Path Curvature in Workspace and in Joint Space: Evidence for Coexisting Coordinative Rules in Aiming

in Motor Control

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Edwin Van Thiel
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Ruud G.J. Meulenbroek
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Wouter Hulstijn
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In this study we tried to establish whether point-to-point aiming movements are planned in workspace, joint space, or both. Eight right-handed subjects performed horizontal, vertical, and diagonal aiming movements on a transversal plane. Movements were performed at several speeds. Curvature variations of the hand and corresponding joint-space paths were investigated as a function of position, direction, and speed. Straightness of hand paths predominated for vertical movements but was systematically violated for horizontal and top-right to bottom-left movements. Furthermore, the hand-path curvature of the latter movements increased with speed. Joint-space paths showed more deviation from a straight line than hand paths except for top-left to bottom-right movements in which the paths were equally curved. A comparison of normalized path curvatures at the hand and joint level indicated that in aiming, the coordinative rule of straight-line production seems to apply to both workspace and joint-space planning. The present findings confirm Kawato's (1996) views that optimization processes operate concurrently at the two control levels of arm-trajectory formation under study.

The authors are with the Nijmegen Institute for Cognition and Information, PO Box 9104, 6500 HE Nijmegen, The Netherlands.

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