Compensation for subliminal timing perturbations in perceptual-motor synchronization

 It is sometimes assumed that limits of temporal discrimination established in psychophysical tasks constrain the timing information available for the control of action. Results from the five perceptual-motor synchronization experiments presented here argue against this assumption. Experiment 1 demonstrates that subliminal (0.8–2%) local changes in interval duration in an otherwise isochronous auditory sequence are rapidly compensated for in the timing of synchronized finger tapping. If this compensation is based on perception of the highly variable synchronization error (SE) rather than of the local change in stimulus period, then it could be based solely on SEs that exceed the temporal order threshold. However, that hypothesis is ruled out by additional analyses of Exp. 1 and the results of Exp. 2, a combined synchronization and temporal order judgment task. Experiments 3–5 further show that three factors that affect the detectability of local deviations from stimulus isochrony do not inhibit effective compensation for such deviations in synchronized tapping. Experiment 5, a combined synchronization and detection task, shows directly that compensation for timing perturbations does not depend on explicit detection. Overall, the results suggest that the automatic processes involved in the temporal control of action have access to more accurate timing information than do the conscious decision processes of auditory temporal judgment. Received: 19 November 1998 / Accepted: 18 March 1999     

Phase correction in sensorimotor synchronization: nonlinearities in voluntary and involuntary responses to perturbations

When finger taps are synchronized with an auditory sequence, both a global phase shift (PS) and a local event onset shift (EOS) in the sequence elicit a phase correction response (PCR) on the next tap. The PCR to an expected PS is intended and large, whereas that to an expected EOS is unintended and smaller. PCR magnitude increases linearly with perturbation magnitude up to about +/-15% of the sequence period (500 milliseconds). With larger perturbations, voluntary PCRs increase more slowly whereas involuntary PCRs reach an asymptote. These results, obtained previously in a blocked design [J. Exp. Psychol. Human Percept. Perform. (in press)], were replicated in a randomized design and in two additional task contexts that varied participants' intentions while neutralizing their expectations. Neither design nor expectations seemed to play a role. However, considerable individual differences were noted. The results confirm that phase correction is partially automatic and partially subject to voluntary control, and they provide empirical estimates of error correction functions that may be useful in formal modeling of sensorimotor synchronization behavior.     

Automaticity and voluntary control of phase correction following event onset shifts in sensorimotor synchronization

Seven experiments show that an event onset shift (EOS) in an auditory sequence causes an involuntary phase correction response (PCR) in synchronized finger tapping. This PCR is (a) equally large in inphase and antiphase tapping; (b) reduced but still present when the EOS occurs in either of two interleaved (target-distractor) sequences; (c) unaffected by increased pitch separation between these sequences; (d) asymptotic in magnitude as EOS magnitude increases, unlike the intentional PCR to expected phase shifts; and (e) enhanced when the EOS precedes the onset of tapping, because of phase resetting. Thus, phase correction is revealed to be partially automatic and partially under voluntary control, and to be based mainly on temporal information derived from simple onset detection.     

Rapid motor adaptations to subliminal frequency shifts during syncopated rhythmic sensorimotor synchronization

The synchronization of rhythmic arm movements to a syncopated metronome cue was studied in a step-change design whereby small tempo shifts were inserted at fixed time points into the metronome frequency. The cueing sequence involved three stimulus types: (1) target contact in synchrony with the metronome beats, (2) syncopated target contact midway in time between audible beats, and (3) syncopated target contact following either a +2% or -2% change in stimulus frequency. Analysis of normalized and aggregated data revealed that (1) during the syncopation condition the response period showed a rapid adaptation to the frequency-incremented stimulus period, (2) response period was less variable during syncopated movement, (3) mean synchronization error and variability, calculated during syncopation relative to the mathematical midpoint of the stimulus cycle, were reduced during syncopated movements, and (4) synchronization error following the frequency increment showed trends to return linearly to pre-increment values which was fully achieved in the -2% change condition only. The results suggest that frequency entrainment to stimulus period was possible during syncopated movement with the response and stimulus onsets 180 degrees out of phase. Most remarkably, 70-80% of the adaptation of the response period to the new stimulus period was immediately attained during the second half cycle of the syncopated movement. Finally, a mathematical model, based on recursion, was introduced that accurately modeled actual data as a function of the previous stimulus and response intervals and a weighted response of period error and synchronization error, which showed dominance of frequency entrainment over phase entrainment during rhythmic synchronization.     

