Rather than using that clumsy TMS technique, Marc and his colleagues decided to target speech-related motor systems using the CNS equivalent of a smart bomb: simply ask participants to make 150 lip or tongue movement over a 10 minute span. The idea is that this will fatigue the system and produce a kind of motor after effect in lip or tongue areas. Perception of lip- or tongue-related speech sounds (/pa/ and /ta/) can then be assessed behavioral. They used syllable discrimination (same-different) both with and without noise.
They calculated d' and beta scores. (WOOHOO!) d' of course is a measure of discrimination performance, corrected for response bias. Beta is a measure of the bias -- specifically, the threshold that a subject is using for deciding that the stimuli are, in this case, same or different.
So what did they find? Fatiguing lip or tongue motor systems had no effect on discrimination (d', top graph) but did have significant effector-specific effects on bias (beta scores, bottom graph).
Sato et al. conclude:
Compared to the control task, both the tongue and lip motor training biased participants’ response, but in opposite directions. These results are consistent with those observed by Meister et al. (2007, Current Biology) and d’Ausilio et al. (2009, Current Biology), obtained by temporarily disrupting the activity of the motor system through transcranial magnetic stimulation.
So what this shows is that motor stimulation/fatigue has no effect on acoustic perception, but on a higher-level decision/categorization process. Now, Marc has indicated that he views this decision/categorization process as "part of speech perception" which is perfectly legit. It certainly is part of speech perception as defined by this task. My own interests in speech perception, however, don't include all the processes involved in performing this task, so I take this as evidence that motor stimulation doesn't affect "speech perception".
Use-induced motor plasticity affects speech perception
Marc Sato1, CA, Krystyna Grabski1, Amélie Brisebois2, Arthur M. Glenberg3, Anahita Basirat1, Lucie Ménard2, Luigi Cattaneo4
1 GIPSA-LAB, CNRS & Grenoble Universités, France - 2 Département de Linguistique, Université du Québec à Montréal, Canada
3 Department of Psychology, Arizona State University, USA - 4 Centro Interdipartimentale Mente e Cervello, Università di Trento, Italy