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

Abstract

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.

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.

Figure 2

A young female chimpanzee, Cleo, performing the visual search task in Experiment 1.

Figure 3

Summary of Experiment 1. (A) Stimuli used in Experiment 1. (B) Mean response times on correct trials under each condition in Experiment 1. Error bars indicate the 95% confidence intervals (CIs) of the differences in means.

Figure 4

Stimuli used in Experiment 2. Note that the gap width depicted in this figure is exaggerated rather than drawn to scale.

Figure 5

Mean response times on correct trials under each condition in Experiment 2. Error bars indicate the 95% CIs in the differences in means. **: p < 0.01, ***: p < 0.001.