FACILITATION OF HUMAN FIRST DORSAL INTEROSSEOUS MUSCLE RESPONSES TO TRANSCRANIAL MAGNETIC STIMULATION DURING VOLUNTARY CONTRACTION OF THE CONTRALATERAL HOMONYMOUS MUSCLE

The size of compound motor evoked potentials (cMEPs) to transcranial m agnetic stimulation of the motor cortex was measured in the relaxed fi rst dorsal interosseous muscle of the nondominant hand (ndFDI) during different levels of voluntary contraction in the homonymous muscle of the dominant hand (dFDI). cMEP responses in the ndFDI became larger wh en the dFDI was contracted to forces ranging 10-70% of maximum volunta ry contraction. Variability in the amplitude of the cMEP responses in ndFDI decreased when dFDI was contracted. Comparison with cMEPs to spi nal cord stimulation suggested a large component of the facilitation w as occurring at a cortical level. The amplitude of cMEP responses in n dFDI also increased when the tibialis anterior muscle of the leg on th e contralateral side was contracted. The observed facilitation of moto neurons during contraction of contralateral muscles might involve a tr anscallosal pathway modulating the excitability of one cortex when the other is activated. (C) 1998 John Wiley & Sons, Inc.