Long-Latency Afferent Inhibition During Selective Finger Movement

1 Human Motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland; 2 Department of Neurology, Medical University of Vienna, Austria; 3 Research Unit of Exercise Science and Sports Medicine, Department of Human Biology, University of Cape Town, South Africa; and 4 Biostatistics Branch, NINDS/NIH, Bethesda, Maryland Submitted 30 March 2005; accepted in final form 19 April 2005 Stimulation of a peripheral nerve of a hand at rest modulates excitability in the motor cortex and, in particular, leads to inhibition when applied at an interval of 200 ms (long-latency afferent inhibition; LAI). Surround inhibition (SI) is the process that inhibits neighboring muscles not involved in a particular task. The neuronal mechanisms of SI are not known, and it is possible that LAI might contribute to it. Using transcranial magnetic stimulation (TMS) with and without movement of the index finger, the motor-evoked potentials (MEPs) were measured of two functionally distinct target muscles of the hand (abductor digiti minimi muscle = ADM, 1st dorsal interosseus muscle = FDI). Electrical stimulation was applied 180 ms before TMS to either the fifth finger or the index finger. Both homotopic and heterotopic finger stimulation resulted in LAI without movement. With index finger movement, motor output further decreased with homo- and heterotopic stimulation in the ADM. In the moving FDI, however, there was no change with either homo- or heterotopic stimulation. Additionally, in the unstimulated movement trials, LAI increased with the amount of unintentional co-activation that occurred despite attempts to maintain the ADM at rest. However, with finger stimulation added, there were almost no increased MEPs despite co-activation. These findings suggest that LAI increases during movement and can enhance SI. Address for reprint requests and other correspondence: M. Hallett, Human Motor Control Section, NINDS/NIH, Bldg. 10/5N226, 10 Center Dr., Bethesda, MD 20892 (E-mail: hallettm{at}ninds.nih.gov )