Cerebral achromatopsia

Cerebral achromatopsia is a type of color-blindness that is caused by damage to the cerebral cortex of the brain, rather than abnormalities in the cells of the eye's retina.

Cerebral achromatopsia differs from other forms of color blindness in subtle but important ways. It is a consequence of cortical damage in lingual and fusiform gyri, near the base of the brain (ventro-medial occipital lobe). This damage is almost always the result of injury or illness. Because of the location of the tissues whose damage is associated with cerebral achromatopsia, and the requirement that it be damaged in both hemispheres of the brain, complete cerebral achromatopsia is very rare.

Patients with cerebral achromatopsia deny having any experience of color when asked and fail standard clinical assessments like the Farnsworth-Munsell 100-hue test (a test of color ordering with no naming requirements). Patients may often not notice their loss of color vision and merely describe the world they see as being "drab". The critical difference between cerebral achromatopsics and people with other forms of colour blindness is that cerebral achromatopsics retain the ability to perceive chromatic borders. For example, they see a red square on a green background effortlessly even when the red and green are equally bright. There are various ways of ruling out a role for unintended luminance differences in accounting for this, e.g. random luminance masking. It even appears that cerebral achromatopsics can discriminate contrasts on the basis of colour direction, but they can't use these contrasts to compare the colour of surfaces that do not adjoin directly. It has been suggested that cerebral achromatopsia might best be seen as a failure in specific colour-constancy mechanisms.

Cerebral achromatopsia illustrates the way in which chomatic information can be used to achieve many goals, only one of which is the perception of surface colour, and that those different goals may be met by distinct pathways within the visual system.

The Case of the Colorblind Painter
The most famous instance of cerebral achromatopsia is that of "Jonathan I." immortalized in a case study by Oliver Sacks and Robert Wasserman, and published as The Case of the Colorblind Painter. The essay tracks Johnathan I.'s experience with cerebral achromatopsia from the point where an injury to his occipital lobe leaves him without the ability to perceive color, through his subsequent struggles to adapt to a black, white and gray world, and finally to his acceptance and even gratefulness for his condition. Especially pertinent is the analysis of how cerebral achromatopsia affects his practice as a painter and artist. Descriptions of cerebral achromatopsia's effects on his psychological health and visual perception are especially striking. For instance, in recounting Mr. I.'s descriptions of flesh and foods, the authors write:

"Mr. I. could hardly bear the changed appearances of people ("like animated gray statues") any more than he could bear his own changed appearance in the mirror: he shunned social intercourse and found sexual intercourse impossible. He saw people's flesh, his wife's flesh, his own flesh, as an abhorrent gray; "flesh-colored" now appeared "rat-colored" to him. This was so even when he closed his eyes, for his preternaturally vivid ("eidetic") visual imagery was preserved but now without color, and forced on him images, forced him to "see" but see internally with the wrongness of his achromatopsia. He found foods disgusting in their grayish, dead appearance and had to close his eyes to eat. But this did not help very much, for the mental image of a tomato was as black as its appearance."