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Abstract:

To achieve a given motor task a single trajectory must be chosen from the infinite set of possibilities consistent with the task. To investigate such motor planning in a natural environment, we examined the kinematics of reaching movements made around a visual obstacle in three-dimensional space. Within each session, the start and end points of the movement were uniformly varied around the obstacle. However, the distribution of the near points, where the paths came closest to the obstacle, showed a strong anisotropy, clustering at the poles of a preferred axis through the center of the obstacle. The preferred axes for movements made with the left and right arms were mirror symmetric about the midsagittal plane, suggesting that the anisotropy stems from intrinsic properties of the arm rather than extrinsic visual factors. One account of these results Movement planning can be considered as the specification of a movement trajectory from the infinite number of possible trajectories that are consistent with a given task (for review, see Wolpert, 1997). Although many theories of motor planning have been proposed, they fall into two main classes: extrinsic and intrinsic. Proponents of extrinsic planning suggest that the process is hierarchical, beginning with the specification of the trajectory of the end point, such as the hand or finger, in extrinsic visual

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