Title Pacman in the sky with shadows : the effect of cast shadows on the perceptual completion of occluded figures by chimpanzees and humans

Background: Humans readily perceive whole shapes as intact when some portions of these shapes are occluded by another object. This type of amodal completion has also been widely reported among nonhuman animals and is related to pictorial depth perception. However, the effect of a cast shadow, a critical pictorial-depth cue for amodal completion has been investigated only rarely from the comparative-cognitive perspective. In the present study, we examined this effect in chimpanzees and humans. Results: Chimpanzees were slower in responding to a Pacman target with an occluding square than to the control condition, suggesting that participants perceptually completed the whole circle. When a cast shadow was added to the square, amodal completion occurred in both species. On the other hand, however, critical differences between the species emerged when the cast shadow was added to the Pacman figure, implying that Pacman was in the sky casting a shadow on the square. The cast shadow prevented, to a significant extent, compulsory amodal completion in humans, but had no effect on chimpanzees. Conclusion: These results suggest that cast shadows played a critical role in enabling humans to infer the spatial relationship between Pacman and the square. For chimpanzees, however, a cast shadow may be perceived as another "object". A limited role for cast shadows in the perception of pictorial depth has also been reported with respect to human cognitive development. Further studies on nonhuman primates using a comparative-developmental perspective will clarify the evolutionary origin of the role of cast shadows in visual perception. Background Humans readily perceive whole shapes as intact when portions of these shapes are occluded by another object (see Figures. 1A and 1F). For example, when we see the objects depicted in Figure 1A, we do not recognize these as a red Pacman and a green square but rather see them as a red full circle occluded by a green square. This phenomenon is often called perceptual completion. According to Kanizsa [1], perceptual completion can be categorized into modal completion, including completion involving subjective contours, and amodal completion, as shown in Figures 1A and 1F. Like modal completion [e.g., [2,3]], amodal completion has been observed not only in human adults, but also in human infants [e.g., [4]], nonhuman primates [5-8], and some species of birds [9,10]. Sato et al. [5] trained one adult female chimpanzee on a matching-to-sample task in which unitary and separated rectangles appeared as stimuli. After training, the chimpanzee was tested with a sample stimulus partly occluded by a large object. Although the sample rectangles were physically separated by this occluder, they would be perceived as unitary if perceptual completion occurred. The results clearly indicated that the chimpanzee achieved perceptual completion; that is, she consistently chose the unitary rectangular stimulus. In the context of these and other results, it would appear that amodal completion is a quite basic visual-perceptual function for visual animals [cf. [11]], although pigeons have exhibited exceptionally negative results[[7,12-16], but see [17,18] for positive results in pigeons]. For example, Fujita and Ushitani [14] tested pigeons and humans under a visual search task in which the target was a Pacman-like lozenge with or without an occluder, and the distractors were intact lozenges. * Correspondence: tomonaga@pri.kyoto-u.ac. jp 1 Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan Full list of author information is available at the end of the article Tomonaga and Imura Behavioral and Brain Functions 2010, 6:38 http://www.behavioralandbrainfunctions. com/content/6/1/38 Page 2 of 9 The response times of humans increased significantly over those obtained under other conditions when some portion of the lozenge target was occluded by another square, showing strong amodal completion. In contrast, however, pigeons demonstrated no such increase in response times, suggesting the absence of amodal completion. Amodal completion is affected by many factors [19], such as (common) motion, regularity of edges (good continuation or so-called relatability, [20]), and various kinds of pictorial depth cues. Motion and regularity cues have been examined extensively in nonhuman primates [e.g., [5,8,11]], although the role of pictorial depth information, such as T-junctions [7,21], linear perspectives [[22], cf. [23,24]], and shadows [cf. [24]] has received relatively less attention from comparative-cognitive researchers. When humans see occluded figures, we infer the three-dimensional spatial relationship between the figure and the occluder (e.g., the occluder superimposed on the figure, as shown in Figure 1D) in addition to completing the whole shape. Working within the context of scarce comparative research in this domain, Fagot et al. [22] tested the effects of linear perspectives on the amodal completion of occluded figures by baboons. Baboons did not show amodal completion in their previous experiments [6]. When the occluded shape was presented on a linearperspective background, however, the baboons perceived a Pacman-like occluded circle as a full circle [22]. Their results may suggest that additional perspective cues facilitated the depth perception implied by T-junctions. The other, less focal, depth cue is shadow information. For example, when we see a picture such as Figure 1B, in which a shadow is cast by a green occluder on a red Pacman, we readily recognize the spatial relationship between these objects and complete the occluded object (the occluder in the sky and a full circle, not a Pacman, on the ground, as in Figure 1D). Other interpretations, such as those depicted in Figures 1C (Pacman and green square on the same ground) and 1E (Pacman floating in the sky), seem implausible. Indeed, cast shadows provide information about the three-dimensional spatial arrangement of objects and background. In addition, the compulsory amodal completion in Figure 1G would have been interrupted and we would have recognized that the Pacman figure in the sky was casting a shadow cast on the green square as soon as the darker area adjacent to Pacman was recognized as a cast shadow. When the cast shadow is removed, as in Figure 1F, we easily complete the occluded portion and perceive the stimulus depicted in Figure 1I, a green square floating above and occluding the full red circle. Perception of shadows by nonhuman animals has not been studied extensively from a comparative perspective. A few studies on the perception of the shading and brightness gradient of the surface of an object have been conducted in birds [25,26] and chimpanzees [27,28]. However, quite a few studies on the perception of cast shadows in nonhuman animals have been conducted [24]. Imura and Tomonaga [24] examined the effects of cast shadows on chimpanzees' perceptions of size constancy in the context of a linear perspective [23]. In this context, the size of an object located near the vanishing point is relatively overestimated, an effect referred to as the "corridor illusion" [29,30]. Imura and Tomonaga examined the role of additional three-dimensional depth information derived from shadows cast on objects. However, the effect of cast shadows was rather limited in chimpanzees, and their performances were modified only when the cast shadows were moving along with moving objects. Moving cast shadows also affected the perception of motion trajectories in Japanese macaque infants [31], chimpanzees [31], and human infants [[32], cf. [33]]. To our knowledge, however, no studies on the effect of cast shadows on amodal completion in human and nonhuman animals have been conducted. The present study examined this effect in chimpanzees and humans from a comparative-cognitive perspective using a visual search task (see Figure 2) similar to that used with pigeons [14]. Figure 1 Amodal completion and pictorial depth perception. Thickness of arrows indicates the plausibility of the percept implied by stimuli A, B, F, and G. For humans, the cast shadow determined the spatial relationship between the occluder and Pacman (or full circle) in a relatively unambiguous fashion. (A) (B)