Broca's area shows a "sentence complexity" effect. It responds more during the comprehension of object relative (OR) constructions than easier to process subject relative (SR) constructions:
OR: The man that the boy pushes is wearing a red shirt
SR: The man that pushes the boy is wearing a red shirt
What is driving the complexity effect? Presumably it is some form of working memory. In the case of OR sentences, you have to hold two items in memory -- the man, the boy -- before you get to the verb that tells you the role these items will play in the sentence. In SR sentences, you only have one item in memory -- the man -- before you encounter the verb which establishes the role of the noun phrase.
But what is the nature of this working memory process? It is a syntactic-specific form of working memory? Or is it an ordinary, domain-general form of working memory, like Baddeley's phonological loop?
David Caplan and colleagues (2000) addressed this issue a while ago in an imaging study that tested for the existence of a sentence complexity effect in Broca's area under conditions of articulatory suppression (subject was continuously articulating a phrase). This manipulation effectively eliminates articulatory rehearsal, a major component of phonological working memory. If the sentence complexity effect in Broca's area was due to phonological working memory -- the articulatory rehearsal component in particular -- then no complexity effect should be evident during articulatory suppression. Caplan et al. reported a complexity effect in Broca's area even during articulatory suppression, however, and so they concluded that the effect was due to "processing syntactic forms themselves" rather than to articulatory rehearsal/phonological working memory.
I like Caplan et al. study, but it doesn't convince me fully that at least part of the complexity effect isn't due to phonological working memory. For one thing, they used written stimuli, and I'm never convinced that reading generalizes to auditory comprehension -- my primary interest. For another thing, the activation focus in Broca's area was very anterior, likely in the pars triangularis/BA45, not in the pars opercularis/BA44 which is more likely a site involved in articulatory rehearsal. Maybe a good chunk of the suppression effect in "Broca's area" -- usually defined as the combination of the two areas noted above -- IS driven by plain old articulatory rehearsal.
Former TB West grad student, Corianne Rogalsky, decided to find out. We first conducted a simple behavioral experiment where undergrads were asked to comprehend sentences either during articulatory suppression, or while performing a control secondary task, finger tapping. During both secondary tasks, subjects were basically at ceiling for active sentences, passive-voice sentences, and SR sentences, but performance dropped off for OR sentences, as expected. Importantly, performance was significantly worse on OR sentences during articulatory suppression compared to finger tapping, indicating that articulatory suppression was interfering with "complex" sentence comprehension above and beyond the general effects of performing two tasks at once. So, articulatory rehearsal seems to be involved in comprehending OR sentences.
What is the role of Broca's area in these effects? We used fMRI to find out. Subjects listened to SR or OR sentences and made semantic plausibility judgments about them during articulatory suppression, finger tapping, or without any secondary task. We also mapped the activation patterns associated with the secondary tasks alone. Without any concurrent task, we found a robust sentence complexity effect in Broca's area, that included both the pars triangularis and pars opercularis:
During articulatory suppression, much of the complexity effect disappeared, leaving only a focus in the pars triangularis similar to Caplan et al.'s finding:
Somewhat unexpectedly, finger tapping also led to an elimination of much of the complexity effect activation in Broca's area, although with a very different distribution. Now the complexity effect is evident in the pars opercularis:
This pars opercularis focus, but not the triangularis focus, was also strongly activated by the articulatory suppression task alone, indicating that this more posterior portion of Broca's region is the IFG correlate of articulatory rehearsal, as previous work has indicated.
So what does all this mean? First, we can conclude that plain old, domain-general articulatory rehearsal contributes to sentence comprehension under high load conditions (i.e., OR sentences), and at least part of the sentence complexity effect in Broca's area is attributable solely to articulatory rehearsal, namely the pars opercularis part. Second, we replicated Caplan's result showing that a portion of Broca's area, the pars triangularis, persists in showing a sentence complexity effect even during articulatory suppression. However, this effect was eliminated during concurrent finger tapping, possibly arguing against a syntactic specific interpretation of the sentence complexity effect in the pars-T. Oddly though, finger tapping alone didn't activate the pars-T, it just reduced the amplitude of the response to the OR sentences (without an obvious drop in behavioral performance). So the story on the pars opercularis seems clear: it supports articulatory rehearsal, which in turn supports the processing of high load sentences. But the story on Mr. T is mysterious. We're open to suggestions.
David Caplan, Nathaniel Alpert, Gloria Waters, Anthony Olivieri (2000). Activation of Broca's area by syntactic processing under conditions of concurrent articulation Human Brain Mapping, 9 (2), 65-71 DOI: 10.1002/(SICI)1097-0193(200002)9:23.0.CO;2-4
Rogalsky C, Matchin W and Hickok G (2008) Broca's area, sentence comprehension, and working memory: an fMRI study. Front. Hum. Neurosci. doi:10.3389/neuro.09.014.2008