Differences in Real and Illusory Shape Perception Revealed by Backward

Vision Research 45 (2005) 91–102 www.elsevier.com/locate/visres

Diﬀerences in real and illusory shape perception revealed by backward masking

Michelle L. Imber a,b,c,*, Robert M. Shapley b, Nava Rubin b

a Department of Psychology, New York University, United States b The Center for Neural Science, New York University, 4 Washington Place, New York, NY 10003, United States c Department of Psychiatry, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02115, United States

Received 4 July 2003; received in revised form 12 July 2004

Abstract

Illusory contours (ICs) are thought to be a result of processes involved in the perceptual recovery of occluded surfaces. Here, we investigate the relationship between real and illusory contour perception using a shape discrimination task and backward masking paradigm. ICs can mask other ICs when times between mask onset and stimulus onset, or SOAs, are very long (300ms), but real contours (RCs) are not similarly effective. Masking is absent for RC masks at perceptually salient contrasts, as well as for those with contrast lowered to match the perceived brightness of the illusory surface. We also find that RCs are not masked at long SOAs, either by ICs or by other RCs. Finally, the masking seen between ICs can occur for different sizes of target and mask. The cross-size masking would not be expected if the masking were at a level sensitive to retinal contour location. The late masking therefore may be related to a higher level of processing of shape categories and surfaces, the level at which shapes defined by ICs and RCs are differentially represented. 2004 Elsevier Ltd. All rights reserved.

Keywords: Illusory contours; Boundary interpolation; Real contours; Size invariance; Shape discrimination

1. Introduction background. A portion of the bounding contour is not supported by a luminance-defined gradient. It is thought Neuroscientists have long tried to understand the that illusory contours result from processes responsible normal processes of the brain by studying what is abnor- for segmentation. These processes are believed to under- mal or unusual. In attempts to dissect the complex com- lie contour completion of occluded and illusory surfaces putations that result in visual recognition of objects, (Kellman & Shipley, 1991). In order to understand these researchers have studied a perceptual illusion known segmentation processes we have studied the percep- as the illusory contour (Kanizsa, 1955; Kanizsa, 1976; tion of shapes bounded by illusory contours and real Petry & Meyer, 1987; Schumann, 1987), or IC. This phe- contours. nomenon, illustrated in Fig. 1, results when observers Numerous psychophysical studies point to perceptual perceive a surface occluding a set of inducing elements interactions between real and illusory contours. For (inducers, or pac-men) over an otherwise homogeneous example, there are interactions between real and illusory lines in a task of vernier acuity (Greene & Brown, 1997) and in versions of famous perceptual phenom- * Corresponding author. Address: Brigham Behavioral Neurol- ogy Group, Brigham and WomenÕs Hospital, 221 Longwood Avenue, ena such as the Poggendorff (Beckett, 1989, 1990)and M-Level, Boston, MA 02115, United States. Bourdon (Walker & Shank, 1988) illusions. Studies have E-mail address: [email protected] (M.L. Imber). been made of common aftereffects involving tilt and

