Abstract
Our ability to maintain a bipedal standing posture across different environments requires the nervous system to accurately estimate the state of the body within the surrounding world and generate the appropriate motor actions needed to balance upright. When balancing in new (potentially unstable) environments, the brain must (1) identify and determine the source of unexpected self-motion, and (2) associate ongoing motor commands with novel sensory feedback to adapt motor control. This thesis examines how humans adapt and perceive their movement behavior to maintain standing balance. In a series of studies, participants balanced in conditions with modified sensorimotor, mechanical and/or environmental factors. Specifically, participants balanced 1) on a unique robotic balance simulator, that allows for the control and manipulation of the neural signals associated with ongoing standing balance, and 2) atop a stand-up paddle board floating on water, which creates unique task challenges. To determine how humans adapt to and perceive these balancing conditions, I measured the orientation and motion of the upright body, sensory-evoked muscle responses and participant perception of self-motion.
There were three key findings from this research. One, the human balance system demonstrates remarkable adaptability, recalibrating motor control to maintain standing balance under a variety of conditions that are initially destabilizing to upright posture. A key example of this adaptation occurred when participants learned to stand with large sensorimotor delays (400 ms) in balance control, which refutes computational models that suggest upright stance cannot be maintained with such delays. Two, the brain can generalize these learned principles of balance control across different directions of movement and muscles contributing to balance control. This suggests that the brain estimates the source of novel whole-body balance relationships and broadly updates its control to balance in vastly different scenarios. Three, the balance system can operate according to its own sensorimotor principles that are not always accessible to our conscious awareness. Consequently, accurate perceptual awareness of ongoing standing motion is not always required for balance adaptation. Nevertheless, the interplay between our balancing actions and perceptual awareness of self-motion can influence how we perceive our body state and control our movements. Collectively, to adapt motor behavior and maintain stable perception for standing balance, the nervous system infers the spatio-temporal relationships between sensory and motor signals to form and update probabilistic estimates of self-motion. These studies have implications for understanding the neuromechanical principles governing human balance, and for developing new methodologies to rehabilitate individuals with balance impairments.