Fig. 1.

Experimental setup and protocol. A: subjects sat in front of a computer display and made reaching movements toward and away from the body while holding the handle of a robotic manipulandum that applied forces to the hand. B: there were 3 trial types: null trials, force-field trials, and error-clamp trials. The motors of the manipulandum were turned off for null trials. During force-field trials, the motors were used to produce forces on the hand (gray arrows) that were proportional in magnitude and perpendicular in direction to the velocity of hand motion (black arrow). Forces were determined as a function of hand velocity: f = Bv. During error-clamp trials, the robot motors were used to constrain movements in a straight line toward the target by counteracting any motion perpendicular to the target direction. C: timeline of the experiment. Only slow movements were cued during the adaptation periods, whereas a mixture of both fast and slow movements was cued during the baseline and testing periods.