TASK: for elucidating the temporal evolution of the illusory (A) percept. Reynolds (1981) applied such a backward α masking technique to the study of illusory contour OR ? ‘‘microgenesis’’ (time course of evolution). Reynolds presented Kanizsa-type triangles for a duration of 50ms. After various stimulus onset asynchronies (SOAs, (B) the duration between onset of the target and onset of the mask), observers were asked to discriminate between a TEST MASK 1 straight-sided triangle, a curved triangle, or no triangle MASK 2 at all. In some stimulus displays, the triangle was in- tercepted by a brick-wall pattern that was logically 83 42 incompatible with the depth information that would 56 Γ correspond with a perceived illusory triangle. Reynolds 306 found that IC perception could take place by 100ms TIME but that the percept disintegrated 50–100ms later when (C) (msec) the brick overlay was present. He interpreted his results to mean that top-down processes were responsible for TEST MASK 1 MASK 2 the disappearance of the illusory surface at relatively late durations (50–100ms), consistent with a hypothesis-testing model of IC perception. However, the top- 83 42 down interpretation has been questioned in subsequent 56 Γ research on this subject (see Parks, 1994, 1995; Petry 306 & Meyer, 1987; Rubin, 2001 for discussion). TIME Ringach & Shapley (1996) devised a shape discrimi- (msec) nation task to study properties of IC perception (Fig. 1(A)). They and others have shown with a variety of Fig. 1. (A) At right, the shape discrimination task used in all methods, including spatial masking remote from the experiments (after Ringach and Shapley, 1996). At left, a represents the degrees rotation of the top left inducer. (B) Experiment 1: The inducers, that good performance in this task (i.e., dis- sequence of events within a trial, for an Illusory Contour (IC) mask. crimination of shapes with small curvature; see Section Each trial was composed of five frames: stimulus, fixation point, 3.1) depends on the ability to perceive ICs (Gold, Mur- ‘‘pinwheels’’ (local orientation) mask, fixation point, and illusory ray, Bennett, & Sekuler, 2000; Kellman, Yin, & Shipley, square mask. The IC stimuli shown here are not drawn to scale; in our 1998; Ringach & Shapley, 1996; Rubin, Nakayama, & experiment, the support ratio (between the inducer diameter and the illusory square side) was only 25%. The duration of each frame in ms is Shapley, 1996, 1997). shown in the lower left-hand corner. The duration of the fourth frame, To investigate the time-evolution of IC formation, C, was varied across trials. (C) Sequence of events for trials in which Ringach & Shapley (1996) double-masked the illusory- Mask 2 was a real square. shape targets. The first mask contained local orientation information that interfered with the local inducersÕ elements, but that did not have a globally defined orientation masking (Paradiso, Shimojo, & Nakayama, shape. This mask reduced performance on IC-defined 1989; Smith & Over, 1976, 1977, 1979); results were al- shape discrimination when flashed at an SOA of less ways comparable for real and illusory contours. Other than 117ms. At longer SOAs the local mask became researchers have found evidence of binocular rivalry be- less and less effective. The second mask in their tween real and illusory shapes (Bradley, 1982). In these double-mask experiment consisted of a Kanizsa-type cases, it appears that the brain treats illusory contours illusory square that overlapped in position and size with like real contours. the target IC shape (except that its bounding ICs In order to analyze the subprocesses that lead to the were straight, not curved; see Fig. 1(B)). The second perception of an illusory contour, researchers have em- (ÔglobalÕ) mask interfered with task performance at ployed visual masking techniques (Gellatly, 1980; latencies as long as 250–300ms (140–200ms after the Muise, LeBlanc, Blanchard, & de Warnaffe, 1993; Parks, presentation of the first, ÔlocalÕ mask). A no-contour 1994; Reynolds, 1981; Ringach & Shapley, 1996; Weis- (NC) control, with all inducers facing outwards, failed stein & Matthews, 1974) in which the processing of a to mask the illusory shape at this latency. Based on target shape is interrupted or impaired by the presenta- their findings, Ringach and Shapley conjectured the tion of a second figure. Masking is thought to enable the existence of two stages in the processing of ICs. In investigator to disrupt the stream of visual processes, the first stage, local luminance features are detected; and to query the system about its current state at the in the second, the illusory boundary is interpolated into time of the disruption. Such paradigms can be useful a global percept of a shape. M.L. Imber et al. / Vision Research 45 (2005) 91–102 93

The aims of the present study were to investigate the 2.2. Stimuli relationship between the processing of shapes defined by real and illusory contours by means of backward A Silicon Graphics Indigo II computer generated all masking, and in particular to study the proposed sec- stimuli and presented them on a 343mm · 274mm ond stage of processing postulated by Ringach & Shap- screen (resolution of 1280 · 1024 pixels). The refresh ley (1996). We measured participantsÕ thresholds for the rate was 72Hz, and the mean luminance was 16.7cd/ amount of curvature needed to discriminate between m2. Participants sat in a darkened chamber, where they the two different categories of illusory shapes, while viewed stimuli binocularly at a distance of 60cm from manipulating the properties of a late-stage mask. The the screen. late mask could be either an IC-bound shape or a shape Illusory shapes were Kanizsa squares or distorted defined by a homogeneous luminance decrement (which versions of Kanizsa squares. They consisted of four we called a ‘‘real’’ contour, or RC). Further, we used a ‘‘pac-man’’ inducers aligned to produce the illusion of cross-masking technique to test the ability of illusory a figure occluding four disks. The corners of the figure masks to reduce performance based on the perception were located at the center of each inducer, such that of RC targets. Finally, we studied the sensitivity of the length of the figureÕs side was approximately the dis- the late stage of masking to the size of the stimulus. tance between inducer centers. Real shapes were identi- The main results of the experiments were (1) there cal to the analogous illusory shapes except that their was always much less masking of ICs by RCs than by surfaces were colored uniformly to make the location other ICs, and (2) IC–IC masking was not dependent of the shape explicit (see Fig. 1(B) and (C) for examples). on stimulus size. One interpretation of these data is that The inducers and local masks were brighter than the the late masking may be related to a higher level of background at +30% contrast (such that they appeared processing of shape categories and surfaces, at which white against a field of gray, with a luminance of 21.7cd/ level shapes defined by ICs and RCs are represented m2). When real squares were presented, the inducers differently. were at +30% contrast while the real surface appeared dark gray. In most cases, real surfaces were presented at a contrast of À15% (luminance = 14.2cd/m2). For 2. General methods Experiment 2, however, the real surface appeared only slightly darker than the background at a contrast of 2.1. Observers À3% (16.2cd/m2). Unless otherwise noted, all figures subtended 15.6 of visual angle. The inducers sub- A total of 23 observers participated in the experi- tended approximately 2, yielding a support ratio of ments reported here. All observers had normal or cor- l = 0.25 (i.e., 25% of each side of the square was ‘‘sup- rected-to-normal vision, and were naı¨ve as to the ported’’ by the presence of the inducers, while 75% purposes of the experiment. Prior to their acceptance was illusory). into this study, observers completed one or more training/screening sessions (adapted from Rubin et al., 1997) 2.3. Procedure to ensure their ability to perform the task properly and consistently. The initial session generally comprised six Subjects discriminated between two types of shapes blocks of trials, where Block 1 and Block 2 contained (Ringach & Shapley, 1996; see Fig. 1(A), right). To gen- fairly easy practice trials designed to familiarize the erate a test shape, the inducing elements of a Kanizsa subject with the task. In Block 3, the subject made a square were rotated by an angle of a. The top-left more difficult discrimination. Blocks 4 and 5 were iden- and bottom-right inducers were rotated by +a, and the tical, and each contained a mixture of difficult and easy top-right and bottom-left inducers were rotated by Àa. trials. Finally, Block 6 was identical to Block 3. Rubin A positive angle denotes counterclockwise rotation. et al. (1997) showed that this procedure leads to an The angle of rotation for the upper-left-hand inducer abrupt, stimulus-specific form of perceptual learning. is taken, by convention, as the a for a particular stimu- Significant improvement from Block 3 to Block 6 was lus. Thus, shapes with a positive value of a appear to a criterion of acceptance to the present study; specifi- have inward-bulging sides and outward-bulging top cally, subjects whose thresholds exceeded 3.0 on Block and bottom, while shapes with a negative a bulge out- 6 or who performed the task in an erratic manner were ward at the sides and inward at the top and bottom. not retained. Across all four of our experiments, Ringach & Shapley (1996) called these two categories approximately half of the individuals who participated ‘‘thin’’ and ‘‘fat’’, respectively. We did not use these ver- in the screening met these inclusion criteria. All partic- bal labels with our observers, but used only the pictorial ipants were compensated for their participation, either example shapes; nevertheless, in the text below we will with payment or with credit hours that filled a course occasionally use the verbal labels for brevity. For each requirement. subject and condition, five values of ±a were presented 94 M.L. Imber et al. / Vision Research 45 (2005) 91–102

