Linear summation of cat motor cortex outputs

J Neurosci. 2006 May 17;26(20):5574-81. doi: 10.1523/JNEUROSCI.5332-05.2006.


Recruitment of movement-related muscle synergies involves the functional linking of motor cortical points. We asked how the outputs of two simultaneously stimulated motor cortical points would interact. To this end, experiments were done in ketamine-anesthetized cats. When prolonged (e.g., 500 ms) trains of intracortical microstimulation were applied in the primary motor cortex, stimulus currents as low as 10-20 microA evoked coordinated movements of the contralateral forelimb. Paw kinematics in three dimensions and the electromyographic (EMG) activity of eight muscles were simultaneously recorded. We show that the EMG outputs of two cortical points simultaneously stimulated are additive. The movements were represented as displacement vectors pointing from initial to final paw position. The displacement vectors resulting from simultaneous stimulation of two cortical points pointed in nearly the same direction as the algebraic resultant vector. Linear summation of outputs was also found when inhibition at one of the cortical points was reduced by GABAA receptor antagonists. A simple principle emerges from these results. Notwithstanding the underlying complex neuronal circuitry, motor cortex outputs combine nearly linearly in terms of movement direction and muscle activation patterns. Importantly, simultaneous activation does not change the nature of the output at each point. An additional implication is that not all possible movements need be explicitly represented in the motor cortex; a large number of different movements may be synthesized from a smaller repertoire.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Algorithms
  • Animals
  • Biomechanical Phenomena / methods
  • Brain Mapping
  • Cats
  • Efferent Pathways / drug effects
  • Efferent Pathways / physiology*
  • Electric Stimulation
  • Electromyography / methods
  • Evoked Potentials, Motor / drug effects
  • Evoked Potentials, Motor / physiology
  • Forelimb / innervation
  • Forelimb / physiology
  • GABA Antagonists / pharmacology
  • GABA-A Receptor Antagonists
  • Linear Models
  • Male
  • Motor Cortex / drug effects
  • Motor Cortex / physiology*
  • Motor Neurons / drug effects
  • Motor Neurons / physiology*
  • Movement / drug effects
  • Movement / physiology*
  • Muscle Contraction / physiology
  • Muscle, Skeletal / innervation
  • Muscle, Skeletal / physiology*
  • Nerve Net / drug effects
  • Nerve Net / physiology
  • Neural Inhibition / drug effects
  • Neural Inhibition / physiology
  • Receptors, GABA-A / metabolism
  • Synaptic Transmission / physiology
  • gamma-Aminobutyric Acid / metabolism


  • GABA Antagonists
  • GABA-A Receptor Antagonists
  • Receptors, GABA-A
  • gamma-Aminobutyric Acid