Functional connectivity and oscillatory neuronal activity in the resting human brain.

Perceptions, thoughts, emotions and actions emerge from interactions between neuronal assemblies distributed across the brain rather than from local computations in restricted brain areas. Indeed, the operation of every cognitive act requires the integration of distributed activity, as implemented through long-range neuronal communication via a network of structural connections. Functional interactions in the brain are very often studied in subjects at rest, since the resting state is a privileged condition in which brain activity is unbiased by any specific goal-directed task. Early resting state studies showed that electrophysiological oscillatory activity in specific frequency bands supports synchronization processes related to long-range neuronal communication. In turn, experimental evidence from neuroimaging studies revealed that the human brain is organized into multiple large-scale networks of regions showing correlated hemodynamic activity. Multimodal studies have begun to disclose relationships between functional connectivity, as revealed by hemodynamic signals, and underlying electrophysiological processes. Furthermore, functional connectivity studies directly based on electrophysiological signals have recently revealed fundamental information regarding long-range neuronal communication at behaviorally relevant time-scales. The integration of different lines of evidence from hemodynamic and electrophysiological studies suggests that rapid changes of synchronized oscillatory activity in distributed brain networks is relevant for the ongoing maintenance and modulation of the task representations that form the basis of our cognitive flexibility.