What Does Broca's Area Activation to Sentences Reflect?

Abstract

There is no doubt that listening to or reading sentences activates the left frontal lobe structures in and around Brocaʼs area. The question is, what process(es) does this activation reflect? In our recent critical review of this literature (Rogalsky & Hickok, 2011) we questioned whether any of this activation reflects processes that are specific to receptive sentence processing. Specifically, we pointed out that sentence activation in the pars opercularis overlaps with activations induced by speech articulation, whereas more anterior (pars triangularis/orbitalis) and dorsal (middle frontal gyrus) sentence-elicited activations overlap with activations induced by cognitive control tasks. Given that sentence processing may be assisted by nonspecific functions such as articulatory rehearsal and cognitive control, this raises serious questions regarding the sentence specificity of activations in Brocaʼs area and vicinity during comprehension. We concluded that the pars opercularis supports sentence comprehension via articulatory rehearsal mechanisms (phonological STM), whereas more anterior and dorsal areas may support sentence processing via some, as yet undetermined, higher-order process such as cognitive control. Additional arguments for this position were also put forward. Federenko and Kanwisher (FK Daneman & Newson, 1992; Slowiaczek & Clifton, 1980; Baddeley, Thomson, & Buchanan, 1975), and an explicit memory probe task may induce articulatory rehearsal. There is little doubt that Brocaʼs area and vicinity play some role in speech production, including sentencerelated processes (e.g., agrammatism is a common correlate of left frontal lesions). Therefore, the differences between sentences and word lists in F&Kʼs data may be a result of covertly producing sentences rather than comprehending them. Our claims specifically target sentence comprehension. A previous study of ours directly assessed the role of production-related processes in driving activations in Brocaʼs area during the performance of a complex sentence comprehension task, semantic plausibility judgment (Rogalsky, Matchin, & Hickok, 2008). We reasoned that sentences that are particularly difficult to understand, such as object-relative clauses, may induce articulatory rehearsal to assist in task performance, compared with simpler sentences, which should rely to a lesser extent on such mechanisms. We found that Brocaʼs area responded more to complex than simple sentences, but that an articulation task, assessed within-subjects, completely overlapped the activation in posterior sectors (pars opercularis) and that the “sentence complexity effect” in this region did not emerge when subjects comprehended sentences during concurrent articulation (articulatory suppression). In other words, articulatory processes explain the Brocaʼs area activation to sentences, at least in the posterior sector. In the anterior sector (pars triangularis), a sentence complexity effect persisted even during concurrent articulation. However, this effect disappeared during a concurrent finger-tapping task, again suggesting that the sentence activations in Brocaʼs area are not sentence specific. University of California-Irvine, University of Southern California