OBJECTIVE Sub-motor threshold 5 Hz repetitive paired associative stimulation (5 Hz-rPAS25ms) produces a long-lasting increase in corticospinal excitability. Assuming a spike-timing dependent plasticity-like (STDP-like) mechanism, we hypothesized that 5 Hz-rPAS at a shorter inter-stimulus interval (ISI) of 15 ms (5 Hz-rPAS15ms) would exert a lasting inhibitory effect on corticospinal excitability. METHODS 20 healthy volunteers received two minutes of 5 Hz-rPAS15ms. Transcranial magnetic stimulation (TMS) was applied over the motor hotspot of the right abductor pollicis brevis muscle at 90% active motor threshold. Sub-motor threshold peripheral electrical stimulation was given to the left median nerve 15 ms before each TMS pulse. We assessed changes in mean amplitude of the unconditioned motor evoked potential (MEP), short-latency intracortical inhibition (SICI), intracortical facilitation (ICF), short-latency afferent inhibition (SAI), long-latency afferent inhibition (LAI), and cortical silent period (CSP) before and for 60 minutes after 5-Hz rPAS15ms. RESULTS Subthreshold 5-Hz rPAS15ms produced a 20-40% decrease in mean MEP amplitude along with an attenuation in SAI, lasting at least 60 minutes. A follow-up experiment revealed that MEP facilitation was spatially restricted to the target muscle. CONCLUSIONS Subthreshold 5-Hz rPAS15ms effectively suppresses corticospinal excitability. Together with the facilitatory effects of subthreshold 5-Hz rPAS25ms (Quartarone et al., J Physiol 2006;575:657-670), the results show that sub-motor threshold 5-Hz rPAS induces STDP-like bidirectional plasticity in the motor cortex. SIGNIFICANCE The results of the present study provide a new short-time paradigm of long term depression (LTD) induction in human sensory-motor cortex.

中文翻译：

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以 THETA 频率为目标的运动手区域的低强度重复配对关联刺激导致皮质脊髓兴奋性持续降低

目的 亚运动阈值 5 Hz 重复配对关联刺激 (5 Hz-rPAS25ms) 可持久增加皮质脊髓兴奋性。假设存在尖峰时间依赖性可塑性（STDP 样）机制，我们假设 5 Hz-rPAS 在 15 ms（5 Hz-rPAS15ms）的较短刺激间间隔（ISI）下会对皮质脊髓产生持久的抑制作用。兴奋性。方法 20 名健康志愿者接受两分钟的 5 Hz-rPAS15ms。经颅磁刺激 (TMS) 应用于右侧拇短外展肌的运动热点，活动运动阈值为 90%。在每个 TMS 脉冲前 15 毫秒，对左侧正中神经进行亚运动阈值外周电刺激。我们评估了无条件运动诱发电位 (MEP) 平均振幅的变化，5Hz rPAS15ms 之前和之后 60 分钟的短延迟皮质内抑制 (SICI)、皮质内促进 (ICF)、短延迟传入抑制 (SAI)、长延迟传入抑制 (LAI) 和皮质静默期 (CSP) . 结果 亚阈值 5-Hz rPAS15ms 使平均 MEP 振幅降低 20-40%，同时 SAI 衰减，持续至少 60 分钟。后续实验表明 MEP 促进在空间上仅限于目标肌肉。结论 亚阈值 5-Hz rPAS15ms 有效抑制皮质脊髓兴奋性。连同亚阈值 5-Hz rPAS25ms (Quartarone et al., J Physiol 2006;575:657-670) 的促进作用，结果表明亚运动阈值 5-Hz rPAS 在运动皮层诱导 STDP 样双向可塑性.