Influences of clustering and switching on the verbal fluency performance of patients with Alzheimer's disease

Several lines of evidence suggest that in Alzheimer's disease (AD) there is a progressive degradation of the hierarchical organization of semantic memory. To evaluate this hypothesis, clustering and switching on phonemic and semantic fluency tasks were studied. For elderly controls, bath clustering and switching were correlated with the numbers of correct words generated on both fluency tests, but the contribution of clustering was greater on the semantic task. Patients with AD generated fewer correct wards and made fewer switches than controls on both fluency tests. The average size of their semantic clusters was smaller and the contribution of clustering to word generation was less than for controls. Severity of dementia was correlated with the numbers of correct words and switches, but not with cluster size. These results are consistent with various hypotheses which maintain that the structure of semantic memory in AD is degraded but provide no evidence that this process is progressive. Instead, progressive worsening of verbal fluency in AD seems to be more strongly associated with the deterioration of mechanisms that govern initiation of search for appropriate subcategories.