(yielding 10 stimulus levels). Each stimulus level was 3.1.2. Procedure shown 20 times in randomized order using the method The events composing each trial are depicted in Fig. of constant stimuli. In the resulting psychometric func- 1(B) and (C). To initiate a trial, the subject pressed a tions, based on 200 trials each, the angle of inducer rota- button on the computer mouse. This produced a fixation tion a was the independent variable while the dependent mark, which remained on the screen throughout the variable was the fraction of trials in which the subject duration of the trial. The first stimulus in the sequence classified the shapes as ‘‘thin’’. Because a within-subjects was the illusory figure with either positive or negative design was used, multiple psychometric functions were a. It remained on the screen for 83ms, and was followed measured for each subject in a counterbalanced order. by a blank screen (mean luminance, with the fixation Subjects fixated on a small black diamond throughout point) for 42ms. At 125ms from trial onset, a ‘‘local’’ the experiment, located at the center of the figure, and mask consisting of four pinwheel shapes was shown were given audible feedback in the form of a computer for a duration of 56ms. Each pinwheel shared an iden- beep for each correct response. tical radius, center position, and contrast with the inducer previously occupying its location. The local mask, rotated randomly from trial to trial, contained orienta- 2.4. Analysis tion information that, according to Ringach & ShapleyÕs (1996) model, served to limit the local processing time For each experiment, thresholds were computed both for the illusory stimulus to under 125ms. A second for individual observers and for averaged observers blank was then presented, for a variable duration based on pooling of the raw data. Psychometric func- (Cms). Finally, a late-stage mask composed of a square tions were fitted to sigmoidal curves of the form (a = 0) plus inducers was presented for 306ms. The y = ([1 + tanh(B(x À A))]/2), with the slope B and the square could be either real or illusory. At this point in bias A as free parameters. The abscissa was the rotation the trial, the fixation mark disappeared and subjects angle a and the ordinate was the fraction of times the pressed one of two buttons to indicate to which category subject (or subjects, for pooled data) classified the shape the shape appeared to belong (Fig. 1(A)). Within each as ‘‘thin’’ at that value of a. The threshold, defined as the block of trials, a and C were varied, but mask condition amount of (illusory) curvature needed to reach 81.6% (real or illusory) was constant. Block order was alter- correct classification, was computed from the best-fit nated such that all odd blocks contained only illusory curve. Bootstrap simulation (Efron & Tibshirani, square masks and all even blocks contained only real 1993) was employed to determine the statistical reliabil- square masks. Each observer contributed data for both ity of the estimated parameters. One thousand simulated mask conditions and for each of five C durations, yield- experiments were performed for each psychometric ing a total of 10 psychometric functions per observer. function fit, weighted by the subjectÕs actual performance at each stimulus level. Thresholds are reported 3.2. Results ±1 standard deviation of the bootstrap distribution. Un- less otherwise noted, figures depict pooled data. Pooled thresholds, computed by fitting threshold curves to the averaged raw data of all seven participants, appear in Fig. 2. The masking function obtained when 3. Experiment 1: Backward masking of illusoryshapes Mask 2 was an IC square is in agreement with the data with real and illusorysquares reported previously (Imber, Shapley, & Rubin, 2000; Ringach & Shapley, 1996). The IC mask causes an eleva- The first goal in this series of experiments was to rep- tion in discrimination threshold that peaks at intermask licate in a larger sample Ringach & ShapleyÕs (1996) durations of 100ms. When Mask 2 is an RC, however, finding that illusory squares are an effective late-stage no masking is seen. Significant differences between IC mask for illusory shapes, and furthermore to determine and RC thresholds are observed at all points prior to whether ‘‘real’’ (luminance-defined) squares are similarly C = 403ms. A baseline threshold value was obtained effective. from the last block of the training session, in which trials included only three frames—a stimulus, blank screen 3.1. Method with fixation, and pinwheels mask. The reader should note that in subsequent experiments (Experiments 3 3.1.1. Observers and 4), we also re-measured the baseline threshold after Seven naı¨ve observers participated in this experiment. the completion of the study. Because for most subjects Each observer was required to attend and pass a train- some degree of additional learning was evident in those ing/screening session as described above. Data for this experiments, it is likely that the baseline estimate de- experiment were collected in three subsequent sessions picted in Fig. 1 is somewhat higher than the ‘‘true’’ of 30–45min each. value. M.L. Imber et al. / Vision Research 45 (2005) 91–102 95

