PSYCHOLOGICAL SCIENCE. Research Article

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1 Research Article MUST FIGUREr43ROUND ORGANIZATION PRECEDE OBJECT RECOGNITION? An Assumption in Peril Mary A. Peterson^ and Bradley S. Gibson^ ^University of Arizona and-^ Johns Hopkins University Aistract The assumption that figure-ground segmentation must precede object or shape recognition has been central to theories of visual perception. We showed that assumption to be incorrect in an experiment in which observers reported the first perceived figure-ground organization of briefly exposed stimuli depicting two regions sharing a figure-ground border. We manipulated the symmetry ofthe two regions and their orientationdependent denotivity (roughly, their meaningfulness), and measured how each of these variables influenced figure-ground reports when the stimuli were exposed for 14, 28, 57, 86, or 100 ms, and followed immediately by a mask. Influences on figureground organization from both symmetry and orientationdependent object recognition processes were found; both were observed first in the 28-ms condition. Object recognition inputs did not dominate symmetry inputs. We suggest that object recognition processes may operate simultaneously on both sides of edges detected before figure-ground relationships are determined. Figure-ground organization entails segmenting the visual field into regions that are figures and regions that are backgrounds. Figures are patently endowed with shape and appear to occlude backgrounds; the latter appear to be shapeless near the borders they share with figures. Since the early 20th century, most theories of visual perception and object recognition have been grounded on the assumption that regions must be registered as figures before they can be matched to representations of objects in memory (Biederman, 1987; Hebb, 1949; Kosslyn, 1987; Marr, 1982; Palmer & Rock, 1994; Rock, 1975; but see Lowe, 1985, who argued that figure-ground organization need not precede object recognition). One reason for the prevalence of the assumption that figure-ground segregation precedes object recognition is that effects of prior experience are thought to be mediated by a similarity match between some description ofthe current stimulus and a representation in memory. Many theorists have argued that this similarity match requires the prior presence of a shape, and, therefore, the prior determination of figure-ground relationships (Epstein & De- Shazo, 1960; Gottschaldt, 1929a, 1929b; Rock, 1962, 1975; Wallach, 1949). Therefore, despite the fact that many other perceptual phenomena have proved to be permeable to influence from memory or other top-down processes (e.g., the wordsuperiority effect; Reicher, 1969), most theorists have continued to maintain that figure-ground organization and depth segregation must be immune to influence from processes reflecting Address correspondence to Mary A. Peterson, Department of Psychology, University of Arizona, Tucson, AZ 85721; tnapeters ccit.arizona.edu. The second author is now at Notre Dame University. past experience (i.e., top-down processes) and must depend only on variables that can be computed from tbe current stimulus array (bottom-up variables). In this article, we report an experiment that challenges the assumption that figure-ground relations must first be resolved before representations of objects in memory can be accessed; in so doing, we challenge the bottom-up disposition regarding figure-ground organization as well. In our experiment, observers viewed brief exposures of figure-ground stimuli and reported the first perceived figure-ground organization. Each stimulus depicted a white region and a black region side by side, sharing a central border (or edge); this is the figure-ground border of interest in this experiment. Samples of the stimuh we used can be seen in Figure I, Immediately after viewing a stimulus, observers pressed a key to their right or their left to indicate which region had appeared to be interposed in front of the other at the central border (i.e., which region had appeared to be figure). We manipulated two variables potentially relevant to figureground organization. First, we manipulated potential top-down inputs to figure-ground organization from object recognition processes. To do so, we designed the two sides of tbe central contour to differ in how explicitly they depicted commonplace objects, as measured in a preliminary study in which observers listed which objects were depicted along the two sides of the contour. For one side (e.g., the black side of each stimulus in Fig. 1), agreement was relatively high (M = 89%), whereas for the other side (e.g., the white side of each stimulus in Fig. 1), agreement was low im = 27%). We refer to the regions that