… we suggest … under some circumstances forward predictions from the motor speech system can modulate the perception of others’ speech. … forward predictions generated via motor commands can function as a top-down attentional modulation of sensory systems. Such attentional modulation may be important for sensory feedback control because it sharpens the perceptual acuity of the sensory system to the relevant range of expected inputs (see below). This ‘‘attentional’’ mechanism might then be easily co-opted for motor-directed modulation of the perception of others’ speech, which would be especially useful under noisy listening conditions, thus explaining the motor speech induced effects of perception (Hickok et al., 2011, Neuron, 69(3), 407-422).
Nonetheless, I still hold the view that current evidence for this claim--including the many high-impact TMS reports--is exceptionally weak. The problems with these reports are primarily methodological. They typically use an unnatural task such as syllable discrimination, which is not something we do in normal speech processing settings, and response bias is not well controlled. See Hickok 2014 for extensive discussion.
A recent paper from Pulvermuller's group (Schomers et al.) reports a nice study that addresses my previous concerns. The study follows up an influential report from the same group showing that stimulation of motor lip versus tongue areas differentially affects perception of syllables with lip- versus tongue-related sound onsets (D'Ausilio et al., 2009). In the new study they again stimulated either lip or tongue motor areas and found a crossover interaction in the speed at which words starting with lip- or tongue-related sounds were recognized (via button press decisions to a matching picture).
It's a nice result, but not without complication. A closer look reveals that:
(1) It is not clear whether the stimulation can be attributed to motor cortex stimulation or to somatosensory cortex stimulation. It would be quite interesting in it's own right if somato stimulation modulated speech perception making the paper important in that sense, but since we are debating the role of the motor system, an ambiguity in whether motor cortex was really the source of the effect weakens the claim quite a bit.
(2) The effect held only in the reaction time data. No effect was observed for accuracy, which seems rather more important for speech processing.
(3) The RT effect held only for tongue-related sounds but not lip-related sounds.
All of these complications just reinforce my skepticism that the motor system is doing anything for speech perception. But maybe I'm just nitpicking. These scientists made a serious effort to address my previous concerns and did a nice job. True, the effects aren't whopping but at least some effects were noted. Combined with the several previous reports perhaps this finally confirms that the motor system has a role to play in speech perception.
OK, so let me bite my tongue (thus temporarily modulating my perception of tongue-related sounds!) and admit that, consistent with my own claim (see above quote), the motor system indeed plays a role in perception.
The next question we must ask, then, is how much of a role does it play. And the answer from the Schomers study is not very much:
--Since these TMS-induced effects only emerge with the speech stimuli are barely detectible anyway (69% accuracy in the Schomers et al. experiment), the motor system contributes only when we are failing to hear a good chunk of the speech signal.
--Under those circumstances, it doesn't even improve perceptual accuracy. All it does is speed up responses.
--It only does this for some speech sounds.
So, fine, I admit that the motor system plays a "causal role" in speech perception. But that role is nearly inconsequential to understanding the neural and computational basis of speech perception.