IC stimulus, IC mask mechanism was behind the diﬀerence in masking eﬀect. IC stimulus, RC mask Alternatively, perhaps the relatively high contrast of IC stimulus, no mask 3 the gray square made the masking square seem irrele- ) s

e vant, and thus easily ignored. In Experiment 2, we tested e r

g the possibility that the RC would be a more eﬀective e d (

mask if its perceived contrast more closely resembled d l

o that of the IC. The contrast of the RC mask was de- h 2 s e

r creased to the smallest just-noticeable-diﬀerence above h T detection threshold that was resolvable by the stimulus delivery system.

1 4.1. Method 100 200 300 400 Intermask Duration (msec) 4.1.1. Observers Fig. 2. Illusory contours are masked by illusory squares but not by Two individuals, who participated in Experiments 1 real squares. When the second mask is an IC, thresholds are elevated and 3 respectively, also served as participants for this for relatively short values of C. No such elevations are seen for real control experiment. masks. A baseline threshold, shown at right, was computed from the last block of the training session, in which the second mask was absent. Thresholds shown were calculated for a ‘‘pooled observer’’ (see text). 4.1.2. Procedure The experimental procedure was identical to that of Experiment 1, except that Mask 2 was a pale gray RC 3.3. Discussion presented at À3% contrast to the background.

This experiment replicates the earlier study of late 4.2. Results masking by IC-defined shapes (Ringach & Shapley, 1996). As previously shown by Ringach and Shapley, Observers reported that the apparent brightness of a the late masking is not explained by masking of inducer pale real square was similar to that of the shape defined elements because a mask that interfered with the induc- by an illusory contour. However, the individual masking ers but did not contain a global shape was ineffective at functions shown in Fig. 3 show that the real square is SOAs longer than 117ms. The results also indicate that not an effective mask for an illusory shape even when RCs do not interfere with IC completion at latencies its apparent brightness is similar. For observer EP, sig- similar to those at which an IC mask is effective. There nificant differences between masking conditions are ob- are a host of possible reasons for this finding. It could be served at C = 97, 125, 137, and 403ms; for observer that different populations of neurons are involved in GE, significant differences are evident at C = 167 and processing IC and RC shapes. Another possibility is that 403ms. the time course of IC and RC processing is drastically different, such that the stimulus and mask do not inter- 4.3. Discussion act temporally at these relatively long masking latencies (with SOAs of 280–350ms). A third alternative is that These data are consistent with pilot data presented the second mask disrupts a process upstream from earlier (Imber et al., 2000), which similarly suggested boundary completion, such as shape representation, rec- that drastically decreasing the contrast of a gray surface ognition, or categorization, and that those processes dif- does not improve its ability to mask an illusory shape. fer for RC- and IC-bound shapes. However, before Because the low-contrast RC-defined shape was similar these questions can be addressed, there remains the pos- in apparent brightness to the IC-defined shape, its inutil- sibility that low-level stimulus properties are preventing ity in masking must derive from some other factor than the RCs from serving as a strong mask. This question apparent brightness. A mechanism relying on perceived was investigated in Experiment 2. contrast can likely be discounted as well. The results of Experiments 1 and 2 imply that there is a fundamental divergence in the processing of ICs and 4. Experiment 2: Backward masking of illusory RCs within the context of the shape discrimination task. shapes with low-contrast real squares This difference could reflect spatial segregation of processing resources, incongruities in time course of Observers reported that the real square appeared ‘‘so processing, or some combination of the two. Another much darker’’ and was thus ‘‘perceptually different’’ vital issue is the point in the time evolution of the task from the illusory shapes of Experiment 1. We thus at which the IC mask causes its disruption. Candidate considered the possibility that a contrast-sensitive stages include boundary completion, shape closure, 96 M.L. Imber et al. / Vision Research 45 (2005) 91–102

Subject EP Subject GE 3 7 IC stimulus, IC mask IC stimulus, IC mask IC stimulus, Pale RC mask IC stimulus, Pale RC mask 6

2 5

1 3 Threshold (degrees) Threshold (degrees)

