BSICoS group, I3A Institute, University of Zaragoza, IIS Aragón, Zaragoza, Spain.
Department of Bioengineering, Imperial College, London, UK.
J Physiol. 2023 Aug;601(15):3187-3199. doi: 10.1113/JP282983. Epub 2022 Jul 13.
Transcranial alternating current stimulation (TACS) is commonly used to synchronize a cortical area and its outputs to the stimulus waveform, but gathering evidence for this based on brain recordings in humans is challenging. The corticospinal tract transmits beta oscillations (∼21 Hz) from the motor cortex to tonically contracted limb muscles linearly. Therefore, muscle activity may be used to measure the level of beta entrainment in the corticospinal tract due to TACS over the motor cortex. Here, we assessed whether TACS is able to modulate the neural inputs to muscles, which would provide indirect evidence for TACS-driven neural entrainment. In the first part of the study, we ran simulations of motor neuron (MN) pools receiving inputs from corticospinal neurons with different levels of beta entrainment. Results suggest that MNs are highly sensitive to changes in corticospinal beta activity. Then, we ran experiments on healthy human subjects (N = 10) in which TACS (at 1 mA) was delivered over the motor cortex at 21 Hz (beta stimulation), or at 7 Hz or 40 Hz (control conditions) while the abductor digiti minimi or the tibialis anterior muscle were tonically contracted. Muscle activity was measured using high-density electromyography, which allowed us to decompose the activity of pools of motor units innervating the muscles. By analysing motor unit pool activity, we observed that none of the TACS conditions could consistently alter the spectral contents of the common neural inputs received by the muscles. These results suggest that 1 mA TACS over the motor cortex given at beta frequencies does not entrain corticospinal activity. KEY POINTS: Transcranial alternating current stimulation (TACS) is commonly used to entrain the communication between brain regions. It is challenging to find direct evidence supporting TACS-driven neural entrainment due to the technical difficulties in recording brain activity during stimulation. Computational simulations of motor neuron pools receiving common inputs in the beta (∼21 Hz) band indicate that motor neurons are highly sensitive to corticospinal beta entrainment. Motor unit activity from human muscles does not support TACS-driven corticospinal entrainment.
经颅交流电刺激（TACS）常用于使皮质区域及其输出与刺激波形同步，但基于人类脑记录来为此收集证据具有挑战性。皮质脊髓束将运动皮层的β 振荡（∼21 Hz）线性传输到持续收缩的肢体肌肉。因此，由于 TACS 对运动皮层的作用，肌肉活动可用于测量皮质脊髓束中β 同步的水平。在这里，我们评估了 TACS 是否能够调节肌肉的神经输入，这将为 TACS 驱动的神经同步提供间接证据。在研究的第一部分，我们对接收来自皮质脊髓神经元的输入的运动神经元（MN）池进行了模拟，这些神经元的β 同步水平不同。结果表明，MN 对皮质脊髓β 活动的变化高度敏感。然后，我们在 10 名健康的人类受试者中进行了实验，其中 TACS（1 mA）以 21 Hz（β 刺激）、7 Hz 或 40 Hz（对照条件）施加于运动皮层，同时使小指展肌或胫骨前肌持续收缩。使用高密度肌电图测量肌肉活动，这使我们能够分解支配肌肉的运动单位池的活动。通过分析运动单位池活动，我们观察到，在任何 TACS 条件下，都不能一致地改变肌肉接收到的常见神经输入的频谱含量。这些结果表明，在β 频率下给予运动皮层的 1 mA TACS 不会使皮质脊髓活动同步。 关键点：经颅交流电刺激（TACS）常用于使大脑区域之间的通讯同步。由于在刺激期间记录大脑活动的技术困难，因此很难找到支持 TACS 驱动的神经同步的直接证据。接收β 频段（∼21 Hz）常见输入的运动神经元池的计算模拟表明，运动神经元对皮质脊髓β 同步非常敏感。来自人类肌肉的运动单位活动不支持 TACS 驱动的皮质脊髓同步。
