"Shane" left an important comment on our Mirror Neuron Results entry, pointing out a couple of papers by Iacoboni's group that address the neuropsychological data relevant to the MN theory of speech perception. Thanks for bringing up these papers, Shane, they are definitely worth discussing here.
Let's start with the Wilson and Iacoboni (2006) paper, which I actually like quite a bit. The fundamental result is that when subjects passively listen to native and non-native phonemes that varied in terms of how readily they can be articulated, fMRI-measured activity in auditory areas covaried with the producibility of non-native phonemes. This suggests that sensory mechanisms are important in guiding speech articulation, as we and others, such as Frank Guenther, have suggested. Wilson and Iacoboni agree, but also argue that the motor system "plays an active role" concluding that "speech perception is neither purely sensory nor motor, but rather a sensorimotor process." I don't think the data from this paper provides crucial evidence supporting a critical role of the motor system, but let's hold that discussion for a subsequent post. What I'd like to address is the point that Shane brought up regarding this paper:
Admirably, Wilson & Iacoboni attempt to deal with the question of Broca's aphasia. In attempting to explain why Broca's aphasics, who have large left frontal lesions yet preserved speech recognition, they suggest, "It is possible that in Broca's aphasia, motor areas in the right hemisphere continue to support speech perception..." (p. 323). This is an odd proposal. Basically, one has to assume that there are motor-speech systems in the right frontal lobe that are neither necessary nor sufficient for speech production, but which can, nonetheless fully support speech perception. This is a strange kind of motor-speech system. More to the point though, if speech perception depends on active participation of the motor speech system, then functional destruction of the motor speech system, as occurs in severe Broca's aphasia, should severely impact speech recognition. It does not. I don't see any theoretical detour around this empirical fact.
Wilson and Iacoboni offer another possibility to explain Broca's aphasia in the context of a motor theory of speech perception. They point out that many such patients indeed have speech perception deficits when assessed using sublexical tasks such as syllable discrimination. This is true, of course, but as we have argued repeatedly (see any Hickok & Poeppel paper, and/or the series of entries on meta-linguistic tasks), performance on these sorts of tasks is not predictive of ecologically valid speech recognition abilities.
Conclusion: motor speech systems are NOT playing any kind of major role in speech recognition. The mirror neuron theory of speech perception, just like its predecessor, the motor theory of speech perception, is wrong in any strong form.
Once we all agree to this, then we are in a position to have an interesting discussion, because we can then begin to ask questions like, Do motor speech systems participate in any, say supportive, aspect of speech recognition? If so, under what conditions? (Perhaps under noisy listening conditions.) What kind of operations might be supported by this system? (How about predictive coding, attentional modulation, etc.?)
So let's start this discussion by looking first for evidence of motor involvement in speech recognition. Shane suggested this paper: Meister et al. (2007). The essential role of premotor cortex in speech perception. Current Biology, 17, 1692-1696, so we'll start here in our next post.