0 1

50 100 150 200 250 300 350 400 450 50 100 150 200 250 300 350 400 450 Intermask Duration (msec) Intermask Duration (msec)

Fig. 3. Data for two individual observers who participated in Experiment 2. When Mask 2 is a very pale RC, with perceived brightness similar to that of the illusory shape, it still proves an ineffective mask in comparison with the Kanizsa square. object representation, or shape categorization. If shape control for the order of exposure to the different contour categorization is important, one might find it problem- classes. atic that the participants were not asked in Experiments 1 and 2 to make shape judgments about the RC class of 5.1.2. Stimuli stimuli. Therefore, RC stimuli may seem irrelevant to The structure of trials in this experiment followed the decision process in these experiments. To control that used in Experiments 1 and 2. A target stimulus for this possibility, and to explore the interaction be- was presented in the first of five frames. The target tween real and illusory shapes further, a new experiment could be illusory, as before, or a filled-in real surface, was devised in which a 2 · 2 design allowed both RCs at À15% contrast compared to the background. The and ICs to function as stimulus and mask. sides of the surface consisted of circular arcs that were tangent to the tips of the inducersÕ mouths. (See Fig. 4(A) for examples of different shapes defined by RCs.) 5. Experiment 3: Cross-masking between real and The second frame was again a blank screen with fixation illusoryshapes point. It was evident, however, that an alternative to the pinwheels was needed in place of the first mask when In Experiments 1 and 2, ICs alone served as targets the target was a real shape. The new mask must inter- for the shape discrimination. We designed a new task fere with the curved edges of the target surface. Accord- using shapes defined by RCs as targets. By connecting ingly, we created a real surface whose sides were defined the inducers with an arc that was tangent at the tips of by a sinusoidal function (see Fig. 4(B) for example). the inducersÕ ‘‘mouths’’, we made RC versions of the This surface was used as Mask 1 only when the target same kind of curved-sided shapes stimuli used in prior was a real shape; if the target was illusory, the pinwheels experiments. With a 2 · 2 design, we were therefore able were used as before. Each side of the new figure varied to compare RCs and ICs in terms of their ability to randomly in phase across trials. The amplitude of the mask and be masked. sinusoidal edges and contrast of the surface were adjusted to produce a similar level of performance for 5.1. Method shape discrimination in the absence of the second mask, for both IC and RC targets (i.e., in the training ses- 5.1.1. Observers sions). As before, the fourth frame was a blank screen A new set of six observers was recruited, and divided which varied in duration across trials, and the fifth into two groups. For this experiment, two training ses- frame was a square that could be either real or illusory sions were required for each observer. One group of in nature. observers completed the standard six-block training session as described previously. The other group completed a similar training session, except that they first learned 5.1.3. Procedure to make the shape discrimination using real instead of Each subject participated in a total of six sessions. illusory shapes. In the second session of the experiment, The first two were training sessions, as described above. observers trained in the alternate contour condition to In all subsequent sessions, there were four conditions M.L. Imber et al. / Vision Research 45 (2005) 91–102 97

5.2. Results

Fig. 4(C) illustrates the dependence of threshold upon C duration. In general, thresholds were the highest for IC targets masked by IC squares. The masking function peaked at 100ms. Once again, as in our previous experiments, the use of RCs as masks yielded a flat masking function; RCs were ineffective masks for ICs. When the RC was the target stimulus, thresholds were overall slightly lower (by 0.5–1). Neither RC nor IC masks elevated thresholds for RC target discrimination at any of the latencies we tested. Masking, therefore, was elicited only when both target and mask were illusory. As before, we measured thresholds in the absence of the second mask, both before and after the experiment. Subjects showed similar performance for IC and RC targets. This performance did not significantly improve over time, although there was a trend towards lower post-experiment thresholds for RCs. Estimates of the baseline threshold pooled across time (before vs. after the experiment) are also indicated for IC and RC shapes (gray symbols on graph). For IC shapes masked by IC squares, thresholds were elevated above baseline at all Cs tested. For IC shapes masked by RC squares, thresholds were slightly elevated relative to baseline only at C = 125ms. For RC targets, no Fig. 4. (A) Sample RC stimuli and (B) the mask used when the elevations above baseline were observed whether discrimination target was a real contour. Note that in the experiment, masks were real or illusory. the mask was presented with pale inducers on a gray background, with surface at À9% contrast to the ground. The edges are defined by three 5.3. Discussion cycles of a sinusoid, with the phase of each side independently and randomly jittered from trial to trial. (C) Pooled thresholds as a function of the duration between Mask 1 and Mask 2. No cross- The results of Experiment 3 generally replicate those masking between illusory and real contours is seen at these latencies. reported in Experiments 1 and 2. One noticeable differ- At extreme right, pooled thresholds from the last block of the ence is the higher overall magnitude of IC thresholds in training session, measured prior to (hexagonal markers) and following Experiment 3 compared with Experiments 1 and 2. This (inverted triangular markers) the experimental blocks. Only Mask 1 was used in these trials. There is no evidence of significant difference is probably a result of inter-group differences between between RC and IC thresholds or between pre-and post-experimental the subjects of the different experiments. However, be- sessions. Gray symbols indicate pooled baseline thresholds collapsed cause the experiments were designed for within-group across time of measurement. comparisons, the inter-group differences do not affect our conclusions. It is well known that visual objects can mask other that allowed for all possible combinations of real and visual objects at shorter latencies (see, e.g., Breitmeyer, illusory targets and masks. Trials within a block all be- 1984). In pilot studies, we found that the pattern de- longed to the same condition, as before, but again varied signed to mask the RC (Fig. 4(B)) was extremely effec- as to intermask duration (C) and angle of inducer rota- tive in raising thresholds for RCs when presented at tion (a). Each block consisted of 100 trials; a subject short SOAs (<100ms; data not shown). At the mask- completed 40 blocks for a total of 4000 trials and the ing latencies used here, however, real contours appear measurement of twenty psychometric functions (five lev- to be immune to interference from a real or illusory els of C · 4 combinations of stimulus and mask). Block mask. It is very likely that the processes involved in order was counterbalanced across conditions. After the making the shape discrimination for the real contours final session of the experiment, observers repeated the are more rapid in time course than those involved in last block of the training sessions for RCs and ICs. the same task for ICs. However, the nature of the These measurements enabled us to determine whether processes occurring at the long latencies at which further learning had taken place over the course of the ICs can be masked (i.e., 280–350ms) remains to be experiment. elucidated. 98 M.L. Imber et al. / Vision Research 45 (2005) 91–102

