Stare-in-the-crowd effect evolved from a necessity for fast and accurate gaze perception for complex social interactions (Emery 2000). According to this model, an eye direction detector (EDD) mechanism - a neurocognitive system responsible for the rapid detection of eyes in the environment and their gaze direction – arose as a function of the geometry created by a dark circular iris relative to the white sclera (Baron-Cohen 1994). Specifically, it is contended that the increased ratio of exposed sclera in the eye outline of humans serves to enhance the velocity and acuity of gaze direction perception (Kobayashi and Kohshima 2001). Baron-Cohen, S. (1994)
Sclera in lobe-finned fishes, various reptiles, and birds. The ring has disappeared in many groups, including living amphibians, some reptiles and fish, and all mammals. The eyes of all non-human primates are dark with small, barely visible sclera. The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are: The sclera is opaque due to the irregularity of the Type I collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a