Different responses of spontaneous and stimulus‐related alpha activity to ambient luminance changes

Alpha oscillations are particularly important in determining our percepts and have been implicated in fundamental brain functions. Oscillatory activity can be spontaneous or stimulus-related. Furthermore, stimulus-related responses can be phase- or non-phase-locked to the stimulus. Non-phase-locked (induced) activity can be identified as the average amplitude changes in response to a stimulation, while phase-locked activity can be measured via reverse correlation techniques (echo function). However, the mechanisms and the functional roles of these oscillations are far from clear. Here, we investigated the effect of ambient luminance changes, known to dramatically modulate neural oscillations, on spontaneous and stimulus-related alpha. We investigated the effect of ambient luminance on EEG alpha during spontaneous human brain activity at rest (experiment 1) and during visual stimulation (experiment 2). Results show that spontaneous alpha amplitude increased by decreasing ambient-luminance, while alpha frequency remained unaffected. In the second experiment, we found that under low-luminance viewing the stimulus-related alpha amplitude was lower, and its frequency was slightly faster. These effects were evident in the phase-locked part of the alpha response (echo function), but weaker or absent in the induced (non-phase-locked) alpha responses. Finally, we explored the possible behavioral correlates of these modulations in a monocular critical flicker frequency task (experiment 3), finding that dark adaptation in the left eye decreased the temporal threshold of the right eye. Overall, we found that ambient luminance changes impact differently on spontaneous and stimulus-related alpha expression. We suggest that stimulus-related alpha activity is crucial in determining human temporal segmentation abilities.<br>

α振荡（Alpha oscillations）在人类感知觉的形成过程中发挥着关键作用，且与大脑的多项基础生理功能密切相关。神经振荡活动可分为自发性振荡与刺激相关性振荡两大类。进一步而言，刺激相关性响应可分为与刺激相位锁相（phase-locked）及非相位锁相（non-phase-locked）两种亚型。其中，非相位锁相（诱发性，induced）活动可通过刺激引发的平均振幅变化进行识别，而相位锁相活动则可借助反向相关技术（回声函数，echo function）进行量化测量。然而，此类振荡的内在机制与功能角色仍远未被充分阐明。
本研究聚焦于环境亮度变化——已知其可显著调控神经振荡活动——对自发性与刺激相关性α振荡的影响。我们分别在静息状态下的自发性人脑活动（实验1）与视觉刺激范式中（实验2），探究了环境亮度对脑电图（Electroencephalogram, EEG）α振荡的调控作用。
实验1结果显示，自发性α振荡的振幅随环境亮度降低而显著升高，但其频率未受亮度变化的影响。实验2中，我们发现低亮度视觉条件下，刺激相关性α振荡的振幅更低，且振荡频率略快。此类效应在α响应的相位锁相部分（回声函数）中表现显著，但在诱发性（非相位锁相）α响应中则相对较弱，甚至未出现可观测的效应。
最后，我们通过单眼临界闪烁频率任务（实验3）探究了上述神经振荡调控效应可能对应的行为关联，实验结果表明左眼的暗适应过程会降低右眼的时间阈值。
总体而言，本研究证实环境亮度变化对自发性与刺激相关性α振荡的表达存在差异化影响。我们认为，刺激相关性α振荡活动对人类的时间分段能力至关重要。