Mark T. Elliott

Being discrete helps keep to the beat

Synchronising our actions with external events is a task we perform without apparent effort. Its foundation relies on accurate temporal control that is widely accepted to take one of two different modes of implementation: explicit timing for discrete actions and implicit timing for smooth continuous movements. Here we assess synchronisation performance for different types of action and test the degree to which each action supports corrective updating following changes in the environment. Participants performed three different finger actions in time with an auditory pacing stimulus allowing us to assess synchronisation performance. Presenting a single perturbation to the otherwise regular metronome allowed us to examine corrections supported by movements varying in their mode of timing implementation. We find that discrete actions are less variable and support faster error correction. As such, discrete actions may be preferred when engaging in time-critical adaptive behaviour with people and objects in a dynamic environment.

Multisensory cues improve sensorimotor synchronisation

Synchronising movements with events in the surrounding environment is an ubiquitous aspect of everyday behaviour. Often, information about a stream of events is available across sensory modalities. While it is clear that we synchronise more accurately to auditory cues than other modalities, little is known about how the brain combines multisensory signals to produce accurately timed actions. Here, we investigate multisensory integration for sensorimotor synchronisation. We extend the prevailing linear phase correction model for movement synchronisation, describing asynchrony variance in terms of sensory, motor and timekeeper components. Then we assess multisensory cue integration, deriving predictions based on the optimal combination of event time, defined across different sensory modalities. Participants tapped in time with metronomes presented via auditory, visual and tactile modalities, under either unimodal or bimodal presentation conditions. Temporal regularity was manipulated between modalities by applying jitter to one of the metronomes. Results matched the model predictions closely for all except high jitter level conditions in audio–visual and audio–tactile combinations, where a bias for auditory signals was observed. We suggest that, in the production of repetitive timed actions, cues are optimally integrated in terms of both sensory and temporal reliability of events. However, when temporal discrepancy between cues is high they are treated independently, with movements timed to the cue with the highest sensory reliability.

MatTAP: A MATLAB toolbox for the control and analysis of movement synchronisation experiments

Investigating movement timing and synchronisation at the sub-second range relies on an experimental setup that has high temporal fidelity, is able to deliver output cues and can capture corresponding responses. Modern, multi-tasking operating systems make this increasingly challenging when using standard PC hardware and programming languages. This paper describes a new free suite of tools (available from http://www.snipurl.com/mattap) for use within the MATLAB programming environment, compatible with Microsoft Windows and a range of data acquisition hardware. The toolbox allows flexible generation of timing cues with high temporal accuracy, the capture and automatic storage of corresponding participant responses and an integrated analysis module for the rapid processing of results. A simple graphical user interface is used to navigate the toolbox and so can be operated easily by users not familiar with programming languages. However, it is also fully extensible and customisable, allowing adaptation for individual experiments and facilitating the addition of new modules in future releases. Here we discuss the relevance of the MatTAP (MATLAB Timing Analysis Package) toolbox to current timing experiments and compare its use to alternative methods. We validate the accuracy of the analysis module through comparison to manual observation methods and replicate a previous sensorimotor synchronisation experiment to demonstrate the versatility of the toolbox features demanded by such movement synchronisation paradigms.

Moving in time: Bayesian causal inference explains movement coordination to auditory beats

Many everyday skilled actions depend on moving in time with signals that are embedded in complex auditory streams (e.g. musical performance, dancing or simply holding a conversation). Such behaviour is apparently effortless; however, it is not known how humans combine auditory signals to support movement production and coordination. Here, we test how participants synchronize their movements when there are potentially conflicting auditory targets to guide their actions. Participants tapped their fingers in time with two simultaneously presented metronomes of equal tempo, but differing in phase and temporal regularity. Synchronization therefore depended on integrating the two timing cues into a single-event estimate or treating the cues as independent and thereby selecting one signal over the other. We show that a Bayesian inference process explains the situations in which participants choose to integrate or separate signals, and predicts motor timing errors. Simulations of this causal inference process demonstrate that this model provides a better description of the data than other plausible models. Our findings suggest that humans exploit a Bayesian inference process to control movement timing in situations where the origin of auditory signals needs to be resolved.

Stepping to phase-perturbed metronome cues: multisensory advantage in movement synchrony but not correction

Humans can synchronize movements with auditory beats or rhythms without apparent effort. This ability to entrain to the beat is considered automatic, such that any perturbations are corrected for, even if the perturbation was not consciously noted. Temporal correction of upper limb (e.g., finger tapping) and lower limb (e.g., stepping) movements to a phase perturbed auditory beat usually results in individuals being back in phase after just a few beats. When a metronome is presented in more than one sensory modality, a multisensory advantage is observed, with reduced temporal variability in finger tapping movements compared to unimodal conditions. Here, we investigate synchronization of lower limb movements (stepping in place) to auditory, visual and combined auditory-visual (AV) metronome cues. In addition, we compare movement corrections to phase advance and phase delay perturbations in the metronome for the three sensory modality conditions. We hypothesized that, as with upper limb movements, there would be a multisensory advantage, with stepping variability being lowest in the bimodal condition. As such, we further expected correction to the phase perturbation to be quickest in the bimodal condition. Our results revealed lower variability in the asynchronies between foot strikes and the metronome beats in the bimodal condition, compared to unimodal conditions. However, while participants corrected substantially quicker to perturbations in auditory compared to visual metronomes, there was no multisensory advantage in the phase correction task—correction under the bimodal condition was almost identical to the auditory-only (AO) condition. On the whole, we noted that corrections in the stepping task were smaller than those previously reported for finger tapping studies. We conclude that temporal corrections are not only affected by the reliability of the sensory information, but also the complexity of the movement itself.

