Sensorimotor synchronization (SMS) is the coordination of rhythmic movement with an external rhythm, ranging from finger tapping in time with a metronome to musical ensemble performance. An earlier review (Repp, 2005) covered tapping studies; two additional reviews (Repp, 2006a, b) focused on music performance and on rate limits of SMS, respectively. The present article supplements and extends these earlier reviews by surveying more recent research in what appears to be a burgeoning field. The article comprises four parts, dealing with (1) conventional tapping studies, (2) other forms of moving in synchrony with external rhythms (including dance and nonhuman animals’ synchronization abilities), (3) interpersonal synchronization (including musical ensemble performance), and (4) the neuroscience of SMS. It is evident that much new knowledge about SMS has been acquired in the last 7 years.

/pdf/sensorimotor-synchronization-a-review-of-recent-research-ymvlozhb1e.pdf

Sensorimotor synchronization: A review of recent research (2006–2012)

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Detection, Estimation, and Modulation Theory

Deciding when and whether to move is critical for survival. Loss of dopamine neurons (DANs) of the substantia nigra pars compacta (SNc) in patients with Parkinson's disease causes deficits in movement initiation and slowness of movement. The role of DANs in self-paced movement has mostly been attributed to their tonic activity, whereas phasic changes in DAN activity have been linked to reward prediction. This model has recently been challenged by studies showing transient changes in DAN activity before or during self-paced movement initiation. Nevertheless, the necessity of this activity for spontaneous movement initiation has not been demonstrated, nor has its relation to initiation versus ongoing movement been described. Here we show that a large proportion of SNc DANs, which did not overlap with reward-responsive DANs, transiently increased their activity before self-paced movement initiation in mice. This activity was not action-specific, and was related to the vigour of future movements. Inhibition of DANs when mice were immobile reduced the probability and vigour of future movements. Conversely, brief activation of DANs when mice were immobile increased the probability and vigour of future movements. Manipulations of dopamine activity after movement initiation did not affect ongoing movements. Similar findings were observed for the initiation and execution of learned action sequences. These findings causally implicate DAN activity before movement initiation in the probability and vigour of future movements.

Dopamine neuron activity before action initiation gates and invigorates future movements

In 1988 we introduced impulsive testing of semicircular canal function measured with scleral search coils and showed that it could accurately and reliably detect impaired function even of a single lateral canal Later we showed that it was also possible to test individual vertical canal function in peripheral and also in central vestibular disorders, and proposed a physiological mechanism for why this might be so For the next 20 years, between 1988 and 2008, impulsive testing of individual semicircular canal function could only be accurately done by a few aficionados with the time and money to support scleral search coil systems – an expensive, complicated and cumbersome, semi-invasive technique that never made the transition from the research lab to the dizzy clinic Then in 2009 and 2013 we introduced a video method of testing function of each of the 6 canals individually Since 2009 the method has been taken up by most dizzy clinics around the world, with now close to 100 refereed articles in PubMed In many dizzy clinics around the world video Head Impulse Testing has supplanted caloric testing as the initial and in some cases the final test of choice in patients with suspected vestibular disorders Here we consider 7 current, interesting and controversial aspects of video Head Impulse Testing: 1 Introduction to the test; 2 the progress from the head impulse protocol (HIMPs) to the new variant – suppression head impulse protocol (SHIMPs); 3 The physiological basis for head impulse testing; 4 Practical aspects and potential pitfalls of video Head Impulse Testing; 5 Problems of vestibulo-ocular reflex gain calculations; 6 Head impulse testing in central vestibular disorders, and 7 To stay right up-to-date – new clinical disease patterns emerging from video head impulse testing With thanks and appreciation we dedicate this article to our friend, colleague and mentor, Dr Bernard Cohen of Mount Sinai Medical School, New York, who since his first article 55 years ago on compensatory eye movements induced by vertical semicircular canal stimulation, has become one of the giants of the vestibular world

/pdf/the-video-head-impulse-test-2c10fcvqqc.pdf

The Video Head Impulse Test.

Motor learning can be framed theoretically as a problem of optimizing a movement policy in a potentially uncertain or changing environment. This is precisely the general problem studied in the field of reinforcement learning. Reinforcement learning theory proposes two distinct approaches to solving this general problem: Model-based approaches first identify the dynamics of the task or environment then use this knowledge to compute the optimal movement policy. Model-free approaches, by contrast, directly identify successful policies through a process of trial and error. Here, we review existing literature on motor control in the light of this distinction. Motor learning research in the last decade has been dominated by studies that elicit learning through adaptation paradigms and find the results to be consistent with a model-based framework. Studying the behavior of patients in such adaptation paradigms has implicated the cerebellum as prime candidate for the neural substrate of the internal models that sub serve model-based control. A growing body of experimental results, however, demonstrates that not all of motor learning in conventional paradigms can be explained within model-based frameworks, but can be understood in terms of an additional component of learning driven by model-free reinforcement of successful actions. We conclude that the brain maintains distinct model-based and model-free learning systems, with distinct neural substrates, which act in competitive balance to direct behavior.

