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The Journal of Neuroscience, May 28, 2008, 28(22):5631-5640; doi:10.1523/JNEUROSCI.0093-08.2008

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Behavioral/Systems/Cognitive
Mechanisms Underlying Functional Changes in the Primary Motor Cortex Ipsilateral to an Active Hand

Monica A. Perez and Leonardo G. Cohen

Human Cortical Physiology Section and Stroke Neurorehabilitation Clinic, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892

Correspondence should be addressed to Dr. Leonardo G. Cohen, Human Cortical Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 Center Drive, Bethesda, MD 20892-1430. Email: cohenl{at}ninds.nih.gov

Performance of a unimanual hand motor task results in functional changes in both primary motor cortices (M1_ipsilateral and M1_{contralateral}). The neuronal mechanisms controlling the corticospinal output originated in M1_ipsilateral and the resting hand during a unimanual task remain unclear. Here, we assessed functional changes within M1_ipsilateral and in interhemispheric inhibition (IHI) associated with parametric increases in unimanual force. We measured motor-evoked potential (MEP) recruitment curves (RCs) and short-interval intracortical inhibition (SICI) in M1_ipsilateral, IHI from M1_{contralateral} to M1_ipsilateral, and the influence of IHI over SICI using transcranial magnetic stimulation at rest and during 10, 30, and 70% of maximal right wrist flexion force. EMG from the left resting flexor carpi radialis (FCR) muscle was comparable across conditions. Left FCR MEP RCs increased, and SICI decreased with increasing right wrist force. Activity-dependent (rest and 10, 30, and 70%) left FCR maximal MEP size correlated with absolute changes in SICI. IHI decreased with increasing force at matched conditioned MEP amplitudes. IHI and SICI were inversely correlated at increasing forces. In the presence of IHI, SICI decreased at rest and 70% force. In summary, we found activity-dependent changes in (1) SICI in M1_ipsilateral, (2) IHI from M1_{contralateral} to M1_ipsilateral, and (3) the influence of IHI over SICI in the left resting hand during force generation by the right hand. Our findings indicate that interactions between GABAergic intracortical circuits mediating SICI and interhemispheric glutamatergic projections between M1s contribute to control activity-dependent changes in corticospinal output to a resting hand during force generation by the opposite hand.

Key words: primary motor cortex; interhemispheric inhibition; transcallosal pathways; intracortical inhibition; transcranial magnetic stimulation; force

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Received Jan. 9, 2008; revised April 20, 2008; accepted April 20, 2008.

Correspondence should be addressed to Dr. Leonardo G. Cohen, Human Cortical Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 Center Drive, Bethesda, MD 20892-1430. Email: cohenl{at}ninds.nih.gov

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