Dynamical primitives of spontaneous activity in the cortex: the role of arousal and brain-body interactions

Spontaneous dynamics in the cortex are not merely reflective of noise. Extensive research has demonstrated that the intrinsically generated activity in cortical networks is not only structured according to task-relevant activity patterns, but that it can also shape behavior, both in the short-term and through the long-term consolidation and generalization of said activity patterns. For this reason, researchers have hypothesized that the spontaneous activity of cortical networks has a predictive role, contributing to the consolidation of patterns relevant for future behavior and the maintenance of the internal world model.

Spontaneous dynamics are also know to vary across multiple timescales, with a particular relevance for shared ultra-slow fluctuations (<0.1Hz), which have been shown to transiently and simultaneously modulate the excitability of widespread cortical areas. Although computational work has shown that such slow fluctuations can be emergent from fast local dynamics in cortical networks alone, an increasing body of evidence suggests a strong coupling of such dynamics with the activity of arousal nuclei (e.g. locus coeruleus and cholinergic basal forebrain) and also bodily signals, such as hear-rate variability and respiratory volume, which also relate to the activity of arousal nuclei. However, the causal relationships between these variables are not yet clear. Do bodily rhythms generate a pacemaking signal on top of which cortical dynamics are organized? Or are these slow fluctuations an emergent property of brain-body interactions that then exert top-down control on both cortical and bodily rhythms?

In this paper, we will review literature on ultra-slow cortical fluctuations, their spatiotemporal organization and their relationship to the neuromodulatory activity of subcortical nuclei related to arousal. In addition, we will review how these signals reflect brain-body coupling and how they might be affected by perturbations that are both intrinsic (body) and extrinsic (environment) to the organism. With this, we aim to formulate a theory of how brain-body interactions establish an emergent dynamical framework for the organization spontaneous activity in the cortex, how this contributes to behavior and how it might be affected in neuropsychiatric disorders.