Prevalence of rhythmic brain oscillations in EEG dynamics

Oscillatory brain dynamics has been studied extensively for a century using electroencephalography, and spatial and temporal characteristics of oscillations have been linked to a variety of physiological and pathophysiological conditions. The underlying recordings are either ongoing electroencephalograms (EEGs) usually of short duration (seconds to minutes) or are obtained during perturbation experiments such as evoked or event-related activities. With modern acquisition systems that allow continuous recordings over days to weeks and beyond, there is an increasing research focus into non-oscillatory brain dynamics, which possibly constitutes the major part of EEG recordings. We aim at adding to the current discussion about the relevance of oscillatory and non-oscillatory brain dynamics and investigated the frequency with which periodic or quasiperiodic oscillatory brain dynamics occurs at which brain site over extended periods of time. To this end, we use our recently proposed metric circulance that sensitively and robustly positions time series along a continuous spectrum from periodicity to randomness. Investigating multi-day EEG recordings from nine subjects with and without disorders of the central nervous system, we observe sustained oscillatory brain dynamics predominantly over bilateral temporo-parieto-occipital brain regions, however, with an overall frequency of only a few percent. The sparseness of sustained periodic or quasiperiodic brain dynamics calls for reconsidering its claimed relevance.