Bruno H. Repp

Sensorimotor synchronization: a review of the tapping literature.

Sensorimotor synchronization (SMS), the rhythmic coordination of perception and action, occurs in many contexts, but most conspicuously in music performance and dance. In the laboratory, it is most often studied in the form of finger tapping to a sequence of auditory stimuli. This review summarizes theories and empirical findings obtained with the tapping task. Its eight sections deal with the role of intention, rate limits, the negative mean asynchrony, variability, models of error correction, perturbation studies, neural correlates of SMS, and SMS in musical contexts. The central theoretical issue is considered to be how best to characterize the perceptual information and the internal processes that enable people to achieve and maintain SMS. Recent research suggests that SMS is controlled jointly by two error correction processes (phase correction and period correction) that differ in their degrees of cognitive control and may be associated with different brain circuits. They exemplify the general distinction between subconscious mechanisms of action regulation and conscious processes involved in perceptual judgment and action planning.

Phonetic trading relations and context effects: new experimental evidence for a speech mode of perception.

This article reviews a variety of experimental findings, most of them obtained in the last few years, that show that the perception of phonetic distinctions relies on a multiplicity of acoustic cues and is sensi tive to the surrounding context in very specific ways. Nearly all of these effects have correspondences in speech production, and they are readily explained by the assumption that listeners make continuous use of their tacit knowledge of speech patterns. A general auditory theory that does not make reference to the specific origin and function of speech can, at best, handle only a small portion of the wealth of phenomena reviewed here. Special emphasis is placed on several recent studies that obtained different patterns of results depending on whether identical stimuli were perceived as speech or as nonspeech. These findings provide strong empirical evidence for the existence of a special speech mode of perception.

Auditory dominance in temporal processing: new evidence from synchronization with simultaneous visual and auditory sequences.

Evidence that audition dominates vision in temporal processing has come from perceptual judgment tasks. This study shows that this auditory dominance extends to the largely subconscious processes involved in sensorimotor coordination. Participants tapped their finger in synchrony with auditory and visual sequences containing an event onset shift (EOS), expected to elicit an involuntary phase correction response (PCR), and also tried to detect the EOS. Sequences were presented in unimodal and bimodal conditions, including one in which auditory and visual EOSs of opposite sign coincided. Unimodal results showed greater variability of taps, smaller PCRs, and poorer EOS detection in vision than in audition. In bimodal conditions, variability of taps was similar to that for unimodal auditory sequences, and PCRs depended more on auditory than on visual information, even though attention was always focused on the visual sequences.

The influence of metricality and modality on synchronization with a beat

The great majority of the world’s music is metrical, i.e., has periodic structure at multiple time scales. Does the metrical structure of a non-isochronous rhythm improve synchronization with a beat compared to synchronization with an isochronous sequence at the beat period? Beat synchronization is usually associated with auditory stimuli, but are people able to extract a beat from rhythmic visual sequences with metrical structure? We addressed these questions by presenting listeners with rhythmic patterns which were either isochronous or non-isochronous in either the auditory or visual modality, and by asking them to tap to the beat, which was prescribed to occur at 800-ms intervals. For auditory patterns, we found that a strongly metrical structure did not improve overall accuracy of synchronization compared with isochronous patterns of the same beat period, though it did influence the higher-level patterning of taps. Synchronization was impaired in weakly metrical patterns in which some beats were silent. For the visual patterns, we found that participants were generally unable to synchronize to metrical non-isochronous rhythms, or to rapid isochronous rhythms. This suggests that beat perception and synchronization have a special affinity with the auditory system.

Influence of vocalic context on perception of the [∫]-[s] distinction

When synthetic fricative noises from a [∫]-[s] continuum are followed by [a] or [u] (with appropriate formant transitions), listeners perceive more instances of [s] in the context of [u] than in the context of [a]. Presumably, this reflects a perceptual adjustment for the coarticulatory effect of rounded vowels on preceding fricatives. In Experiment 1, we found that varying the duration of the fricative noise leaves the perceptual context effect unchanged, whereas insertion of a silent interval following the noise reduces the effect substantially. Experiment 2 suggested that it is temporal separation rather than the perception of an intervening stop consonant that is responsible for this reduction, in agreement with recent, analogous observations on anticipatory coarticulation. In Experiment 3, we showed that the vowel context effect disappears when the periodic stimulus portion is synthesized so as to contain no formant transitions. To dissociate the contribution of formant transitions from contextual effects due to vowel quality per se, Experiment 4 employed synthetic fricative noises followed by periodic portions excerpted from naturally produced [∫a], [sa], [∫u], and [su]. The results showed strong and largely independent effects of formant transitions and vowel quality on fricative perception. In addition, we found a strong speaker (male vs. female) normalization effect. All three influences on fricative perception were reduced by temporal separation of noise and periodic stimulus portions. Although no single hypothesis can explain all of our results, they are generally supportive of the view that some knowledge of the dynamics of speech production has a role in speech perception.

Top-Down Control of Rhythm Perception Modulates Early Auditory Responses

Our perceptions are shaped by both extrinsic stimuli and intrinsic interpretation. The perceptual experience of a simple rhythm, for example, depends upon its metrical interpretation (where one hears the beat). Such interpretation can be altered at will, providing a model to study the interaction of endogenous and exogenous influences in the cognitive organization of perception. Using magnetoencephalography (MEG), we measured brain responses evoked by a repeating, rhythmically ambiguous phrase (two tones followed by a rest). In separate trials listeners were instructed to impose different metrical organizations on the rhythm by mentally placing the downbeat on either the first or the second tone. Since the stimulus was invariant, differences in brain activity between the two conditions should relate to endogenous metrical interpretation. Metrical interpretation influenced early evoked neural responses to tones, specifically in the upper beta range (20–30 Hz). Beta response was stronger (by 64% on average) when a tone was imagined to be the beat, compared to when it was not. A second experiment established that the beta increase closely resembles that due to physical accents, and thus may represent the genesis of a subjective accent. The results demonstrate endogenous modulation of early auditory responses, and suggest a unique role for the beta band in linking of endogenous and exogenous processing. Given the suggested role of beta in motor processing and long‐range intracortical coordination, it is hypothesized that the motor system influences metrical interpretation of sound, even in the absence of overt movement.

Rate Limits in Sensorimotor Synchronization With Auditory and Visual Sequences: The Synchronization Threshold and the Benefits and Costs of Interval Subdivision

Abstract 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.

Phase Correction, Phase Resetting, and Phase Shifts After Subliminal Timing Perturbations in Sensorimotor Synchronization

Recent studies of synchronized finger tapping have shown that perceptually subliminal phase shifts in an auditory sequence are rapidly compensated for in the motor activity (B. H. Repp, 2000a). Experiment 1 used a continuation-tapping task to confirm that this compensation is indeed a phase correction, not an adjustment of the central timekeeper period. Experiments 2-5 revealed that this phase correction occurs even when there is no ordinary sensorimotor asynchrony--when the finger taps are in antiphase or arbitrary phase relative to the auditory sequence (Experiments 2 and 3) or when the tap coinciding with the sequence phase shift is withheld (Experiments 4 and 5). The phase correction observed in the latter conditions was instantaneous, which suggests that phase resetting occurs when the motor activity is discontinuous. A prolonged phase shift suggestive of overcompensation was observed in some conditions, which poses a challenge to pure phase correction models.

Compensation for subliminal timing perturbations in perceptual-motor synchronization

Abstract 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.

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.