Short answer: We don't know. Despite the title of a recent paper by Riikka Mottonen and Kate Watkins' in The Journal of Neuroscience, Motor Representations of Articulators Contribute to Categorical Perception of Speech Sounds, the data reported are, unfortunately, uninterpretable.
Here's the long answer: Mottonen and Watkins asked subjects to perform syllable identification (which of two syllables did you hear?) and syllable discrimination (are the two syllables you just heard same or different). The stimuli were place or voice onset time continua and the design followed a standard categorical perception (CP) experimental design. These tasks were performed either pre-rTMS of after rTMS was applied to the lip area of left motor cortex. I will focus here on the discrimination task. The critical condition was when the subjects discriminated a lip-related sound (ba or pa) from a non-lip-related sound (da or ga). They report that discrimination across a category boundary was less accurate after TMS to motor lip areas than before. Specifically, for the ba-da stimuli, the mean proportion of "different responses" to cross category (i.e., physically different) syllables was .73 pre-TMS and .58 post-TMS; similar findings are reported for the pa-ta stimuli. Discriminations that did not involve lip-related sounds (ka-ga or da-ga) were not affected by stimulation, nor was lip-related speech sound discrimination affected by motor hand area stimulation.
So how is this uninterpretable? At risk of becoming a methods curmudgeon (it's probably too late), this paper, like many I've discussed here, used the wrong analysis. Instead of using an unbiased measure, d-prime, they used a biased measure, proportion of different responses. This renders the data uninterpretable because we can't actually tell whether TMS affected the perception of the speech sounds (an interesting possibility) or the subjects' response biases (a less interesting possibility). It's not the authors' fault though. Speech studies of this sort have used the wrong measure for decades. Does this make the entire field uninterpretable? Well... I'll let you draw your own conclusions.
To illustrate the problem, consider the graph below which plots d-prime values as a function of a range of possible hit rate values (hits in this study would be correctly identifying same trials as same) for two constant false alarm (FA) rates corresponding to the pre- and post-TMS values reported by Mottonen and Watkins. FA was calculated simply by subtracting the proportion of different response values from 1, i.e., proportion of different responses (to different trials) is the proportion of correct rejections (the two stimuli are not the same), and FAs is just the inverse of correct rejections. d-prime is a bias corrected measure of discrimination where 0=chance and anything over 3 is real good.
Notice from the graph that depending on the proportion of hits, the d-prime value can range anywhere between 0 to almost 5! But this is true for any constant FA rate, of course, as discrimination ability is only meaningful by looking at the relation between hits and FAs. Think of it this way. If you have a FA rate of 0 (perfect performance on different trials) that seems fantastic, but at the same time, if your hit rate is also 0, well then you clearly just like to say "different" all the time no matter what you hear. A FA rate of 0 is only meaningful if you have a higher than 0 hit rate and the higher the hit rate, the higher your d-prime score.
Notice too that because a given FA value can result any number of d-prime values depending on the hit rate there is virtually complete overlap in d-prime values for the two curves (except for the upper end). This means that TMS could have had no affect whatsoever on the ability of subjects to discriminate lip-related speech sounds. For example, if the hit rate for pre-TMS was .7 and hit rate for post-TMS was .8 then the d-prime in both cases is approximately 2.5 -- no difference. It is also possible that discrimination was worse prior to TMS than after TMS (e.g., if hit rates were .6 and .8 respectively)!
I'm not saying that the study is necessarily wrong, or that TMS didn't affect the perception of lip-related speech sounds. What I'm saying is we can't tell from these data. Maybe once they calculate d-prime the data will show an even more impressive effect. But since we only have half of the information there is no way to know whether perception was affected: the findings are uninterpretable. T
This study is potentially really important and very interesting. As such, I would like to urge the authors to redo the analysis and publish an addendum in J. Neuroscience. I certainly would like to know if it actually worked!
Mottonen, R., & Watkins, K. (2009). Motor Representations of Articulators Contribute to Categorical Perception of Speech Sounds Journal of Neuroscience, 29 (31), 9819-9825 DOI: 10.1523/JNEUROSCI.6018-08.2009