This is a very interesting article with a clever design. Recent work complements its results, perhaps suggesting that conspicuity to instantiate eye contact as a pre-requisite for effective gaze-following in humans: https://www.researchgate.... (https://www.nature.com/ar...).

However, focusing on human uniqueness in eye morphology to motivate research (ie., Kobayashi and Koshima's work) in this area is increasingly inadequate, as new sources of evidence directly contradict this assumption:

1- In gorillas ( https://doi.org/10.1002/a...)
2- In orangutans ( https://academic.oup.com/...)
3- In bonobos and, perhaps, chimpanzees (https://www.pnas.org/cont...)

As intuitively attractive as the idea presented by Kobayashi and Koshima may seem, enough evidence has accumulated that we should be more wary of upholding saltationist claims about eye morphology at this point.

In addition to the inaccuracy of the statement that 99% of nonhuman primates have eye morphology that is less conspicuous" (based on the evidence accumulated since then), the experimental design in this study assumes that a darker sclera is always cryptic, despite the contrast between the iris and sclera being the same. This may be due to the lack of direct measure of conspicuity, despite the availability of such measurements (in humans: https://www.tandfonline.c... https://doi.org/10.1080/1... and other ape species https://www.pnas.org/cont... https://doi.org/10.1073/p...). As such, the results presented in this study do not point to perceptual conspicuity (ie., objective contrast) or scleral brightness (dark-light), but to a specific combination of scleral and iridal brightness (what was termed Type 1 in https://www.pnas.org/cont...) as relevant for effective gaze perception in humans.

In short, whereas this article does provide interesting information regarding perceptual mechanisms of gaze perception in humans, a lack of comprehensive literature research undermine the rationale motivating its execution (human uniquenes), and the interpretation of its results (that it is scleral color and not a specific combination of scleral and iridal brightness).

I invite the authors and anyone interested in the truth of scleral coloration in primates to search google images of ie., barbary macaques, sulawesi macaques, white-faced sakis, gelada baboons, yunnan snub-nosed monkeys, gee's golden langurs, or many other species to show the pervasiveness of white sclerae among primate species and the factual inaccuracy of Kobayashi and Koshima's article that has become so fixed in the literature.