THE MOTOR SYSTEM DOES NOT LEARN THE DYNAMICS OF THE ARM BY ROTE MEMORIZATION OF PAST EXPERIENCE

The purpose of this study was to investigate the learning mechanisms u nderlying motor adaptation of ann movements to externally applied pert urbing forces. We considered two alternative hypotheses. According to one, adaptation occurs through the learning of a mapping between the s lates (positions and velocities) visited by the arm and the forces exp erienced at those states. The alternative hypothesis is that adaptatio n occurs through the memorization of the temporal sequence of forces e xperienced along specific trajectories. The first mechanism correspond s to developing a model of the dynamics of the environment, whereas th e second is a form of ''rote learning.'' Both types of learning would lead to the recovery of the unperturbed performance. We have tested th ese hypotheses by examining how adaptation is transferred across diffe rent types of movements. Our results indicate that 1) adaptation to an externally applied force field occurs with different classes of movem ents Including but not limited to reaching movements and 2) adaptation generalizes across different movements that visit the same regions of the external field. These findings are not compatible with the hypoth esis of rote learning. Instead, they are consistent with the hypothesi s that adaptation to changes in movement dynamics is achieved by a mod ule that learns to reproduce the structure of the environmental field as an association between visited states and experienced forces, indep endent of the kinematics of the movements made during adaptation.