Sensitivity and complexity of adaptive models in human motor control

Motor adaptation is a form of learning in which the execution of movements is adjusted in a predictive manner in order to compensate for external perturbations. By examining the mechanisms underlying human movement, motor adaptation studies provide information that may increase our ability to diagnose and treat neurological injuries and inspire the design of dexterous robots. In this thesis I present the results of three psychophysical experiments, each of which investigates a particular feature of motor adaptation. The first experiment examined the sensitivity of the adaptive mechanism, in the context of a debate centring on the equilibrium point (EP) hypothesis of motor control. Specifically, it has been argued that results contradictory to the EP hypothesis reported in a study of movements made in a Coriolis field stem from voluntary corrections elicited by the magnitude and destabilizing nature of the field perturbations. That is, it has been suggested that a perturbation threshold exists, above which adaptive corrections are necessitated. I tested the existence of an adaptation threshold in normal speed reaches made in perturbation fields ranging in strength from small to significant levels. The results of the experiment demonstrated a substantial adaptation response over the entire range of field strengths examined, indicating that adaptive response does not display threshold behaviour. The second experiment examined motor adaptation to perturbation fields of varying spatial complexity. The results demonstrated that subjects were able to rapidly adapt to spatially complex fields using a combination of increased impedance and internal model formation. Adaptation aftereffects of both simple and complex form were detected, indicating that complex internal model representations may be gradually developed over the course of adaptation. Alternatively, simple aftereffects detected for the fields with the greatest degree of spatial complexity examined may result from an inability to faithfully represent them, due to the wide tuning functions of motor primitives. The third experiment examined motor adaptation to divergent force fields of spatially varying instability. I tested the hypothesis that subjects would modulate impedance during movement in a manner consistent with the stability characteristics of the reaching environments. The results demonstrated that subjects were able to at least partially adapt to these fields through increases in impedance. A trend showing impedance modulation was also detected, however modulation was matched to spatial changes in kinematics created by field instabilities, rather than to changes in field instabilities, per se. The overall results of the thesis indicate that the adaptive process is highly sensitive, elicited in a wide variety of perturbation environments, and achieves the 'best possible' result through the flexible combination of internal modeling and impedance modulation.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat