Comparative Color Categories

The entire range of human color perception, in all its vibrancy, is due to the operation of just three types of photosensitive retinal cone cells and the neural mechanisms responsible for interpreting their signals. These cone cells contain photosensitive opsin pigments that preferentially absorb light at different wavelengths. The short-wavelength (S) pigment has peak absorption of light at about 430 nm, roughly corresponding to blue light; the medium-wavelength (M) pigment has peak absorption at about 530 nm, corresponding to green light; and the long-wavelength (L) pigment has peak absorption at about 560 nm, corresponding to red light (see Fig. 1). Dysfunction in the cone cells or in the coding of their pigments leads to various types of color blindness. In humans, genes for S-type cones are located on chromosome 7, while those for the Mand L-type cones are located on the X chromosome. This largely explains why color blindness is a much more common phenomenon in men, as women possess independent copies of the Mand L-pigment genes on their two X chromosomes. The Mand L-type pigments are nearly identical in their genetic specifications. The small difference in their light-absorption profiles is due to a difference in only 3 out of 364 amino acids that code for their respective proteins [8]. The S-type pigment, on the other hand, is more distinct on the molecular level. Each type of cone cell responds over a range of wavelengths, and the intensity of a cone’s response to a light signal depends not only on that signal’s wavelength but also its intensity. This creates a situation in which the wavelength of a light signal cannot be uniquely identified based on the response of a single type of cone cell. Unique color information is only recoverable through the simultaneous and concerted action of different types of cone cells operating within a small neighborhood of each other. In a sense, the response profile of each type of cone cell can be viewed as a “basis vector” that, along with the other cones that make up the basis, defines a multidimensional color space. This