The Role of Motor Planning in Reaching and the Impact of StrokePublic
Motor planning is fundamental to the performance of everyday reaching movements. The influence of planning is not limited to voluntary movements but extends to involuntary movements initiated in response to sensory stimuli, such as postural perturbations applied to the arm. Stroke alters voluntary reaching and the involuntary response to perturbations, but the contributions of planning impairments are not well understood. Measures of motor planning can be made by quantifying neural activity in the preparatory period preceding movement onset. However, planning cannot be distinguished from initiation which also occurs during this period. Sensory stimuli can evoke the involuntary initiation of a planned movement with consistent latency linked to the time of the stimulus. This allows planning to be investigated separate from initiation. However, it is not known if the planning processes that influence involuntary movements are the same as those that influence voluntary movements. The objective of this dissertation was to investigate the contributions of motor planning to voluntarily and involuntarily initiated reaches. This objective was met by assessing the influence of temporal factors on motor planning and investigating the impact of stroke. First, I demonstrated that uncertainty in when to move influenced the time course of planning as well as the timing and magnitude of the involuntary response evoked by a perturbation. I found the influence of uncertainty on involuntary responses resembled that previously reported for voluntary movements. Second, I compared the time required for a motor plan to influence movements initiated voluntarily and involuntarily. I found that a motor plan was prepared and accessible in ~160 ms for voluntary and involuntary initiation. Third, I assessed whether impairments in the time required for motor planning contributed to voluntary reach dysfunction following stroke. I found that the time required to plan a ballistic reach was not affected by stroke. Overall, this dissertation demonstrates that motor planning can similarly influence reaches initiated voluntarily and involuntarily in response to sensory stimuli. Temporal factors, including the time available and uncertainty in when to move, influence planning, but the time required for motor planning does not contribute to the effect of stroke on voluntary reaching.
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