Sensory and non-sensory visual disorders in man and monkey

The posterior third of the cerebral cortex in monkeys consists of a patchwork of visual areas in each of which there is a 'map' of the retina. The details of the 'map' vary considerably from one area to another and one notable variation concerns the optimal visual feature to which the cells respond. Orientation, disparity, colour and movement are emphasized in separate areas that appear to be concerned with sensory analysis. Their existence and the possibility that brain damage is occasionally restricted chiefly to one such area may explain the rare highly selective visual sensory impairments that can follow posterior cerebral damage in man. Other areas are notable for having little or no retinotopic representation. Here the cells may have huge receptive fields and complex trigger features. When such regions are removed, the animal's visual sensory abilities are intact but its recognition of patterns and objects is not. This condition resembles human visual agnosia.