Individualized Brain Inhibition and Excitation Profile in Response to Paired-Pulse TMS

Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) are generated from paired-pulse transcranial magnetic stimulations (ppTMS) using certain interstimulus intervals (ISIs). ppTMS provides an accessible technique to evaluate inhibitory and facilitatory motor neural circuits. However, SICI and ICF are highly variable such that individual variability is not captured by any one static ISI. The authors hypothesized that individuals may have individualized and relatively stable pattern of SICI-ICF profiles. They tested SICI and ICF profiles using ISIs from 1 to 500ms, on 2 occasions about 3 weeks apart, and the test-retest reliability, in 23 healthy controls. Moderate-to-good test-retest reliabilities were found at ppTMS with 1 and 3ms ISIs (SICI) and with 12, 15, 18, and 21ms ISIs (ICF), but not with other control ISIs. A similar pattern of results was obtained for men and women. Interestingly, the peak facilitation, peak inhibition, and maximum inhibition and facilitation ranges were individualized, such that they varied considerably across individuals but had high repeatability within individual (Cronbach's = 0.76 to 0.85). Therefore, individuals appear to have unique inhibition-facilitation profiles that are relatively stable. Although the functional implications of individualized profiles are currently unknown, the relatively stable profiles may index underlying neural inhibition and excitation traits.