Title: Representation of space and time for the perception and control of standing balance
Thesis Supervisor: Dr. Jean-Sébastien Blouin
Committee members: Dr. Patrick Forbes, Dr. Calvin Kuo
Defence Chair: Romeo Chua
Abstract: While balancing upright, dynamic changes in body spatial representation occur as time progresses. Although space and time are often considered independent processes, I propose there is no separation of these dimensions in the brain as our sensors encode self-motion through neural action potentials changing properties (e.g. rate or timing) with respect to a constant time vector. I explored this possibility by characterizing how body dynamics and time influenced the perception and control of standing balance. First, simulations revealed that inertia- and viscosity-induced changes in balance control stability were similar to those induced by sensorimotor delays. I exposed 20 healthy participants to alterations in body inertia, viscosity and sensorimotor delays using a unique robotic system. Compared to baseline balance control, participants exhibited lower time within the virtual robotic limits (1.40-21.40% reduction), larger CoM average speed (1.22-4.09×) and sway velocity variance (2.43-38.86×) in the AP and ML directions with inertia below unity and negative viscosities, replicating the direction of balance effects induced by imposed sensorimotor delays. When asked to perceive how they balanced, participants matched their control of balance with 200ms imposed delay by choosing 0.50× (0.26-0.73) their inertia or -47.33× (-56.02 to -38.63) their viscosity. In addition, participants perceived that combining their control of balance with a 200ms imposed delay with either 3.30× (2.43-4.17) inertia or 53.05× (44.47-61.63) viscosity matched their control of balance in the control (no delay) condition. Based on these results, I exposed ten naïve participants to baseline balance control, 200ms imposed delay, 3.30× inertia with 200ms delay and 53.05× viscosity with 200ms delay conditions to determine if body dynamics could compensate for delay-induced instability in balance control. Compared to the 200ms delay condition, participants showed an increase in time in limits (29.20-32.40%) and a decrease in CoM average speed (76-81%) and sway velocity variance (90-97%) when balancing with a 200ms delay combined with body dynamics representing 3.30× their inertia or 53.05× their viscosity. The findings from this study provide novel insight into the way that the brain processes incoming sensory information to build internal representations of space and time for the perception and control of standing balance.