Cortical and subcortical influences on clustering and switching in the performance of verbal fluency tasks.

Impairments on lexical and semantic fluency tasks occur in both cortical and subcortical dementia. Recent reports that the average size of phonemic and semantic clusters is reduced in Alzheimer's disease (AD), but not in Parkinson's disease (PD) could support the hypothesis that in AD verbal fluency deficits arise from degraded memory storage while in PD the same impairments result from defective retrieval. In the present study, patients with AD, PD with dementia, or Huntington's disease produced fewer words, fewer switching responses and smaller semantic cluster sizes. Patients with multiple sclerosis, regardless of whether or not they were demented, produced fewer words and switching responses, but normal size clusters, and patients with PD without dementia performed normally on all fluency measures. These results indicate that reductions in cluster size on verbal fluency tests are best interpreted as changes in the efficiency of access to lexical and semantic memory stores. The findings are also consistent with the idea that patterns of cognitive impairment may differ among diseases that result in subcortical dementia.