Multidimensional scaling reveals a color dimension unique to 'color-deficient' observers

Normal color vision depends on the relative rates at which photons are absorbed in three types of retinal cone:short-wave (S), middle-wave (M) and long-wave (L) cones, maximally sensitive near 430, 530 and 560nm, respectively. But 6% of men exhibit an X-linked variant form of color vision called deuteranomaly [ 1]. Their color vision is thought to depend on S cones and two forms of long-wave cone (L, L ') [ 2,3]. The two types of L cone contain photopigments that are maximally sensitive near 560nm, 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 need three primary lights if they are to match all possible spectral power distributions, but the matches they make are different from those of the normal. Here we use multidimensional scaling (MDS) [ 4,5] to reveal the color dimension that is private to the deuteranomalous observer.