BOLD correlates of EEG alpha phase-locking and the fMRI default mode network

Synchronous oscillations in the brain, particularly alpha-band activity observed in EEG, are linked to resting state networks (RSNs) seen in fMRI BOLD signals. This study reports evidence that global EEG synchronization in the alpha frequency band correlates with BOLD activity in brain areas involved in specific RSNs, such as the default mode network. The results confirm that specific RSNs are organized by long-range synchronization in the alpha band. Overlap was found between areas where BOLD correlates of global EEG synchronization and associated RSNs are located, suggesting that the spatio-temporal organization of EEG rhythms, not just spectral dynamics, is important for understanding brain network coordination.

Study at a glance

Abstract

Phase locking or synchronization of brain areas is a key concept of information processing in the brain. Synchronous oscillations have been observed and investigated extensively in EEG during the past decades. EEG oscillations occur over a wide frequency range. In EEG, a prominent type of oscillations is alpha-band activity, present typically when a subject is awake, but at rest with closed eyes. The spectral power of alpha rhythms has recently been investigated in simultaneous EEG/fMRI recordings, establishing a wide-range cortico-thalamic network. However, spectral power and synchronization are different measures and little is known about the correlations between BOLD effects and EEG synchronization. Interestingly, the fMRI BOLD signal also displays synchronous oscillations across different brain regions. These oscillations delineate so-called resting state networks (RSNs) that resemble the correlation patterns of simultaneous EEG/fMRI recordings. However, the nature of these BOLD oscillations and their relations to EEG activity is still poorly understood. One hypothesis is that the subunits constituting a specific RSN may be coordinated by different EEG rhythms. In this study we report on evidence for this hypothesis. The BOLD correlates of global EEG synchronization (GFS) in the alpha frequency band are located in brain areas involved in specific RSNs, e.g. the 'default mode network'. Furthermore, our results confirm the hypothesis that specific RSNs are organized by long-range synchronization at least in the alpha frequency band. Finally, we could localize specific areas where the GFS BOLD correlates and the associated RSN overlap. Thus, we claim that not only the spectral dynamics of EEG are important, but also their spatio-temporal organization.