Verbal fluency in Huntington's disease: a longitudinal analysis of phonemic and semantic clustering and switching

Two underlying components of verbal fluency tasks have been identified as clustering (the ability to generate successive words within a sub-category) and switching (the ability to shift from one sub-category to another). Selective impairment of switching ability occurs in patients with frontostriatal pathology, whilst clustering ability is compromised with temporal lobe dysfunction. Letter fluency tasks have been shown to be especially sensitive to frontostriatal deficits, whereas, category fluency tasks tend to be compromised by temporal lobe pathology. This study examined two types of verbal fluency task (letter fluency and category fluency) using two levels of analysis (phonemic and semantic) for clustering and switching measures. The performance of 21 frontostriatally compromised Huntington's disease (HD) patients was followed over an average of 3.5 annual follow-up visits. HD patients showed a significant reduction of correctly generated words over time, together with a significant increase in word repetitions. Phonemic switching decreased significantly over time for both letter and category fluency. Semantic switching, however, remained stable over time for both verbal fluency tasks. Clustering (both semantic and phonemic) likewise remained stable and did not vary longitudinally for either letter or category fluency. Hence, phonemic switching alone drove verbal fluency performance and this selective impairment can be explained by the progressive involvement of frontostriatal circuitry in the natural progression of HD.