Aggarwal Srishty, Ray Supratim
Department of Physics, Indian Institute of Science, Bengaluru, India.
Centre for Neuroscience, Indian Institute of Science, Bengaluru, India.
Eur J Neurosci. 2025 Jul;62(2):e70193. doi: 10.1111/ejn.70193.
Nonlinear dynamical methods such as Higuchi fractal dimension (HFD) are often used to study the complexities of brain activity. In human electroencephalogram (EEG), although power in the gamma band (30-70 Hz) and the slope of the power spectral density (PSD) have been shown to reduce with healthy ageing, there are conflicting findings regarding how HFD and other measures of complexity vary with ageing. Further, the dependence of HFD on features obtained from PSD (such as gamma power and slope) has not been thoroughly probed. To address these issues, we computed time- and frequency-resolved HFD for EEG data collected from an elderly population (N = 217), aged between 50 and 88 years, for baseline (BL) eyes open state and during a fixation task in which visual grating stimuli that induce strong gamma oscillations were presented. During BL, HFD increased with age for frequencies up to 150 Hz but surprisingly showed an opposite trend at higher frequencies. Interestingly, this change in HFD was opposite to the age-related change in PSD 1/f slope. Further, stimulus-related changes in HFD were anticorrelated with the changes in oscillatory power. However, stimulus- and age-related changes in HFD persisted even after normalization with surrogates, showing the effect of nonlinear dynamics on HFD. Further, age classification using HFD was slightly better than classification using spectral features (power and slope). Therefore, HFD could be jointly sensitive to various spectral features as well as some nonlinearities not captured using spectral analysis, which could enhance our understanding of brain dynamics underlying healthy ageing.
诸如 Higuchi 分形维数（HFD）等非线性动力学方法常用于研究大脑活动的复杂性。在人类脑电图（EEG）中，尽管已表明γ频段（30 - 70 Hz）的功率和功率谱密度（PSD）的斜率会随着健康衰老而降低，但关于 HFD 和其他复杂性度量如何随衰老而变化，存在相互矛盾的研究结果。此外，HFD 对从 PSD 获得的特征（如γ功率和斜率）的依赖性尚未得到充分探究。为了解决这些问题，我们计算了从 217 名年龄在 50 至 88 岁之间的老年人群收集的 EEG 数据在基线（BL）睁眼状态和进行固定任务期间的时间和频率分辨 HFD，在固定任务中呈现了诱发强烈γ振荡的视觉光栅刺激。在 BL 期间，高达 150 Hz 的频率下，HFD 随年龄增加，但令人惊讶的是，在更高频率下呈现相反趋势。有趣的是，HFD 的这种变化与 PSD 1/f 斜率的年龄相关变化相反。此外，HFD 与刺激相关的变化与振荡功率的变化呈反相关。然而，即使在用替代物进行归一化后，HFD 与刺激和年龄相关的变化仍然存在，表明非线性动力学对 HFD 的影响。此外，使用 HFD 进行年龄分类略优于使用频谱特征（功率和斜率）进行分类。因此，HFD 可能对各种频谱特征以及一些频谱分析未捕获的非线性因素具有联合敏感性，这可以增强我们对健康衰老背后大脑动力学的理解。
