Here's why I love it. There are a number of claims in the literature on the neuroscience of language that I disagree with. One is Sophie Scott's claim that speech recognition is a left hemisphere function that primarily involves anterior temporal regions. Another is Angela Friederici's claim that a portion of Broca's area, BA44, is critical for "hierarchical structure processing". In the study reported in this new paper, Friederici and Scott have teamed up and proven both of these claims to be incorrect. This I like.
What I hate about the paper is that the authors don't seem to recognize that their new data provide strong evidence against their previous claims, and in fact argue that it supports their view(s).
So what did they do? The experiment is a nice combination of the intelligibility studies that Scott has published and the syntactic processing studies that come out of Friederici's lab. It was a 2x2 design: grammatical sentences versus ungrammatical sentences x intelligible versus unintelligible (spectrally rotated) sentences.
What did they find? The intelligible minus unintelligible contrast showed bilateral activation up and down the length of the STG/STS, i.e., not just in the left hemisphere and not just anterior to Heschl's gyrus. This contradicts previous studies from Scott's group, particularly with respect to the right hemisphere activation, as the current paper correctly pointed out:
...the right-hemispheric activation in response to increasingly intelligible speech deviates from the original papers on intelligibility [Narain et al., 2003; Scott et al., 2000]. (p. 6)
In short, the primary bit of data that has been driving claims for a left anterior pathway for intelligible speech has been shown to be inaccurate. This is not terribly surprising as those previous studies were severely under powered.
Conclusion #1: the "pathway for intelligible speech" is bilateral and involves both anterior and more posterior portions of the STS/STG.
What about Broca's area and hierarchical structure building? In fairness, most of the paper was about the STG/STS and not about Broca's area, but the role of Broca's area was addressed and of course it is perfectly fair to use data from this study to address a hypothesis proposed by Friederici in other papers. If Broca's area is involved in hierarchical structure building, then it should activate during the comprehension of sentences, which surely are hierarchically structured. Thus, the intelligible (structured) minus unintelligible (unstructured) contrast should result in activation of Broca's area. Yet it did not. The contrast between intelligible and unintelligible sentences resulted only in activation in the superior temporal lobes.
Conclusion #2: Hierarchical structure building can be achieved without Broca's area involvement.
So in light of these findings, how does one maintain the view that intelligible speech primarily involves the left hemisphere and that syntactic (hierarchical) processing involves Broca's area? It all hinges on the response to those pesky ungrammatical sentences.
Here's the assumption on which their argument relies: syntactic processing is really only revealed during the processing of ungrammatical sentences. They don't state it in these terms, but this is what you have to assume for their arguments to work. Right off the bat we have a problem with this assumption. When you listen to an ungrammatical sentence, not only does this mess up syntactic processing, but it also increases the load on semantic integrative processes and who knows what other meta-cognitive processes are invoked by hearing a sentence like, "The pizza was in the eaten", which is an example of the kind of violation they used. In fact, one might even argue that processing an ungrammatical sentence causes the syntactic processing mechanism to shut down and instead crank up cognitive interpretation strategies. Thus rather than highlighting syntax, such a manipulation may highlight non-syntactic comprehension strategies!
So what happens when you listen to ungrammatical sentences and spectrally rotated ungrammatical sentences?
Ungrammatical sentences minus grammatical sentences (intelligible only) resulted in activation the left and right superior temporal lobe, Broca's area (left BA 44), and the left thalamus. So the "syntactic" effect is bilateral in the superior temporal lobe, but at least we now have Broca's area active.
The authors then took these seven ROIs defined in the two main contrasts (intell-unintell and gramm-ungramm), extracted percent signal change around the peaks and performed subsequent ANOVAs to assess interactions. These interactions are what really drives their argument. However, we now have another problem, namely that the data that defined the ROIs is not independent of the data that were subsequently analyzed using ANOVAs. We therefore can't be sure the reported effects are valid. Nonetheless, let's pretend they are see if the conclusions make sense.
Here is a graph of the interactions:
The claim here is that "syntax" (i.e., greater response to ungrammatical) and intelligibility (i.e., greater response to intelligible) significantly interacted only in the left hemisphere ROIs, and indeed in all of them, including BA 44 and the thalamus. Therefore these regions represent the critical network, according the Friederici et al., because they are responding to the syntactic features in intelligible speech and not merely acoustic differences which are present in the unintelligible speech as well. Something is very wrong with this logic even beyond the possible invalid assumption and analysis methods noted above.
Consider the response pattern in BA 44. Zero response to normal syntactically structured sentences (which presumably requires some degree of syntactic processing), significant activation to intelligible ungrammatical sentences, significant (or so it seems) activation to UNINTELLIGIBLE versions of grammatical sentences, and no activation to unintelligible versions of ungrammatical sentences. What possible syntactic computation could be invoked BOTH by a grammatical violation and unintelligible noises but not by grammatical sentences? And this pattern is considered part of the intelligible speech/syntactic processing system whereas the right anterior STS, which shows a very robust intelligibility effect and no obvious effect of violation is not. I would suggest instead that because the right STS area is actually responding to sentences and not just broken sentences or spectrotemporal noise patterns that the right STS is more likely involved in sentence processing.
In the end, Friederici et al.'s entire argument rests on (i) a possibly invalid assumption about their "syntactic" manipulation, (ii) a possibly contaminated statistical analysis, and (iii) a logically questionable definition of what counts as a region involved in the processing of these language stimuli.
The basic findings are extremely important though because they confirm that speech recognition and now the "pathway for intelligible speech" is bilateral and that Broca's area is silent during normal sentence comprehension and therefore is not involved in basic syntactic/hierarchical structure building.
Friederici AD, Kotz SA, Scott SK, & Obleser J (2009). Disentangling syntax and intelligibility in auditory language comprehension. Human brain mapping PMID: 19718654
Narain, C., Scott, S. K., Wise, R. J., Rosen, S., Leff, A., Iversen, S. D., & Matthews, P. M. (2003). Defining a left-lateralized response specific to intelligible speech using fMRI. Cereb Cortex, 13(12), 1362-1368.
Scott, S. K., Blank, C. C., Rosen, S., & Wise, R. J. S. (2000). Identification of a pathway for intelligible speech in the left temporal lobe. Brain, 123, 2400-2406.
Scott, S. K., & Wise, R. J. (2004). The functional neuroanatomy of prelexical processing in speech perception. Cognition, 92(1-2), 13-45.