Perceptual and semantic contributions to episodic memory: evidence from semantic dementia and Alzheimer's disease

Previous group studies involving patients with semantic dementia, who have impaired semantic memory associated with temporal lobe atrophy, have documented the preservation of pictorial recognition memory, in contrast to patients with early Alzheimer's disease, who characteristically exhibit amnesia. The present study replicated this general pattern, although four of the semantic dementia patients with the most severe semantic deficit additionally had impaired recognition memory. Three factors that might contribute to this pattern of memory performance were examined: atrophic damage to medial temporal lobe regions, degradation of semantic representations, and disruption to visuoperceptual processes. Assessment of MRI scans revealed that atrophy affecting the perirhinal cortex region accurately predicted the recognition memory deficit seen at advanced stages of semantic dementia, but there was no evidence that it could be attributed directly either to degraded semantic knowledge or disrupted perceptual processing. In Alzheimer's disease, evidence suggested that visuoperceptual impairment might be involved in the poor recognition memory typically seen in the disorder. These results have implications for the differential diagnosis of semantic dementia and Alzheimer's disease and for cognitive and neural theories of human long-term memory. (C) 2002 Elsevier Science (USA). All rights reserved.