The Defect of Function in Visual Agnosia

CLINICAL neurology has revealed several examples of that uncommon disorder, visual agnosia, but only to increase the disagreement regarding the traditional concept. This concept had its origin with the extirpation experiments of Munk (1881) on dogs, which, after partial extirpation of their occipital cortex, appeared to see and avoid obstacles though apparently unable to recognize them—a "mind blindness, " or object visual agnosia. Comparable clinical experience led to the formulation of ideas regarding visual agnosia as a defect of a specific gnostic function, predicating intact primary sensory processes and no dementia. Yet in the literature only one patient with visual agnosia (Bay, 1953) had normal visual acuity and visual fields and no dementia, but even in this case more careful testing revealed defects attributed to the primary sensory process. Siemerling (1890) had early questioned the classical concept of visual agnosia and believed that a similar state could be produced in normal people by altering the illumination of the environment with a sodium lamp and reducing the visual acuity to 1/30 by wearing glasses. Bay (1953) denies that there is a specific gnostic function or a disorder of that function, but that the defect of recognition is a combination of imperfect vision and impaired general psychological qualities (dementia of some degree). However, in his case without any dementia, he considered that the defects in the dynamic processes of "Funktionswandel " indicated an impairment of sensory activity which fully explained the gnostic disturbances. The complication of agnostic disorders by quantitative visual and intellectual defects and emotional reactions, renders their study one of great difficulty. Moreover, the number of adequately documented cases in the literature is very small and, as illustrated by the famous