Speech Motor System Mediates Phonetic Categorization

Phonemes, the elementary units of speech, greatly vary in their acoustic structure when produced by different speakers in different contexts. The listener's brain therefore faces a fundamental challenge of mapping highly variable acoustic signals to discrete phonetic categories. Whether the speech motor system, primarily engaged in speech production, is involved in solving this perceptual categorization challenge is still under debate. This thesis investigated the influence that the speech motor system exerts on the processing of speech sounds by recording brain activity with magnetoencephalography (MEG). Considering that the perceptual processes leading to the categorization of speech sounds unfold within 200 ms after the sound onset, MEG, as a direct measure of neuronal activity with millisecond temporal accuracy, provides an ideal non-invasive brain imaging method for the purpose of this thesis. Several methodological approaches were used, including equivalent current dipoles and minimum-norm estimates for modeling the cortical activity, stimulus-specific adaptation of evoked responses for inferring neuronal stimulus selectivity, and phase synchrony of neural oscillations for examining connectivity between brain areas. The results show that 1) the auditory information of a speech sound is integrated with the listener's motor information of how the speech sound is articulated within 200 ms after the sound onset, 2) this sensorimotor integration facilitates the categorization of speech sounds, 3) the speech motor system contains categorical representations of speech sounds, and 4) these categorical representations depend on attentive listening to the sounds. Together, these results demonstrate that when humans attend to speech, the speech motor system constrains the acoustic-phonetic interpretation of speech sounds into discrete categories through the integration of auditory and motor information. This thesis therefore expands the knowledge on the role of the speech motor system in phonetic processing and adds evidence to the view that, rather than being a purely sensory process, speech perception is sensorimotor in nature.