Color vision: Opsins and options
- Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, United Kingdom
A recent estimate suggests that we can potentially distinguish 2.3 million colors (1), and yet we achieve this by comparing the rates at which photons are absorbed in just three classes of retinal photopigment (Fig. 1 Lower). The photopigments consist of 11-cis-retinal bound to different “opsins,” which are members of the large family of G-protein-coupled receptors or heptahelicals. But our exquisite discrimination of hue requires that the three different opsins should be cleanly segregated into different cone cells in the retina. A new paper by Yanshu Wang et al. (2) bears on how such segregation may be maintained. The research group is led by Jeremy Nathans, whose now classic papers laid the basis of the molecular genetics of the cone pigments (3, 4).
(Lower) The sensitivities of the long-wave (L), middle-wave (M), and short-wave (S) photopigments found in the retinae of humans and of Old World primates. (Upper) The sensitivities of the pigments thought to have been present in ancestral mammals.
Old and New Subsystems of Color Vision.
Our own trichromatic color vision depends on two subsystems, a phylogenetically recent one overlaid on a much more ancient one (5). Most mammals are dichromatic, having only two types of cone receptor in the retina (6). For the main business of vision—the detection of movement and form—they rely on a single class of cone, with its peak sensitivity at long wavelengths (500–570 nm). Among these cones lies a second sparser population of cones with peak sensitivity at short wavelengths (Fig. 1 Upper). A basic color vision is gained by comparing the rate of quantum catch in the short-wave cones with that in the long-wave cones. This ancient mammalian subsystem has its own morphological substrate in early stages of the visual system (Fig. 2): its signals are carried by the “blue cone” bipolar, by the small bistratified …