Quantifying phase correction in sensorimotor synchronization: empirical comparison of three paradigms

Tapping in synchrony with a metronome requires phase error correction, a process often described by a single-parameter autoregressive model. The parameter (α) is a measure of sensorimotor coupling strength. This study compares α estimates obtained from three experimental paradigms: synchronization with (1) a perfectly regular metronome (RM), (2) a perturbed metronome containing phase shifts (PS), and (3) an "adaptively timed" metronome (AT). Musically trained participants performed in each paradigm at four tempi, with baseline interval durations ranging from 400 to 1300 ms. Two estimation methods were applied to each data set. Results showed that all α estimates increased with interval duration. However, the PS paradigm yielded much larger α values than did the AT paradigm, with those from the RM paradigm falling in between. Positional analysis of the PS data revealed that α increased immediately following a phase shift and then decreased sharply. Unexpectedly, all PS α estimates were uncorrelated with the RM and AT estimates, which were strongly correlated. These results suggest that abruptly perturbed sequences engage a different mechanism of phase correction than do regular or continuously modulated sequences.     

Temporal control of movements in sensorimotor synchronization.

Under conditions in which the temporal structure of events (e.g., a sequence of tones) is predictable, performing movements in synchrony with this sequence of events (e.g., dancing) is an easy task. A rather simplified version of this task is studied in the sensorimotor synchronization paradigm. Participants are instructed to synchronize their finger taps with an isochronous sequence of signals (e.g., clicks). Although this is an easy task, a systematic error is observed: Taps usually precede clicks by several tens of milliseconds. Different models have been proposed to account for this effect ("negative asynchrony" or "synchronization error"). One group of explanations is based on the idea that synchrony is established at the level of central representations (and not at the level of external events), and that the timing of an action is determined by the (anticipated) action effect. These assumptions are tested by manipulating the amount of sensory feedback available from the tap as well as its temporal characteristics. This article presents an overview of these representational models and the empirical evidence supporting them. It also discusses other accounts briefly in the light of further evidence.     

Perfect phase correction in synchronization with slow auditory sequences

Phase correction during synchronization (finger tapping) with an isochronous auditory sequence is typically imperfect, requiring several taps to complete. However, two independent hypotheses predict that phase correction should approach perfection when the sequence tempo is slow. The present results confirm this prediction. The experiment used a phase perturbation method and a group of musically trained participants. As the sequence interonset interval increased from 300 to 1200 ms, the phase correction response to perturbations increased and approached instantaneous phase resetting between 700 and 1200 ms, depending on the individual. A possible explanation of this finding is that emergent timing of the periodic finger movement vanishes as the movement frequency decreases and thus ceases to compete with event-based timing.     

Phase attraction in sensorimotor synchronization with auditory sequences: effects of single and periodic distractors on synchronization accuracy

Four experiments showed that both single and periodic distractor tones affected the timing of finger taps produced in synchrony with an isochronous auditory target sequence. Single distractors had only small effects, but periodic distractors occurring at various fixed or changing phase relationships exerted strong phase attraction. The attraction was asymmetric, being stronger when distractors preceded target tones than when they lagged behind. A large pitch difference between target and distractor tones (20 vs. 3 semitones) did not reduce phase attraction substantially, although in the case of continuously changing phase relationships it did prevent complete capture of the taps by the distractors. The results support the hypothesis that phase attraction is an automatic process that is sensitive primarily to event onsets.     

