Title: The role of uncertainty in the vestibular control of balance during locomotion
Thesis Supervisor: Dr. Jean-Sébastien Blouin
Committee members: Dr. Romeo Chua, Dr. Calvin Kuo
Defence Chair: Dr. Mark Carpenter
Abstract: Uncertainty is always present in the sensory information that we receive, worsening confidence in our self-motion estimates. This includes information from the vestibular system, which detects head motion in space and evokes whole-body balance responses during locomotion. These responses are attenuated with increasing step cadence and gait speed, but the neural mechanisms behind this modulation are not clear. One model suggests that the ratio (Vres) between the motor command variability and vestibular noise drives these changes. However, there is contradictory evidence concerning the physiological underpinnings of this model and its alignment with human behaviour. Twelve participants walked outside at step cadences 40-140% of their preferred cadence. We calculated coherence between electrical vestibular stimuli and mediolateral linear accelerations from inertial measurement units (IMUs) on the back, right ankle, and left ankle to infer the vestibular control of balance during locomotion. We also calculated Vres using the linear accelerations and angular velocities from an IMU on the head. We extracted peak coherences and mean Vres measures. To compare how these changed at faster step cadences, we performed paired T-tests between the 100, 120, and 140% cadence conditions and fitted exponential decay and 2nd degree polynomial functions to the data. Peak coherences decreased between the 100% and 140% cadence conditions (p < 0.025) while most linear acceleration Vres measures increased at cadences faster than 100% (multiple p values < 0.025). There were no significant changes to the angular velocity Vres measures (all p values > 0.025). Furthermore, the changes in peak coherence as a function of step cadence were best fitted to an exponential decay function (adjusted R2 = 0.682-0.711) while the changes to the Vres were all best fitted to a polynomial (adjusted R2 = 0.359-0.891), with minima near the preferred step cadence (91-123 steps/min). These results demonstrate that Vres does not predict vestibular control at faster step cadences and suggest that head kinematic variability may be optimized at the preferred cadence. This understanding has key implications for sensorimotor processing, which can inform computational modelling and how sensory information is impaired in clinical conditions.