elicited high agreement as high-denotative regions and the regions that elicited low agreement as low-denotative regions. (We have found that it is virtually impossible to create a region that fails to elicit an interpretation held in common by at least some ofthe observers. The critical difference between the highand low-denotative regions is tbe extent to which observers agree about what object is depicted.) To assess whether object recognition processes contribute to figure-ground organization, we presented the stimuli in both upright and inverted orientations and compared the likelihood of choosing the high-denotative region as figure in tbe two orientations. It is well documented that inverting stimuli that have a canonical upright increases the time required for recognition (or precludes recognition), presumably because memory representations code for a specifically oriented object, and a timeconsuming correction must be applied before misoriented objects can be matched to those representations (Johcoeur, 1988; Rock & DiVita, 1987; Tarr & Pinker, 1989), Our orientation manipulation depends on the time-consuming nature ofthe correction process: If the correction process is not finished before figure-^ound relationships are determined, inputs to figureground organization signaling the goodness of fit between the VOL. 5, NO. 5, SEPTEMBER 1994 Copyright 1994 American Psychological Society 253

2 Object Recognition manipulation changes only the goodness of the initial match between the high-denotative regions and their best-fitting representations in shape memory. Thus, if symmetry alone infiuences figure-ground organization, stimulus orientation should be irrelevant. The same basic pattern of symmetry efi'ects should be obtained for both upright and inverted stimuli. The critical evidence for object recognition contributions to the first perceived figure-ground organization would be that observers report seeing high-denotative regions as figure more often in upright than in inverted stimuli. One important question is whether infiuences of object recognition, if present, can be observed at exposure durations as short as those at which influences of symmetry, which do not require access to representations in memory, are evident. METHOD Fig. 1. Upright versions of the stimuli used in this experiment. (Inverted versions can be seen by rotating tbe page 180.) Crossing symmetry of the low-denotative (LD) region with symmetry of the high-denotative (HD) region yields a 2 x 2 table of stimuli. In this figure, HD regions are black and located on the left of each stimulus; LD regions are white and located on the right of each stimulus. In the experiment, HD regions were black half of the time and white the other half of the time, and on the right half of the time and on the left the other half of the time. S = symmetric; A = asymmetric. high-denotative regions and their best-fitting representations in object memory may be present for upright stimuli but absent for inverted stimuli. Thus, if orientation-dependent object recognition processes contribute to figure-ground organization, we expect that the high-denotative regions will be seen as figure more often when the stimuli are upright than when they are inverted. (The logic of this manipulation has been confirmed in previous experiments that, unhke the present experiment, examined reversals of figure-ground organization; Peterson & Gibson, 1993, in press; Peterson, Harvey, & Weidenbacher, 1991, Reversals do not necessarily provide evidence regarding initial organization, which is the theoretically critical state, whereas the current experiment does.) The second variable we manipulated was the symmetry of the two regions sharing an edge. We considered a region to be symmetric if a vertical line drawn through its center divided it into two regions that were mirror images of each other. Since the time of the gestalt psychologists, symmetry has been counted among the variables that contribute to perceived figure-ground organization (e.g., Harrower, 1936). Symmetric regions are more likely to be seen as figure than are asymmetric regions. Despite its configural nature, symmetry has been considered to be a bottom-up variable because it can be extracted from the current display. If inouences of symmetry on figureground organization can be observed in our task, symmetric regions should be seen as figure more often when paired with asymmetric regions than when paired with symmetric regions. The orientation manipulation leaves unchanged all bottomup inputs to figure-ground organization (e.g., regarding the relative symmetry, convexity, or area of the two regions); this Subjects Each of a total of 240 observers viewed the stimuli for one of five exposure durations: 14, 28, 57, 86, or 100 ms (48 