Experimental evidence for the gaze-signaling hypothesis: White sclera enhances the visibility of eye-gaze direction in humans and chimpanzees

Abstract

Hallmark social activities of humans, such as cooperation and cultural learning, involve eye-gaze signaling through joint attentional interaction and ostensive communication. The gaze-signaling and related cooperative-eye hypotheses posit that humans evolved unique external eye morphology, including exposed white sclera (the white of the eye), to enhance the visibility of eye-gaze for conspecifics. However, experimental evidence is still lacking. This study tested the ability of human and chimpanzee participants to detect the eye-gaze directions of human and chimpanzee images in computerized tasks. We varied the level of brightness and size in the stimulus images to examine the robustness of the eye-gaze directional signal against visually challenging conditions. We found that both humans and chimpanzees detected gaze directions of the human eye better than that of the chimpanzee eye, particularly when eye stimuli were darker and smaller. Also, participants of both species detected gaze direction of the chimpanzee eye better when its color was inverted compared to when its color was normal; namely, when the chimpanzee eye has artificial white sclera. White sclera thus enhances the visibility of eye-gaze direction even across species, particularly in visually challenging conditions. Our findings supported but also critically updated the central premises of the gaze-signaling hypothesis.

The gaze of a social monkey is perceptible to conspecifics and predators but not prey

“…Recently, Kano et al . [ 31 ] demonstrated that captive chimpanzees ( Pan troglodytes ) can discriminate between directed and averted eyes [ 31 ]. In sum, nonhuman primates use the information of conspecific gaze, but the cues that they use to make these judgements are little understood and not easily investigated in laboratory settings.…”

“…In a design that used digital images of rhesus macaques with averted eyes or heads as predictive cues for eye saccades to a target stimulus, Deaner & Platt [28] reported that averted eyes were as effective a cue as an averted head for this species (although targets to the cued side were detected only approximately 5 ms faster than targets to the uncued side). Recently, Kano et al [31] demonstrated that captive chimpanzees (Pan troglodytes) can discriminate between directed and averted eyes [31]. In sum, nonhuman primates use the information of conspecific gaze, but the cues that they use to make these judgements are little understood and not easily investigated in laboratory settings.…”