The relationship between variability of intertap intervals and interval duration

Subjects tracked intervals in a synchronization paradigm at interval durations of 180, 210, 240, 270, 300, 400, 500, 600, 700, 800, 900, and 1,000 ms. The variability of intertap intervals (ITIs) shows a sudden increase near 300 ms. This increase is interpreted as indicating the transition from automatic to controlled movement. It is suggested that the sudden change in the variability of ITIs does not reflect the operation of different timing mechanisms at short and long intervals, but differences in the way in which attentional processes come to bear on movement initiation for different interval durations. In contrast to previous findings reported in the literature, a U-shaped function between interval duration and variability in the 300-to-l,000-ms range was not observed.