Speech motor mechanisms as an adaptive reflexive gain control system in non-native speech perception

Simulation accounts posit that speech is covertly imitated to support perception in a top-down manner. Behaviourally, covert imitation can be measured through the stimulus-response compatibility (SRC) task. In a speech SRC task, participants produce a target speech syllable whilst perceiving a speech distractor that either matches the target (compatible condition) or does not (incompatible condition). The degree to which the distractor is covertly imitated is captured by the automatic imitation effect, computed as the difference in response times (RTs) between compatible and incompatible trials. Simulation accounts disagree on whether covert imitation is enhanced when speech perception is more challenging or instead when the speech is familiar. To dissociate between these two accounts, we conducted two experiments, one behavioural and one using Transcranial Magnetic Stimulation (TMS), in which participants completed SRC tasks with native and non-native contrasts. In Experiment 1, participants performed an SRC task with two native and two non-native contrasts, received perception training with the non-native contrasts, and performed the SRC task again. Results showed that non-native distractors were linked to higher automatic imitation than the native distractors before and after the training, but that the automatic imitation effect for the non-native distractors decreased more after training. Experiment 2 demonstrated that speech primary motor cortex showed higher excitability during passive perception of non-native compared to native contrasts but showed no correlation with automatic imitation. Our results highlight that speech motor cortex acts as an adaptive reflexive gain control mechanism.