Visual Search for Human Gaze Direction by a Chimpanzee (Pan troglodytes)

Tomonaga, Masaki; Imura, Tomoko · 2010 · Crossref

DOI: 10.1371/journal.pone.0009131

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Summary

This study investigates whether chimpanzees (*Pan troglodytes*) exhibit the “stare-in-the-crowd” effect—a visual search asymmetry where direct gazes are detected more efficiently than averted gazes—and identifies the specific visual cues used for gaze discrimination. While humans possess high-contrast eyes (dark iris, white sclera) that facilitate gaze perception, chimpanzees have low-contrast eyes (dark sclera), raising questions about how they process human gaze cues. The researchers aimed to determine if a chimpanzee could discriminate human gaze directions and whether her processing mechanisms mirrored those of humans or relied on different features. The experiments involved an adult female chimpanzee, Chloe, trained on computer-controlled tasks. In Experiment 1, Chloe performed a visual search task, locating a target human face (direct or averted gaze) among distractors. Stimuli included upright, inverted, and scrambled faces to test robustness. Experiments 2–4 utilized three-item oddity discrimination tasks to isolate specific cues. Experiment 2 manipulated eye contrast polarity and introduced “bloodshot” illusions (darkened sclera) to test if gaze perception relied on iris-sclera contrast. Experiment 3 examined the transfer of gaze discrimination to three-quarter-view faces and tested whether Chloe relied on head orientation or solely on iris position. Experiment 4 further analyzed the role of head orientation versus eye position. The results demonstrated that Chloe exhibited a significant stare-in-the-crowd effect, detecting direct-gaze targets faster than averted-gaze targets. This superiority persisted even when faces were inverted or scrambled, indicating that holistic facial configuration was not required. Crucially, Chloe’s gaze perception was controlled by the contrast between the iris and sclera, similar to humans. She exhibited the “bloodshot” illusion, perceiving gaze direction as shifting toward the darker side of the sclera. However, her ability to discriminate gaze deteriorated significantly when stimuli changed from front-view to three-quarter-view faces. Further testing revealed that Chloe attended only to the position of the iris within the eye, ignoring head orientation cues that humans typically integrate to judge gaze direction. These findings indicate that chimpanzees can discriminate human gaze directions and are more sensitive to direct gazes, likely due to their extensive exposure to humans. However, their gaze perception is limited compared to humans; they rely primarily on iris position and contrast polarity rather than integrating head orientation. This suggests that while chimpanzees possess the basic capacity for gaze discrimination, their processing mechanisms differ from humans, potentially reflecting evolutionary differences in social communication cues.

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