Neurophysiological experiments using transcranial magnetic stimulation (TMS) have sought to probe the function of the motor division of the corpus callosum. Primary motor cortex sends projections via the corpus callosum with a net inhibitory influence on the homologous region of the opposite hemisphere. Interhemispheric inhibition (IHI) experiments probe this inhibitory pathway. A test stimulus (TS) delivered to the motor cortex in one hemisphere elicits motor evoked potentials (MEPs) in a target muscle, while a conditioning stimulus (CS) applied to the homologous region of the opposite hemisphere modulates the effect of the TS. We predicted that large CS MEPs would be associated with increased IHI since they should be a reliable index of how effectively contralateral motor cortex was stimulated and therefore of the magnitude of interhemispheric inhibition. However, we observed a strong tendency for larger CS MEPs to be associated with reduced interhemispheric inhibition which in the extreme lead to a net effect of facilitation. This surprising effect was large, systematic, and observed in nearly all participants. We outline several hypotheses for mechanisms which may underlie this phenomenon to guide future research.

使用经颅磁刺激 (TMS) 的神经生理学实验试图探索胼胝体运动分裂的功能。初级运动皮层通过胼胝体发送投射，对另一半球的同源区域产生净抑制影响。大脑半球间抑制 (IHI) 实验探索了这种抑制途径。传递到一个半球运动皮层的测试刺激 (TS) 在目标肌肉中引发运动诱发电位 (MEP)，而施加到另一半球同源区域的条件刺激 (CS) 则调节 TS 的效果。我们预测大型 CS MEP 将与 IHI 增加有关，因为它们应该是对侧运动皮层被刺激的有效程度以及大脑半球间抑制程度的可靠指标。然而，我们观察到较大的 CS MEP 与减少的半球间抑制有关的强烈趋势，这在极端情况下会导致促进的净效应。这种令人惊讶的效果是巨大的、系统的，并且在几乎所有参与者中都观察到了。我们概述了可能构成这一现象的机制的几个假设，以指导未来的研究。并在几乎所有参与者中观察到。我们概述了可能构成这一现象的机制的几个假设，以指导未来的研究。并在几乎所有参与者中观察到。我们概述了可能构成这一现象的机制的几个假设，以指导未来的研究。