observers per group). Stimuli and Apparatus Each stimulus consisted of one black region and one white region sharing a central border. These stimuh were displayed against a perceptually medium-gray background (determined by pretest). Each stimulus was constructed so that one side ofthe central border between the two paired regions was high in denotivity and the other was relatively low in denotivity, as determined by pretesting (see the introduction). The highdenotative side of the central border always depicted an object that had a vertical axis of elongation or symmetry, and that had a typical upright orientation. We systematically varied whether the high- and low-denotative regions were asymmetric (A) or symmetric (S): In AA stimuli (upper left quadrant of Fig, 1), both regions were asymmetric; in SS stimuli (lower right quadrant of Fig. 1), both regions were symmetric; in AS and SA stimuli (the remaining quadrants of Fig, 1), one region was symmetric and the other asymmetric (the first letter refers to the asymmetry or symmetry of the high-denotative region, and the second letter refers to the asymmetry or symmetry of the lowdenotative region). The two regions of each stimulus were approximately equal in total area and in summed local convexities across each edge. Convexity has been shown to be a powerful cue to figureground organization (Kanizsa & Gerbino, 1976; Stevens & Brookes, 1988); it was therefore critical to equate the two regions for convexity. We were able to construct only 12 stimuli in which the areas and local convexities of the two contiguous regions were approximately equal, yet one region was low in denotivity whereas the other was high in denotivity, depicting an object with a typical orientation. The symmetric highdenotative regions depicted a lamp, a house, and a wine glass; the asymmetric high-denotative regions depicted a wrench, a seahorse, and a profile of a woman's head. Each of the highdenotative regions was paired once with an asymmetric low- 254 VOL. 5, NO. 5, SEPTEMBER 1994

3 Mary A. Peterson and Bradley S. Gibson denotative region and once with a symmetric low-denotative region. Upright and inverted versions of these stimuli were created. ' We used this first set of stimuli, containing curved edges, to create a second set of straight-edge stimuli: Following a procedure used by Attneave (1954), we identified the minima and maxima of curvature in the contours of the curved-edge stimuli and connected these points by straight line segments. Curvededge and straight-edge versions of one of the stimuh are shown in Figure 2. Contour curvattire and local convexities were eliminated in this second set of stimuli, which was included to allow us to evaluate the role of those variables in the object recognition processes under examination. The masking stimulus was a rectangular field of thick lines and splotches that concealed the minima and maxima of contour curvature when the stimulus and the mask were viewed simultaneously. The stimuli were displayed on a Princeton monitor controlled by a Compaq 386/25e. The stimuli subtended 2.5 to 4.2 in width and 2.9 to 3.1 in height from the observer's viewing distance (130 cm). Tbe mask subtended 7.9 by 4.5. Procedure Observers were instructed about figure-ground organization while viewing the Rubin vase-faces stimulus and a sample stimulus similar to the stimuli in the experimental set. They were asked to make figure-ground decisions regarding the central border, reporting as figure whichever region appeared to be the occluding region at the central border. Observers were encouraged to guess randomly when they could not determine which region appeared to be figure. They were told that they would see many novel shapes among the stimuli they were viewing. Before each stimulus was exposed, observers fixed their eyes on a cross that was centered on a point through which the central contour of the figure-ground stimulus would pass when it was subsequently displayed. The fixation cross was exposed for 500 ms and was followed by the stimulus, which was exposed for 14, 28, 57, 86, or 100 ms. Immediately following the timed stimulus exposure, the masking stimulus was presented for 200 ms. Observers responded by pressing a key to their right or to tbeir left to indicate whether the region to the right or to the left of the central contour had appeared to occlude the other region (i.e., had appeared to be figure). Observers' responses were summarized as the proportion of choices of the highdenotative regions as figure. Each subject viewed each ofthe 24 stimuli (12 curved-edge stimuli, 12 straight-edge stimuli) once upright and once in- I. Symmetric low-denotative regions sharing a central contour with high-denotative regions necessarily had the property of depicting highdenotative regions along both their vertical edges. Therefore, when possible, we designed the asymmetric low-denotative regions to have this same property. Thus, the contours of low-denotative regions depicted high-denotative shapes on their converse sides, and the contours of high-denotative regions depicted low-denotative shapes on their converse sides. If figures only are matched to representations in shape memory, this property should never influencefigure-groundorganization. Fig. 2. The curved-edge version (a) and the straight-edge version (b) of a stimulus with two asymmetric regions. (The bighdenotative region, on the right in each version, depicts a wrench.) verted.