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Loes Janssen, Céline Crajé, Matthias Weigelt and Bert Steenbergen

We examined anticipatory motor planning and the interaction among both hands in a discrete bimanual task. To this end, participants had to grasp and manipulate two cylindrical objects simultaneously under varying conditions in which (a) the grip selection requirements, i.e., orientation of the to-be-grasped objects, differed between the two hands and (b) the type of grip for one hand was preinstructed, while the grip for the other hand was free choice. Results showed that participants, when grasping for two bars with a free grip choice, prioritized planning for comfortable end postures over symmetry of movement execution. Furthermore, when participants were free to choose a grip for their left hand, but were instructed on how to grasp an object with their right hand, we found no interaction between the grip selections of both hands, suggesting that motor planning proceeds independently for both hands.

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Winston D. Byblow, Jeffery J. Summers, Andras Semjen, Irina J. Wuyts and Richard G. Carson

Two experiments required right-handed subjects to trace circular trajectories while complying with either a symmetric or asymmetric pattern. In symmetric patterns, circles were traced in a mirror image either inward or outward. In asymmetric patterns, circles were traced in the same direction either clockwise or counterclockwise. Subjects were instructed to trace with spatial accuracy while maintaining a strict temporal relationship to a metronome that scaled movement rates from 1.25 to 3 Hz. The symmetric patterns were more stable than asymmetric patterns; the circularity of trajectories was greater for the dominant side; and there were spontaneous reversals in the direction of circling in the nondominant limb when performing asymmetric patterns. The second experiment examined the same subjects under the instruction of intentionally changing the pattern by reversing the left or right limb circling direction when cued to do so. The degree of interlimb interference was highly asymmetric and contingent on the direction of pattern change. Intentional direction reversals were more expedient and with less disruption to the contralateral limb when asymmetric to symmetric pattern changes were effected through a reversal in the direction of nondominant side. The results are interpreted with reference to evidence that the supplementary motor area mediates descending input to the upper limbs during disparate bimanual actions, but not during symmetric actions.