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

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 0.1 Hz. Results: RTMS completely abolished the significant increase of motor-evoked potential (MEP) amplitudes after PAS_25ms (PAS_25ms only: 1.05 ± 0.14 to 1.76 ± 0.66 mV, p = 0.001; rTMS + PAS_25ms: 1.08 ± 0.18 to 1.02 ± 0.44 mV, n.s.) and also abolished the significant decrease of MEP amplitudes after PAS_10ms (PAS_10ms only: 1.00 ± 0.14 to 0.73 ± 0.32 mV; rTMS + PAS_10ms: 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.