Here are a few thoughts...
1. Are unilateral lesions a strong enough test to reject the motor theory of speech perception? Well first let's be very clear about what the motor theory claims. I quote from Liberman & Mattingly (1985):
the objects of speech perception are the intended phonetic gestures of the speaker, represented in the brain as invariant motor commands that call for movements of the articulators through certain linguistically significant configurations. These gestural commands are the physical reality underlying the traditional phonetic notions-for example, ‘tongue backing, ’ ‘lip rounding,’ and ‘jaw raising’-that provide the basis for phonetic categories. They are the elementary events of speech production and perception.” (p. 2)
So if we could find a patient with damage to these "motor commands that call for movements of the articulators" then such a patient should not be able to perceive speech sounds. Now how do we know whether we have found such a patient? Well, it seems to me if you find someone who no longer can call up these motor commands to produce speech, then you have found the critical case. Of course, you want to make sure the deficit is not simply a peripheral disease, etc., and really affects motor planning. Severe Broca's aphasia fits the bill here, and as we all know, they can comprehend speech quite well. Therefore, the motor theory is wrong.
As far as I can tell -- please someone correct me if I'm wrong! -- the only way out of this argument against the motor theory is to point to the word intended in Liberman & Mattingly's definition:
the objects of speech perception are the intended phonetic gestures of the speaker [italics added]
Maybe intended gestures are coded more abstractly and are separate from the motor planning areas that are often damaged in Broca's aphasia (note, this is often most of the left lateral frontal and even parietal lobe!). I would actually agree with this assessment (note, this means that intended gestures are probably not stored in the frontal or even parietal lobe, i.e., the mirror system!). The reason I agree with this is that it admits the possibility that intended gestures are not motor in nature but sensory. That is, suppose that the goal or intention of a speech act is not a movement but a sound. The target of such a goal or intention would then be coded in auditory cortex. This is essentially the view we've been promoting: a sensory theory of speech production rather than a motor theory of speech perception.
2. Does the undeniable linkage between speech perception and speech production mean that the motor theory is correct? I agree that there is undeniable evidence that speech perception and speech production are linked. Without such a linkage, we wouldn't be able to talk. However, it puzzles me why motor theorists assume (perhaps tacitly) that the only explanation for this linkage is that speech is perceived via the motor system. Why does the linkage have to hold only in that direction, namely that action determines perception? Why can't the reverse hold, namely that perception determines action? In fact we know that the later relation has to hold for some abilities. Learning to produce the speech sounds and words of your language is a motor learning task -- you have to figure out how to move your vocal tract to make the sounds and words that you are hearing. The input to this motor learning task is auditory. Therefore, there must be a mechanism for using auditory information to inform motor speech gestures. We know further that this auditory to motor influence persists throughout life because of the effects of late onset deafness, which causes articulatory decline, and various studies on effects of altered auditory feedback on speech production. So we have plenty of justification for why sensory and motor systems are linked without invoking a motor theory. It is true that motor-related activity can influence perception, but these effects too can be explained without invoking a full blown motor theory. All one has to assume is that the motor system can prime sensory speech systems, top-down style, perhaps in the form of forward models. Now we can explain everything the motor theory does and in addition explain why Broca's aphasia doesn't destroy the ability to understand speech.
3. Does anyone believe the motor theory as it was presented 50 years ago? Well, no, but that's because the motor theory came with a lot of baggage like the modularity of the system (the "speech is special" claim). The central claim of the theory, that speech is perceived by mapping acoustic input onto motor representations is a core belief of many neuroscientists who dabble in speech. For example, here is the concluding sentence from the abstract of a forthcoming paper in Current Biology titled, "The Motor Somatotopy of Speech Perception by D'Ausilio, Pulvermuller, Salmas, Bufalari, Begliomini, & Fadiga:
We discuss our results in light of a modified ‘motor theory of speech perception’ according to which speech comprehension is grounded in motor circuits not exclusively involved in speech production.
So yes, people believe in the core tenet of the motor theory -- despite the fact that the theory makes no sense in light of Broca's aphasia.
Thanks much to Patti for her very important comments!
Alessandro D'Ausilio, Friedemann Pulvermüller, Paola Salmas, Ilaria Bufalari, Chiara Begliomini, Luciano Fadiga (2009). The Motor Somatotopy of Speech Perception Current Biology DOI: 10.1016/j.cub.2009.01.017
Liberman, A.M., and Mattingly, I.G. (1985). The motor theory of speech perception revised. Cognition 21, 1-36.
I greatly enjoyed reading this paper, it was about time a paper like this appeared, especially after 10 years of people explaining everything in terms of mirror neurons as the solution to everything. However, I find that your review of the evidence against the motor theory of speech perception seems rather harsh. You state that the motor theory predicts that damage to the structures supporting production should damage perception and vice versa. And as no compelling evidence to this effect was found, you conclude that the motor theory is not true (in its strong form). But most of the lesion studies involve patiens with unilateral lesions, and it could very well be the case that only bilateral lesions lead to deficits in perception or production. (Note that I'm making the same argument as Rizzolatti & Craighero (2004) that you discuss on page 4.) It's been argued on many occasions that lesion studies cannot provide a conclusive answer for this type of argumentation. Maybe TMS is the answer here? Also, Galantucci et al also argue that, while the strongest claims of the motor theory are obviously false, it is undeniably the case that speech perception and production are linked. (Even though you state this as well at the bottom of page 11 right column). I don't think that there's any researcher in the speech field who still supports the motor theory in its strong form. Anyway, I can see your point and I believe you're right, I just think that it misrepresents the field to assume that anyone still believes in the motor theory as it was presented 50 years ago... (Now I'll read your TiCS paper ;-) )