Abstract
The central nervous system (CNS) may produce coordinated motor outputs via the combination of motor modules representable as muscle synergies. Identification of muscle synergies has hitherto relied on applying factorization algorithms to multimuscle electromyographic data (EMGs) recorded during motor behaviors. Recent studies have attempted to validate the neural basis of the muscle synergies identified by independently retrieving the muscle synergies through CNS manipulations and analytic techniques such as spike-triggered averaging of EMGs. Experimental data have demonstrated the pivotal role of the spinal premotor interneurons in the synergies' organization and the presence of motor cortical loci whose stimulations offer access to the synergies, but whether the motor cortex is also involved in organizing the synergies has remained unsettled. We argue that one difficulty inherent in current approaches to probing the synergies' neural basis is that the EMG generative model based on linear combination of synergies and the decomposition algorithms used for synergy identification are not grounded on enough prior knowledge from neurophysiology. Progress may be facilitated by constraining or updating the model and algorithms with knowledge derived directly from CNS manipulations or recordings. An investigative framework based on evaluating the relevance of neurophysiologically constrained models of muscle synergies to natural motor behaviors will allow a more sophisticated understanding of motor modularity, which will help the community move forward from the current debate on the neural versus nonneural origin of muscle synergies.
Highlights
In one of his first contributions to the philosophy of the brain, Roger Sperry [1] argued eloquently that “the principal function of the nervous system is the coordinated innervation of the musculature. . .[and] the sole product of brain function is motor coordination” (p. 297)
A muscle synergy identified by the algorithm may or may not correspond to a motor module, which refers to the hypothesized discrete neuromotor control unit that functions as a neural constraint on motor output
We have reviewed recent works that address the neural basis of muscle synergies by correlating outputs from the neural and muscular domains, but other studies have attempted to establish a causal relationship between a specific neural structure and the organization of certain muscle synergies by evaluating how the synergies are altered by manipulating the neural structure
Summary
In one of his first contributions to the philosophy of the brain, Roger Sperry [1] argued eloquently that “the principal function of the nervous system is the coordinated innervation of the musculature. . .[and] the sole product of brain function is motor coordination” (p. 297). In SpTA, activities of CNS neurons and the EMGs from multiple muscles are recorded while the subject performs a motor behavior.
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