Disorders of semantic memory.

It is now established that selective disorders of semantic memory may arise after focal cerebral lesions. Debate and dissension remain on three principal issues: category specificity, the status of modality-dependent knowledge, and the stability and sufficiency of stored information. Theories of category specificity have focused on the frequently reported dissociation between living things and man-made objects. However, other dimensions need theoretical integration. Impairments can be both finer-grain and broader in range. A second variable of importance is stimulus modality. Reciprocal interactive dissociations between vision and language and between animals and objects will be described. These indicate that the derivation of semantic information is constrained by input modality: we appear to have evolved separable databases for the visual and the verbal world. Thirdly, an orthogonal distinction has been drawn between degradation disorders, where representations are insufficient for comprehension, and access deficits, in which representations have become unstable. These issues may have their parallel in the acquisition of knowledge by the developing child.