Many studies have investigated the causal relevance of brain oscillations using rhythmic stimulation, either through direct-brain or sensory stimulation. Yet, how intrinsic rhythms interact with the externally generated rhythm is largely unknown. We presented a flickered (60 Hz) visual grating or its correspondent unflickered stimulus in a psychophysical change detection task during simultaneous magnetoencephalography recordings to humans to test the effect of visual entrainment on induced gamma oscillations. Notably, we generally observed the coexistence of the broadband induced gamma rhythm with the entrained flicker rhythm (reliably measured in each participant), with the peak frequency of the induced response remaining unaltered in approximately half of participants-relatively independently of their native frequency. However, flicker increased broadband induced gamma power, and this was stronger in participants with a native frequency closer to the flicker frequency (resonance) and led to strong phase entrainment. Presence of flicker did not change behavior itself but profoundly altered brain behavior correlates across the sample: While broadband induced gamma oscillations correlated with reaction times for unflickered stimuli (as known previously), for the flicker, the amplitude of the entrained flicker rhythm (but no more the induced oscillation) correlated with reaction times. This, however, strongly depended on whether a participant's peak frequency shifted to the entrained rhythm. Our results suggests that rhythmic brain stimulation leads to a coexistence of two partially independent oscillations with heterogeneous effects across participants on the downstream relevance of these rhythms for behavior. This may explain the inconsistency of findings related to external entrainment of brain oscillations and poses further questions toward causal manipulations of brain oscillations in general.

中文翻译：

---

可以通过有节奏的大脑刺激来研究大脑振荡的因果作用吗？

许多研究已经通过直接大脑或感觉刺激使用有节奏的刺激来调查大脑振荡的因果关系。然而，内在节奏如何与外部产生的节奏相互作用在很大程度上是未知的。我们在向人类同步脑磁图记录期间在心理物理变化检测任务中展示了闪烁 (60 Hz) 视觉光栅或其对应的不闪烁刺激，以测试视觉夹带对诱导伽马振荡的影响。值得注意的是，我们通常观察到宽带诱导的伽马节律与夹带的闪烁节律（在每个参与者中可靠地测量）共存，诱导反应的峰值频率在大约一半的参与者中保持不变——相对独立于他们的原生频率。然而，闪烁增加了宽带诱导的伽马功率，这在本机频率更接近闪烁频率（共振）的参与者中更强，并导致强烈的相位夹带。闪烁的存在并不会改变行为本身，但会深刻改变整个样本的大脑行为：虽然宽带诱导的伽马振荡与非闪烁刺激的反应时间相关（如前所述），但对于闪烁，夹带的闪烁节律的幅度（但没有更多的诱导振荡）与反应时间相关。然而，这在很大程度上取决于参与者的峰值频率是否转移到夹带节奏。我们的研究结果表明，有节奏的大脑刺激导致两种部分独立的振荡共存，参与者之间对这些节律与行为的下游相关性产生不同的影响。这可能解释了与大脑振荡的外部夹带相关的发现的不一致，并对一般的大脑振荡的因果操纵提出了进一步的问题。