Clustering and switching on verbal fluency: the effects of focal frontal- and temporal-lobe lesions.

We examined the hypothesis that, on verbal fluency, clustering (i.e. generating words within subcategories) is related to temporal-lobe functioning, whereas switching (i.e. shifting between subcategories) is related to frontal-lobe functioning. Tests of phonemic and semantic fluency were administered to 53 patients with focal frontal-lobe lesions (FL), 23 patients with unilateral temporal-lobe lesions (TL) and 55 matched controls. Performance by FL patients was consistent with our hypothesis: in comparison to controls, patients with left-dorsolateral or superior-medial frontal lesions switched less frequently and produced normal cluster sizes on both phonemic and semantic fluency. Performance by TL patients was not consistent across fluency tasks and provided partial support for our hypothesis. On phonemic fluency, TL patients were unimpaired on both switching and clustering. On semantic fluency, TL patients were impaired on switching in comparison to controls and left TL patients produced smaller clusters than right TL patients. The best indices for discriminating the patient groups, therefore, were phonemic-fluency switching (impaired only with frontal lesions) and semantic-fluency clustering (impaired only with temporal-lobe lesions).