Multidimensional scaling reveals a color dimension unique to ‘color-deficient’ observers

Introduction Normal color vision depends on the relative rates at which photons are absorbed in three types of retinal cones, which have peak sensitivities in different regions of the spectrum. The shortwave (S), middle-wave (M) and long-wave (L) cones are maximally sensitive near 430, 530 and 560 nm respectively. However, some 6% of men exhibit a variant form of color vision called deuteranomaly. Their color vision is thought to depend on shortwave cones (S) and two forms of long-wave cone (L, L') [1-3]. The two types of long-wave cone contain photopigments that both peak in the same part of the spectrum, near 560 nm, but their spectral sensitivities are different enough that the ratio of their activations gives a useful chromatic signal. Like color-normal observers, deuteranomalous observers are formally trichromatic, in that they require three primary lights if they are to match all possible spectral power distributions in a color-matching experiment, but the matches they make are different from those of the normal. Deuteranomaly is rare in women, since the genes that encode the middle-and long-wave pigments are on the X-chromosome [2] and thus a woman must inherit two anomalous chromosomes if she is to exhibit deuteranomaly.