Linking the spatial dimension to the timing control of rhythmic movements

Rhythmically timed movements are integral to a wide range of human behaviors, including the production of fluent speech, music performance, dance, and sports, to name a few. Deficits in the timing of rhythmic movements have been observed in numerous neurological conditions, which can be devastating for an individual's quality of life. One common method to investigate mechanisms underpinning the timing of rhythmic actions is a finger tapping task in which individuals synchronize finger taps with a series of tones that establish a target inter-tap interval (ITI) and then continue to produce the target interval after the tones stop. The data of interest in these studies have almost exclusively consisted of the time series of tap onsets and associated sequence of ITIs. Because all movements are inherently spatiotemporal, the thesis investigated herein is that our understanding of timing processes is incomplete because previous work ignores the contributions of spatial elements of an individual's movements to their timing control. The experiments reported here fill this gap by using continuous motion tracking to measure the spatiotemporal dynamics of paced and unpaced rhythmic finger tapping and by considering the relation between spatiotemporal measures and timing accuracy and precision. Five experiments tested a series of hypotheses about contributions of spatiotemporal factors to the timing control of rhythmic movements. Experiment 1 tested a preferred velocity hypothesis that integrates amplitude and tempo for unpaced tapping. Participants completed unpaced tapping tasks that separately assessed preferred movement amplitude (finger height) and tempo (Mean ITI). In support of this hypothesis, participants produced similar amplitudes and tempi regardless of instructions for either preferred amplitude or tempo. Experiments 2 and 3 tested an amplitude control hypothesis for paced tapping where participants matched a wide range of target ITIs. Consistent with this hypothesis, individuals decreased tap amplitude with shorter target ITIs and variability in amplitudes predicted variability in ITIs. Further supporting this hypothesis, forcing participants to produce low and high amplitudes during paced tapping interfered with timing accuracy and precision in a manner consistent with amplitude as a parameter in timing control. The preferred velocity hypothesis was further supported by results showing that timing was less variable for conditions where participants tapped at target amplitudes and tempi that, in combination, were closer to their preferred velocity. Experiment 4 extended this line of work to timing control of tapping at slow tempi (near the temporal boundary where perceived rhythm breaks down). Of primary interest was a dwell time hypothesis, which proposes that at slow target ITI when amplitude cannot be increased further to lengthen intervals, participants increase dwell time (how long their finger rests on the table) to produce longer ITIs. Providing initial support for this hypothesis, participants kept tap amplitude constant and increased tap dwell time to produce longer ITIs. At the slowest target ITI, a bimodal distribution in tap dwell times also was observed, reflecting individual differences in dwell time strategy where some participants kept a constant proportion of dwell time to target ITI, while others increased the proportion of dwell time at slower tempi (longer ITIs). As a follow-up, Experiment 5 manipulated tap dwell time during paced tapping at comfortable and very slow tempi (ITIs). Participants successfully lengthened or shortened their dwell time at the slow ITI, regardless of their preferred dwell time. Timing accuracy and precision at the slow ITI were particularly poor when participants were instructed to produce short dwell times, suggesting that longer dwell times at slow tempi facilitate temporal accuracy and precision. Altogether, results provide novel evidence of contributions of spatial characteristics of rhythmic movements to their temporal control and lays the foundations for a new theory of timing and temporal control that links the dimensions of space and time.\