Synchronization of movements with a sequence of external events has been studied for a long time. The simplest task, synchronization with a metronome that produces auditory sounds equally spaced in time, is well understood.

Synchronization becomes a more demanding task when the metronome’s tempo changes systematically. In this case, phase correction alone is not sufficient for keeping synchrony. Rather, the internal timekeeper must be adjusted to the changing period of the metronome. Several models have been proposed for period adjustment. Mates (1994a, 1994b) has suggested that timekeeper adjustments are based on the discrepancy between the current timekeeper interval and the previous metronome interval. This error signal is used to adjust the timekeeper by adding or subtracting a fixed proportion of the interval difference. A simple alternative that we pursue in this article is that, like phase, the timekeeper period is adjusted on the basis of the asynchronies, that is, the temporal difference between perceived events, rather than the difference between time intervals.