On the nature of phase attraction in sensorimotor synchronization with interleaved auditory sequences

In a task that requires in-phase synchronization of finger taps with an isochronous sequence of target tones that is interleaved with a sequence of distractor tones at various fixed phase relationships, the taps tend to be attracted to the distractor tones, especially when the distractor tones closely precede the target tones [Repp, B. H. (2003a). Phase attraction in sensorimotor synchronization with auditory sequences: Effects of single and periodic distractors on synchronization accuracy. Journal of Experimental Psychology: Human Perception and Performance, 29, 290-309]. The present research addressed two related questions about this distractor effect: (1) Is it a function of the absolute temporal separation or of the relative phase of the two stimulus sequences? (2) Is it the result of perceptual grouping (integration) of target and distractor tones or of simultaneous attraction to two independent sequences? In three experiments, distractor effects were compared across two different sequence rates. The results suggest that absolute temporal separation, not relative phase, is the critical variable. Experiment 3 also included an anti-phase tapping task that addressed the second question directly. The results suggest that the attraction of taps to distractor tones is caused mainly by temporal integration of target and distractor tones within a fixed window of 100-150 ms duration, with the earlier-occurring tone being weighted more strongly than the later-occurring one.     

Linear phase correction models for synchronization: parameter identification and estimation of parameters.

Linear phase correction models for synchronized tapping and their stochastic properties are presented. In the most general form they include a central timer, a motor execution, and a phase correction mechanism that acts on the physical or the perceived asynchrony. A central issue of the article is how to identify and estimate the model parameters from the data. Monte Carlo simulations show serious problems of parameter interdependency.     

Comments on "Rapid motor adaptations to subliminal frequency shifts during syncopated rhythmic sensorimotor synchronization" by Michael H. Thaut and Gary P. Kenyon (Human Movement Science 22 [2003] 321-338)

Thaut and Kenyon [Human Movement Sci. 22 (2003) 321] have shown that, in a task requiring tapping in antiphase with a metronome, the response period adapts rapidly to a small (+/-2%) change in the stimulus period, whereas the relative phase between stimulus and response returns to its pre-change value only very gradually. On the basis of these and earlier findings, Thaut and Kenyon argue that period adaptation is rapid and subconscious, whereas phase adaptation is slow and dependent on awareness of a phase error. This interpretation is at variance with results in the literature suggesting that phase correction is rapid and subconscious, whereas period correction is slow and dependent on awareness of a period mismatch. Although differences in terminology (adaptation versus correction) play a role in this conflict, it primarily reflects different conceptions of sensorimotor synchronization and different interpretations of empirical findings. By excluding from their model a central timekeeper or oscillator with a flexible period, Thaut and Kenyon have omitted an essential component of human timing control that is needed for a proper explanation of their results.     

Extremity shifts,risk shifts and attitude shifts after group discussion

Moscovici and Zavalloni (1969) suggest that both risk shifts and attitude shifts after group discussion are examples of a general group tendency to polarize opinions. In the present experiment, using both attitude and risk items, group discussion did not make individual opinions more extreme; only the group average became more extreme. This group extremity increase was not simply a more general way of conceptualizing the directional shifts in attitude and risk; group extremity increase appeared to be an effect of discussion that was independent of the risk and attitude shifts. Also, subjects in the co-working pretest of the standard risk-shift paradigm were found to be less extreme and more ‘agreeing’ than pretest subjects who were truly alone. This co-working/alone difference persisted after discussion and was not related to group extremity increase. On both attitude and risk items, group extremity increase was strongly correlated with group opinion convergence. It is argued from this correlation that group extremity increase may be an effect of some aspect of conformity influence.     

Adaptation to tempo changes in sensorimotor synchronization: Effects of intention, attention, and awareness

Adaptation to tempo changes in sensorimotor synchronization is hypothesized to rest on two processes, one (phase correction) being largely automatic and the other (period correction) requiring conscious awareness and attention. In this study, participants tapped their finger in synchrony with auditory sequences containing a tempo change and continued tapping after sequence termination. Their intention to adapt or not to adapt to the tempo change was manipulated through instructions, their attentional resources were varied by introducing a concurrent secondary task (mental arithmetic), and their awareness of the tempo changes was assessed through perceptual judgements. As predicted, period correction was found to be strongly dependent on all three variables, whereas phase correction depended only on intention.     

