Occlusion of bidirectional plasticity by preceding low-frequency stimulation in the human motor cortex

Abstract: 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 10ms 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 PAS25ms (PAS25ms only: 1.05±0.14 to 1.76±0.66mV, p =0.001; rTMS+PAS25ms: 1.08±0.18 to 1.02±0.44mV, n.s.) and also abolished the significant decrease of MEP amplitudes after PAS10ms (PAS10ms only: 1.00±0.14 to 0.73±0.32mV; rTMS+PAS10ms: 1.15±0.35 to 1.25±0.43mV, 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. [Copyright &y& Elsevier]