Human subjects can readily adapt their movement trajectories to different dynamic or visuomotor environments. The focus of the current study was to determine whether subjects could simultaneously adapt to multiple dynamic environments. Subjects (n=5) drew ellipses continuously for 70 s using a torquable manipulandum under six distinct dynamic conditions, representing the combination of load type (spring, viscous, and inertia) and load direction (assisting and opposing). Each subject performed two control, ten load, and five washout trials. A significant effect of force condition on the trajectory of the movement was found in 26 of 30 cases (6 conditions × 5 subjects); the magnitude of the distortion differed across the conditions. The extent of adaptation also differed across the loads. Opposing inertia and viscosity led to fast adaptation. However, assisting inertia and viscosity were associated with relatively slow adaptation. The results of adaptation to the stiffness conditions were not consistent. Following sudden removal of the load we saw an additional disturbance of the trajectory (after-effect), which was often the mirror image of the original distortion. The shape and size of the after-effect were different across load conditions. These results show that human subjects can adapt to a variety of different dynamic transformations and that the time-course of adaptation is dependent on both the state space and the direction of the load.
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Acknowledgements We thank J.F. Soechting and B. Amirikian for helpful discussions, and G. Pellizzer and J. Konczak for useful comments on an earlier version of the manuscript. The work was supported in part by the US National Science Foundation (IBN-9604555), a Merit Review award from the Department of Veterans Affairs, and the American Legion Chair in Brain Sciences. T. Fukushi was a research fellow of the Uehara Memorial Foundation.
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