Infra-slow brain fluctuations: A taxonomy of mechanistic hypotheses

Researchers have been intrigued by infra-slow fluctuations in neural recordings for many decades, uncovering important aspects such as the presence of spatial structure and a relation to behavior. Functionally distinct brain systems that are timely recruited to meet internal and external demands, also display a degree of coordination at infra-slow time scales, but the reason why infra-slow activity fluctuates has been challenging to explain. In recent years, a number of conceptual views have been advanced to explain variability in infra-slow activity. Some highlight the benefits of variability, others emphasize a direct role in cognition, while others focus on "offline" functions. The present review first offers an overview of the current perspectives which originate from diverse methodologies and approaches. We then review recent evidence indicating that infra-slow macroscopic activity involves global coordinated changes of neural activity, neuromodulatory systems, autonomic signals, and vascular and lymphatic flow. We highlight the possibility that much of the current observations on infra-slow dynamics may in fact represent a complex system operating as a homeostatic mechanism with specific behavioral and psychological signatures during wakefulness.