosi, 1999).Several studies indicate that this object processing needs no

llusory contours to be activated, but only an interpolation basedn the definition of a salient space. For instance, N1 (Brodeur etl., 2006; Murray, Foxe, Javitt, & Foxe, 2004; Murray et al.,002; Yoshino et al., 2006) and the LOC activities (Stanley

Rubin, 2003) are still greater for incomplete figures thanor controls even in the absence of illusory contours in incom-lete figures. These illusory contours would rather be activatedetroactively through feedbacks projecting within lower-levelisual areas such as V2 (Halgren et al., 2003; Yoshino et al.,006). This model is not inconsistent with the results obtainedn the present study since no earlier modulation or VEPs to illu-ory contours was found prior to the N1 with the classic modef presentation. However, supposing that the processing of illu-ory contours follows the object perception thrusts aside theationale according to which the P1 illusory contour signal isverwhelmed by the inducer signal in the classic block.

The N1 effects are very different in the notch block with theodal square evoking the smallest N1. In the perspective of the

bject processing model and in accordance with our hypothe-is, it can be assumed that the N1 to the amodal square wasncreased relative to the modal square because the notch modef presentation considerably facilitated the interpolation of thequare. This square is difficult to perceive in the classic blockecause the arc-lines added to the inducers reduce the likelihoodf connecting the notches/corners. As explained in the methods,resenting only the notches at onset increases the likelihood ofssociating the notches to the same object but also increases theikelihood of separating these notches from the elements that doot share the same temporal properties, namely the rest of thenducers. The fact that the amodal figure benefited from a moremportant reduction of RT with the notch mode of presentations consistent with this view. This idea is supported by previoustudies using different kinds of stimuli. For instance, perceivinggroup of target tilted lines within an array of vertical irrelevant

ines is easier when the appearances of the tilted and vertical linesre shifted rather than simultaneous (see Blake & Lee, 2005 forreview). Interestingly, the beneficial effect provided by this

hift does not occur if the targets are already easy to perceiveecause they obviously differ from the irrelevant elements. Fornstance, perception of target horizontal lines presented withinn array of irrelevant vertical lines will not benefit much from ahift of their respective appearance (Leonards, Singer, & Fahle,996). The impact of temporal cues on the perception of theodal square could thus not occur because this square already

enefits from excellent spatial cues.The object processing model cannot fully account for some

f the other N1 differences found in the notch block, particu-arly the greater N1s to the controls. In fact, these differences

ppear to be a consequence of new factors that influence allgures except for the modal square. The N1 effect to object pro-essing may therefore still be present, but in conjunction withther simultaneous modulations that have a stronger impact on

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he amplitude of the VEPs. In support to this claim, it can beecalled that because the N1s evoked in both blocks did notave exactly the same scalp distribution, they might reflect theolicitation of different processes. As a possible factor, one mayall to mind a variable related to a perceptual problem. Figuralnformation in the modal control is incompatible with the tem-oral information provided by the appearance of the notches. Onhe one hand, figural information suggests that the notches arendependent from one another because they are located on thexternal part of the inducers and therefore, cannot be intercon-ected. On the other hand, notches tend to be grouped togetherecause they share the same onset and offset. Elements thathare similar temporal features (motion, direction, appearance)re normally grouped together and distinguished from the ele-ents characterized by different temporal features (Alais, Blake,Lee, 1998; Fahle, 1993; Kojima, 1998; Lee & Blake, 2001;

eonards et al., 1996; Usher & Donnelly, 1998). The greatest1 to the modal control relative to the modal square could there-

ore reflect the additional resources allocated in response to thiserceptual problem. The modal square does not present suchncompatibility because its notches are placed in a way thatavours their connection. The amodal square is more ambigu-us. Connection is favoured by the placement of the notches buthe enclosure disrupts it. This ambiguity would explain why the1 evoked by the amodal square is larger than those evoked by

he modal square and smaller than those evoked by the amodalontrol.

In conclusion, P1 and N1 differences obtained with the notchode of presentation represent a major change compared to the

esults obtained with the classic mode. These new results thusntroduce important new constraints in the interpretation of theisual potentials evoked by illusory figures. Further investiga-ions are necessary to test the hypotheses pertaining to the preciseunctional significance of each of these differences.

cknowledgement

Mathieu Brodeur was supported by fellowship MDR-54627rom the Canadian Institute for Health Research (CIHR); J.runo Debruille by scholarship MSH-40304 from the same

nstitute. Franco Lepore holds a Canadian Research Chair of theational Sciences and Engineering Research Council of Canada

NSERC). The study was supported by grant 96997-10 from theonds de la Recherche en Sante du Quebec allocated to J. Brunoebruille.

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