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 cortic al and subcortical dementia. Recent reports that the average size of p honemic 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 pr esent study, patients with AD, PD with dementia, or Huntington's disea se produced fewer words, fewer switching responses and smaller semanti c cluster sizes. Patients with multiple sclerosis, regardless of wheth er or not they were demented, produced fewer words and switching respo nses, but normal size dusters, and patients with PD without dementia p erformed normally on all fluency measures. These results indicate that reductions in cluster size on verbal fluency tests are best interpret ed as changes in the efficiency of access to lexical and semantic memo ry stores. The findings are also consistent with the idea that pattern s of cognitive impairment may differ among diseases that result in sub cortical dementia. (C) 1998 Elsevier Science Ltd. All rights reserved.