^ The different types of stimuli (curved vs. straight edge) were viewed in different blocks; half the subjects viewed the curved-edge stimuli first, and the other half viewed the straightedge stimuli first. Observers viewed upright versions of half the stimuli of a given type first, and inverted versions second. Across all observers, a given stimulus was viewed first an equal number of times in its upright orientation and in its inverted orientation. No stimulus of a given type was seen for a second time until all stimuli of that type had been seen once (either upright or inverted). The lightness (black vs. wbite) and the location (right vs. left) ofthe high-denotative region were counterbalanced within and between subjects, which required a cell size of 24 subjects. (For each subject, the lightness and location of a given stimulus were maintained across upright and inverted orientations and across curved- and straight-edge versions.) ln order to permit an independent test of effects of order of viewing the different stimulus types, we tested 24 observers in each group with curved-edge stimuli first and 24 with straight-edge stimuli first. RESULTS AND DISCUSSION The straight-edge stimuli were included to allow us to assess the infiuence of curvature and local convexity on figure-ground organization in general and on contributions of object recognition to figure-ground organization in particular. The order in which the observers viewed the straight-edge versus curvededge stimuli did not infiuence responses (F < 1 for the main 2. Two of the 48 observers in each exposure duration failed to see one of the curved upright stimuli in the AS condition and one of the curved inverted stimuli in the SA condition. VOL. 5, NO. 5, SEPTEMBER

4 Object Recogtiition effect of block order and ps >,20 for two-way interactions involving order). Stimulus type did make a difference, albeit a small one: Overall, bigh-denotative regions of curved-edge stimuli were seen as figure more often than high-denotative regions of straight-edge stimuli (.62 vs..58), as revealed by a main effect of stimulus type, Fil, 235) = 20.50, /? < Despite this main effect of stimulus type, the pattem of results obtained for straight-edge stimuli was similar to that obtained for curved-edge stimuli. Edge curvature seemed to have little infiuence on the object recognition processes under study here (see also Attneave, 1954). The two types of stimuli are discussed separately only where differences occurred. The main question of interest was whether orientationdependent object recognition processes would influence the first perceived figure-ground organization. We hypothesized that if there were such an infiuence, high-denotative regions would be seen as figure more often when the stimuli were exposed in an upright than in an inverted orientation. Consistent with this hypothesis, we found that for stimuli exposed for 28 ms or longer, observers saw the high-denotative regions as figure more often when they viewed upright stimuli than when they viewed inverted stimuli (see Fig. 3a). The overall analysis of variance (ANOVA) showed a main effect of orientation, F(l, 235) = 98.18, p <.0001, and an interaction between orientation and exposure duration, F(4, 235) = 9.95, p < In separate ANOVAs for the various exposure durations, main effects of orientation were obtained for all exposure durations except 14 ms: For the 28-ms, 57-ms, 86-ms, and 100-ms groups, all ps <,002; for the 14-ms group, p >,20. The finding that highdenotative regions were chosen as figure more often when they were upright than inverted demonstrates that object recognition processes contribute to the first perceived figure-ground organization for stimuli exposed for 28 ms or longer. Effects of symmetry (which, at least theoretically, require no access to representations of objects in memory) were also observed first in the 28-ms condition. High-denotative regions were more likely to be seen as figure when they were symmetric rather than asymmetric, as revealed by a main effect of the symmetry ofthe high-denotative region, F(l, 235) = 331.5, p <.0001 (see Fig. 