Phonological and lexical motor facilitation during speech listening: a transcranial magnetic stimulation study

In the present study, we used transcranial magnetic stimulation (TMS) to investigate the influence of phonological and lexical properties of verbal items on the excitability of the tongue's cortical motor representation during passive listening. In particular, we aimed to clarify if the difference in tongue motor excitability found during listening to words and pseudo-words [Fadiga, L., Craighero, L., Buccino, G., Rizzolatti, G., 2002. Speech listening specifically modulates the excitability of tongue muscles: a TMS study. European Journal of Neuroscience 15, 399-402] is due to lexical frequency or to the presence of a meaning per se. In order to do this, we investigated the time-course of tongue motor-evoked potentials (MEPs) during listening to frequent words, rare words, and pseudo-words embedded with a double consonant requiring relevant tongue movements for its pronunciation. Results showed that at the later stimulation intervals (200 and 300 ms from the double consonant) listening to rare words evoked much larger MEPs than listening to frequent words. Moreover, by comparing pseudo-words embedded with a double consonant requiring or not tongue movements, we found that a pure phonological motor resonance was present only 100 ms after the double consonant. Thus, while the phonological motor resonance appears very early, the lexical-dependent motor facilitation takes more time to appear and depends on the frequency of the stimuli. The present results indicate that the motor system responsible for phonoarticulatory movements during speech production is also involved during speech listening in a strictly specific way. This motor facilitation reflects both the difference in the phonoarticulatory characteristics and the difference in the frequency of occurrence of the verbal material.