Specialisation in Broca's region for semantic, phonological, and syntactic fluency?

The literature suggests that that semantic and phonological fluency tasks selectively activate left Brodmann's area (BA) 45 and 44, respectively, in Broca's speech region. We used functional MRI to test this hypothesis. Subjects performed a semantic and a phonological fluency task. In addition, a syntactic fluency task (e.g. "Generate nouns with masculine gender") was included. Resting blocks were included as a low-level control condition. The exact localisation of the effects was tested with cytoarchitectonic probability maps of BA 44 and BA 45. Participants generated fewer words in the syntactic than in both the semantic and the phonological condition, which did not differ from each other. Compared to rest, all language tasks activated the well-known language network in the left hemisphere including both left BA 44 and BA 45. In the direct contrasts between the different verbal fluency tasks, phonological fluency activated BA 44 more strongly than semantic or syntactic fluency. However, semantic fluency did not elicit higher activation than the phonological fluency tasks in any part of Broca's region. No differences were observed between syntactic and semantic fluency. Thus, the activation in BA 45 observed during verbal fluency tasks seems to be not restricted to semantic processing as suggested by the literature. In contrast, phonological verbal fluency additionally involved the left BA 44. In conclusion, different parts of Broca's region support task-specific and more general processes in verbal fluency.