3b). In addition, higb-denotative regions were less likely to be seen as figure when the low-denotative regions were symmetric rather than asymmetric, as revealed by a main effect ofthe symmetry of the low-denotative region, F(l, 235) = , p <,0001 (see Fig. 3c). In ANOVAs conducted separately for each condition of exposure, main effects of the symmetry ofthe high-denotative region and ofthe symmetry of the low-denotative region were obtained in all conditions except the 14-ms condition: For the 28-ms, 57-ms, 86-ms, and l(k)-ms conditions, all ps <,01; for the 14-ms condition, ps >.40. The effects of the symmetry of the low-denotative region increased with exposure duration up to 86 ms, for both types of stimuli. The effects of the symmetry of the high-denotative region increased with exposure duration up to 57 ms for the curved-edge stimuli and up to 86 ms for the straight-edge stimuli. (This effect was evident in a three-way interaction junong exposure duration, the symmetry of the high-denotative region, and the symmetry ofthe tow-denotative region, F[4, 235] = 2.%, p <,03.) Orientation modulated the magnitude of the symmetry effects, as can be seen in Table 1. The orientation-dependent S Exposure Duration (ms) Exposure Duration (ms) Fig, 3, The proportions of choices of the high-denotative (HD) region as figure as a function of exposure duration; standard errors were,02 or.03. The panels show the effects of (a) orientation, (b) the symmetry of the high-denotative region, and (c) the symmetry of the low-denotative region. Asterisks indicate that the difference between the dark and light columns was statistically significant VOL. 5, NO. 5, SEPTEMBER 1994

5 Mary A. Peterson and Bradley S. Gibson Table 1. Proportions of choices ofthe high-denotative region as figure High-denotative region Asymmetric Symmetric Mean Asymmetric Symmetric Mean Low-denotative region Asymmetric Symmetric Upright stimuli Inverted stimuli Note. Values are averaged from all exposure duration conditions except 14 ms. Mean increase in the likelihood of seeing the high-denotative region as figure was larger when the high-denotative region was symmetric than when it was asymmetric. This finding suggests that orientation-dependent matches to representations of objects in memory may be facilitated for symmetric relative to asymmetric high-denotative regions (see also McMullen & Farah, 1991; Tarr & Pinker, 1990). In addition, the orientation-dependent increase in the likelihood of seeing the high-denotative region as figure was larger when the low-denotative region was symmetric than when it was asymmetric (see Table 1), at least for curved stimuli exposed for 57 ms or longer. This latter finding may simply reflect the strong influence of symmetry in the inverted condition. These effects were shown to be significant by an interaction between orientation and the symmetry of the higb-denotative region, F(l, 235) = 10.35, p <.002; by a threeway interaction among exposure duration, orientation, and the symmetry of the low-denotative region, F(4, 235) = 4.81, p <.(Wl; and by a three-way interaction among type, orientation, and the symmetry ofthe low-denotative region, F(l, 235) = 5.14, p <.03.^ Evidence regarding how symmetry inputs and object recognition inputs combine can be obtained by examining stimuli in which an asymmetric high-denotative region was paired with a symmetric low-denotative region (AS stimuli). As expected if symmetry inputs are present but object recognition inputs are absent or diminished for inverted stimuli, the high-denotative 3. The stimuli were displayed so that the central contour separating the high- and low-denotative regions passed through the point on which the fixation cross had been located. Hence, regions lying to the right and regions lying to the left of the central contour may have been analyzed first by the left and right hemispheres, respectively (although the central contour may have been analyzed initially by both hemispheres, given that it lay predominantly on the fovea). A question raised by one of the reviewers led us to test whether any differences were observed as a function of right-versus-left location. We found that for inverted SS stimuli, high-denotative regions were chosen as figure more often when located to the right than to the left of the central contour; such differences were not observed for upright SS stimuli. No other differences due to left-versus-ri^t location relative tofixationwere found. region of inverted versions of AS stimuli was chosen as figure significantly less often than predicted by chance factors alone (.38; see Table 1); hence, the symmetric low-denotative region was preferred as the figure. This symmetry-driven preference was not evident in responses about upright AS stimuli, however; in this