Studies of chromatic discrimination have historically varied in their experimental procedures and in the units in which thresholds are expressed, but by 1950, it was clear that thresholds depend both on differences in the ratios of cone excitations and on the properties of post-receptoral channels. Discrimination was found to be optimal at the chromaticity to which the observer was currently adapted. Consequently, the most famous dataset in this field, MacAdam's discrimination ellipses, cannot validly be used to construct a uniform color diagram or to estimate the number of discernible colors: MacAdam's single observer, Perley Nutting, would have been in different states of adaptation when matching different colors. Furthermore, a sensation of difference is not the same as a difference of sensations: Some color discriminations may depend on channels that extract differences between adjacent regions, and different pairs of chromaticities could then give rise to the same sensation of difference.
Mollon et al. (Thu,) studied this question.
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