the disorders













visual form agnosia


"a neurological virus"

"a unifying vision"

"an active process"

The perception of color confers an obvious evolutionary advantage. Finding fruit in a tree, for instance, is much easier if the fruit is perceived as red and the leaves of the tree perceived as green, rather than having leaves and fruit both perceived simply as shades of gray. Similarly, finding a tiger in the brush is much easier if the tiger is perceived as orange and black, and the brush perceived as brown, rather than having tiger and brush both perceived as shades of gray. Such evolutionary pressures have shaped our perception of color to be fast and very accurate. There are some well-known genetic deficits in color vision, such as the red-green color blindness that affects men more often than women. This color blindness is due to a mutation of the gene for the red cone photopigment which resides on the X chromosome. Since men have only one X chromosome, whereas women have two, men are more likely to suffer from this color blindness. The opposite deficit, called heterochromatopsia, in which colors are seen where there are no colors, occurs less frequently and is due to mutations both on the X chromosome and on chromosome 11.

The heterochromatopsic patient even sees colors where normal observers would see only a uniform white visual field. Heterochromatopsics report an anomalous glowing field of color in the white region, and describe it as being similar in quality to the glow of a neon sign. Fortunately, heterochromatopsia can be easily corrected with specially crafted polarized glasses. Such treatment is now prescribed as soon as the diagnosis of heterochromatopsia is confirmed, since the condition otherwise might progress and lead to hallucinatory percepts in other aspects of vision, such as in motion and depth. In rare cases, untreated heterochromatopsia can evolve into even more widespread perceptual hallucinations, and even to schizophrenia. There is some speculation by researchers studying heterochromatopsia that this might have been the condition from which the artist van Gogh suffered, though of course such speculation, while plausible, cannot be confirmed.