Anomia: a doubly typical signature of semantic dementia.

This study was designed to explore the nature of the anomia that is a defining feature of semantic dementia. Using a pool of 225 sets of picture naming data from 78 patients, we assessed the effects on naming accuracy of several characteristics of the target objects or their names: familiarity, frequency, age of acquisition and semantic domain (living/non-living). We also analysed the distribution of different error types according to the severity of the naming deficit. A particular focus of the study was the impact on naming of a previously unconsidered variable: the typicality of an object within its semantic category. This factor had a major influence both on naming success and on the proportions of different error types. Moreover, and increasingly so with declining naming accuracy, the patients' single-word incorrect responses were more typical than the target names. The observed effects of typicality sit well within models of semantic memory that represent concepts in terms of patterns of co-occurrence of constituent features. The results add to a growing body of evidence that, throughout the progressive deterioration of conceptual knowledge that characterises semantic dementia, both accuracy of performance and the nature of error responses are increasingly determined by the domain-specific aspects of typicality relevant to the task in question.