Vocal effort and within-speaker coordination in speech production : effects on postural control

This thesis probes the joint role of respiration in speech motor control and postural control by examining the effect of increasing the loudness of speech production, or vocal effort, on within-speaker coordination. Specifically, this work tested the dual hypothesis that the functional demands of speech production at increasingly higher levels of vocal effort would result in increasingly rigid coordination across multiple bodily subsystems, and that this entrainment would ultimately affect postural control, resulting in a loss of balance An interactive spontaneous speech task was used to elicit speech at multiple levels of vocal effort by increasing the intended communication distance. Data from acoustic and kinematic measurement domains, including speech, rigid body motion of the head, 2d motion of the body extracted from video, and postural forces and torques measured at the feet, were collected simultaneously. These data were analyzed using a unique collection of techniques for the analysis of non-stationary time-series, which included methods for assessing cross-domain correspondence, system dimensionality, and fluctuation characteristics. The results of these analyses show convergent evidence for both hypotheses. Coordination among kinematic and acoustic measurement domains both strengthens and simplifies at high levels of vocal effort, and evidence of postural instability was found at the highest levels of vocal effort. Subsystem fluctuation characteristics show a direct relationship to the observed effects on coordination, both in terms of their intrinsic properties and in terms of changes due to increased vocal effort. Although this study did not include a direct measure of respiration, these results highlight the necessity of expanding our understanding of respiration’s role in speech motor control, especially insofar as the inevitable crossover between speech and other task domains, such as postural control, is concerned. The methodology used in this study can be straightforwardly expanded towards these ends, and provides a potentially useful in-roads to research in this direction. Even in the absence of a respiratory measure, these results will be of potential interest to clinicians working on the treatment of patients with speech disorders associated with neurological dysfunction, as occur, for example, in Parkinson’s disease.