David commented previously on Grodzinsky and Santi's (2008) TICS paper on the function of Broca's area. Justifiably in my view, David expressed his disappointment that an obviously multi-functional region was being linked to a single function, syntactic movement. In addition, a recent reply to the paper in TICS by Roel Willems and Peter Hagoort (2009) highlighted the possible role of Broca's area in semantic processing, an issue that was not addressed at all in the Grodzinsky and Santi paper. So there is no shortage of critics of the opinion piece. I might as well join the party...
By way of reminder, Grodzinsky and Santi discuss four main hypotheses regarding the function of Broca's area: action comprehension, working memory, syntactic complexity, and syntactic movement. They argue for the last as being the one that is best supported by the data. Having dabbled in theoretical explanations of the pattern of comprehension deficits in Broca's aphasia, I'm sympathetic to the syntactic movement account. However, there are two problems with the proposed link to Broca's area (BA 44/45). One is that the argument against the working memory theory is exceptionally weak and the other is that the comprehension deficit does not appear to be linked specifically with Broca's area.
The working memory theory. The idea behind the working memory theory is that sentences containing syntactic movement require additional working memory resources to process and Broca's region (or more accurately, the lesions that produce Broca's aphasia and agrammatic comprehension) is(are) critical for working memory. G&S argue against this position on the basis of "preliminary studies" (p. 477) of the comprehension of reflexive constructions, such as Mama Bear touched herself, which they suggest requires working memory. Six Broca's aphasics were able to comprehend such sentences "contrary to the prediction of a WM deficit account" (p. 477). Well, what if the amount of working memory required to comprehend Mama Bear touched herself is less than the amount of working memory required to comprehend The cat that the dog chased was very big? If there are working memory differences between these constructions, which seems a priori plausible, then the argument against a working memory explanation is invalid. Further, our own work suggest that working memory may account for at least a portion of the comprehension pattern attributed to patients with left frontal convexity lesions (see this post).
Broca's area is not specifically implicated in agrammatic comprehension. It is well known (well, maybe not well known, but well established) that damage restricted to Broca's area does not cause Broca's aphasia (Mohr, 1976; Mohr, et al. 1978). Agrammatic comprehension -- i.e., the pattern of comprehension deficits that Grodzinsky and Santi are trying to explain -- is associated with Broca's aphasia, not with Broca's area lesions. From this we can infer that lesions to Broca's area alone do not cause syntactic movement deficits. Add to this the observation that conduction aphasics, i.e., patients with posterior lesions, also tend to exhibit agrammatic comprehension, and you have a one-two punch: Lesions to restricted to Broca's area don't cause agrammatic comprehension and lesions to other brain regions (and sparing Broca's area) can cause agrammatic comprehension. From this we conclude that Broca's area plays no special role in syntactic movement (Hickok, 2000).
One of the complications here is that there are probably a number ways to cause deficits on the comprehension of sentences with long-distance dependencies. These structures tend to be harder to comprehend in control subjects. As such, one would expect that any disruption of processing efficiency, e.g., working memory or attentional deficits, could cause "impairments" in the comprehension of these types of sentences. Of course, a specific disruption of the syntactic operation computing long-distance dependencies could also disrupt comprehension on these sentences, but other possible sources of the deficit are rarely assessed, let alone ruled out when testing such patients.
We still don't understand the role of Broca's area in sentence comprehension, working memory, action processing, semantic processing, or any of its other possible functions.
Y GRODZINSKY, A SANTI (2008). The battle for Broca’s region Trends in Cognitive Sciences, 12 (12), 474-480 DOI: 10.1016/j.tics.2008.09.001
Roel M. Willems, Peter Hagoort (2009). Broca's region: battles are not won by ignoring half of the facts Trends in Cognitive Sciences, 13 (3), 101-101 DOI: 10.1016/j.tics.2008.12.001
Hickok, G. (2000). The left frontal convolution plays no special role in syntactic processing. Behavioral and Brain Sciences, 23, 35-36.
Mohr, J. P. (1976). Broca's area and Broca's aphasia. In H. Whitaker & H. A. Whitaker (Eds.), Studies in neurolinguistics, vol. 1 (pp. 201-235). New York: Academic Press.
Mohr, J. P., Pessin, M. S., Finkelstein, S., Funkenstein, H. H., Duncan, G. W., & Davis, K. R. (1978). Broca's aphasia: Pathological and clinical. Neurology, 28, 311-324.