Abstract
BACKGROUND: Paired associative stimulation (PAS), with stimulus interval of 21.5 or 25 ms, using transcranial magnetic stimulation in the posterior-anterior (PA) current direction, produces a long-term-potentiation-like effect. Stimulation with PA directed current generates both early and late indirect (I)-waves while that in anterior-posterior (AP) current predominantly elicits late I-waves. Short interval intracortical inhibition (SICI) inhibits late I-waves but not early I-waves. OBJECTIVE: To investigate how cortical inhibition modulates the effects of PAS. METHODS: PAS at stimulus interval of 21.5 ms conditioned by SICI (SICI-PAS) was compared to PAS alone with both PA and AP directed currents. RESULTS: PAS with both current directions increased cortical excitability. SICI-PAS increased cortical excitability in the PA but not the AP current direction. CONCLUSIONS: Both early and late I-waves circuits can mediate cortical PAS plasticity under different conditions. Plasticity induction with the late but not the early I-wave circuits is blocked by SICI.