Scale-free brain activity: past, present, and future

•Arrhythmic, scale-free brain activity is distinct from brain oscillations.•Scale-free brain activity contains rich temporal structures beyond power spectrum.•Scale-free brain activity is relevant to task performance and arousal state.•Scale-free brain activity is altered in developmental and disease processes.•Computational modeling has shed light on the potential generative mechanisms of scale-free brain activity.

Brain activity observed at many spatiotemporal scales exhibits a 1/f-like power spectrum, including neuronal membrane potentials, neural field potentials, noninvasive electroencephalography (EEG), magnetoencephalography (MEG), and functional magnetic resonance imaging (fMRI) signals. A 1/f-like power spectrum is indicative of arrhythmic brain activity that does not contain a predominant temporal scale (hence, ‘scale-free’). This characteristic of scale-free brain activity distinguishes it from brain oscillations. Although scale-free brain activity and brain oscillations coexist, our understanding of the former remains limited. Recent research has shed light on the spatiotemporal organization, functional significance, and potential generative mechanisms of scale-free brain activity, as well as its developmental and clinical relevance. A deeper understanding of this prevalent brain signal should provide new insights into, and analytical tools for, cognitive neuroscience.