On distinguishing a visual signal from noise by patients with visual agnosia and visual hallucinations

Characteristics of the process of distinguishing a visual signal from visual noise by patients with cerebral hemisphere lesions as well as by patients with visual and auditory hallucinations were investigated. Visual noise was formed by a special programme on a computer by realizations of first-order statistics with probability of appearance of black elements p=0·35, 0·25 and 0·15. Against each of these noises outlined designs of the same eight objects were exposed for identification. Visual system capacity was calculated in bits per second. In the group of patients with local brain pathology only occipital cases manifested disorders in the information processing. These disorders were evident even at the highest signal/noise ratio and practically remained persistent. The problem was mainly of identification of a signal, with comparatively preserved capacity for detection. Patients with visual hallucinations also manifested distinct disorders of visual information processing. However, the revealed disorders in these patients, in contrast to those with local lesions in the occipital cortex, were transient, less intensive, and in this case they were found in detection as well as in identification. The role of the occipital cortex and deep cerebral structures in visual information processing is discussed.