RAPID COMMUNICATION The Motor System Does Not Learn the Dynamics of the Arm by Rote Memorization of Past Experience

Conditt, Michael A., Francesca Gandolfo, and Ferdinando A. rasso 1981). These kinematics were drastically altered when Mussa-Ivaldi. The motor system does not learn the dynamics of the manipulandum generated a disturbing perturbation. In the arm by rote memorization of past experience. J. Neurophysiol. the experiments of Shadmehr and Mussa-Ivaldi (1994) the 78: 554–560, 1997. The purpose of this study was to investigate manipulandum was programmed to generate forces whose the learning mechanisms underlying motor adaptation of arm amplitude and direction depended linearly on the measured movements to externally applied perturbing forces. We considered velocity of the hand. These forces altered, in a specific way, two alternative hypotheses. According to one, adaptation occurs the dynamics of the system controlled by the subject’s motor through the learning of a mapping between the states (positions activities. Nevertheless, after a period of repeated moveand velocities) visited by the arm and the forces experienced at ments with the perturbed dynamics, the subject recovered the those states. The alternative hypothesis is that adaptation occurs through the memorization of the temporal sequence of forces expeoriginal kinematics. As in the case of visuomotor adaptation, rienced along specific trajectories. The first mechanism correadaptation to a disturbing force field occurred in parallel sponds to developing a model of the dynamics of the environment, with the development of aftereffects: when the disturbing whereas the second is a form of ‘‘rote learning.’’ Both types of field was suddenly removed, after prolonged exposure, the learning would lead to the recovery of the unperturbed perforresulting kinematics displayed a change opposite to that inmance. We have tested these hypotheses by examining how adaptaduced initially by the same disturbing field. Shadmehr and tion is transferred across different types of movements. Our results Mussa-Ivaldi considered these aftereffects as evidence that indicate that 1) adaptation to an externally applied force field ocsubjects adapted to a disturbing field by adding to their motor curs with different classes of movements including but not limited command a preprogrammed force component that was equal to reaching movements and 2) adaptation generalizes across different movements that visit the same regions of the external field. and opposite to the field. They suggested that adaptation to These findings are not compatible with the hypothesis of rote learna novel force field occurs by creating an ‘‘internal model’’ ing. Instead, they are consistent with the hypothesis that adaptation of this field. The forces generated by this internal model are to changes in movement dynamics is achieved by a module that added to the normal, unperturbed motor commands so as to learns to reproduce the structure of the environmental field as an restore the original movements. This theory of adaptation association between visited states and experienced forces, indepenis appealing because it implies that after a change in the dent of the kinematics of the movements made during adaptation. environment dynamics, it is not necessary for the control system to relearn the entire repertoire of previously acquired motor skills. These skills can be recovered by superimposing I N T R O D U C T I O N