Adaptation to visuomotor rotation in isometric reaching is similar to movement adaptation

Isometric reaching, in which the arm remains stationary and the user controls a virtual cursor via force input, is a motor task that has not been thoroughly compared to real reaching. In this study, we ask if isometric adaptation to a kinematic perturbation is similar to adaptation in movement, and if the type of isometric mapping (position or velocity control) influences learning. Healthy subjects made real and virtual reaches with the arm in plane. In some trials, the cursor was rotated counter clockwise by 45°to perturb the kinematic mapping. To assess adaptation, the angular error of cursor movement at 150 ms from movement onset was measured for each reach; error was averaged across subjects and a two-state learning mode was fit to error data. For movement and isometric groups, average angular error peaked at perturbation onset, reduced over 200 reaches, and reversed direction when the perturbation was removed. We show that subjects are able to adapt to a visuomotor rotation in both position- and velocity-based cursor control, and that the time course of adaptation resembles that of movement adaptation. Training of virtual reaching using force/torque input could be particularly applicable for stroke patients with significant movement deficits, who could benefit from intensive treatments using simple, cost-effective devices.