Color Vision Defects

The biological basis of color vision can be divided into two stages: the first is the light-sensitive cone photoreceptor cells in the retina and the second is the neural components that process information about wavelength gathered by the photoreceptors. Humans with normal color vision have three populations of cone photoreceptors that are classed according to their relative spectral sensitivities as short-, mediumand long-wavelength sensitive, and abbreviated S, M and L respectively. Red–green color vision is mediated by neural circuitry that compares the quantal catches of L and M cones; blue–yellow color vision is mediated by circuitry that compares the summed quantal catches of L and M cones with the quantal catch of S cones. People with normal color vision can distinguish more than a million colors, but people who are color blind see many fewer colors. Color blindness can be inherited or acquired. Inherited color blindness affects the first stage of color vision and is commonly caused by rearrangements, deletions and mutations of genes that encode the light-absorbing photopigment molecules in cones. Acquired color blindness arises through insult to the visual system, for example through drug or chemical toxicity, disease or trauma. (See Visual Pigment Genes: Evolution.)