Listener-speaker perceived distance predicts the degree of motor contribution to speech perception

Listening speech sounds activates motor and premotor areas in addition to temporal and parietal brain regions. These activations are somatotopically localized according to the effectors recruited in the production of particular phonemes. Previous work demonstrated that transcranial magnetic stimulation (TMS) of speech motor centers somatotopically altered speech perception, suggesting a role for the motor system. However, these effects seemed to occur only under adverse listening conditions, suggesting that degraded speech may stimulate listeners to adopt unnatural neural strategies relying on motor centers. Here, we investigated whether naturally occurring interspeaker variability, which did not affect task difficulty, made a speech discrimination task sensitive to TMS interference. In this paradigm, TMS over tongue and lips motor representations somatotopically altered the discrimination time of speech. Furthermore, the TMS-induced effect correlated with listeners' similarity judgments between listeners' and speakers' speech productions. Thus, the degree of motor recruitment depends on the perceived distance between listener and speaker. This result supports the claim that discriminating others' speech pattern requires the contribution of the listener's own motor repertoire. We conclude that motor recruitment in speech perception can be a natural product of discriminating speech in a normally variable and unpredictable environment, not merely related to task difficulty.