Apperceptive agnosia and the visual recognition of object categories in dementia of the Alzheimer type

Abstract

This paper reports the case of a woman (NA) with a clinical diagnosis of Alzheimer's disease and a modality-specific disorder affecting the visual recognition of living things. Investigations revealed early visual processing deficits suggestive of visual apperceptive agnosia. Visually related naming errors were numerous, but NA's ability to describe the physical properties of visually similar objects suggested that the structural descriptions of objects were preserved. There were no effects of structural similarity, visual complexity, concept familiarity, name frequency or age of acquisition on her naming of visual objects, and her ability to define the names of objects was normal. Errors were consistent over repeated tests. Purely visual errors were particularly likely to occur (a) when the object was presented from a non-canonical view, (b) when parts of objects were presented out of context and (c) when objects could be readily segmented into separate parts; the latter were particularly common for man-made objects. In contrast, category-related visual errors were more likely to reflect the overall shape of the object, and were most common to living things. The results are argued to show that category-specific deficits, usually considered to be semantic in origin, can arise from perceptual impairments occurring at an early stage of visual processing.