Objective: Low-frequency stimulation, which does not induce long-term potentiation (LTP) or long-term potentiation (LTD) by itself, suppresses consecutive LTP or LTD induction in vitro. We tested whether a similar interaction occurs in the human motor cortex.
Methods: LTP- or LTD-like plasticity was induced using paired associative stimulation (PAS) with 25 and 10 ms interstimulus interval and conditioned by suprathreshold repetitive transcranial magnetic stimulation (rTMS) at a frequency of 0.1Hz.
Results: RTMS completely abolished the significant increase of motor-evoked potential (MEP) amplitudes after PAS(25 ms) (PAS(25 ms) only: 1.05+/-0.14 to 1.76+/-0.66 mV, p=0.001; rTMS+PAS(25 ms): 1.08+/-0.18 to 1.02+/-0.44 mV, n.s.) and also abolished the significant decrease of MEP amplitudes after PAS(10 ms) (PAS(10 ms) only: 1.00+/-0.14 to 0.73+/-0.32 mV; rTMS+PAS(10 ms): 1.15+/-0.35 to 1.25+/-0.43 mV, p=0.006). RTMS alone did not significantly alter MEP amplitudes but increased SICI and LICI.
Conclusions: Low frequency stimulation increases intracortical inhibition and occludes LTP- and LTD-like plasticity in the human motor cortex.
Significance: This finding supports the concept that metaplasticity in the human motor cortex follows similar rules as metaplasticity in in vitro experiments.
2009 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.