Tapping in synchrony with a perturbed metronome: the phase correction response to small and large phase shifts as a function of tempo

When tapping is paced by an auditory sequence containing small phase shift (PS) perturbations, the phase correction response (PCR) of the tap following a PS increases with the baseline interonset interval (IOI), leading eventually to overcorrection (B. H. Repp, 2008). Experiment 1 shows that this holds even for fixed-size PSs that become imperceptible as the IOI increases (here, from 400 to 1200 ms). Earlier research has also shown (but only for IOI=500 ms) that the PCR is proportionally smaller for large than for small PSs (B. H. Repp, 2002a, 2002b). Experiment 2 introduced large PSs and found smaller PCRs than in Experiment 1, at all of the same IOIs. In Experiments 3A and 3B, the author investigated whether the change in slope of the sigmoid function relating PCR and PS magnitudes occurs at a fixed absolute or relative PS magnitude across different IOIs (600, 1000, 1400 ms). The results suggest no clear answer; the exact shape of the function may depend on the range of PSs used in an experiment. Experiment 4 examined the PCR in the IOI range from 1000 to 2000 ms and found overcorrection throughout, but with the PCR increasing much more gradually than in Experiment 1. These results provide important new information about the phase correction process and pose challenges for models of sensorimotor synchronization, which presently cannot explain nonlinear PCR functions and overcorrection.     

Rate limits in sensorimotor synchronization with auditory and visual sequences: the synchronization threshold and the benefits and costs of interval subdivision

Synchronization of finger taps with an isochronous event sequence becomes difficult when the event rate exceeds a certain limit. In Experiment 1, the synchronization threshold was reached at interonset intervals (IOIs) above 100 ms with auditory tone sequences (in a 1:4 tapping task) but at IOIs above 400 ms with visual flash sequences (1:1 tapping). Using IOIs above those limits, the author investigated in Experiment 2 the reduction in the variability of asynchronies that tends to occur when the intervals between target events are subdivided by additional identical events (1:1 vs. 1:n tapping). The subdivision benefit was found to decrease with IOI duration and to turn into a cost at IOIs of 200-250 ms in auditory sequences and at IOIs of 450-500 ms in visual sequences. The auditory results are relevant to the limits of metrical subdivision and beat rate in music. The visual results demonstrate the remarkably weak rhythmicity of (nonmoving) visual stimuli.     

Perception in action: The impact of sensory information on sensorimotor synchronization in musicians and non-musicians

The present study aimed at investigating to what extent sensorimotor synchronization is related to (i) musical specialization, (ii) perceptual discrimination, and (iii) the movement’s trajectory. To this end, musicians with different musical expertise (drummers, professional pianists, amateur pianists, singers, and non-musicians) performed an auditory and visual synchronization and a cross-modal temporal discrimination task. During auditory synchronization drummers performed less variably than amateur pianists, singers and non-musicians. In the cross-modal discrimination task drummers showed superior discrimination abilities which were correlated with synchronization variability as well as with the trajectory. These data suggest that (i) the type of specialized musical instrument affects synchronization abilities and (ii) synchronization accuracy is related to perceptual discrimination abilities as well as to (iii) the movement’s trajectory. Since particularly synchronization variability was affected by musical expertise, the present data imply that the type of instrument improves accuracy of timekeeping mechanisms.     

