Residual vision in patients with retrogeniculate lesions of the visual pathways

Twenty-five subjects have been studied who, as a result of damage to the striate cortex, were 'blind' in extensive areas (scotomata) of the visual fields. Of these 25 subjects, 5 exhibited residual vision in response to transient lights presented within the scotoma, which enabled them to locate the stimuli by hand-reaching or by eye movements; the latter have been measured by electro-oculography. The residual vision underlying their responses was characterized by low flicker-fusion and by sensitivity in detection of movement which increased as target speed was raised. Discrimination for the direction of target movement was poor, but spatial resolution in the discrimination of target displacement was essentially normal. The subjects were unable to recognize or discriminate the spatial structure of targets located within the 'blind' field, and the observed dissociation between spatial discrimination of displacement and pattern is examined in relation to the 'two systems' hypothesis of visual function. There is no obvious correlation between the extent of neuronal damage as revealed by CT scans and the existence of residual vision.