case, the high- and low-denotative regions were chosen as figure equally often (.48 reports of the highdenotative region as figure; see Table 1). The differences between these two conditions were significant for each condition of exposure duration from 28 ms through 100 ms (all ps <.05). This finding suggests that object recognition inputs to figureground organization can alter the figure-ground process, rather than simply facilitate it. The responses about upright AS stimuli suggest funher that, when present, object recognition inputs to figure-ground organization do not dominate symmetry inputs. The prevailing preference for bottom-up explanations of early visual processes has been motivated in part by an implicit assumption that top-down processes, should they intervene, would necessarily dominate bottom-up processes. Further evidence regarding bow object recognition inputs combine with symmetry inputs can be obtained by comparing the figure-ground organization perceived for upright AS stimuli and upright AA stimuli. Stimuli in these two conditions consist ofthe same asymmetric high-denotative regions, but AS stimuli consist of symmetric low-denotative regions, whereas AA stimuli consist of asymmetric low-denotative regions. If object recognition inputs dominate symmetry-determined organizations, then the likelihood of seeing a given upright high-denotative region as figure should not depend on the symmetry of the low-denotative region with which it shares a border. Alternatively, if object recognition inputs are simply one of many cues to figure-ground organization (Peterson, in press), then highdenotative regions should be chosen as figure less often in upright AS stimuli, in which symmetry and object recognition inputs to figure-ground organization compete, than in upright AA stimuli, in which no competition exists. Consistent with the latter view, we found that high-denotative regions were seen as figure significantly less often in upright AS stimuli than in upright AA stimuli (.48 vs..61, respectively; see Table 1). This difference was significant in an analysis including all conditions of exposure together and in tests conducted on data from the 28-ms, 86-ms, and 100-ms conditions individually, all ps <.003; the difference was marginally significant for the 57-ms condition, p <.08. Thus, our results demonstrate that object recognition processes do not dominate inputs regarding the symmetry ofthe two regions sharing a border. Instead, object recognition inputs simply appear to provide additional information about which of the two regions on opposite sides of a contour is most likely to be the figure. SUBSTRATE FOR OBJECT RECOGNITION INFLUENCES These results, demonstrating that object recognition processes operate before figure-ground organization, revive an old conundrum: How can object recognition processes operate before figure-ground organization provides shaped regions to serve as their substrate? VOL. 5, NO. 5, SEPTEMBER

6 Object Recognition Prefigtirat Recognition Processes We have proposed that the object recognition processes that contribute tofigure-groundorganization iprefigural recognition processes) operate on edges per se and not necessarily on the edges of regions already determined to be figure (Peterson, in press; Peterson et al., 1991; Peterson & Gibson, 1993, in press). Many modem theorists of object recognition assume that parts of an object can be extracted by analyzing the object's contours (e.g., Biederman, 1987; Marr & Nishihara, 1978). Nevertheless, they have retained the "figure-ground first" assumption, partly because an object's parts are thought to be delimited by successive minima of curvature along the object's contour. Those segments of contour that are minima of curvature from one side are maxima of curvature from the other side. Therefore, contour-parsing mechanisms using minima of curvature wiu identify different object parts (and hence, different objects) along the two sides of a contour (Hoffman & Richards, 1985). Because of this fact, it has been assumed that the side of origin for the contour-parsing mechanisms must be determined before they can operate; and it has been assumed tbat figureground organization serves that function. Acknowledging that different sets of parts are dehmited along the two sides of a contour, we have proposed that prefigural shape recognition processes carve those edges that can be detected early in visual processing into parts from both sides simultaneously, tberefore indicating different sets of parts along an edge's two sides. We have proposed further that these part sets are used to access in parallel those representations of objects in memory that best fit the opposite sides of an edge or contour. The output of these activated representations in memory