The influence of frequency, concreteness, and grammatical category on semantic processing and word retrieval in probable Alzheimer's disease

Word retrieval difficulties hamper the communicative abilities of individuals with Alzheimer's disease (AD), but clinicians and researchers know little about why certain words are difficult for persons with AD to produce and comprehend during conversational speech. Identification of variables that influence word retrieval performance in AD is an essential first step in the development of models of lexical access in AD. The influential models of lexical access and semantic processing in nonimpaired adults developed by Levelt and colleagues (Levelt, 1989; Levelt, Roelefs, and Meyer, 1999) and Damasio and colleagues (Damasio, A. R., 1990; Damasio, H., Grabowski, Tranel, Hichwa, and Damasio, A. R., 1996) suggest that target word frequency, concreteness, and grammatical category may play important roles in determining lexical retrieval and semantic processing success in AD.; Retrieval and processing of a core set of target words selected on the basis of their frequency, concreteness, and grammatical category characteristics was assessed via word-nonword decision, definition, sentence completion, and naming to definition tasks. Participants in the primary study included twenty elderly controls, ten mildly impaired AD subjects, and seven moderately impaired AD subjects. As predicted by the Levelt and Damasio models, concreteness had a greater impact upon AD performance for the definition and naming to definition tasks, which required effortful processing of the semantic representations of target words. Frequency exerted a greater influence on AD performance for the task that required minimal semantic processing, word-nonword decision, and the tasks in which the orthographic form of the target word was not given, sentence completion and naming to definition. Interactions between concreteness and frequency in the results of the sentence completion tasks together with the effects of concreteness on definition and naming to definition performance suggest a connection between semantic and episodic with AD and normal aging has its root in a failure to subconsciously access words. Analysis of individual performance patterns suggests that loss of the semantic representations of specific words is not a typical feature of mild to moderate AD.