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.