Department of Electronics, Information and Bioengineering, Politecnico di Milano, Milan, Italy.
J Neural Eng. 2019 Sep 17;16(5):056028. doi: 10.1088/1741-2552/ab296e.
Flickering visual stimulation is known to evoke rhythmic oscillations in the electroencephalographic (EEG) activity, called steady-state visually evoked potentials (SSVEP). The presence of harmonic components in the EEG signals during SSVEP suggests the non-linearity of the visual-system response to rhythmic stimulation, but the nature of this behavior has not been deeply understood. The aim of this study is the quantitative evaluation and characterization of this non-linear phenomenon and its interference with the physiological alpha rhythm by means of spectral and higher order spectral analysis.
EEG signals were acquired in a group of 12 healthy subjects during a pattern-reversal stimulation protocol at three different driving frequencies (7.5 Hz, 15 Hz and 24 Hz). Spectral power values were estimated, after Laplacian spatial filtering, to quantitatively evaluate the changes in the power of the individual alpha and stimulation frequencies related harmonic components. Bicoherence measure were employed to assess the presence of quadratic phase coupling (QPC) at each channel location.
Our analysis confirmed a strong non-linear response to the rhythmic stimulus principally over the parieto-occipital channel locations and a simultaneous significant alpha power suppression during 7.5 Hz and 15 Hz stimulation. A prominent sub-harmonic component characterized the resonance behavior of the 24 Hz stimulation.
The findings presented suggest that bicoherence is a useful tool for the identification of QPC interactions between stimulus-related frequency components within the same signal and the characterization of the non-linearity of SSVEP-induced harmonics generation. In addition, the applied methodology demonstrates the presence of coupled EEG rhythms (harmonics of the main oscillation) both in resting condition and during stimulation, with different characteristics in the distinct brain areas.
闪烁视觉刺激会在脑电图（EEG）活动中引起有节奏的振荡，称为稳态视觉诱发电位（SSVEP）。在 SSVEP 期间 EEG 信号中存在谐波分量表明视觉系统对节律刺激的非线性响应，但这种行为的性质尚未得到深入理解。本研究的目的是通过频谱和高阶谱分析定量评估和表征这种非线性现象及其对生理 alpha 节律的干扰。
在三组不同的驱动频率（7.5 Hz、15 Hz 和 24 Hz）下，对 12 名健康受试者进行模式反转刺激方案，采集 EEG 信号。在进行拉普拉斯空间滤波后，估计频谱功率值，以定量评估个体 alpha 功率和刺激频率相关谐波分量的变化。采用双相干测度来评估每个通道位置的二次相位耦合（QPC）的存在。
我们的分析证实了对节律刺激的强烈非线性响应，主要是在顶枕部通道位置，并且在 7.5 Hz 和 15 Hz 刺激期间同时显著抑制 alpha 功率。24 Hz 刺激的特征是突出的次谐波分量，表现出共振行为。
提出的研究结果表明，双相干测度是识别同一信号中与刺激相关的频率分量之间 QPC 相互作用以及表征 SSVEP 诱导谐波产生的非线性的有用工具。此外，所应用的方法学证明了在休息状态和刺激期间存在耦合的 EEG 节律（主要振荡的谐波），并且在不同的大脑区域具有不同的特征。
