Central representation of time during motor learning

This study stemmed from the observation that the brain of human as well as nonhuman primates is capable of forming and memorizing remarkably accurate internal representations of the dynamics of the arm. These dynamics establi sh a functional relation between applied force and ensuing arm motion, a re lation that generally is quite complex and nonlinear. Current evidence show s that the motor control system is capable of adapting to perturbing forces that depend on motion variables such as position, velocity, and accelerati on. The experiments we report here were aimed at establishing whether or no t the motor system also may adapt to forces that depend explicitly on time rather than on motion variables, Surprisingly, the experiments suggest a ne gative answer. When asked to compensate for a predictable and repeated time -varying pattern of disturbing forces, subjects learned to counteract the d isturbance by producing forces that did not depend on time but on the veloc ity and the position of the arm. We conclude from this evidence that time a nd time-dependent dynamics are not explicitly represented within the neural structures that lire responsible for motor adaptation. Although our findin gs are not sufficient to rule out the presence of a timing structure within the central nervous system, they are consistent with other investigations that conspicuously failed to find evidence for such a central clock.