Hearing a melody in different ways: multistability of metrical interpretation, reflected in rate limits of sensorimotor synchronization

Music commonly induces the feeling of a regular beat (i.e., a metrical structure) in listeners. However, musicians can also intentionally impose a beat (i.e., a metrical interpretation) on a metrically ambiguous passage. The present study aimed to provide objective evidence for this little-studied mental ability. Participants were prompted with musical notation to adopt different metrical interpretations of a cyclically repeated isochronous 12-note melody while tapping in synchrony with specified target tones in the melody. The target tones either coincided with the imposed beat (on-beat tapping) or did not (off-beat tapping). An adaptive staircase method was employed to determine the fastest tempo at which each synchronization task could be performed. For each metrical interpretation, a significant advantage for on-beat over off-beat tapping was obtained - except in a condition in which participants, instead of synchronizing, were in control of the target tones. By showing that a self-imposed beat can affect sensorimotor synchronization, the present results provide objective evidence for endogenous perceptual organization of metrical sequences. It is hypothesized that metrical interpretation rests upon covert rhythmic action.     

Coordination modes in sensorimotor synchronization of whole-body movement: a study of street dancers and non-dancers

This study investigated whole-body sensorimotor synchronization (SMS) in street dancers and non-dancers. Two kinds of knee bending movement in a standing position to a metronome beat were explored in terms of stability under different movement frequencies: down-movement condition (knee flexion on the beat) and up-movement condition (knee extension on the beat). Analyses of phase relation between movement and beat revealed several distinct differences between the down- and up-movement conditions, and between dancers and non-dancers. In both groups under the up-movement condition, deviation from intended phase relation at higher beat rates, and enhanced fluctuations were observed. The deviation from intended phase relation under up-movement condition, and movement fluctuations were greater in non-dancers than in dancers. Moreover, subjective difficulty rating revealed that both groups felt that the up-movement condition was more difficult at higher beat rates. These findings suggest that down and up movements are two distinguishable coordination modes in whole-body coordination, and that street dancers have superior whole-body SMS ability.     

Effects of feedback from active and passive body parts on spatial and temporal parameters in sensorimotor synchronization

Previous research on sensorimotor synchronization has manipulated the somatosensory information received from the tapping finger to investigate how feedback from an active effector affects temporal coordination. The current study explored the role of feedback from passive body parts in the regulation of spatiotemporal motor control parameters by employing a task that required finger tapping on one’s own skin at anatomical locations of varying tactile sensitivity. A motion capture system recorded participants’ movements as they synchronized with an auditory pacing signal by tapping with the right index finger on either their left index fingertip (Finger/Finger) or forearm (Finger/Forearm). Results indicated that tap timing was more variable, and movement amplitude was larger and more variable, when tapping on the finger than when tapping on the less sensitive forearm. Finger/Finger tapping may be impaired relative to Finger/Forearm tapping due to ambiguity arising through overlap in neural activity associated with tactile feedback from the active and the passive limb in the former. To compensate, the control system may strengthen the assignment of tap-related feedback to the active finger by generating correlated noise in movement kinematics and tap dynamics.     

The timing effects of accent production in synchronization and continuation tasks performed by musicians and nonmusicians

Two groups of subjects differing in their musical expertise produced periodic finger-tapping sequences involving a pattern of accentuation. In some situations, the taps were synchronized with the clicks of a metronome. We recorded the trajectory of the subjects' finger displacement in the vertical plane, and the force and the moment of occurrence of the taps on the response key. Musicians tended to equalize the durations of the downstrokes at all positions in the sequence. Nonmusicians moved their finger quickly to produce the accent, and more slowly to produce the subsequent tap. These variations in the movement-execution time were partly compensated by opposite variations in the onsets of the movements, e.g., the short-duration movements were delayed. Despite these differences in their movement strategies, musicians and nonmusicians generated very similar tap-timing profiles. The intertap interval after the accent was lengthened regardless of the subjects' musical expertise and the metronome conditions (metronome present or absent). The lengthening did not depend on whether the interval before the accent was shortened (without the metronome) or not (with the metronome). It is suggested that an internal timekeeper may generate temporal goal points at which the keytaps should occur. The lengthening of the interval after the accent is attributed to transient changes in the working of the internal clock.