Brain modulation is a modification process of brain activity through external stimulations. However, which condition can induce the activation is still unclear. Therefore, we aimed to identify brain activation conditions using 40 Hz monaural beat (MB). Under this stimulation, auditory sense status which is determined by frequency and power range is the condition to consider. Hence, we designed five sessions to compare; no stimulation, audible (AB), inaudible in frequency, inaudible in power, and inaudible in frequency and power. Ten healthy participants underwent each stimulation session for ten minutes with electroencephalogram (EEG) recording. For analysis, we calculated the power spectral density (PSD) of EEG for each session and compared them in frequency, time, and five brain regions. As a result, we observed the prominent power peak at 40 Hz in only AB. The induced EEG amplitude increase started at one minute and increased until the end of the session. These results of AB had significant differences in frontal, central, temporal, parietal, and occipital regions compared to other stimulations. From the statistical analysis, the PSD of the right temporal region was significantly higher than the left. We figure out the role that the auditory sense is important to lead brain activation. These findings help to understand the neurophysiological principle and effects of auditory stimulation.

翻译：脑调控是通过外部刺激对大脑活动进行调节的过程。然而，何种条件能够诱发大脑激活仍不明确。因此，本研究旨在利用40赫兹单耳节拍（MB）识别大脑激活条件。在该刺激条件下，由频率与功率范围决定的听觉感官状态是需要考虑的条件。为此，我们设计了五个对比实验组：无刺激组、可听组（AB）、频率不可听组、功率不可听组，以及频率与功率均不可听组。十名健康参与者接受了每次十分钟的不同刺激实验，并记录脑电图（EEG）。分析中，我们计算了各组EEG的功率谱密度（PSD），并从频率、时间及五个脑区维度进行对比。结果表明，仅在可听组中观察到40赫兹处显著的功率峰值。诱发的EEG振幅增加始于刺激开始后一分钟，并持续增强至实验结束。相较于其他刺激组，可听组在额叶、中央区、颞叶、顶叶及枕叶区域均呈现显著差异。统计分析显示，右侧颞叶区域的PSD显著高于左侧。我们明确了听觉感官在引导大脑激活中的关键作用。这些发现有助于理解听觉刺激的神经生理原理及其效应。