Keeping with the beat: movement trajectories contribute to movement timing.

Previous studies of paced repetitive movements with respect to an external beat have either emphasised (a) the form of movement trajectories or (b) timing errors made with respect to the external beat. The question of what kinds of movement trajectories assist timing accuracy has not previously been addressed. In an experiment involving synchronisation or syncopation with an external auditory metronome we show that the nervous system produces trajectories that are asymmetric with respect to time and velocity in the out and return phases of the repeating movement cycle. This asymmetry is task specific and is independent of motor implementation details (finger flexion vs. extension). Additionally, we found that timed trajectories are less smooth (higher mean squared jerk) than unpaced ones. The degree of asymmetry in the flexion and extension movement times is positively correlated with timing accuracy. Negative correlations were observed between synchronisation timing error and the movement time of the ensuing return phase, suggesting that late arrival of the finger is compensated by a shorter return phase and conversely for early arrival. We suggest that movement asymmetry in repetitive timing tasks helps satisfy requirements of precision and accuracy relative to a target event.