6. Experiment 4: Cross-masking between illusory C, 40 blocks of trials, and 20 resultant psychometric shapes of different sizes functions per observer. Block order was randomized and counterbalanced. Again, thresholds from the train- At latencies of 280–400 ms, processes occur that are ing sessions were re-measured after the completion of integral to the task of illusory shape discrimination. The the experiment to test for additional perceptual learning. mechanisms involved appear not to be active when the target is an RC. Where these processes occur in the hier- 6.2. Results archy of the visual system, however, remains unclear. If the masking interferes with a process occurring in early Prior to the experimental blocks, thresholds were visual areas, then it is likely to be driven by the physical comparable for large and small Kanizsa shapes, properties of the stimulus such as retinal size. If the although subjects performed slightly better with the masking is affecting later stages of processing, such as smaller shapes. The results of the experiment (Fig. 5) stages in which the surface is represented or the discrim- indicate that the thresholds were somewhat elevated ination judgment is made, then the way observers cogni- above respective baselines for all experimental condi- tively categorize or represent the stimulus might be more tions, indicating that ICs of different size are capable important to masking efficacy than its low-level physical of masking each other at these long SOAs. Following properties. One way to approach this question is to see if the experiment, some improvement in the perception smaller illusory shapes can mask larger ones, and vice- of both stimuli but especially the small ICs became evi- versa. If they can, it would suggest that the masking is dent, indicating that subjects continued to learn over the mediated by something other than contour interference. course of the study (see Fig. 5). If they cannot, it might suggest that the masking is affecting contour-completion processes, and that those 6.3. Discussion differ between real and illusory shapes. The fact that masking by same-size ICs is equivalent 6.1. Method to masking by different-sized ICs suggests that the late- stage masking is insensitive to the exact positioning of 6.1.1. Observers the masking contours. This lends support to the idea A new group of ten observers participated in this that the late-stage masking involves a level separate experiment. Observers were trained in the same shape from contour-completion processes. discrimination task over the course of two sessions, as in Experiment 3. One group of observers completed the standard six-block training session with Kanizsa targets as described previously. The other group completed 4.0 a similar training session, except that the ICs were of smaller size (70%) than those seen by the first group. 3.5 )

In the second session of the experiment, observers s

e 3.0 e switched to train with the other size. r g e

d 2.5 (

d 6.1.2. Procedure l o

h 2.0 This experiment was in design analogous to Experi- s e r ment 3, which compared cross-masking ability for ICs h T and RCs. Instead of contour type, however, what varied 1.5 across conditions was the size of the illusory shape used 1.0 as Mask 2. The large shapes used in previous conditions (side length = 15.6; inducer radius = 2.0) were paired 100 150 200 250 300 350 400 with smaller IC shapes that were 70% the size of the Intermask duration (msec) originals (side length = 11; inducer radius = 1.4), mak- Large stimulus, Large mask Large stimulus, no mask (pre-experiment) ing them just small enough so that neither the inducers Large stimulus, small mask Large stimulus, no mask (post-experiment) Small stimulus, large mask Small stimulus, no mask (pre-experiment) nor the completed contour overlapped with the larger Small stimulus, small mask Small stimulus, no mask (post-experiment) ICs. The support ratio of l = 0.25 was maintained in both conditions. Mask 1 was always the pinwheel pat- Fig. 5. Pooled thresholds for Experiment 4, illustrating cross-masking tern, adjusted to match the size of its antecedent target between ICs of diﬀerent sizes (subtending 15.6 and 11, respectively). stimulus. This again led to a 2 · 2 design, with four com- At extreme right of graph, data from the last block of the training session, collected just prior to and just following completion of the binations of the stimulus and the second mask (large experiment, indicate some improvement in threshold that is most masked by large, large masked by small, small masked prominent for the smaller shapes. Once again, gray symbols indicate by large, and small masked by small), ﬁve durations of pooled baseline thresholds collapsed across time of measurement. M.L. Imber et al. / Vision Research 45 (2005) 91–102 99

