D’Ausilio and colleagues report a very nice new study showing that stimulation of human motor cortex (via TMS) directly affects the perception of speech sounds. TMS was applied to the lip or tongue areas of M1 while participants were asked to identify speech sounds that either involved prominent lip articulation, [b] and [p], or prominent tongue articulation, [d] or [t]. They found a double-dissociation: relative to a non-stimulation baseline, participants were faster to indicate that they heard a lip-related sound when TMS was applied to motor lip areas, and faster to indicate that they heard a tongue-related sound when TMS was applied to motor tongue areas. The authors conclude that “motor structures provide a specific functional contribution to the perception of speech sounds” and go on to propose “a modified ‘motor theory of speech perception’ according to which speech comprehension is grounded in motor circuits…”.
I strongly disagree with this conclusion and I have convinced the editors of Current Biology to let me tell you why. Since I don't have the cool flashy experiment to report, I only get my piece on their website, not in print, but I'll take it nonetheless. Fadiga and company are writing a response to my arguments. Have a look when it comes out and let me know what you think.
D'Ausilio, A., Pulvermüller, F., Salmas, P., Bufalari, I., Begliomini, C., & Fadiga, L. (2009). The Motor Somatotopy of Speech Perception Current Biology, 19 (5), 381-385 DOI: 10.1016/j.cub.2009.01.017