Central
representation of time during motor learning
MICHAEL A. CONDITT † AND
FERDINANDO A. MUSSA-IVALDI ‡§
† Department of Biomedical
Engineering, Northwestern University, Sensory Motor Performance Program,
Rehabilitation Institute of Chicago, 345 East Superior, Suite 1406, Chicago, IL
60611;
‡ Departments of Physiology
and Physical Medicine and Rehabilitation, Northwestern University Medical
School, 303 East Chicago Avenue, M211, Chicago, IL 60611-3008
Proc. Natl.
Acad. Sci. USA
Vol. 96,
pp. 11625–11630, September 1999
Neurobiology
Communicated
by Emilio Bizzi, Massachusetts Institute of Technology, Cambridge, MA, July 12,
1999 (received for review October 15, 1998)
ABSTRACT
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 establish a
functional relation between applied force and ensuing arm motion, a relation
that generally is quite complex and nonlinear. Current evidence shows that the
motor control system is capable of adapting to perturbing forces that depend on
motion variables such as position, velocity, and acceleration. The experiments
we report here were aimed at establishing whether or not the motor system also
may adapt to forces that depend explicitly on time rather than on motion
variables. Surprisingly, the experiments sug-gest a negative answer. When asked
to compensate for a predictable and repeated time-varying pattern of disturbing
forces, subjects learned to counteract the disturbance by producing forces that
did not depend on time but on the velocity and the position of the arm. We
conclude from this evidence that time and time-dependent dynamics are not
explicitly represented within the neural structures that are responsible for
motor adaptation. Although our findings 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.