The Motor System Does Not Learn the Dynamics of the
Arm by Rote
Memorization of Past Experience
MICHAEL A. CONDITT, FRANCESCA GANDOLFO, AND FERDINANDO A. MUSSA-IVALDI
Sensory
Motor Performance Program, Rehabilitation Institute of Chicago, Northwestern
University, Chicago, Illinois
60611;
and Department of Brain and Cognitive Sciences, Massachusetts Institute of
Technology, Cambridge,
Massachusetts
02139
J. Neurophysiol. 78: 554–560, 1997.
ABSTRACT
The
purpose of this study was to investigate the learning mechanisms underlying
motor adaptation of arm movements to externally applied perturbing forces. We considered two alternative
hypotheses. According to one
hypothesis, adaptation occurs through the learning of a mapping between the
states (positions and velocities) visited by the arm and the forces experienced
at those states. The alternative hypothesis
is that adaptation occurs through the memorization of the temporal sequence of
forces experienced along specific trajectories. The first mechanism corresponds to developing a model of the
dynamics of the environment, while the second is a form of "rote
learning". Both types of learning
would lead to the recovery of the unperturbed performance. We have tested these hypotheses by examining
how adaptation is transferred across different types of movements. Our results indicate that (a) adaptation to
an externally applied force field occurs with different classes of movements
including but not limited to reaching movements, and (b) adaptation generalizes
across different movements which visit the same regions of the external field. These findings are not compatible with the
hypothesis of rote learning. Instead,
they are consistent with the hypothesis that adaptation to changes in movement
dynamics is achieved by a module that learns to reproduce the structure of the
environmental field as an association between visited states and experienced
forces, independent of the kinematics of the movements made during adaptation.