Effect of conditioning transcranial stimulation on motor evoked potentials

Motor evoked potentials (MEPs) were recorded from extensor carpi radialis muscle in response to paired transcranial magnetic stimuli (interstimulus intervals (ISI) of 3 and 13 ms) with a test stimulus intensity of 120%. The intensity of the subthreshold conditioning stimulus ranged from 55% to 85% of the motor threshold. The threshold of intracortical inhibition (ICI: ISI--3 ms) was significantly lower than the threshold of intracortical facilitation (ICF: ISI--13 ms). The values of test MEP area at 3 ms ISI showed U-shaped dependence on the conditioning intensity while the values of test MEP area at 13 ms ISI recorded over the ICF threshold were augmented with increasing conditioning intensities. The changes of MEP latencies were polyphasic and in one and the same direction for ISIs of 3 and 13 ms. The results are suggesting a simultaneous action of ICF and ICI mechanisms.