Normal color vision depends on the integrity of cone cells, which are most numerous in the macular region. When activated, they convey information to special columns of cells in the striate cortex. Three different cone pigments with optimal sensitivities to short (blue), middle (green), and long (orange-yellow) wavelengths are said to characterize these cells. Transmission to higher centers for the perception of color is effected by neurons and axons that encode at least two pairs of complementary colors: red-green in one system and yellow-blue in the other. In the optic nerves and tracts, the fibers for color are of small caliber and seem to be preferentially sensitive to certain noxious agents and to pressure; thus, dyschromatopsia is a common feature of many optic neuropathies. The visual fields for blue-yellow are smaller than those for white light, and the red and green fields are smaller than those for blue-yellow.
Diseases may affect color vision by abolishing it completely (achromatopsia) or partially by quantitatively reducing one or more of the three attributes of color—brightness, hue, and saturation. Alternatively, only one of the complementary pairs of colors may be lost or reduced, usually the red-green axis. The disorder may be congenital and hereditary or acquired. The most common form, and the one to which the term color blindness is usually applied, is a male sex-linked inability to see red and green while normal visual acuity is retained. The main problem arises in relation to traffic lights, but patients learn to use the position of the light as a guide. Several other genetic abnormalities of cone pigments and their phototransduction have been identified as causes of achromatopsia. The defect cannot be seen by inspecting the retina. A failure of the cones to develop or a degeneration of cones may cause a loss of color vision, but in these conditions visual acuity is often diminished, a central scotoma may be present, and, although the macula also appears to be normal ophthalmoscopically, fluorescein angiography shows the pigment epithelium to be defective. Whereas congenital color vision defects are usually protan (red) or detan (green), leaving yellow-blue color vision intact, most acquired lesions affect all colors, at times disparately. Lesions of the optic nerves usually affect red-green more than blue-yellow; the opposite is true of retinal lesions. An exception is a rare, dominantly inherited, optic atrophy, in which the scotoma mapped by a large blue target is larger than that for red.
Damasio has drawn attention to a group of acquired deficits of color perception with preservation of form vision, the result of focal damage (usually infarction) of the visual association cortex, and subjacent white matter. Color vision may be lost in a quadrant, half of the visual field, or the entire field. The latter, or full-field achromatopsia, is the result of bilateral occipitotemporal lesions involving the fusiform and lingual gyri, a localization that accounts for its frequent association with visual agnosia (especially prosopagnosia), and some degree of visual field defect. A lesion restricted to the inferior part of the one occipitotemporal lobe, sparing both the optic radiations and striate cortex, may cause a contralateral hemiachromatopsia. With a left-sided lesion in this location, alexia may be associated with the right hemiachromatopsia.