Transcranial alternating current Stimulation (tACS) induced aftereffects in the individual Alpha frequency band: an EEG/fNIRS study

经颅交流电刺激(TACS)是一种非常有趣的调节脑电节律的新方法。电极被放置在头部，选定频率内的最小交流电(1 MA)通过大脑。据报道，这种方法可以在人类身上使用，没有任何风险。然而，它可能改变或调节大脑节律，并与靶区神经元的放电模式同时进行。人类最重要的脑震荡是阿尔法节律，频率约为10赫兹。由于不同受试者的阿尔法频率可能不同，为了刺激和分析，使用了单独调整的阿尔法频率(IAF)。最近的研究表明，经过IAF调整的TAC可以在长达90分钟的时间内增强脑电(EEG)的幅度(Kasten等人，2016)b)如果在Tacs期间执行内隐运动学习范式，作为一项“连续反应时间任务”(SRTT)，这可以提高学习成绩(Pollok等人)。基于这些发现，我们将进行一项研究，包括根据IAF调整的20分钟TACS。我们还将包括SRTT，并将在刺激后的六个小时内分析影响。40名男性参与者，双盲分为刺激(STIM)组和假刺激(Sham)组，将自愿参加实验。我们将重点研究IAF-TAC对脑电幅度、学习成绩和脑内血液氧合的影响，正如功能近红外光谱(FNIRS)所评估的那样。将回答三个主要问题：i)据报道，IAF-TAC可以在此频率范围内增强脑电幅度长达90分钟。在一项初步研究中，我们观察到刺激后三个多小时的波幅增加。因此，我们将分析这种后遗症在tACs后长达六个小时的持续时间。ii)内隐运动学习范式，因为如果与伴随的TAC一起训练，SRTT会产生更好的结果。如果这种积极作用是由于Tacs诱导的幅度增加所致，那么只要幅度增加，学习有效性也应该增加。因此，参与者将在Tacs后重复进行SRTT(假设有升高的幅度)，我们将分析学习效率。iii)到目前为止，还没有关于Tacs期间脑电幅度的增加与伴随的脑活动增加或减少有关的单一报告。因此，我们将通过fNIRS记录脑血氧含量，作为衡量大脑活动的一种手段。FNIR将在TAC之前、期间和之后被记录下来。总而言之，这是本研究将回答的三个高度相关的主题。这些结果对于理解Tacs的基本机制以及潜在的治疗应用都具有非常重要的意义。

Transcranial alternating current stimulation (tACS) is a highly interesting, new method to modulate brain rhythms. Electrodes are placed on the head and a minimal alternating current (1mA) within a selected frequency is sent through the brain. As far as reported this method can be used in humans without any risk. However, it may alter or modulate brain rhythms and concurrent the discharge pattern of the neurons in the targeted area. The most important brain oszillation in humans is the alpha rhythm with a frequency of about 10 Hz. Because alpha frequency can vary between subjects, for stimulation and analysis the individually adjusted alpha frequency (IAF) is used.Recent studies indicated that a) IAF adjusted tACS can enhance the amplitudes of the electroencephalogram (EEG) in this frequency for up to 90 minutes (Kasten et al.2016) b) if an implicit motor learning paradigm as a “serial reaction time task” (SRTT) is executed during tACS, this can increase learning performance (Pollok et al. 2015).Based on these findings we will perform a study including a 20 minute tACS adjusted to the IAF. We will also include a SRTT and we will analyze the effects for six hours after the stimulation. Forty male participants, double-blind divided in a stimulation (Stim) and a sham stimulation (Sham) group, will voluntarily take part in the experiment. We will focus on the effects of IAF-tACS on the EEG amplitudes, on learning performance and on the blood oxygenation in the brain, as assessed by functional near infrared spectroscopy (fNIRS). Three main questions will be answered:i) It has been reported that IAF-tACS can enhance the EEG amplitudes in this frequency range for up to 90 minutes. In a pilot study we have observed increased amplitudes for more than three hours after the stimulation. Thus, we will analyze the duration of this after effect for up to six hours after the tACS.ii) implicit motor learning paradigms as the SRTT yield better results if trained with accompanying tACS. If this positive effect is due to the tACS induced enhanced amplitudes, learning effectivity should be increased too, as long as the amplitudes are enhanced. Thus, the participants will perform the SRTT repeatedly after the tACS (assuming there are enlevated amplitudes) and we will analyze the learning efficiency.iii) Up to now there is no single report about whether the increase of the EEG amplitudes during tACS is correlated to an accompanying increased or decreased brain activity. Thus, we will record brain blood oxygenation by the means of fNIRS, as a measure of brain activity. fNIRS will be recorded before, during and after tACS.Taken together these are three highly relevant topics that will be answered with this study. The results are of high importance both, for the understanding of the basic mechanisms of tACS as well as for potential therapeutic applications.