Motor learning and sensory plasticity in healthy adults and Parkinson's disease

We use multiple sources of sensory information to guide goal-directed movements, such as reaching. When information from multiple modalities (i.e. vision, proprioception) is incongruent, one learns to adapt his or her movements and recalibrate one sense to more closely match the other; simply put, one begins to perceive his/her hand where one sees it. This thesis attempts to better characterize this sensory recalibration (termed 'proprioceptive recalibration') following adaptation to a visuomotor distortion under a variety of contexts, and contributes to the existing literature that describes sensory plasticity associated with motor learning. Specifically, chapter two describes the effect of initial exposure to a visuomotor distortion and the dominance of the hand trained on proprioceptive recalibration. In. this study, participants used their dominant right or non-dominant left hand to reach to targets with visual feedback of hand position that was abruptly rotated clockwise relative to their unseen hand. Proprioceptive recalibration was then assessed and found to be comparable in the two hands and consistent with previous studies employing a gradual perturbation; these findings suggest that neither the initial error signal nor dominance of the hand trained influence recalibration. Chapter three describes how the magnitude of the visuomotor distortion affects the magnitude of recalibration, and how this is related to changes in reach aftereffects. Changes in reach aftereffects and proprioception were measured following adaptation to increasingly misaligned visual hand feedback; these changes were found to increase systematically as a function of the distortion magnitude. However, while these changes were directly correlated with the distortion magnitude, they were not correlated with each other, which suggests that these two processes may be mediated by simultaneous yet separate underlying mechanisms. Chapter four similarly describes how the magnitude of a cross-sensory error signal (generated in the absence of a visuomotor signal derived from goal-directed movement) affects the magnitude of recalibration, and how this is related to changes in reach aftereffects. Participants moved their unseen hand along a grooved path while viewing a cursor that moved towards a target; the position of the path was gradually rotated counter-clockwise with respect to the cursor. Following this cross-sensory adaptation, changes in reach aftereffects and proprioception were both found to saturate at a small distortion as no further changes were observed with training with increasing misalignment. Furthermore, these changes were not correlated with the magnitude of the misalignment. However, in contrast to the findings in chapter three, these changes were correlated with each other, suggesting that the cross-sensory discrepancy drives changes in both reach aftereffects (partially) and proprioception. This study helps to characterize the contribution of different error signals to changes in motor and sensory systems. Lastly, chapter five describes how damage to central nervous system structures integral to sensorimotor integration (i.e. the basal ganglia) affects proprioceptive recalibration. Patients with Parkinson's disease were able to learn to reach to targets with gradually rotated and translated visual feedback of hand positions comparably to healthy older adults. Patients also recalibrated proprioception comparably to healthy older adults, although the trend for greater recalibration in patients suggests that they may depend more on salient visual information of hand position than proprioceptive feedback to guide movement.