Semantic memory disruption in dementia of Alzheimer's type; Analysis of disruptions in the structure of semantic memory and comparisons to a case of semantic dementia and a case with selective semantic memory deficits resulting from herpes simplex virus encephalitis

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  • アルツハイマー型痴呆の意味記憶障害―障害構造の分析と意味痴呆・選択的意味記憶障害例との比較―

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Abstract

   Disruption in the structure of semantic memory in the early stages of this type of memory deficit was investigated in 20 patients with mild to moderate dementia of Alzheimer's type and 20 healthy normal controls. A semantic deficit for both a verbal representation (a word) and a visual representation (a line drawing) observed in the DAT group did not relate to the clinical disease severity, an attention deficit, visuospatial dysfunction, and cognitive dysfunction evaluated on the MMSE score. Moreover, a semantic deficit having a significant correspondence between the same individual items was most frequently found to exist for both representations. These results suggest that semantic memory itself (semantic contents meant by each representation) is disrupted in DAT and that the semantic memory system is amodal or single.<br>   Disruption of semantic memory structure was also investigated in a case (case 1) involving selective semantic memory deficits due to the left-sided temporal lobe lesions resulting from herpes simplex virus encephalitis, and a case (case 2) of semantic dementia in which remarkable atrophy was seen in both temporal lobes. Disruption of semantic memory structure observed in case 1 was similar to that found in the DAT group, suggesting that semantic memory itself is also disrupted in case 1. On the other hand, from the perspective of a single semantic memory system, case 2 involves the severe impairment of access between a representation and semantic memory rather than the disruption of semantic memory per se. Disruption of semantic memory itself may be caused by localized damages of the left temporal lobe such as those seen in case 1. In contrast, the severe impairment in access to semantic memory may be caused by degeneration in both temporal lobes such as that seen in case 2.

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