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The Journal of Neuroscience, October 8, 2008, 28(41):10363-10369; doi:10.1523/JNEUROSCI.3564-08.2008

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Behavioral/Systems/Cognitive
Short Intracortical and Surround Inhibition Are Selectively Reduced during Movement Initiation in Focal Hand Dystonia

Sandra Beck,^1,2 Sarah Pirio Richardson,^1,3 Ejaz A. Shamim,¹ Nguyet Dang,¹ Martin Schubert,⁴ and Mark Hallett¹

¹Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892-1428, ²Department of Clinical Neurology and Neurophysiology, University of Freiburg, D-70196 Freiburg, Germany, ³Department of Neurology, University of New Mexico, Albuquerque, New Mexico 87106, and ⁴Spinal Cord Injury Centre, University Hospital Balgrist, 8008 Zurich, Switzerland

Correspondence should be addressed to Dr. Sandra Beck, Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Building 10/5N240, 10 Center Drive, Bethesda, MD 20892-1428. Email: becksa{at}ninds.nih.gov

In patients with focal hand dystonia (FHD), pathological overflow activation occurs in muscles not involved in the movement. Surround inhibition is a neural mechanism that can sharpen desired movement by inhibiting unwanted movement in adjacent muscles. To further establish the phenomenon of surround inhibition and to determine whether short intracortical inhibition (SICI) reflecting inhibition from the local interneurons in primary motor cortex (M1), might play a role in its genesis, single- and paired-pulse transcranial magnetic stimulation (TMS), and Hoffmann reflex testing were applied to evaluate the excitability of the relaxed abductor pollicis brevis muscle (APB) at various intervals during a movement of the index finger in 16 patients with FHD and 20 controls. Whereas controls showed inhibition of APB motor-evoked potential (MEP) size during movement initiation and facilitation of APB MEP size during the maintenance phase, FHD patients did not modulate APB MEP size. In contrast, SICI remained constant in controls, but FHD patients showed reduced SICI during movement initiation. The H_max/M_max ratio in control subjects increased during movement initiation. The results provide additional evidence for the presence of surround inhibition in M1, where it occurs only during movement initiation, indicating that different mechanisms underlie movement initiation and maintenance. Thus, surround inhibition is sculpted both in time and space and may be an important neural mechanism during movement initiation to counteract increased spinal excitability. SICI may contribute to its generation, because in patients with FHD, the lack of depression of APB MEP size is accompanied by a reduction in SICI.

Key words: dystonia; motor control; motor cortex; inhibition; human; coordination

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Received July 29, 2008; revised Aug. 27, 2008; accepted Aug. 28, 2008.

Correspondence should be addressed to Dr. Sandra Beck, Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Building 10/5N240, 10 Center Drive, Bethesda, MD 20892-1428. Email: becksa{at}ninds.nih.gov

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