Friday, March 28, 2008

The Motor Theory of Speech Perception Reviewed

Ok, if you intend to do any research in the field of speech/language, here is a paper you have to read:

If you don't read this paper, you might as well get out of the field. Bye! The paper presents a brilliant review of the Motor Theory past and present, summarizing the motivation for its initial development, changes to the theory over the years, and presenting an almost thorough review of existing evidence for each of the three main claims of the theory, (1) speech processing is special, (2) perceiving speech is perceiving gestures, and (3) the motor system is recruited for perceiving speech.  The review covers an impressively broad range of data, from the classic Motor Theory results (McGurk Effect, duplex perception, etc.) to mirror neurons and the role of the motor system in perception beyond speech.  The authors suggest that claim #1 should probably be retired, but claims 2 & 3 remain viable, likely even true.  

I say "almost thorough review" because there is not one line of text dedicated to aphasia, which in my view provides some of the best evidence against claims 2&3.  I've discussed this issue before in the context of mirror neurons (click here or here) but it applies with equal force in the context of the Motor Theory (mirror neuron-related theories of perception are basically motor theories of perception).  Let me reiterate the relevant aphasia evidence: 

Destruction of the motor systems controlling the ability to produce speech does not produce a commensurate destruction of the ability to recognize/comprehend speech.  

This means that speech can be recognized/comprehended without the speech-motor system, which in turn, shows that any strong interpretation of the Motor Theory or mirror neuron-like accounts of speech perception is WRONG.  

As I read this paper, I was struck generally by the fact that aphasia data seems to have been largely ignored in the decades of debate over the Motor Theory.  I haven't done an exhaustive lit search, but a cursory search didn't turn up anything, and the Galantucci et al. review, which is very thorough otherwise, didn't even allude to it.  Why?  Critical and obvious evidence from Broca's aphasia (bad production, good comprehension) has been around even longer than the Motor Theory.  I seriously don't understand the omission.

So why is the paper mandatory reading if it missed such a critical piece of evidence?  Because except for that one omission, it really is a brilliant review.  Of particular importance, is the case the paper makes for sensory-motor interaction in speech, which in my view is absolutely dead-on correct, as our work on the topic attests.  I would suggest that it is this aspect of the Motor Theory -- it's emphasis on the connection between perception and production -- that is the most accurate and enduring aspect of the theory.  It's just that the Motor Theory weights the motor side of the interaction too heavily in its theory of perception.  


Brad Buchsbaum said...

what if the "motor system" extends all the way back in to "sensory" cortex?

In other words lesions to frontal systems not causing speech perception deficits is only definitive if the motor system is synonymous with frontal structures.

But what about Spt? Perched way back there in posterior-land but nevertheless -- don't you think? -- possessing a motor soul!

Greg Hickok said...

Yes, absolutely right: the motor system (and mirror neurons, if I'm not mistaken) is not exclusively frontal. But the question is, does damage to any aspect of this system cause speech perception deficits? The answer seems to be, no. Conduction aphasia, which we have argued results from damage to the posterior part of the "motor system" (Spt/sensory-motor integration), is primarily a production deficit, with minimal consequences for speech recognition.

dwi eny said...

thanks, this article helped much.