/pdf/model-based-and-model-free-mechanisms-of-human-motor-1fl71gjb5d.pdf

Model-Based and Model-Free Mechanisms of Human Motor Learning

Neural systems that control movement maintain accuracy by adaptively altering motor commands in response to errors. It is often assumed that the error signal that drives adaptation is equivalent to the sensory error observed at the conclusion of a movement; for saccades, this is typically the visual (retinal) error. However, we instead propose that the adaptation error signal is derived as the difference between the observed visual error and a realistic prediction of movement outcome. Using a modified saccade-adaptation task in human subjects, we precisely controlled the amount of error experienced at the conclusion of a movement by back-stepping the target so that the saccade is hypometric (positive retinal error), but less hypometric than if the target had not moved (smaller retinal error than expected). This separates prediction error from both visual errors and motor corrections. Despite positive visual errors and forward-directed motor corrections, we found an adaptive decrease in saccade amplitudes, a finding that is well-explained by the employment of a prediction-based error signal. Furthermore, adaptive changes in movement size were linearly correlated to the disparity between the predicted and observed movement outcomes, in agreement with the forward-model hypothesis of motor learning, which states that adaptation error signals incorporate predictions of motor outcomes computed using a copy of the motor command (efference copy).

https://www.physiology.org/doi/pdf/10.1152/jn.00394.2010

Sensorimotor adaptation error signals are derived from realistic predictions of movement outcomes.

In an environment full of potential goals, how does the brain determine which movement to execute? Existing theories posit that the motor system prepares for all potential goals by generating several motor plans in parallel. One major line of evidence for such theories is that presenting two competing goals often results in a movement intermediate between them. These intermediate movements are thought to reflect an unintentional averaging of the competing plans. However, normative theories suggest instead that intermediate movements might actually be deliberate, generated because they improve task performance over a random guessing strategy. To test this hypothesis, we vary the benefit of making an intermediate movement by changing movement speed. We find that participants generate intermediate movements only at (slower) speeds where they measurably improve performance. Our findings support the normative view that the motor system selects only a single, flexible motor plan, optimized for uncertain goals. It is thought that, when goals are uncertain, actions are generated by averaging multiple possible movement plans. Here the authors show that movement planning under uncertainty instead varies flexibly depending on the speed of the movement in order to maximize success.

/pdf/motor-planning-flexibly-optimizes-performance-under-q2p5j8w782.pdf

Motor planning flexibly optimizes performance under uncertainty about task goals.

ObjectiveTo describe vestibulo-ocular function in the immediate postoperative period after unilateral vestibular deafferentation from vestibular schwannoma resection.Study DesignProspective longitudinal study.SettingTertiary medical center.PatientsFive patients who underwent vestibular schwannoma re

Early adaptation and compensation of clinical vestibular responses after unilateral vestibular deafferentation surgery.

Motor skill learning involves a practice-induced improvement in the speed and/or accuracy of a discrete movement. It is often thought that paradigms involving repetitive practice of discrete movements performed in a fixed sequence result in a further enhancement of skill beyond practice of the individual movements in a random order. Sequence-specific performance improvements could, however, arise without practice as a result of knowledge of the sequence order; knowledge could operate by either enabling advanced motor planning of the known sequence elements or by increasing overall motivation. Here, we examined how knowledge and practice contribute to performance of a sequence of movements. We found that explicit knowledge provided through instruction produced practice-independent improvements in reaction time and execution quality. These performance improvements occurred even for random elements within a partially known sequence, indicative of a general motivational effect rather than a sequence-specific effect of advanced planning. This motivational effect suggests that knowledge influences performance in a manner analogous to reward. Additionally, practice led to similar improvements in execution quality for both known and random sequences. The lack of interaction between knowledge and practice suggests that any skill acquisition occurring during discrete sequence tasks arises solely from practice of the individual movement elements, independent of their order. We conclude that performance improvements in discrete sequence tasks arise from the combination of knowledge-based motivation and sequence-independent practice; investigating this interplay between cognition and movement may facilitate a greater understanding of the acquisition of skilled behavior.

/pdf/explicit-knowledge-enhances-motor-vigor-and-performance-40mi5ccqoz.pdf

Explicit knowledge enhances motor vigor and performance: motivation versus practice in sequence tasks.

The mechanisms by which the human brain controls eye movements are reasonably well understood, but those for the head less so. Here, we show that the mechanisms for keeping the head aimed at a stationary target follow strategies similar to those for holding the eyes steady on stationary targets. Specifically, we applied the neural integrator hypothesis that originally was developed for holding the eyes still in eccentric gaze positions to describe how the head is held still when turned toward an eccentric target. We found that normal humans make head movements consistent with the neural integrator hypothesis, except that additional sensory feedback is needed, from proprioceptors in the neck, to keep the head on target. We also show that the complicated patterns of head movements in patients with cervical dystonia can be predicted by deficits in a neural integrator for head motor control. These results support ideas originally developed from animal studies that suggest fundamental similarities between oculomotor and cephalomotor control, as well as a conceptual framework for cervical dystonia that departs considerably from current clinical views.

https://www.jneurosci.org/content/jneuro/33/27/11281.full.pdf

Aaron L. Wong

Papers

Sensorimotor adaptation error signals are derived from realistic predictions of movement outcomes.

Motor planning flexibly optimizes performance under uncertainty about task goals.

Early adaptation and compensation of clinical vestibular responses after unilateral vestibular deafferentation surgery.

Explicit knowledge enhances motor vigor and performance: motivation versus practice in sequence tasks.

Keeping Your Head On Target