Auditory-motor interactions during speech perceptions and production

Speech perception and production are two central aspects of human language. It remains unclear, however, how sensory and motor systems interact with each other during these two processes. The overall aim of this thesis was to investigate auditory-motor interactions during both speech perception and production. There were two lines of research. The first explored sensorimotor adaptation in speech production, which plays an important role in the development and maintenance of fluent speech production. In study 1 (chapter 3), I examined the compensatory responses of speakers when they experienced perturbations to the auditory feedback of their own speech. The results showed feedback perturbation induced changes in subsequent speech production that were evident not only in the shifted formant, but also in the unaltered one. Moreover, such combined changes resulted in moving the produced speech token closer to a known pattern of speech production for an existing vowel category. This study confirmed the utility of this specific experimental paradigm, and that the apparatus used to apply real-time feedback perturbation was effective. It allowed the development of additional research questions to be tackled using the speech feedback perturbation paradigm. Following the successful implementation of the paradigm in study 1, I carried out a brain stimulation study (chapter 4) exploring the neural basis of sensorimotor adaptation in speech using transcranial magnetic stimulation (TMS), which temporarily inhibits function in stimulated brain areas. This allowed testing of causal hypotheses concerning the role of different parts of the motor system in speech motor adaptation. Here, I aimed to determine whether temporary disruption of the articulatory representation in left primary motor cortex (M1) by repetitive transcranial magnetic stimulation (rTMS) impairs sensorimotor adaptation in speech. Results showed that disruption of tongue, but not hand representation in left M1 almost abolished the expected compensatory response for the upwards F1 shift in auditory feedback, namely a decrease in F1 frequency alongside an increase in F2 frequency. This is the first study to address the key question as to whether cortical motor areas that control movements of articulators causally contribute to speech motor adaptation. The second line of research explored in this thesis was focused on the asymmetry of auditory-motor speech processing in tonal and non-tonal language speakers (study 3, chapter 5). To achieve this, I temporarily disrupted activity in either the left or right motor cortex using the same form of rTMS used in Study 2 and measured the impact of these disruptions on auditory processing of phoneme and tone changes in sequences of syllables using electroencephalography (EEG). I found that the effect of motor disruptions on processing of tone changes differed in tonal and nontonal language speakers: disruption of the right speech motor cortex affected processing of tone changes in non-tonal but not tonal language speakers, whereas disruption of the left speech motor cortex significantly impaired processing of tone changes in tonal language speakers. In contrast, disruption of the speech motor cortex in the left, but not the right hemisphere, impaired processing of phoneme changes in both language groups equally. For the first time, I provide causal evidence that auditory-motor speech processing asymmetries are shaped by the functional role of the acoustic cues in the listener’s native language. Overall, the work presented in this thesis emphasizes the importance of understanding the interactions between auditory and motor systems when investigating speech perception and production. It also calls for more comparative studies between speakers of different languages, since ignoring the diversity of spoken languages can to lead to an incomplete understanding of the neurobiology of speech and language.