White Sclera Enhances Eye Gaze Communication

Article: The language of the eyes
Source: University of Konstanz (Germany)
Published: March 8, 2022

A uniform white sclera in humans
enhances visibility of eye gaze direction

The sclera, the tough protective layer that provides structure to the shape of the eyeball, is white in humans. But this is not uniformly the case in animals, even among primates such as chimpanzees, humans' closest relatives. Researchers studying the significance of the sclera's color discovered that the white of the eye contributes decisively to the visibility of eye gaze direction through its basic color properties. In the experiments, humans and chimpanzees—the cross-species comparison enabling novel differentiation among competing hypotheses, the researchers note—were presented with images on a computer screen, varied in brightness and size of stimulus against varied background shading and distancing ("visually challenging conditions"), and tasked with distinguishing gaze direction for both groups. They found that both humans and chimpanzees distinguished eye-gaze directions of humans better than those of chimpanzees, especially in the visually challenging conditions. Furthermore, when the contrast polarity of the eyes of chimpanzees was reversed compared to what is normal, that is, showing a white sclera and a darker iris, both humans and chimpanzees were able to distinguish gaze direction of the chimpanzees better, indicating that uniform whiteness of the sclera facilitates the visibility of eye gaze direction even across species. Sometimes taken for granted in daily life, the ability to understand the language of the eyes is something we rely on in social activities and when interacting with one another face to face, where eye-gaze signaling is used for communication such as joint attention. First author of the study comments, "We owe this to the formation of the whites of the eye. Humans might have evolved this distinguishing eye feature for conspecific communication...In doing so, humans have probably evolved a unique communicative style critical for their hallmark social activities." 

Human and chimpanzee stimulus eyes in direct and averted (20°) gaze
in normal and reversed contrast polarities, varied in brightness and size

My rating of this study: ⭐

Kano F, Kawaguchi Y and Hanling Y. "Experimental evidence that uniformly white sclera enhances the visibility of eye-gaze direction in humans and chimpanzees." eLife.  8 March 2022. https://doi.org/10.7554/eLife.74086