CLUSTERING AND SWITCHING ON VERBAL FLUENCY - THE EFFECTS OF FOCAL FRONTAL-LOBE AND TEMPORAL-LOBE LESIONS

We examined the hypothesis that, on verbal fluency, clustering (i.e. g enerating words within subcategories) is related to temporal-lobe func tioning, whereas switching (i.e. shifting between subcategories) is re lated to frontal-lobe functioning. Tests of phonemic and semantic flue ncy were administered to 53 patients with focal frontal-lobe lesions ( FL), 23 patients with unilateral temporal-lobe lesions (TL) and 55 mat ched controls. Performance by FL patients was consistent with our hypo thesis: 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. Performanc e by TL patients was not consistent across fluency tasks and provided partial support for our hypothesis. On phonemic fluency, TI, 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 p honemic-fluency switching (impaired only with frontal lesions) and sem antic-fluency clustering (impaired only with temporal-lobe lesions). ( C) 1998 Elsevier Science Ltd. All rights reserved.