Dissertation Defense: Kelly Michaelis
Candidate Name: Kelly Michaelis
Advisors: Peter E. Turkeltaub, M.D., Ph.D. and Andrei Medvedev, Ph.D.
Title: The Role of the Motor System in Speech Perception and the Neural Substrates of Audiovisual Speech Integration
Perceiving and comprehending spoken language is a complex process, but for most of us, it is something we do effortlessly. How does the brain transform the acoustic and visual signals of speech into meaningful percepts? In this dissertation I examine two important aspects of speech perception: the role of the motor system in speech perception, and the neural substrates governing the integration of auditory and visual speech cues. Research has shown that frontal motor areas responsible for speech production are also active during speech perception, however, the broader literature on speech perception clearly implicates temporal lobe pathways, not motor systems, as the major structures underlying speech perception.
The first study aims to clarify the role of the motor system in speech perception by investigating the stimulus features that engage motor processing and answering several outstanding questions in the literature. We use electroencephalography (EEG) and a novel behavioral task to test 1) whether motor activity during perception is specific to speech stimuli as opposed to non-speech sounds, 2) how motor activity changes in response to different types of speech (e.g. sublexical vs. lexical, auditory-only vs. audiovisual), and 3) whether motor activity is evoked specifically for speech perception or is simply a reflection of domain-general processes like attention or decision-making. Our results demonstrate that activity in dorsal stream motor regions is specific to speech stimuli and cannot be explained by domain-general processing, and that motor regions are flexibly engaged and variably effective depending on the features of the stimulus.
The second study focuses on the neural substrates necessary for the integration of auditory and visual speech cues and examines two factors known to impact the audiovisual integration process: the relative timing of auditory and visual inputs and participant age. We find a strong positive relationship between aging and audiovisual integration. Using multivariate lesion-symptom mapping, we show that lesions to left hemisphere dorsal stream regions in the inferior parietal lobe relate to impaired temporal acuity during audiovisual speech perception. Together, these studies provide new evidence regarding the brain regions underlying the processing of speech and the task contexts in which they are engaged.
Friday, July 19 at 11:00am to 1:00pm
Medical and Dental Building, Proctor Harvey Amphitheater
3900 Reservoir Road, N.W., Washington