Constraining theories of semantic memory processing: Evidence from Dementia

Abstract

In this paper we analyse the performance of ten patients with Dementia of the Alzheimer's Type (D.A.T.) who show a pattern of performance suggesting a deficit at the level of semantic memory in the face of normal visual perceptual processing. We use the results of their performance on probe questions for pictures and words to evaluate several hypotheses arising from recent theories concerning semantic memory. We assess whether these patients demonstrate better performance on pictures than words (they do), and whether this can be explained away as a by-product of the perceptual nature of the items tested; pictures whose items have many discernible object parts would tend to contact more residual information in semantic memory, thus producing apparent superior performance from pictures. In fact, we find no support for this explanation. Rather, we are able to demonstrate, in the semantic category of animals, that it is only the items that are correctly identified (as a whole) that will give rise to evidence of better performance on pictures. We then go on to demonstrate that, in contrast to theories suggesting the presence of multiple stores within semantic memory, error analysis of our patients suggests that associative conceptual knowledge is stored amodally … loss of such functional knowledge for pictures is usually accompanied by equal loss of the same information for words. In the discussion section, we present theoretical arguments for a distinction between two kinds of semantic processing; the routines involved in the identification and categorisation of visual instances of concepts, and the associative framework that constitutes our full knowledge of such concepts. We suggest that such a framework is most consistent with the data from our patients. Despite arguments that present theories of semantics are too vague for empirical study, it is possible to test particular elements of such theories and provide data that supports or opposes them.