Disorders of higher visual processing.

A significant proportion of human cortex is involved in visual processing. Cerebral regions beyond striate cortex show specialization for specific stimulus properties: damage to these regions leads to syndromes that also reflect this specialization. These syndromes can be grouped into two broad categories. In the ventral group, there is damage to medial occipitotemporal structures that participate in object identification and recognition. This can result in a variety of syndromes, including achromatopsia, general visual object agnosia, or more selective object recognition deficits such as prosopagnosia, alexia, and certain forms of topographagnosia, such as landmark agnosia. In the second, dorsal group, there is damage to lateral occipitoparietal structures that participate in visuospatial processing and localization. Patients with such damage may have akinetopsia (impaired motion processing), various components of Bálint syndrome, or astereopsis. Finally, it is also possible to find patients in whom residual visual processing is evident despite damage to the geniculostriate pathway; this blindsight may reflect residual function of either subcortical structures, such as the superior colliculus, or alternate afferent routes to surviving extrastriate cortex.