Cerebral Achromatopsia

There are rare exceptions-to the tolerance of color discrimination despite large losses in acuity. An occasional subject loses color vision but maintains normal visual acuity after acquired damage of visual cortex. The cases that have been studied most completely with the entire gamut of color testing methods confirm major but not complete loss of color discrimination. They appear to be able to use wavelength contrast to detect objects but are unable to perceive colors from these cues. Such a subject can distinguish the shape and achromatic brightness differences of traffic lights but cannot see them as red, green, or yellow. They appear as washed-out objects of white and gray. Testing reveals a greater tendency for a blue-yellow (tritanopic) than a red-green deficiency. In general, the recovery from damage to visual cortex will usually include a stage in which white and grays are seen first before colors return. The first color to return is usually red. Increasing the size of the stimulus also tends to improve color perception. Nevertheless, such subjects are indeed very deficient in color vision and retain normal acuity.

These clinical findings imply that there is a significant anatomical separation of chromatic and achromatic contrast processing in the cerebral cortex. This is consistent with a cortical area devoted exclusively to the perception of color, but it is not proof. All of these patients invariably have prosopagnosia, the inability to recognize faces, and a scotomatous area, usually in their superior visual field. There are no reports of isolated acquired achromatopsia without other concurrent visual deficits. It is possible that the perception of color requires feedback from prestriate to striate cortex that facilitates the fusion of chromatic contrast with achromatic contrast perception. This feedback may be more important for incorporating the relatively sparse chromatic processing centers in striate cortex into visual perception.

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