Fig. 1.

Study protocol and performance of typical subjects. A: subjects participated in 2 experiments: abrupt and gradual introduction of a force field. They held the handle of a lightweight robotic arm and made shooting movements to a target, crossing it, and hitting a virtual pillow beyond the target. In the first 170 trials the robot produced a null field (no forces). In the subsequent 240 trials, a curl force field was introduced (field A or field B), perturbing the hand perpendicular to its direction of motion. The gray bars represent “error-clamp” trials during which the robot produced a stiff channel, guiding the hand to the target. These error-clamp trials allowed us to measure the subject's motor output perpendicular to the direction of motion. Vertical dashed lines indicate brief set breaks. B: representative trajectories from the end of the null and the beginning and end of the adaptation periods for an individual subject from the control, mild (#9), and severe (#6) groups. The movement starts at (0,0) and the target is at (0,10).