Distinct coordinate systems for adaptations of movement direction and extent

Learned compensations for perturbed visual feedback of movement extent and direction generalize differently to unpractised movement directions, which suggests different underlying neural mechanisms. Here we investigated whether gain and rotation adaptations are consistent with representation in different coordinate systems. Subjects performed a force aiming task with the wrist, and learned different gains or rotations for different force directions. Generalization was tested without visual feedback for the same extrinsic directions but with the forearm in a different pronation-supination orientation. When the change in forearm orientation caused the adapted visuomotor map to conflict in extrinsic and joint-based coordinates, rotation generalization occurred in extrinsic coordinates but with reduced magnitude. In contrast, gain generalization appeared reduced and phase-shifted. When the forearm was rotated further such that all imposed perturbations aligned in both joint-based and extrinsic coordinates in both postures, rotation generalization was further reduced, whereas there was neither reduction nor phase shift in the pattern of extent generalization. These results show that rotation generalization was expressed in extrinsic coordinates, and that generalization magnitude was modulated by posture. In contrast, gain generalization appeared to depend on target direction defined by an integrated combination of extrinsic and joint-based coordinates, and was not reduced substantially by posture changes alone. Although the quality of the model fit underlying our interpretation prevents us from making strong conclusions, the data suggest that adaptations of movement direction and extent are represented according to distinct coordinate systems.  

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