Fast aiming movements with the left and right arm: evidence for two-process theories of motor control.

Abstract

The kinematics of leftward and rightward movements are different. The question is raised whether these differences in both arms are best accounted for in terms of the flexion-extension dimension or the leftward-rightward dimension. In a simple step-tracking experiment the acceleration-time curves of fast flexions and extensions of the left and of the right elbow joint in a horizontal plane were recorded. The durations of individual segments of the acceleration curves are best predicted by spatial direction: that is, the differences between leftward and rightward movements are either both positive or both negative in both arms. This holds only for the first part of the movement until maximum deceleration is reached. From then on the timing is best predicted by the flexion-extension dimension. This is taken as evidence for a two-process theory of motor control according to which a central process is first in command and a lower-level, muscle-related process then takes over. Although a change which corresponds to the one observed in the time variables is not seen in the maxima of the acceleration curves, this seeming disparity can be reconciled with a two-process theory.