A disorder of lightness discrimination in a case of visual form agnosia.

Benson and Greenberg (1969) described an "agnosic" patient whose severe visual recognition disorder could be accounted for in terms of a deficit in the perception of shape. We report here on a recent case of this disorder (visual form agnosia), and have found that she performs very poorly on tasks of discriminating shades of grey, although she is able to discriminate between hues. This sensory deficit helps to explain some of her perceptual difficulties; it also provides further evidence for parallel feature-processing in the human brain complementary to and consistent with recent physiological data on the visual cortex in animals.