The understanding of normal hemispheric functions in visual information processing relies on inferences from deficits displayed by unilaterally brain-damaged patients. Recent findings in the neuroanatomy and physiology of the visual system and cerebral structures suggest that the brain is an interactive system whose two hemispheres participate conjointly in the production of behavior, and that damage in one component inevitably affects other components. The implications of these properties for research on unilaterally brain-damaged patients are outlined, and empirical evidence of sensory impairments following extrastriate lesion is presented. It is suggested that the effects of such lesions on the neural descriptions of a visual stimulus is to produce deprivation of some aspects of the incoming information, particularly those on which the left hemisphere normally operates. The visuospatial competence of the left hemisphere may not be displayed in such conditions because the representations of the visual world, as elaborated in a damaged brain, are not compatible with its specific neural sensitivity, although the right hemisphere can accommodate itself to such representations. In addition, an examination of relevant findings from the normal population indicates that the critical contribution of the right hemisphere to visual pattern recognition prevails under conditions of substantial functional sensory deficits resulting from the particular viewing conditions inherent in lateral tachistoscopic presentations. It is suggested that research on brain-damaged patients and normal subjects in visual information processing yields findings that must be interpreted within the context of, and in terms appropriate to, brain functioning for valid inferences about the normal functions of the cerebral hemispheres.