LEARNING A NEW VISUOMOTOR TRANSFORMATION - ERROR-CORRECTION AND GENERALIZATION

The use of an aiming tool requires learning a new transformation betwe en visual and proprioceptive information and motor command. We have ex amined this question by quantifying the kinematics of the movement dur ing the transitory phase of adaptation to a rotational bias (60 degree s counterclockwise, then clockwise) added to a standard mouse-cursor d evice in the plane of the screen. Control-aiming movements were almost linear with a bell-shaped velocity profile. The bias induced an equiv alent initial directional error which was usually corrected within 20 trials. The learning trajectories were combinations of spirals and fas t or slow straight movements. The posture of the hand was slightly (le ss than 10 degrees) modified by the bias. These features suggest three corrective processes: on-line continuous correction based on evaluati on of the relative cursor-to-target position, discrete correction base d on assessment of the discrepancy angle between the cursor-to-target direction and the effective cursor direction, and memorization of tria l-to-trial correction. These results are interpreted in the light of n europhysiological 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 th at 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 st ored in relation to the coding of the target direction and that moveme nt towards a new direction is computed as a projection of the previous ly learned bias on the new visual direction.