What is unique about the human eye? Comparative image analysis on the external eye morphology of human and nonhuman great apes

Experimental evidence that uniformly white sclera enhances the visibility of eye-gaze direction in humans and chimpanzees

“…H2 posits the perceptual advantage of the iris-sclera color difference. As the iris-sclera color difference (note the difference between the contrast and difference measures employed by previous studies; Perea-García et al, 2019 ; Caspar et al, 2021 ; Mearing and Koops, 2021 ; Kano et al, 2021 ) was similar between chimpanzees and humans in general ( Kano et al, 2021 ) and did not differ between our chimpanzee and human stimuli with both positive and negative eyes (see Appendix 3—figure 2 for the quantitative evaluation of our stimuli), we predicted no performance difference between the stimulus types in participants of both species. H3 posits the perceptual advantage of the horizontally elongated shape.…”

“…We developed five sets of hypotheses and predictions ( Table 1 ). H1 is our key hypothesis, which posits the perceptual advantage of uniformly white sclera ( Kobayashi and Kohshima, 2001 ; Kobayashi and Kohshima, 1997 ; Kano et al, 2021 ) and thus predicted increased performance for both human and chimpanzee positive eyes in participants of both species. H2 posits the perceptual advantage of the iris-sclera color difference.…”

“…Despite the widespread popularity of the gaze-signaling and related cooperative-eye hypotheses in the literature, these hypotheses have been severely challenged by recent quantitative morphological studies showing that certain eye features of humans are not necessarily unique among great ape species ( Mayhew and Gómez, 2015 ; Perea-García et al, 2019 ; Caspar et al, 2021 ; Mearing and Koops, 2021 ; Kano et al, 2021 ). Accordingly, there is currently a heated debate over whether the external eye morphology of humans has any communicative function.…”

“…Specifically, those studies have shown that (1) the sclera of humans is not necessarily more exposed than that of other great ape species ( Mayhew and Gómez, 2015 ; Caspar et al, 2021 ; Kano et al, 2021 ), that (2) the color contrast/difference between the iris and the sclera is similar between humans and other great apes ( Perea-García et al, 2019 ; Caspar et al, 2021 ; Mearing and Koops, 2021 ; Kano et al, 2021 ), and that (3) there is substantial individual variation in the extent to which sclera is unpigmented among some nonhuman ape species ( Mayhew and Gómez, 2015 ; Caspar et al, 2021 ; Mearing and Koops, 2021 ; Kano et al, 2021 ; Perea García, 2016 ). Given these new findings, one recent study suggested that one (and possibly only) eye feature that both distinguishes humans from other great apes and contributes to the visibility of eye-gaze direction is uniform whiteness in humans’ exposed sclera; i.e., depigmentation all the way from the iris edge to the eye corners ( Kano et al, 2021 ). This previous study used image analysis and computer vision techniques and identified that uniformly white sclera characterizes clear visibilities of both iris and eye-outline edges – the two essential features that contribute to the visibility of eye-gaze directions (see Appendix 3—figure 1 for the illustration of this aspect).…”

“…Our experimental procedures followed a previous study adopting training procedures ( Tomonaga and Imura, 2010 ) instead of spontaneous gaze-following/cueing procedures for chimpanzees ( Tomasello et al, 2007 ; Tomonaga, 2007 ) because, while the former study successfully trained a chimpanzee to discriminate eye-gaze directions of a stimulus (human) face, the latter studies reported that chimpanzees predominantly rely on head-directional cues but not eye-gaze directional cues in the spontaneous tasks. Also, as the strength of the visual signal generally depends on its degradation caused by natural noises ( Endler, 1990 ), we tested the robustness of eye-gaze signals against shading and distancing ( Figure 1A ), namely the simulated visual conditions where stimulus eyes were presented darker (in shadows) and smaller (in the distance), following a previous study on great ape eye color ( Kano et al, 2021 ).…”