Learning a new visuomotor transformation: error correction and generalization.

The use of an aiming tool requires learning a new transformation between visual and proprioceptive information and motor command. We have examined this question by quantifying the kinematics of the movement during the transitory phase of adaptation to a rotational bias (60 degrees counterclockwise, then clockwise) added to a standard mouse-cursor device in the plane of the screen. Control-aiming movements were almost linear with a bell-shaped velocity profile. The bias induced an equivalent initial directional error which was usually corrected within 20 trials. The learning trajectories were combinations of spirals and fast or slow straight movements. The posture of the hand was slightly (less than 10 degrees) modified by the bias. These features suggest three corrective processes: on-line continuous correction based on evaluation of the relative cursor-to-target position, discrete correction based on assessment of the discrepancy angle between the cursor-to-target direction and the effective cursor direction, and memorization of trial-to-trial correction. These results are interpreted in the light of neurophysiological data and neural net modeling, which suggest that the visuomotor transformation performed by cortical areas for reaching is effected by projecting the visual information on a reference frame that rotates with the arm. The initial directional error reappeared when the direction of the target was changed and increased with degree of change. The limited generalization suggests that bias correction is stored in relation to the coding of the target direction and that movement towards a new direction is computed as a projection of the previously learned bias on the new visual direction.