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The Journal of Neuroscience, May 17, 2006, 26(20):5574-5581; doi:10.1523/JNEUROSCI.5332-05.2006

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Behavioral/Systems/Cognitive
Linear Summation of Cat Motor Cortex Outputs

Christian Ethier,¹ Laurent Brizzi,¹ Warren G. Darling,² and Charles Capaday¹

¹Department of Anatomy and Physiology, Faculty of Medicine, Université Laval, Québec, Québec, Canada G1J 2G3, and ²Department of Exercise Science, The University of Iowa, Iowa City, Iowa 52242

Correspondence should be addressed to Dr. C. Capaday, Centre de Recherche Université Laval Robert Giffard, Brain and Movement Laboratory, F-6500, 2601 de la Canardière, Québec, Québec, Canada G1J 1G3. Email: charles.capaday{at}anm.ulaval.ca

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 µA 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 GABA_A 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.

Key words: population vector hypothesis; motor cortex; cortical circuits; directional motor control; motor cortical function; neuronal ensembles

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Received Dec. 14, 2005; revised April 12, 2006; accepted April 14, 2006.

Correspondence should be addressed to Dr. C. Capaday, Centre de Recherche Université Laval Robert Giffard, Brain and Movement Laboratory, F-6500, 2601 de la Canardière, Québec, Québec, Canada G1J 1G3. Email: charles.capaday{at}anm.ulaval.ca

This article has been cited by other articles:

				A. d'Avella, L. Fernandez, A. Portone, and F. Lacquaniti Modulation of Phasic and Tonic Muscle Synergies With Reaching Direction and Speed J Neurophysiol, September 1, 2008; 100(3): 1433 - 1454. [Abstract] [Full Text] [PDF]

				B. R. Townsend, L. Paninski, and R. N. Lemon Linear Encoding of Muscle Activity in Primary Motor Cortex and Cerebellum J Neurophysiol, November 1, 2006; 96(5): 2578 - 2592. [Abstract] [Full Text] [PDF]

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