Clustering and switching processes in semantic verbal fluency in the course of Alzheimer's disease subjects: results from the PAQUID longitudinal study.

Reduced semantic fluency performances have been reported in the preclinical phase of Alzheimer's disease (AD). To investigate the cognitive processes underlying this early deficit, this study analyzed the verbal production of predemented subjects for the animals category with the qualitative parameters related to clustering (i.e. the ability to generate words belonging to semantic subcategories of animals) and switching (i.e. the ability to shift from one subcategory to another) proposed by Troyer. This qualitative analysis was applied to the PAQUID (Personnes Agées QUID) cohort, a 17-year longitudinal population-based study. The performances on the animal verbal fluency task of 51 incident cases of possible and probable AD were analyzed at the onset of dementia, 2 years and 5 years before dementia onset. Each case was matched for age, sex and education to two control subjects leading to a sample of 153 subjects. The mean cluster size and the raw number of switches were compared in the two samples. The results revealed a significantly lower switching index in the future AD subjects than in the elderly controls including 5 years before dementia incidence. A significant decline in this parameter was evidenced all along the prodromal phase until the clinical diagnosis of dementia. In contrast, the mean cluster size could not discriminate the two groups. Therefore the results support the hypothesis that impaired shifting abilities - rather than semantic memory storage degradation - could explain the early decline in semantic fluency performance occurring in the predementia phase of AD.