Attentional fluctuations in a timing task

In studies where long sequences of movements were made to large targets, feedforward control was engaged and movement distance time series had a pink-noise structure. In contrast, when targets were small, visuomotor feedback processes guided the hand to the target, and a white-noise time series was observed. The feedforward-to-feedback shift reflected an internal-to-external shift in attention. Here, we examine how such shifts influence time-series structure in timing: Participants produced 300 consecutive key presses at two interstimulus intervals (ISIs = 500 & 2000 ms) within synchronization (SYNC) and continuation (CONT) timing tasks. During SYNC, presses were made in time with a metronome; that provided the opportunity to engage feedback control by attending to the metronome ISI and adjusting the intertap interval (ITI) accordingly. Feedback engagement should increase with the increase in ISI. Therefore, we predict that ISI lengthening should induce ITI time series whitening. During CONT, although the metronome was disabled, participants were instructed to produce ITIs that matched the target ISI; that forced reliance on feedforward control of timing under both ISIs. Thus, we predict pink-noise ITI time series under both ISIs.