Changes in Higuchi's fractal dimension across age in healthy human EEG are anti-correlated with changes in oscillatory power and 1/f slope

Non-linear dynamical methods such as Higuchis fractal dimension (HFD) are often used to study the complexities of brain activity. In human electroencephalogram (EEG), while power in the gamma band (30-70 Hz) and the slope of the power spectral density (PSD) have been shown to reduce with healthy aging, there are conflicting findings regarding how HFD and other measures of complexity vary with aging. 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 elderly population (N=217), aged between 50-88 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 anti-correlated with the changes in oscillatory power. However, age classification using HFD was slightly better than classification using spectral features (power and slope). Further, stimulus and age-related changes in HFD persisted even after normalization with surrogates, showing the effect of non-linear dynamics on HFD. Therefore, HFD could be jointly sensitive to various spectral features as well as some non-linearities not captured using spectral analysis, which could enhance our understanding of brain dynamics underlying healthy aging.