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1 Division of Biology, California Institue of Technology, Pasadena, CA, USA
2 Department of Cognitive Science, University of California, San Diego, La Jolla, CA, USA
* To whom correspondence should be addressed. E-mail: etorres{at}vis.caltech.edu.
In reach-to-grasp movements, the interaction between the hand changes in position and those in orientation is poorly understood. A theoretical approach previously proposed (Torres and Zipser 2002) assumes that motion strategies are resolved in space independently from the temporal dynamics of the motion, and predicts the co-articulation of the hand transport and rotation along the path. The model implies that this simultaneous control is independent of variations in speed, initial posture and required matching orientation. This paper presents experimental data from human subjects that confirms the model's predictions in the context of realistic-unconstrained orientation-matching motions. Speed independence is quantified in the similarity of the postural and endpoint position-orientation paths obtained under 3 different speeds. Significant differences in hand and joint kinematics are shown in response to changes in initial posture and target orientation. The robustness of co-articulation under all 3 experimental conditions supports the idea of an intermediate stage that resolves the geometry of the motion independent of its temporal dynamics.
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