can serve as input to figure-ground computations, along with the output of processes assessing depth cues and gestalt cues. Feed-Forward aad Feedback Processes Other people may prefer to explain our results in terms of feed-forward and feedback mechanisms such as those that have been used to explain the word-superiority effect (e.g., McClelland & Rumelhart, 1981). Feed-forward and feedback processes of some type must mediate the efi'ects we have found. However, our understanding is that the feed-forward and feedback processes currently instantiated in parallel distributed processing models preserve the assumption that figure-ground comes fu-st by facilitating the figure-ground determination that might otherwise be chosen on the basis of bottom-up variables alone but not by altering the course of organization. Our finding that the presence of strong object recognition influences for upright stimuli could alter the figure-ground organization (e,g., compare inverted and upright AS stimuli) requires an explanation in terms of a different sort of feedback process. Unconsciotis Organization and Reorganization in Search of Good Matches to Representations in Memory Alternatively, one might argue that the first perceived figure-^ound organization is not necessarily the first organization arrived at in the course of visual processing, and that the effects we have measured occur only after an initial unconscious figure-ground organization has been determined on the basis of bottom-up variables only. An explanation like this has been proposed (Epstein & DeShazo, 1960; Rock,!975), but its details have not been worked out. As articulated, it entails in sequence: unconscious determination of figure and ground relationships, matching of the figure to object representations, unconscious reversal of figure and ground relationships, matching ofthe new figure to object representations, evaluation ofthe relative figural goodness of the two regions, and, finally, conscious perception of figure and ground. Although a model such as this is consistent with our data, it appears to be designed only to preserve the traditional ordering of figure-ground organization and shape recognition. Moreover, this approach is nonparsimonious, requiring a number of preconscious organizational stages. CONCLUSION Our results demonstrate that both symmetry and orientationdependent object recognition processes infltience figure-ground organization. These infiuences are refiected in main effects of symmetry and orientation, respectively. Moreover, both infiuences were first evident at the same exposure duration (28 ms). These results seem to contradict the conventional bottom-up approach to figure-ground organization and suggest a different functional view of this critical early visual process. We have proposed that object recognition processes treat both sides of edges in parallel prior to figure-ground organization. This proposal has deep implications for current theories of perceptual organization (e.g.. Palmer & Rock, 1994) and for object recognition models that currently work with the parts of a single shape only (e.g.. Hummel & Biederman, 1992). It might be reasonable to claim that prefigural recognition processes operate on regions rather than on edges, and one goal of future research will be to distinguish between these views. Phenomena we have observed and reported elsewhere lead us to favor the view that prefigural object recognition processes operate on edges rather than on regions. In summary, the results reported here reveal an infiuence from object recognition processes on the first perceived figtireground organization. In other experiments, we have obtained converging evidence using paradigms in which observers viewed both two-dimensional and three-dimensional stimuli for long durations and reported about reversals of perceived figureground organization (Peterson et al., 1991; Peterson & Gibson, 1993, in press). Therefore, our findings generalize beyond twodimensional stimuli and beyond the conditions of brief exposure and masking employed in the present experiment. Acknowledgments This research was supported by a grant to the first author from the National Science Foundation (BNS ) and the Air Force Office of Scientific Research. The second author's participation in writing the manuscript was supported by a postdoctoral fellowship from the National Institute of Mental Health (T32-MHI8215). We thank Karen Wynn, Lynn Nadel, Paul Bloom, Bill Ittelson, Julian Hochberg, Kerry Green, and Peter Gerhardstein for valuable discussion and comments and Peter Schnittman, Susan Hacker, Janeen Burroughs, Robin Erie, Shawn Singer, and Mary Padilla for their help conducting this experiment. 258 VOL, 5, NO. 5, SEPTEMBER 1994

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