| To investigate the role of gravitational cues in the learning of balance control, we strapped blindfolded humans into a Multi-Axis Rotation System (MARS) that was programmed to behave like an inverted pendulum and instructed them to balance with minimal oscillations around the direction of balance. In Chapter One, subjects balanced in the upright roll plane where they used both gravitational cues (tilt from gravitational vertical detected by otoliths and somatosensory receptors) and dynamic cues (angular acceleration detected by semicircular canals and somatosensory receptors) to perform the task. Because subjects were seated while balancing, they could not use reflexes and the biomechanical properties of their legs. To quantify learning and performance, we developed metrics derived from joystick movements, phase portraits, and stabilogram diffusion functions. We found that, among other things, subjects learned to reduce destabilizing joystick deflections, maintain an anti-phasic relationship between joystick deflection and their angular position, wait longer before making changes to their movement and adopt a more intermittent style of control. These results suggest that the central contributions of balance control may operate in an intermittent manner, in contrast to the continuous nature of muscle stiffness and postural reflexes. In Chapter Two, subjects balanced in the supine roll plane, where because they were always perpendicular to the gravitational vertical, they could not use gravitational cues to determine their angular position and instead had rely primarily on dynamic cues detected by the semicircular canals and somatosensory system to perform the task. Even after two exposures on consecutive days, the supine subjects showed minimal learning, poor performance and a characteristic pattern of positional drifting. To confirm that these results were not caused by being in the supine orientation, in Chapter Three, subjects balanced about the vertical yaw axis, where they did not have relevant gravitational cues and another group balanced about the horizontal yaw axis where they did have relevant gravitational cues. We found consistent results, where the subjects without relevant gravitational cues showed minimal learning, poor performance and positional drifting regardless of orientation, suggesting that dynamic cues (angular acceleration signals) alone are not sufficient for balancing. Additionally, in Chapter Two, subjects that balanced in the upright roll condition on the first day and in the supine roll condition on the second day showed no enhanced learning. In contrast, subjects who first balanced in the supine roll condition showed enhanced learning on the next day when balancing in the upright roll condition, suggesting that human balance control consists of two dissociable mechanisms: balancing at the direction of gravity using static gravitational cues and balancing at the direction of balance using dynamic cues. Finally, in Chapter 4 we provided an auditory reference point, where subjects heard a beep every time they were at the direction of balance when balancing in the supine roll plane where they did not have relevant gravitational cues. This did not lead to any enhanced learning or better performance, suggesting that a singular reference point is not sufficient to recover proper balancing. In summary, in Chapter One we examined the learning of balance control in the absence of peripheral mechanisms. In Chapters Two and Three we examined the learning of balance control in the absence of relevant gravitational cues. In Chapter Four we examined whether auditory cues could restore proper balance control in the absence of relevant gravitational cues. |
