Sensory modulation during speech movement planning in stuttering and nonstuttering adults

Abstract

Stuttering is a complex neurodevelopmental communication disorder with an approximate prevalence of one percent, affecting about eighty million people worldwide. To date, the specific neural mechanisms associated with stuttering and its overt symptoms are still largely unknown, despite evidence of speech and nonspeech sensorimotor differences and extensive sensorimotor network involvement. This dissertation presents novel results from three electrophysiological experiments that aimed to improve our understanding of the sensorimotor mechanisms underlying stuttering. First, I extended our previously established pre-speech auditory modulation (PSAM) paradigm to investigate the modulatory influence of speech movement planning on auditory processing (assessed with auditory evoked potentials) prior to the production of full sentences rather than just isolated words. Second, I investigated whether pre-speech sensory modulation can also be detected in the somatosensory domain. For this study, sensory processing during speech movement planning was assessed by means of somatosensory-evoked potentials in response to experimentally applied orofacial skin stretches. Third, I explored the functional relevance of PSAM in a preliminary study evaluating the correlation between speakers’ extent of PSAM and kinematics-based estimates of their reliance on feedforward versus feedback control. The results from these experiments replicated key findings from our laboratory’s prior work and provide several new insights into the phenomenon of pre-speech sensory modulation in stuttering and nonstuttering speakers. Specifically, Study 1 revealed substantial auditory modulation during speech movement planning for both words and sentences. Study 2 found no pre-speech somatosensory modulation (PSSM) for either typically fluent speakers or speakers who stutter. Thus, the specific cutaneous mechanoreceptors stimulated with the present experimental technique may not be subject to modulation by pre-motor systems involved in speech planning. Study 3 provided preliminary evidence that PSAM is associated with neural mechanisms directly involved in feedforward mechanisms rather than preparing auditory cortex for optimized feedback monitoring. Taken together, the overall findings confirm that adults who stutter show atypical motor-to-auditory interactions during speech movement planning and suggest that this observation relates to a fundamental limitation in the use of feedforward control mechanisms for speech production. The results suggest new testable hypotheses for further research into the neural mechanisms underlying stuttering and speech sensorimotor control in general.