Interaction between interpersonal and postural coordination during frequency scaled rhythmic sway: the role of dance expertise

Light fingertip touch between partners swaying rhythmically side by side evokes interpersonal synchrony. In non-dancers and dancers swaying to a metronome, we examined the effects of frequency scaling and touch between the partners on both postural (ankle-hip) and inter-personal coordination. In both groups, touch did not interfere with the ankle-hip coordination. In non-dancers but not dancers, increasing frequency resulted in a loss of the ankle-hip coupling that was accompanied by a reduction of the touch mediated interpersonal synchrony. It is suggested that the effect of touch on interpersonal synchrony depends on the reliability of the haptic information sensed at the fingertip and assumes an in phase ankle-hip coupling. These findings have implications in clinical practice when using touch to help balance impaired individuals.

Modelling single-person and multi-person event-based synchronisation

A linear phase correction model has been shown to accurately reflect the corrective processes involved in synchronising motor actions to an external rhythmic cue. The model originated from studies of finger tapping to an isochronous metronome beat and is based on the time series of asynchronies between the metronome and corresponding finger tap onsets, along with their associated intervals. Over recent years the model has evolved and been applied to more complex scenarios, including phase perturbed cues, tempo variations and, most recently, timing within groups. Here, we review the studies that have contributed to the development of the linear phase correction model and the associated findings related to human timing performance. The review provides a background to the studies examining single-person timing to simple metronome cues. We then further expand on the more complex analyses of motor timing to phase and tempo shifted cues. Finally, recent studies investigating inter-personal synchronisation between groups of two or more individuals are discussed, along with a brief overview on the implications of these studies for social interactions. We conclude with a discussion on future areas of research that will be important for understanding corrective timing processes between people.

A smart sensing platform for the classification of ambulatory patterns

Abstract This paper describes an innovative sensing approach allowing capture, discrimination, and classification of transients automatically in gait. A walking platform is described, which offers an alternative design to that of standard force plates with advantages that include mechanical simplicity and less restriction on dimensions. The scope of the work is to investigate as an experiment the sensitivity of the distributive tactile sensing method with the potential to address flexibility on gait assessment, including patient targeting and the extension to a variety of ambulatory applications. Using infrared sensors to measure plate deflection, gait patterns are compared with stored templates using a pattern recognition algorithm. This information is input into a neural network to classify normal and affected walking events, with a classification accuracy of just under 90 per cent achieved. The system developed has potential applications in gait analysis and rehabilitation, whereby it can be used as a tool for early diagnosis of walking disorders or to determine changes between pre- and post-operative gait.

Quantifying sway through surface deflection patterns : a novel approach using distributive tactile sensing

Abstract This paper describes an experiment that extends the distributive sensing approach to identify the three-dimensional location of an object in constant motion. Distributive sensing has previously been successful in the identification of size and location of statically placed objects. Here, a novel system is developed to measure balance or sway in patients. The experimental set-up consisted of a pendulum structure positioned on a supported steel plate. Three low-cost deflection sensors were positioned under the plate with the resulting signals used as inputs to a neural network implemented on a field-programmable gate array. The results show that the embedded system can accurately track the pendulum position in real time with a mean tracking error of around 6 per cent in all three dimensions. This evidence indicates that the technique is sufficiently sensitive and could be implemented in a pragmatic configuration for discriminating between balance and sway.

Cue properties change timing strategies in group movement synchronisation

To maintain synchrony in group activities, each individual within the group must continuously correct their movements to remain in time with the temporal cues available. Cues might originate from one or more members of the group. Current research suggests that when synchronising movements, individuals optimise their performance in terms of minimising variability of timing errors (asynchronies) between external cues and their own movements. However, the cost of this is an increase in the timing variability of their own movements. Here we investigate whether an individual’s timing strategy changes according to the task, in a group scenario. To investigate this, we employed a novel paradigm that positioned six individuals to form two chains with common origin and termination on the circumference of a circle. We found that participants with access to timing cues from only one other member used a strategy to minimise their asynchrony variance. In contrast, the participant at the common termination of the two chains, who was required to integrate timing cues from two members, used a strategy that minimised movement variability. We conclude that humans are able to flexibly switch timekeeping strategies to maintain task demands and thus optimise the temporal performance of their movements.