An Examination of the Factors that Dictate the Relative Weighting of Feedback and Feedforward Input for Speech Motor Control

Speech is arguably the most important form of human communication. Fluent speech production relies on auditory feedback for the planning, execution, and monitoring of speech movements. Auditory feedback is particularly important during the acquisition of speech, however, it has been suggested that over time speakers rely less on auditory feedback as they develop robust sensorimotor representations that allow speech motor commands to be executed in a feedforward manner. The studies reported in this thesis recorded speaker’s vocal and neural responses to altered auditory feedback in order to explore the factors that dictate the relative importance of auditory feedback for speech motor control. More specifically, studies 1 through 3 examined how the role of auditory feedback changes throughout development, while studies 4 and 5 examined the relationship between vocal variability and auditory feedback control, and lastly study 6 looked at how the predictability of auditory feedback errors influences the role of auditory feedback for speech motor control. Results of the first study demonstrated that toddlers use auditory feedback to regulate their speech motor commands, supporting the long held notion that auditory feedback is important during the acquisition of speech. While mapping out the developmental trajectory of vocal and event related potential responses to altered auditory feedback, the second study demonstrated that vocal variability, rather than age, best predicts responses to altered auditory feedback. Importantly, this suggests that the maturation of the speech motor control system is not strictly dependent on age. The third study in this thesis demonstrated that children and adults show similar rates of sensorimotor adaptation, suggesting that once speech is acquired, speakers are proficient at using sensory information to modify the planning of future speech motor commands. However, since adults produced larger compensatory responses, these results also suggested that adults are more proficient at comparing incoming auditory feedback with the feedback predicted by their sensorimotor representations, as a result of possessing more precisely mapped sensorimotor representations. The results of studies four and five demonstrated that vocal variability can be used to predict the size of compensatory responses and sensorimotor adaptation to changes in one’s auditory feedback, respectively. Furthermore, these studies demonstrated that increased variability was related to increased auditory feedback control of speech. Finally, the sixth study in this thesis demonstrated that experimentally induced predictability and variability can be used to induce increases in feedforward and auditory feedback control, respectively. In conclusion, the results reported in this thesis demonstrate that age and vocal variability, both naturally occurring and experimentally induced, are important determinants of the role of auditory feedback in speech motor control