The purpose of this dissertation was to investigate interference during bimanual movements in healthy individuals, and those with impaired movement. During complex bimanual movements, interference can occur, where one hand influences the action of the contralateral hand. Interference likely results from conflicting sensorimotor information shared between brain regions controlling hand movements via neural crosstalk. However, how visual and dynamic feedback processes interact with each other during bimanual reaching movements is not well understood. This dissertation reports two studies that address mechanisms underlying interference in healthy individuals, and one in individuals with dystonia. In the first study, groups experienced either a visuomotor perturbation, dynamic perturbation, combined visuomotor and dynamic perturbation, or no perturbation in their right hand during bimanual reaches. The left hand was examined for interference. The results indicated that the visuomotor and combined perturbations showed greater interference than the dynamic perturbation, but that the combined and visuomotor perturbations were equivalent with one another. This suggests that dynamic and visuomotor sensorimotor processes do not interact between hemisphere-hand systems, and that primarily visuomotor processes result in interference between the hands.The results of Experiment 1 could be explained by visuomotor and dynamic perturbations being coordinated in reference frames that were differentially shared between hemisphere-hand systems. Reference frames are a theoretical construct which explain how the motor system coordinates movements relative to the environment and/or itself. Research suggests that unimanual responses to dynamic perturbations are coordinated in unilateral intrinsic joint-centered reference frames, while visuomotor perturbations are coordinated in bilateral extrinsic reference frames. Little is known about the role of reference frames in interference during bimanual movements. As such, interference may only occur when movements of each hand share a reference frame. In Experiment 2, two groups made bimanual reaching movements while their right hand experienced a dynamic perturbation. In one group, separate cursors represented each hand. In the other group, both hands shared control of a single cursor. Shared control was hypothesized to compel the system to coordinate both hands with a shared representation. The results indicated that the shared-cursor group demonstrated more interference than the dual-cursors group, suggesting that a shared reference frame may induce greater interference.Finally, motor coordination is often disrupted in individuals with movement disorders, such as cervical dystonia (CD). Additionally, patients with CD can show "mirror movements", in which voluntary actions of one effector cause involuntary actions in another effector, suggesting the presence of abnormal sensorimotor integration and neural inhibition. However, how the coordination of bimanual actions and interference are different in cervical dystonia has been unexplored. In Experiment 3, patients with CD and healthy controls performed a bimanual interference task before and after treatment with botulinum toxin. Brain activity was simultaneously recorded with EEG. Results indicated that overall, movements were coordinated similarly between patients and controls. However, greater event-related desynchronization was found in patients, particularly in the post-treatment session. This suggests that bimanual coordination necessitates greater neural resources for successful coordination in CD patients. Together, these studies advance the understanding of how bimanual coordination and interference occurs in healthy individuals, and those with cervical dystonia.
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