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Edited by Mark L. Latash

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Mark L. Latash

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Mark L. Latash

The problem of motor redundancy has been one of the fundamental, albeit elusive, problems in motor control. Traditionally, it has been viewed as a computational problem for the brain, solved with either optimization methods or by introducing additional constraints to motor tasks. This review suggests that the problem was wrongly formulated, and that the abundant degrees of freedom are not to be eliminated but used to ensure dynamic stability of motor performance, which is vital given the unpredictable intrinsic states and external forces. The idea of synergies as mechanisms ensuring action stability is introduced based on the uncontrolled manifold hypothesis and the theory of control with spatial referent coordinates. The importance of controlled stability is illustrated with the phenomena of anticipatory synergy adjustments. This approach is productive for both basic and applied fields as illustrated, in particular, by changes in motor synergies with neurological disorder and exercise.

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Edited by Mark L. Latash

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Mark L. Latash

The concept of primitives has been used in motor control both as a theoretical construct and as a means of describing the results of experimental studies involving multiple moving elements. This concept is close to Bernstein’s notion of engrams and level of synergies. Performance primitives have been explored in spaces of peripheral variables but interpreted in terms of neural control primitives. Performance primitives reflect a variety of mechanisms ranging from body mechanics to spinal mechanisms and to supraspinal circuitry. This review suggests that primitives originate at the task level as preferred time functions of spatial referent coordinates or at mappings from higher level referent coordinates to lower level, frequently abundant, referent coordinate sets. Different patterns of performance primitives can emerge depending, in particular, on the external force field.

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Mark L. Latash

Living systems may be defined as systems able to organize new, biology-specific, laws of physics and modify their parameters for specific tasks. Examples include the force-length muscle dependence mediated by the stretch reflex, and the control of movements with modification of the spatial referent coordinates for salient performance variables. Low-dimensional sets of referent coordinates at a task level are transformed to higher-dimensional sets at lower hierarchical levels in a way that ensures stability of performance. Stability of actions can be controlled independently of the actions (e.g., anticipatory synergy adjustments). Unintentional actions reflect relaxation processes leading to drifts of corresponding referent coordinates in the absence of changes in external load. Implications of this general framework for movement disorders, motor development, motor skill acquisition, and even philosophy are discussed.

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Mark L. Latash

The article offers a way to unite three recent developments in the field of motor control and coordination: (1) The notion of synergies is introduced based on the principle of motor abundance; (2) The uncontrolled manifold hypothesis is described as offering a computational framework to identify and quantify synergies; and (3) The equilibrium-point hypothesis is described for a single muscle, single joint, and multijoint systems. Merging these concepts into a single coherent scheme requires focusing on control variables rather than performance variables. The principle of minimal final action is formulated as the guiding principle within the referent configuration hypothesis. Motor actions are associated with setting two types of variables by a controller, those that ultimately define average performance patterns and those that define associated synergies. Predictions of the suggested scheme are reviewed, such as the phenomenon of anticipatory synergy adjustments, quick actions without changes in synergies, atypical synergies, and changes in synergies with practice. A few models are briefly reviewed.

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Mark L. Latash

Motor control is a young and aspiring field of natural science. Over the past 40 years, it has become an established field of study with several important theoretical developments, including the equilibrium-point hypothesis and its more recent version known as the control with referent spatial coordinates, the principle of abundance, the uncontrolled manifold hypothesis, and the concept of dynamic neural field as the means of task formulation. Important experimental advances have included the exploration of the notion of synergies, the links between descending signals from the brain and referent coordinates of the effectors, and applications of motor control principles to analysis of disordered movements. Further maturation of motor control requires focusing on theory-driven studies. It promises fruitful applications to applied fields such as movement disorders and rehabilitation.

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Mark L. Latash

The target article presents a review of the neural control of the human hand. The review emphasizes the physical approach to motor control. It focuses on such concepts as equilibrium-point control, control with referent body configurations, uncontrolled manifold hypothesis, principle of abundance, hierarchical control, multidigit synergies, and anticipatory synergy adjustments. Changes in aspects of the hand neural control with age and neurological disorder are discussed. The target article is followed by six commentaries written by Alexander Aruin, Kelly Cole, Monica Perez, Robert Sainburg, Marco Sanello, and Wei Zhang.

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Mark L. Latash

This review of movement stability, optimality, and agility is based on the theory of motor control with changes in spatial referent coordinates for the effectors, the principle of abundance, and the uncontrolled manifold hypothesis. A new optimality principle is suggested based on the concept of optimal sharing corresponding to a vector in the space of elemental variables locally orthogonal to the uncontrolled manifold. Motion along this direction is associated with minimal components along the relatively unstable directions within the uncontrolled manifold leading to a minimal motor equivalent motion. For well-practiced actions, this task-specific criterion is followed in spaces of referent coordinates. Consequences of the suggested framework include trade-offs among stability, optimality, and agility, unintentional changes in performance, hand dominance, finger specialization, individual traits in performance, and movement disorders in neurological patients.