Chapter 5 - From broccoli to the brain—Scale-free activity

Abstract

The brain is an organ of nature. Nature includes different time and space scales including shorter and longer ones. The brain exhibits a restricted repertoire of time- and space-scales relative to the much broader range in the natural world. How are time-space scales manifest in the brain? The brain exhibits fluctuations in its neural activity which operate in smaller or longer time scales. Importantly, these fluctuations show certain similar structure across the shorter and longer timescales. This is described as scale-free activity which can be found in the brain but is also ubiquitous in nature. This chapter introduces scale-free activity in general and how it is manifest in the brain. Moreover, we show it can be measured in different ways as by the power law exponent in the frequency domain that measures the relative relationship of slow-to-fast frequency power. While in the time domain, detrended fluctuation analysis is used to quantify the irregular fluctuations of the oscillations as distinguished from their regular oscillatory cycles. Together, scale-free activity is a key feature of the brain's neural activity which characterizes its temporal and spatial structure (second chapter) and strongly shapes our perception and cognition (third and fourth chapter).