Department of Human Physiology, University of Oregon, Eugene, Oregon.
J Neurophysiol. 2021 Feb 1;125(2):523-532. doi: 10.1152/jn.00390.2020. Epub 2020 Dec 23.
Action preparation involves widespread modulation of motor system excitability, but the precise mechanisms are unknown. In this study, we investigated whether intracortical inhibition changes in task-irrelevant muscle representations during action preparation. We used transcranial magnetic stimulation (TMS) combined with electromyography in healthy human adults to measure motor-evoked potentials (MEPs) and cortical silent periods (CSPs) in task-irrelevant muscles during the preparatory period of simple delayed response tasks. In , participants responded with the left index finger in one task condition and the right index finger in another task condition, whereas MEPs and CSPs were measured from the contralateral nonresponding and tonically contracted index finger. During , participants responded with the right pinky finger whereas MEPs and CSPs were measured from the tonically contracted left index finger. In both experiments, MEPs and CSPs were compared between the task preparatory period and a resting intertrial baseline. The CSP duration during response preparation decreased from baseline in every case. A laterality difference was also observed in , with a greater CSP reduction during the preparation of left finger responses compared to right finger responses. Despite reductions in CSP duration, consistent with a release of intracortical inhibition, MEP amplitudes were smaller during action preparation when accounting for background levels of muscle activity, consistent with earlier studies that reported decreased corticospinal excitability. These findings indicate that intracortical inhibition associated with task-irrelevant muscles is transiently released during action preparation and implicate a novel mechanism for the controlled and coordinated release of motor cortex inhibition. In this study, we observed the first evidence of a release of intracortical inhibition in task-irrelevant muscle representations during response preparation. We applied transcranial magnetic stimulation to elicit cortical silent periods in task-irrelevant muscles during response preparation, and observed a consistent decrease in the silent period duration relative to a resting baseline. These findings address the question of whether cortical mechanisms underlie widespread modulation in motor excitability during response preparation.
动作准备涉及运动系统兴奋性的广泛调节，但确切的机制尚不清楚。在这项研究中，我们研究了在动作准备过程中，任务无关肌肉代表中是否存在皮质内抑制的变化。我们使用经颅磁刺激（TMS）结合肌电图，在健康成年人中测量了简单延迟反应任务准备期间任务无关肌肉的运动诱发电位（MEPs）和皮质静息期（CSPs）。在任务中，参与者在一种任务条件下用左手食指反应，在另一种任务条件下用右手食指反应，而 MEPs 和 CSPs 则是从对侧非反应性和持续收缩的食指测量的。在任务中，参与者用右手小指反应，而 MEPs 和 CSPs 则是从持续收缩的左手食指测量的。在这两个实验中，MEPs 和 CSPs 都是在任务准备期与休息期的基线进行比较的。在每种情况下，反应准备期间的 CSP 持续时间都从基线减少。在任务中也观察到了侧偏差异，与右手食指反应相比，左手食指反应的准备过程中 CSP 减少更大。尽管 CSP 持续时间减少，这与皮质内抑制的释放一致，但考虑到肌肉活动的背景水平，MEP 幅度在动作准备期间更小，这与先前报告的皮质脊髓兴奋性降低的研究一致。这些发现表明，在动作准备期间，与任务无关的肌肉相关的皮质内抑制会暂时释放，并暗示了一种新的机制，可以控制和协调运动皮层抑制的释放。在这项研究中，我们观察到在反应准备过程中，任务无关肌肉代表中皮质内抑制释放的第一个证据。我们应用经颅磁刺激来诱发出现在反应准备过程中任务无关肌肉的皮质静息期，并观察到相对于静息基线，静息期持续时间的一致减少。这些发现解决了皮质机制是否是反应准备期间运动兴奋性广泛调节的基础这一问题。
