Listening to speech modulates activity in human motor cortex. It is unclear, however, whether the motor cortex has an essential role in speech perception. Here, we aimed to determine whether the motor representations of articulators contribute to categorical perception of speech sounds. Categorization of continuously variable acoustic signals into discrete phonemes is a fundamental feature of speech communication. We used repetitive transcranial magnetic stimulation (rTMS) to temporarily disrupt the lip representation in the left primary motor cortex. This disruption impaired categorical perception of artificial acoustic continua ranging between two speech sounds that differed in place of articulation, in that the vocal tract is opened and closed rapidly either with the lips or the tip of the tongue (/ba/-/da/ and /pa/-/ta/). In contrast, it did not impair categorical perception of continua ranging between speech sounds that do not involve the lips in their articulation (/ka/-/ga/ and /da/-/ga/). Furthermore, an rTMS-induced disruption of the hand representation had no effect on categorical perception of either of the tested continua (/ba/-da/ and /ka/-/ga/). These findings indicate that motor circuits controlling production of speech sounds also contribute to their perception. Mapping acoustically highly variable speech sounds onto less variable motor representations may facilitate their phonemic categorization and be important for robust speech perception.

Recent studies have yielded contradictory evidence on whether visual speech perception (watching articulatory gestures) can activate the human primary auditory cortex. To circumvent confounds due to inter-individual anatomical variation, we defined our subjects' Heschl's gyri and assessed blood oxygenation-dependent signal changes at 3 T within this confined region during visual speech perception and observation of moving circles. Visual speech perception activated Heschl's gyri in nine subjects, with activation in seven of them extending to the area of primary auditory cortex. Activation was significantly stronger during visual speech perception than during observation of the moving circles. Further, a significant hemisphere by stimulus interaction occurred, suggesting left Heschl's gyrus specialization for visual speech processing.

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The motor regions that control movements of the articulators activate during listening to speech and contribute to performance in demanding speech recognition and discrimination tasks. Whether the articulatory motor cortex modulates auditory processing of speech sounds is unknown. Here, we aimed to determine whether the articulatory motor cortex affects the auditory mechanisms underlying discrimination of speech sounds in the absence of demanding speech tasks. Using electroencephalography, we recorded responses to changes in sound sequences, while participants watched a silent video. We also disrupted the lip or the hand representation in left motor cortex using transcranial magnetic stimulation. Disruption of the lip representation suppressed responses to changes in speech sounds, but not piano tones. In contrast, disruption of the hand representation had no effect on responses to changes in speech sounds. These findings show that disruptions within, but not outside, the articulatory motor cortex impair automatic auditory discrimination of speech sounds. The findings provide evidence for the importance of auditory-motor processes in efficient neural analysis of speech sounds.

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