fMRI studies of Broca's area in sentence comprehension

Broca's Area (ie, Brodmann Areas (BA) 44 and 45) has been associated with syntactic comprehension since the 1970s. In particular, aphasia and neuroimaging studies have shown that Broca's area distinguishes between sentences with subject extraction (ie, The horse that ▪ is kicking the bear is brown) compared to object extraction (ie, The horse that the bear is kicking ▪ is brown). The extracted argument is referred to as a filler and the position from which it is extracted is referred to as a gap and the relationship between them as a filler-gap dependency. Based on these results, functional accounts of Broca's area in sentence comprehension have been formulated that range from verbal Working Memory (WM) to General-Syntactic to Specific-Syntactic. Predictions of these three types of accounts were tested across 4 fMRI studies. Study I contrasted long-distance argument extraction to another syntactic dependency---Reflexive Binding---to see if there was activation overlap in Broca's area for the two dependencies in accordance with a general WM (verbal or syntactic) account. Study II presented a methodological improvement on Study I by parametrically varying the distance of the dependent elements in both dependencies. The results from Study I and II indicate that Broca's area is sensitive to WM demands in sentences with extraction but not those with Reflexive Binding. Study III investigated fMRI adaptation to the two syntactic complexity factors---extraction type (object vs subject) and relative clause position (center-embedded vs right-branching). While anterior Broca's area (BA 45) adapted to extraction type only, posterior Broca's area (BA 44) adapted to both complexity factors. Finally, Study IV used parasitic gap constructions with their pronoun-containing alternative to investigate whether Broca's area is sensitive to gap-filling in addition to filler-to-gap distance. There was no evidence that Broca's area is sensitive to gap-filling in addition to filler-to-gap distance. Collectively, the results indicate that anterior aspects of Broca's area are specific to filler-gap dependencies and particularly to the filler-to-gap distance (ie, storage processes for the filler), whereas posterior aspects of Broca's area are more generally engaged in syntactic processing