Trends in Cognitive Sciences
ReviewThe computational and neural basis of voluntary motor control and planning
Section snippets
Changing views on the use of sensory feedback for voluntary control
It is easy to recognize that sensory signals, such as from our eyes, skin or muscles, help us perceive our body and the world around us. It is harder to understand how these same sensory signals help guide and correct our body movements. Sensory feedback is clearly involved when someone accidentally bumps your arm at a cocktail party leading to rapid motor corrections to avoid spilling a drink held in the hand. Less obvious is whether and how sensory feedback is used continuously to adjust
Optimal Feedback Control (OFC) as a theory of voluntary control
Optimality principles have been common in motor control with various possible objectives, such as minimizing jerk, torque change, or the influence of noise [4]. In 2002, Todorov and Jordan [5] proposed OFC as a theory of voluntary control (Figure 1a). Delayed sensory feedback that makes servo-control unstable is overcome by using optimal state estimation, a Kalman filter integrating efference copy signals with delayed sensory feedback. The derivation of the optimal control policy (i.e.,
Perturbations as a probe of voluntary control
Because feedback is an essential feature of OFC, a key approach to probe control is through perturbations, either visual or mechanical. The use of mechanical perturbations is not new – this approach has been used for many decades to observe muscle stretch responses (Box 1). What OFC clarifies is that control is highly specific to the ongoing behavioural goal. Perturbations do not just elicit stereotypical motor reflexes, but also voluntary control processes directly [21]. Thus, perturbations
Spinal and transcortical feedback
As a theory to describe behaviour, OFC is agnostic as to how voluntary motor control is generated by the spinal cord and brain. The spinal cord provides the first level of feedback processing. In many species, spinal circuits support sophisticated control, such as scratching, wiping, and basic locomotor patterns 36, 61, 62. Analyses of these movements highlight many of the key characteristics predicted by OFC, notably success with variability. The spinal cord also provides phase-dependent
Mapping OFC onto brain circuits
OFC-like control is speculated as being generated by cortical and sub-cortical brain circuits 21, 80. Figure 1b provides an overview of the putative contributions of various brain regions to voluntary control. It is at best a sketch that will evolve as experimentation helps to delineate further the contribution of different brain regions to control. The three basic processes, task selection, control policy, and state estimation, are each generated by highly distributed cortical circuits. The
Concluding remarks
The use of OFC to interpret voluntary movement has emphasized the importance of sensory feedback for control. Visual and mechanical perturbations during motor behaviour constitute an important scientific approach to probe the sophistication of feedback processing, providing a window into voluntary control processes. Recent studies reveal that corrective responses are highly adaptable based on the behavioural goal and consider the many complexities inherent in multi-joint movements. The basic
Acknowledgements
This work was supported by grants from the National Science and Engineering Research Council of Canada, Canadian Institutes of Health Research (CIHR), and a GlaxoSmithKlein-CIHR chair in Neurosciences. The author would like to thank members of the Limb Lab for helpful feedback on this article.
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