Loss of high-level perceptual knowledge of object structure in DAT.

Visual object recognition and naming deficits in patients with dementia of the Alzheimer type (DAT) have typically been attributed to deficits in semantic processing. On a visual object naming test, a group of 10 mild, early stage DAT patients (mean MMSE=23.8) were found to suffer from anomia, compared to a group of 10 age-matched control participants. DAT naming errors were typically within category (commission), associative or circumlocutory errors. Performance on tests of low level visuo-spatial ability fell within the normal range. Together these results suggested that anomia resulted from a dysfunctional semantic system with intact visual perception. However, in a naming task using visually degraded images of familiar objects, the recognition threshold in DAT patients was significantly higher, indicating the need for a more visually complete object representation, before it could be accurately recognised. In a matched task using words visually degraded in an identical manner, the recognition threshold for DAT patients was very similar to that of the control group. It is argued that these results support the idea that impaired structural descriptions of objects (i.e., pre-semantic representation of an object within the visual perceptual system) combines with degraded semantic representations to produce anomia in mild early stage DAT.