Title: Characterizing sensory integration mechanisms in open- and closed-loop models of human balance control
Thesis Supervisor: Dr. Mark Carpenter
Committee Members: Dr. Tim Inglis, Dr. Romeo Chua
External Examiner: Dr. Kei Masani
University Examiners: Dr. Hyosub Kim, Dr. Bryan Gick
Chair: Dr. Michael Gordon
Abstract: Human standing balance is functionally organized as a closed-loop feedback system, which enhances system stability in response to perturbations and sensory deficits but complicates the identification of underlying mechanisms. Mechanistic models of balance control represent hypotheses about these mechanisms that require testing and validation. This thesis encompasses five experiments aiming to address uncertainties within the literature concerning sensory feedback mechanisms and their representation in mechanistic models of balance control. Studies 1 and 2 investigated the properties of a sensory torque feedback mechanism believed to reduce the effort required to stand and play a role in body alignment with gravity. Study 1 used an experimental and simulated forced lean paradigm to test behavioural predictions of this mechanism. In Study 2, multiple balance control models containing various physiological interpretations of slow dynamics were fit to body sway responses to long-duration support surface tilts. These studies provide experimental evidence for a torque feedback mechanism contributing to upright standing balance. Study 3 investigated the velocity dependence of the sensory reweighting mechanism. Support surface tilt sequences were designed with different amplitudes while keeping velocities identical (and vice versa). Estimates of the relative sensory contributions to balance control were obtained from experimental body sway responses to fit a sensory reweighting model. Sensory contributions to balance control varied as a function of tilt stimulus velocity rather than stimulus amplitude, suggesting that the mechanism underlying reweighting is based on velocity cues. Studies 4 and 5 investigated standing behaviour in an artificial open-loop system. Specifically, Study 4 examined whether a simple linear feedback model could account for standing behaviour observed when the body is artificially immobilized. Based on model predictions from Study 4, Study 5 investigated the effect of altered sensory feedback on standing behaviour during artificial immobilization. These studies indicate that behaviour in an artificial open-loop system cannot be fully explained by a simple linear feedback model.
This thesis provides novel insights into the sensorimotor control mechanisms of the human balance system. These insights can contribute to advancements in developing and refining mechanistic balance control models and robotics, which will ultimately improve strategies for identifying and rehabilitating balance deficits.