The Continuity of Movements

Abstract

A study was made of self-initiated and self-terminated elbow movements made by Cebus monkeys during performance of a self-paced step-tracking task. Movements were termed “continuous” if their acceleration traces crossed the zero line only once, and “discontinuous” if there was more than one zero crossing. Parameters of continuous and discontinuous movements were measured. Acceleration in continuous movements could oscillate at 7–8 Hz (“deviations”), and in discontinuous movements at 3–4 Hz (“steps”). Steps of discontinuous movements became smaller as movements approached their targets. Amplitudes of continuous movements and of steps were highly correlated to their durations, and fitted the same regression line. It is concluded that the variations of acceleration of continuous and discontinuous movements appear locked into harmonics of the same frequency. Their relationship to physiological tremor is discussed.