The effects of plausibility of thematic role assignment and syntactic structure on blood oxygenation level dependent (BOLD) signal were studied using event-related functional magnetic resonance imaging by orthogonally varying syntactic structure (subject-vs. object-extracted relative clauses) and the plausibility of nouns playing thematic roles (constrained vs. unconstrained sentences) in a plausibility judgment task. In plausible sentences, BOLD signal increased for object-compared to subject-extracted clauses in unconstrained sentences in left middle temporal and left inferior frontal areas, for this contrast in constrained sentences in left middle temporal but not left inferior frontal areas, and for constrained subject-extracted sentences compared to unconstrained subject-extracted sentences in the left inferior frontal gyrus and the left dorsolateral prefrontal cortex. We relate these areas of activation to the assignment of the syntactic structure of object-compared to subject-extracted structures and the process of checking which thematic roles activated in the course of processing a sentence are licensed by the syntactic structure of the sentence.

Positron emission tomography (PET) was used to determine regional cerebral blood flow (rCBF) as a function of the syntactic form and propositional density of sentences. rCBF increased in the left pars opercularis, part of Broca's area, when subjects processed syntactically more complex sentences. There were no differences in rCBF in the perisylvian association cortex traditionally associated with language processing when subjects made plausibility judgments about sentences with two propositions as compared to sentences with one proposition, but rCBF increased in infero-posterior brain regions. These results suggest that there is a specialization of neural tissue in Broca's area for constructing aspects of the syntactic form of sentences to determine sentence meaning. They also suggest that this specialization is separate from the brain systems that are involved in utilizing the meaning of a sentence that has been understood to accomplish a task.