One contribution of 15 to a Theo Murphy meeting issue 'Interpreting BOLD: a dialogue between cognitive and cellular neuroscience'. Initially regarded as 'noise', spontaneous (intrinsic) activity accounts for a large portion of the brain's metabolic cost. Moreover, it is now widely known that infra-slow (less than 0.1 Hz) spontaneous activity, measured using resting state functional magnetic resonance imaging of the blood oxygen level-dependent (BOLD) signal, is correlated within functionally defined resting state networks (RSNs). However, despite these advances, the temporal organization of spontaneous BOLD fluctuations has remained elusive. By studying temporal lags in the resting state BOLD signal, we have recently shown that spontaneous BOLD fluctuations consist of remarkably reproducible patterns of whole brain propagation. Embedded in these propagation patterns are unidirectional 'motifs' which, in turn, give rise to RSNs. Additionally, propagation patterns are markedly altered as a function of state, whether physiological or pathological. Understanding such propagation patterns will likely yield deeper insights into the role of spontaneous activity in brain function in health and disease.This article is part of the themed issue 'Interpreting blood oxygen level-dependent: a dialogue between cognitive and cellular neuroscience'.
Importance of intrinsic activityAs observed by Hans Berger, in reporting on the first measurements of the human electroencephalogram, spontaneous (intrinsic) neural fluctuations are a dominant feature of the brain's electrical activity [1]. In the context of studying task-evoked neural responses, spontaneous activity was long considered merely to be 'noise'. Thus, most early studies of neural activity employed computational strategies to suppress spontaneous activity. More recently, it has been appreciated that investigating spontaneous activity is essential for understanding brain function [2][3][4]. At the systems level, this paradigm shift was prompted by two major findings. First, although the human brain represents only 2% of total body mass, its intrinsic activity consumes 20% of the body's energy, most of which is used to support ongoing neuronal signalling ([5-9], but see also [10]). Task-related increases in neuronal metabolism are generally small (less than 5%) when compared with this large intrinsic energy consumption (for a recent review, see [9]). Thus, to understand how the brain operates, we must take into account the component that consumes most of the brain's energy: spontaneous activity.The second set of findings has been derived from resting state functional magnetic resonance imaging (rs-fMRI) of the blood oxygen level-dependent (BOLD) signal [11]. Biswal et al. used human rs-fMRI to discover that spontaneous infraslow (less than 0.1 Hz) fluctuations of the BOLD signal are highly correlated within the somatomotor system [12]. This basic result has since been extended to multiple functional networks spanning the entire brain ([13-17]; figure 1a,b). Spatial correlat...