Eye color for a plausible argument stating that some eyes can or do change, based on chemical reactions and hormonal changes within the body. Studies on Caucasian twins, both fraternal and identical, have shown that eye color over time can be subject to change, and major demelanization of the iris may also be genetically determined. Most eye-color changes have been observed or reported in the Caucasian population with hazel and amber eyes. Carleton Coon created a chart by the original Martin scale. The numbering is reversed on the scale below in the (later) Martin–Schultz scale, which is (still) used in physical anthropology.
Filling-in the ring (Krauskopf 1967). These stimuli where intermixed with control stimuli, in which the physical colour of the disk was gradually changed to that of the ring. The animals were instructed to signal a colour change, and their responses to control stimuli and to test stimuli were compared in order to determine if monkeys perceive colour filling-in under steady fixation like humans. The authors recorded the activity of surface- and edge-cells (cells whose receptive fields pointed either to the filled-in surface or to the border between the disk and the ring) in the visual cortices V1 and V2 while the