7. General discussion than their illusory counterparts. LOC seems to be needed for surface completion and object recognition We investigated the late-stage masking of illusory (Grill-Spector, Kushnir, Edelman, Itzchak, & Malach, contours. Ringach & Shapley (1996) found that per- 1998a, 1998b; Mendola et al., 1999; Stanley & Rubin, fect-square IC stimuli could serve as masks in an IC 2003). Electrophysiological studies have linked the shape discrimination task. We confirmed their results LOC to a late EEG component ( 290ms, the ‘‘Ncl’’) for a large range of latencies (100–400ms), and applied known to be associated with an effortful, pre-semantic this paradigm to new classes of stimuli. Real contours level of object completion and recognition (Doniger were not effective as late-stage masking stimuli. In con- et al., 2001; Murray et al., 2002; Tulving & Schacter, trast, ICs were effective late-stage masks even when 1990), possibly involving surface representation (Men- they did not overlap spatially with the target ICs. A dola et al., 1999; Nakayama & Shimojo, 1992; Sajda & likely interpretation is that the second-stage masking Finkel, 1995). If copious cortical resources, for instance, studied here is not acting at a site of boundary com- LOC activation, are required for observers to complete pletion, as conjectured by Ringach & Shapley (1996), and to discriminate classes of illusory surfaces, a mask but rather at processing stages in the visual cortex that that appears during the critical time period of the late are involved in shape categorization. Thus, although component is likely to disrupt the task. The ineffective- we have been using the traditional term illusory ‘‘con- ness of RC masks could be a consequence of the fact tours’’ to describe our stimuli, our data suggest that that such stimuli do not activate LOC, and therefore the late masking we observe is actually due to interfer- cannot interfere with the prolonged processing of IC- ence at the representational level of the illusory surface defined shapes. bounded by these contours. This level of representa- The absence of masking of RC-defined shapes can be tion appears to be independent of size and retinal explained in a similar manner. If RCs require less of an location. investment of resources than do ICs, they may generate a smaller and quicker response that is therefore not 7.1. Real and illusory contours prone to masking at late durations. In support of this idea is our finding that RCs are not effectively masked That an illusory mask is required for successful dis- by other RCs at these relatively long latencies. However, ruption of IC processing at this stage, and that it cannot real contours can mask other real contours at much be replaced effectively by an RC mask, suggest that there shorter durations (Breitmeyer, 1984; Weisstein, 1966). are functional differences in the way real and illusory shapes are processed in the visual system. The diver- 7.2. Late masking and attention gence could, in principle, be spatial, temporal, or both. That is, separate populations of neurons could be active While it is by no means identical, the late masking we in perception of ICs and RCs; the RCs could be proc- observed has some characteristics similar to those of essed at a different time scale; or perhaps segregated ‘‘object substitution masking’’ (OSM; Di Lollo, Enns, neural mechanisms are involved at disparate times. Sev- & Rensink, 2000; Enns & Di Lollo, 2001). In the OSM eral psychophysical studies have found evidence for paradigm, a new stimulus (the mask) can perceptually interactions and similarities between real and illusory ‘‘replace’’ the target in visual short-term memory, such contour perception (e.g., Fahle & Palm, 1991; Gegen- that simple visual judgments about the target are no furtner, Brown, & Rieger, 1997; Paradiso et al., 1989; longer available. Two characteristics that differentiate Ringach & Shapley, 1996; Vuilleumier, Valenza, & Lan- OSM from pattern masking (see Breitmeyer, 1984) are dis, 2001; Walker & Shank, 1988; Weisstein & Mat- that the target and mask do not have to share contours, thews, 1974). Physiological studies, however, have and that the mask can lag the target by long delays. found some differences in the relative strengths with Both of these characteristics were found in the late IC which RCs and ICs activate the same cortical areas masking presented here. There is an important differ- (Larsson et al., 1999; Mendola, Dale, Fischl, Liu, & ence, however, between OSM and the IC–IC masking Tootell, 1999), and in the properties of neurons respond- studied in this paper. For OSM to occur, spatial atten- ing to each contour type within an area (e.g., Ramsden, tion needs to be distributed between the target item Hung, & Roe, 2001; von der Heydt & Peterhans, 1989). and other items in the display just before the masking For example, regions of the lateral occipital complex occurs; if attention is focused on the target (e.g. by (LOC) do seem to be activated more weakly by RCs pre-cueing) OSM is greatly attenuated or altogether than by ICs (Larsson et al., 1999; Mendola et al., 1999). eliminated. Consequently, OSM is thought to reflect One plausible explanation for the observed differ- limitations in attentional mechanisms, specifically in ences between IC and RC processing is that RCs might how rapidly observers can shift from a state of distrib- not require LOC to participate in shape recognition. uted attention (many un-cued items) to focused atten- Furthermore, RCs are probably processed more rapidly tion on the (now masked) target. 100 M.L. Imber et al. / Vision Research 45 (2005) 91–102

