CROSSED AND DIRECT EFFECTS OF DIGITAL NERVES STIMULATION ON MOTOR EVOKED-POTENTIAL - A STUDY WITH MAGNETIC BRAIN-STIMULATION

We studied the influence of contralateral and ipsilateral cutaneous di gital nerve stimulation on mater evoked potentials (MEPs) elicited in hand muscles by transcranial magnetic stimulation (TMS). We tested the effect of different magnetic stimulus intensities on MEPs recorded fr om the thenar eminence (TE) muscles of the right hand while an electri cal conditioning stimulus was delivered to the second finger of the sa me band with an intensity four times above the sensory threshold. Ampl itude decrement of conditioned MEPs as a function of magnetic stimulus intensity was observed. The lowest TMS stimulus intensity produced th e largest decrease in conditioned MEPs. Moreover, we investigated the effects of ipsilateral and contralateral electrical digital stimulatio n on MEPs elicited in the right TE and biceps muscle using an intensit y 10% above the threshold. Marked MEP inhibition in TE muscles followi ng bath ipsilateral and contralateral digital stimulation is the main finding of this study. The decrease in conditioned MEPs amplitude to i psilateral stimulation reached a level of 50% of unconditioned MEP amp litude with the circular coil and 30% with the focal coil. The amplitu de-of conditioned MEPs to contralateral digital stimulation showed a d ecrease of 60% with the circular coil and more than 50% with the focal coil, The onset of the inhibitory effect of contralateral stimulation using the focal coil occurred at a mean of 15 ms later than that of i psilateral stimulation, No MEP inhibition was observed when recording from proximal muscles. Ipsilateral and contralateral digital stimulati on had no effect on F wave at appropriate interstimulus intervals, whe re the main MEP suppression was noted, We stress the importance of sel ecting an appropriate test stimulus intensity to evaluate MEP inhibiti on by digital nerves stimulation. Spinal and cortical sites of sensori motor integration are adduced to explain the direct and crossed MEP in hibition following digital nerves stimulation. (C) 1997 Elsevier Scien ce Ireland Ltd.