Changes in Higuchi Fractal Dimension Across Age in Healthy Human EEG Are Anticorrelated With Changes in Oscillatory Power and 1/f Slope

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.