Semantic memory disorders.

Semantic memory encompasses knowledge of objects, facts and words. A number of brain regions are probably involved, but the left infero-lateral temporal lobe appears to play a key role. The separability of semantic memory from episodic (or autobiographical) memory is a focus of current debate. Impaired semantic memory is a common feature of Alzheimer's disease but is invariably overshadowed by a profound deficit in episodic memory. In semantic dementia, a rarer disorder associated with focal temporal-lobe atrophy, there is selective loss of semantic memory, characterized by preservation of superordinate knowledge of words, and objects, but loss of finer-grained information. This pattern can be interpreted as a degradation of features from a distributed network of semantic representations. Following Herpes simplex encephalitis, patients sometimes show disproportionate loss of knowledge for natural kinds (e.g. animals) with relative preservation of knowledge about artefacts, this may reflect differential damage to neural systems critical for perceptual as opposed to functional features, perceptual properties being more salient in knowledge about natural kinds.