title: Representation of Muscle Synergies in the Primate Brain.
abstract: Evidence suggests that the CNS uses motor primitives to simplify movement control, but whether it actually stores primitives instead of computing solutions on the fly to satisfy task demands is a controversial and still-unanswered possibility. Also in contention is whether these primitives take the form of time-invariant muscle coactivations ("spatial" synergies) or time-varying muscle commands ("spatiotemporal" synergies). Here, we examined forelimb muscle patterns and motor cortical spiking data in rhesus macaques (Macaca mulatta) handling objects of variable shape and size. From these data, we extracted both spatiotemporal and spatial synergies using non-negative decomposition. Each spatiotemporal synergy represents a sequence of muscular or neural activations that appeared to recur frequently during the animals' behavior. Key features of the spatiotemporal synergies (including their dimensionality, timing, and amplitude modulation) were independently observed in the muscular and neural data. In addition, both at the muscular and neural levels, these spatiotemporal synergies could be readily reconstructed as sequential activations of spatial synergies (a subset of those extracted independently from the task data), suggestive of a hierarchical relationship between the two levels of synergies. The possibility that motor cortex may execute even complex skill using spatiotemporal synergies has novel implications for the design of neuroprosthetic devices, which could gain computational efficiency by adopting the discrete and low-dimensional control that these primitives imply.
  
  
  CONCLUSIONS
  We studied the motor cortical and forearm muscular activity of rhesus macaques (Macaca mulatta) as they reached, grasped, and carried objects of varied shape and size. We applied non-negative matrix factorization separately to the cortical and muscular data to reduce their dimensionality to a smaller set of time-varying "spatiotemporal" synergies. Each synergy represents a sequence of cortical or muscular activity that recurred frequently during the animals' behavior. Salient features of the synergies (including their dimensionality, timing, and amplitude modulation) were observed at both the cortical and muscular levels. The possibility that the brain may execute even complex behaviors using spatiotemporal synergies has implications for neuroprosthetic algorithm design, which could become more computationally efficient by adopting the discrete and low-dimensional control that they afford.
authors.affiliation: Department of Electrical Engineering and Computer Sciences, overduin@mit.edu.
authors.forename: Simon A
authors.identifier: http://orcid.org/0000-0003-0903-9473
authors.initials: SA
authors.lastname: Overduin
authors.affiliation: Department of Biomedical Sciences and Morphological and Functional Images, University of Messina, 98125 Messina, Italy, Laboratory of Neuromotor Physiology, Santa Lucia Foundation, 00179 Rome, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: d'Avella
authors.affiliation: Department of Kinesiology, Temple University, Philadelphia, Pennsylvania 19122, Department of Physical Medicine and Rehabilitation, Northwestern University, Chicago, Illinois 60611, and.
authors.forename: Jinsook
authors.initials: J
authors.lastname: Roh
authors.affiliation: Department of Electrical Engineering and Computer Sciences, Helen Wills Neuroscience Institute, and University of California Berkeley-University of California San Francisco Joint Graduate Group in Bioengineering, University of California, Berkeley, California 94720.
authors.forename: Jose M
authors.initials: JM
authors.lastname: Carmena
authors.affiliation: Department of Brain and Cognitive Sciences and McGovern Institute for Brain Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
authors.forename: Emilio
authors.initials: E
authors.lastname: Bizzi
mesh_terms: D000200:Action Potentials; D000818:Animals; D001132:Arm / Q000502:physiology*; D001522:Behavior, Animal / Q000502:physiology*; D001931:Brain Mapping; D003399:Craniotomy; D004558:Electric Stimulation; D004567:Electrodes, Implanted; D004576:Electromyography; D005260:Female; D018737:Hand Strength / Q000502:physiology; D008253:Macaca mulatta / Q000502:physiology*; D008297:Male; D008839:Microelectrodes; D009043:Motor Activity / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology*; D009068:Movement / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D013997:Time Factors
keywords: cortex; grasp; hand; motor; movement; muscle