Besides stimulus intensities and interstimulus intervals (ISI), the electric field (E-field) orientation is known to affect both short-interval intracortical inhibition (SICI) and facilitation (SICF) in paired-pulse transcranial magnetic stimulation (TMS). However, it has yet to be established how distinct orientations of the conditioning (CS) and test stimuli (TS) affect the SICI and SICF generation. With the use of a multi-channel TMS transducer that provides electronic control of the stimulus orientation and intensity, we aimed to investigate how changes in the CS and TS orientation affect the strength of SICI and SICF. We hypothesized that the CS orientation would play a major role for SICF than for SICI, whereas the CS intensity would be more critical for SICI than for SICF. In eight healthy subjects, we tested two ISIs (1.5 and 2.7 ms), two CS and TS orientations (anteromedial (AM) and posteromedial (PM)), and four CS intensities (50, 70, 90, and 110% of the resting motor threshold (RMT)). The TS intensity was fixed at 110% RMT. The intensities were adjusted to the corresponding RMT in the AM and PM orientations. SICI and SICF were observed in all tested CS and TS orientations. SICI depended on the CS intensity in a U-shaped manner in any combination of the CS and TS orientations. With 70% and 90% RMT CS intensities, stronger PM-oriented CS induced stronger inhibition than weaker AM-oriented CS. Similar SICF was observed for any CS orientation. Neither SICI nor SICF depended on the TS orientation. We demonstrated that SICI and SICF could be elicited by the CS perpendicular to the TS, which indicates that these stimuli affected either overlapping or strongly connected neuronal populations. We concluded that SICI is primarily sensitive to the CS intensity and that CS intensity adjustment resulted in similar SICF for different CS orientations.

中文翻译：

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刺激方向和强度对短间隔皮质内抑制 (SICI) 和促进 (SICF) 的影响：一项多通道经颅磁刺激研究。

除了刺激强度和刺激间隔 (ISI)，已知电场 (E-field) 方向会影响配对脉冲经颅磁刺激 (TMS) 中的短间隔皮质内抑制 (SICI) 和促进 (SICF)。然而，尚未确定调节 (CS) 和测试刺激 (TS) 的不同方向如何影响 SICI 和 SICF 生成。通过使用提供刺激方向和强度电子控制的多通道 TMS 换能器，我们旨在研究 CS 和 TS 方向的变化如何影响 SICI 和 SICF 的强度。我们假设 CS 方向对 SICF 比对 SICI 起主要作用，而 CS 强度对 SICI 比对 SICF 更重要。在八名健康受试者中，我们测试了两个 ISI（1.5 和 2.7 毫秒），两个 CS 和 TS 方向（前内侧 (AM) 和后内侧 (PM)）和四个 CS 强度（静息运动阈值 (RMT) 的 50、70、90 和 110%）。TS 强度固定在 110% RMT。在 AM 和 PM 方向将强度调整为相应的 RMT。在所有测试的 CS 和 TS 方向都观察到 SICI 和 SICF。SICI 在 CS 和 TS 方向的任何组合中以 U 形方式依赖于 CS 强度。在 70% 和 90% 的 RMT CS 强度下，更强的 PM 导向的 CS 比弱的 AM 导向的 CS 诱导更强的抑制。对于任何 CS 方向都观察到类似的 SICF。SICI 和 SICF 都不依赖于 TS 方向。我们证明了 SICI 和 SICF 可以由垂直于 TS 的 CS 引起，这表明这些刺激影响了重叠或强连接的神经元群。我们得出结论，SICI 主要对 CS 强度敏感，并且 CS 强度调整导致不同 CS 方向的相似 SICF。