Transcranial paired - Pulse magnetic stimulation for assessing corticocortical inhibition and facilitation

Paired-pulse transcranial magnetic stimulation (TMS) may be used to evaluat e intracortical inhibitory and facilitatory mechanisms non-invasively. The subthreshold conditioning transcranial magnetic stimulus is followed by a s uprathreshold test stimulus to evoke MEPs (motor evoked potentials). Short interstimulus-intervals (ISI < 6 ms) are able to inhibit MEPs whereas ISI b etween 6 and 20 ms may facilitate MEPs. Methods: We examined 10 healthy vol unteers (mean age 35 years) using a paired-pulse paradigm (ISI 3 vs. 13 ms) . The conditioning stimulus was 60, 70, or 80%, and the test stimulus 120 % of motor threshold. Results: With a conditioning stimulus of 70 % and an I SI of 3 ms MEPs were depressed to 64% of baseline (p < 0.01), whereas a fac ilitation of 150% (p < 0.05) could be observed with an ISI of 13 ms. Discus sion: We conclude that these parameters are suitable to evaluate intracorti cal inhibitory and facilitatory mechanisms. The paradigm may be used to exa mine cortical excitability in a variety of neurological diseases.