Wilt, Hannah; (2025) The role of covert imitation in speech perception: an investigation into motor activation during native and non-native speech perception. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A core issue in speech perception research is understanding how listeners recognise highly acoustically variable input. Simulation theories propose that a speech signal is covertly imitated by the listener to inform predictive models of the speaker’s vocal tract gestures, supporting perception in a top-down manner. In line with these theories, substantial evidence demonstrates that speech production processes are involved in speech perception, and in a complementary rather than critical manner. The specific conditions eliciting covert imitation and hence the role that it plays in speech perception have yet to be established. One view suggests that covert imitation plays a compensatory role in speech perception and is enhanced under challenging listening conditions. An alternative position is that covert imitation scales to perceptuomotor experience with the perceived signal. To disambiguate between these two accounts and clarify the role of covert imitation in speech perception, I conducted five experiments investigating the implication of motor processes in native and non-native speech perception. Non-native sounds constitute a perceptual challenge and are unfamiliar to listeners, hence they comprise a critical test case for determining whether covert imitation is primarily driven by perceptual difficulty or by perceptuomotor experience. First, I developed an online speech stimulus-response compatibility (SRC) task to investigate covert imitation of native and non-native speech sounds online (Experiment 1). In three further SRC studies, I established whether covert imitation is enhanced during native or non-native speech perception (Experiments 2-3), and how covert imitation of non-native speech evolves through perceptual training (Experiment 4). Finally, I conducted a transcranial magnetic stimulation (TMS) study to probe motor activation during passive listening to native and non-native speech sounds (Experiment 5). Results showed that covert imitation is enhanced during the perception of non-native speech, supporting a compensatory role of covert imitation in speech perception and refuting an experience-dependent account.

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