Double dissociation of letter and category fluency following left frontal and temporal lobe lesions

Abstract

Background: A number of studies have suggested that temporal cortex is critical for the ability to generate exemplars belonging to a particular semantic category (i.e., category fluency), while frontal cortex is critical for generating words beginning with a particular letter (i.e., letter or phonemic fluency). However, previous studies have often focused on relative, quantitative differences in performance across groups of patients and primarily in the oral domain. Aims: The current study analysed verbal fluency data from two individuals with chronic aphasia, one with a large left temporal lobe lesion and a severe Wernicke's aphasia, and a second individual with a large left frontal lobe lesion and a moderately severe non-fluent aphasia. The goal of the study was to do both qualitative and quantitative analyses of letter and category fluency performance in these two individuals across oral and written fluency domains. Methods & Procedures: Participants were administered both oral and written versions of letter fluency (FAS) and category fluency conditions (fruits, animals, and supermarket items). Participants were given 90 seconds to generate as many items as possible, and their responses were scored for both overall output, as well as qualitative structure using a clustering analysis. Outcomes & Results: The individual with an extensive temporal lobe lesion generated a large number of exemplars on the letter fluency task. Some of these items were neologisms (e.g., frest, anth, and swink), but remarkably all adhered to the phonological constraints of the task. However, this individual was very poor at generating items belonging to semantic categories. In contrast, the individual with a large frontal lesion generated many exemplars on the category fluency task but only two items on the letter fluency task. The same pattern was replicated in the written domain in both individuals, and results from the clustering analysis paralleled these quantitative findings. Conclusions: This report describes two individuals with aphasia who exhibited a double dissociation between letter and category fluency performance in both oral and written domains. The current findings provide further evidence for the notion that left frontal cortex is critical for word retrieval based on phonology, while left temporal cortex is critical for word retrieval based on semantics. The findings have implications for the types of strategies that may be most effective in individuals with lesions to these brain regions critical for word retrieval.