Although our late IC–IC masking paradigm did not bilateral component of the evoked waveform which at- involve an explicit manipulation of attention, it is rea- tained maximal amplitude with successful closure of sonable to assume that subjects attended to the target the figure; the component, which they termed the shape. At the same time, the relatively large size of Ncl, onset at 232ms and peaked at 290ms over our IC shapes and the need for spatial integration of the lateral occipital scalp (presumably because of acti- the inducers to recover the ICs would favor a state vation of the lateral occipital complex, or LOC). A of spatially distributed, rather than focused, attention, component with similar temporal and spatial proper- at least within the initial phase of each trial. Thus, if ties has been reported during detection of other coher- the act of discriminating the IC shapes itself requires ent, meaningful objects (Murray et al., 2002; Vanni focused attention, the deficit caused by the late IC mask et al., 1996). MEG studies also indicate early activa- may also be related to limitations in mechanisms of tion of LOC in response to IC stimuli (Halgren, Men- attentional shifts. dola, Chong, & Dale, 2003). Functional neuroimaging Using the event-related potential (ERP) technique, studies also have implicated regions of the LOC in ob- Woodman & Luck (2003) were able to demonstrate that ject recognition (Malach et al., 1995) and in IC in OSM the target is, in fact, processed and identified by processing as well (ffytche & Zeki, 1996; Hirsch the visual system—the inability to report the target is et al., 1995; Larsson et al., 1999; Mendola et al., caused only at a later stage, when the observer shifts 1999; but see Stanley & Rubin, 2003). This area is attention to the location where the target was but finds thought to contain neurons with size-invariant recep- the mask instead. Interestingly, this scenario resembles tive fields (Grill-Spector et al., 1998b; Malach et al., the subjective experience in our late IC masking para- 1995; Tootell et al., 1998) which therefore respond in digm: observers often have a sense that they ‘‘saw’’ the a size-invariant manner to ICs (Mendola et al., curved IC shape clearly for a fleeting moment, but nev- 1999). The agreement between the late onset of the ertheless are unable to answer questions about it (specif- Ncl, the long-lasting nature of the masking function ically, in which direction the sides were curved). It would described here, and our finding of size-invariance, to- therefore be interesting to test whether in the IC–IC gether suggest a connection between the processes dis- masking paradigm, too, ERP may reveal there is actu- rupted by the late illusory mask and the generators of ally information in the system about the curved IC the Ncl in lateral occipital cortex. shape which observers are unable to access because of Lateral occipital cortex is known to be active in object the IC masking. recognition processing, including the perception of com- It is believed that OSM ‘‘...defies explanation by cur- plete objects (Bar et al., 2001; Grill-Spector et al., 1999; rent feed-forward theories’’ and instead involves ‘‘a mis- Grill-Spector et al., 1998a; Grill-Spector et al., 1998b; match between the reentrant visual representation and Ishai, Ungerleider, Martin, & Haxby, 2000; Kanwisher, the ongoing lower-level activity produced by current sen- McDermott, & Chun, 1997; Malach et al., 1995). Our sory input’’ (Di Lollo et al., 2000). The hypothesized present understanding of object recognition is highly involvement of feedback in perceptual completion and concordant with a multi-stage model (e.g., Grossberg, IC formation (Murray et al., 2002; Stanley & Rubin, Mingolla, & Ross, 1997; Ullman, 1995), which could en- 2003) suggests another possible link between late IC tail multiple volleys of information through the lateral masking and OSM, at the level of the underlying neural occipital region. Furthermore, the LOC may be special- mechanisms. In summary, there are a number of similar- ized to perform routines designed to detect shape units ities at many levels that invite further exploration of the from incomplete information. Previously it has been pro- relations between attentional control, neural feedback, posed that feedback from higher visual areas including and the late IC–IC masking phenomenon reported here. LOC to V1 may be involved in processing of illusory surfaces and ICs (Murray et al., 2002; Stanley & Rubin, 7.3. Masking and possible brain mechanisms 2003). The late masking we observed could have disrupted this feedback pathway and thereby worsened per- Electrophysiological studies in human subjects sup- formance in the shape discrimination task. port the hypothesis that IC–IC masking is occurring In summary, we find evidence that illusory shapes at the level of processing of object shapes, and that are processed differently from complete real shapes. the masking likely depends upon activity in LOC. The neural mechanisms involved in completed-shape The time delay of the observed IC–IC masking is in discrimination are size-invariant, and are disrupted by close accord with that reported for a negative electrical another illusory mask at durations up to 400ms after potential associated with the closure and recognition stimulus onset. Our findings are consistent with a multi- of incomplete objects (Doniger et al., 2000). Doniger stage, recursive model of visual processing, with illu- and colleagues recorded event-related potentials in sory shape completion and recognition dependent human subjects who viewed and attempted to identify upon signal processing in the bilateral lateral occipital fragmented line drawings of objects. They found a cortex. M.L. Imber et al. / Vision Research 45 (2005) 91–102 101

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