Disorders of visual behavior following bilateral posterior cerebral lesions

SummaryFive patients with severe visual disability, associated with bilateral lesions of the posterior cerebrum, were given a battery of experimental tests of visual function. At least three types of impaired visual behavior seemed to vary relatively independently across the five subjects. These were: 1) Elevated thresholds for light detection in central and peripheral visual fields, 2) Inability to distribute visual attention over an array, 3) Inability to identify drawings of objects.We conclude that:A)Elevated thresholds for light detection, even in association with dementia, are not sufficient to account for all cases of visual misidentification or impairments of visual attention.B)Bilateral posterior cerebral damage may result in a severe disorder of visual attention. It involves both the division of visual attention over multiple items and the focusing of attention on a particular item in an array. The disorder is evident in tachistoscopic exposures and is, therefore, not secondary to impaired eye movement. It is independent of both elevated thresholds and impaired identification of single objects or line drawings. Mild disorders in spatial distribution of visual attention may occur with unilateral lesions or with interhemispheric disconnection, but the severe form requires bilateral lesions.C)Bilateral posterior cerebral damage may result in impaired identification of single visual stimuli that cannot be explained in terms of elevated thresholds or disorders of visual attention. The disorder is largely non-specific with regard to the conceptual category of the stimulus and depends on the difficulty of the required discrimination. However, identification of arrays of letters is a function lateralized to the left hemisphere and may be impaired relatively independently of identification of single line drawings, a function which is less lateralized.