News and views on the neural organization of language moderated by Greg Hickok and David Poeppel
moderated by Greg Hickok and David Poeppel
Friday, November 30, 2007
Mirror Neuron Survey Results
Ok, the results are in! A majority (62%) of Talking Brains readers shun mirror neurons as the primary substrate for speech perception. Only 13% believe these cells play a critical role, and 23% are not sure. I personally, would love to have a discussion here between those of us who don't believe the mirror neuron theory of speech perception and those who do. It doesn't have to get nasty. I has some great face to face discussions with my former post doc Stephen Wilson about this stuff. Stephen came from Iacoboni's lab, which has published on the role of motor areas in speech perception. We ended up coming to a reasonable consensus, I think. (Of course, he did leave for UCSF so...) So speak up on the topic! New survey coming soon.
Posted by Greg Hickok at 12:59 PM
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Before we get into a discussion (that is definitely worth having) on how mirror neurons contribute to speech perception, I would like to know one thing about mirror neurons: what do they NOT contribute to? Are there any domains of human experience for which mirror neurons are irrelevant?
Once we have narrowed down the candidates, then I'm ready to play. But I need to know whether speech perception, the evolution of language, empathy/theory of mind/social cognition, autism, aging, imitation etc. are all in play?
I think one way to address David's comment above might be to distinguish between results of empirical studies vs claims made in reviews. There seem to be almost as many review articles/chapters on mirror neurons out there as there are empirical studies (perhaps more). Do others think this is the case? So, as a rule of thumb, let's assume that mirror neurons aren't involved in something unless some empirical data supports the contention in question? Of course, what do we consider supporting evidence? ROI overlap doesn't seem to be a very reliable measure -see the article from Michael Tarr's lab (J Cog Neurosci, 2007, 19, 1-16).
That definitely is my impression regarding the ratio of theoretical speculation vs. empirical papers. The empirical work is all good and interesting. I like the idea of following each theoretical claim with this question: What is the evidence? And you can't say, "Monkey PFC neurons fire when they see someone grab a peanut."
How about we forget the complicated stuff for now, and stick to what we know: speech. What is the evidence that mirror neurons participate in speech perception? My view is that the first hurdle, before talking about any functional activation data, is to explain why damage to the "mirror system" for speech affects production and not speech recognition. If someone can come up with a satisfying answer to this data point, then the mirror neuron hypothesis has a chance. So far, I've seen no explanation. In fact, the question is completely ignored.
Have you seen this paper?:
Meister, I. G., Wilson, S. M., Deblieck, C., Wu, A. D., & Iacoboni, M. (2007). The Essential Role of Premotor Cortex in Speech Perception. Current Biology, 17(19), 1692-1696.
It doesn't address the neuropsychological literature, which is surprising since they use TMS, but is interesting reading in light of your claims about the role of premotor cortex in speech perception - i.e. the activity found is task dependent (e.g. categorical judgements to sublexical information) and not necessary for normal speech perception. However, I have seen an attempt to deal with the question in this paper:
Wilson, S. M., & Iacoboni, M. (2006). Neural responses to non-native phonemes varying in producibility: Evidence for the sensorimotor nature of speech perception. NeuroImage, 33(1), 316-325.
They suggest that perhaps motor areas contributing to speech perception are bilateral, and most of the case studies deal with left hemisphere damage only. But they do cite you guys, and mention the neuropsychological evidence that motor involvement in speech perception seems to be important for sublexical speech perception tasks.
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