Electrophysiological signatures of resting state networks in the human brain

Abstract

Functional neuroimaging and electrophysiological studies have documented a dynamic baseline of intrinsic (not stimulus-or task-evoked) brain activity during resting wakefulness. This baseline is characterized by slow (<0.1 Hz) fluctuations of functional imaging signals that are topographically organized in discrete brain networks, and by much faster (1-80 Hz) electrical oscillations. To investigate the relationship between hemodynamic and electrical oscillations, we have adopted a completely data-driven approach that combines information from simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Using independent component analysis on the fMRI data, we identified six widely distributed resting state networks. The blood oxygenation level-dependent signal fluctuations associated with each network were correlated with the EEG power variations of delta, theta, alpha, beta, and gamma rhythms. Each functional network was characterized by a specific electrophysiological signature that involved the combination of different brain rhythms. Moreover, the joint EEG/fMRI analysis afforded a finer physiological fractionation of brain networks in the resting human brain. This result supports for the first time in humans the coalescence of several brain rhythms within large-scale brain networks as suggested by biophysical studies.brain rhythms ͉ multimodal imaging ͉ resting fluctuations A fundamental issue in neuroscience is understanding how large neuronal assemblies cooperate in the brain, and what mechanisms underlie this cooperation, that is the basis for all sensory, cognitive, and motor activities. Traditional physiological models of brain function emphasize the importance of spike rate as a medium for encoding and transferring signals in the brain, and often delineate electrophysiological spontaneous activity as internal noise (1, 2). More recent models propose that spontaneous activity may also play an important functional role by providing important endogenous or top-down constraints to sensory-, cognitive-, or motor-driven activity or temporal windows of opportunity for long-range communication (3)(4)(5).Functional neuroimaging studies have provided evidence for a baseline of neuronal ongoing activity, from which transient changes induced by specific perceptual and cognitive tasks, generally named activations, arise (6, 7). Interestingly, spontaneous activity, as measured with blood oxygen level-dependent (BOLD) functional MRI (fMRI) in the resting awake or anesthetized brain, is organized in multiple highly specific functional anatomical networks (resting state networks, RSNs) (8, 9). These RSNs fluctuate at frequencies between 0.01 and 0.1 Hz, and strongly overlap with sensory-motor, visual, auditory, attention, language, and default networks that are commonly modulated during active behavioral tasks (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20).A critical step toward understanding the functional role of spontaneous activity is to clarify the neurophysiological basis of these RSN...

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