Hippocampal sharp-wave ripples, brief high-frequency oscillations linked to memory consolidation, are followed by a dramatic increase in fMRI signal within the default mode network (DMN) of anesthetized monkeys. This effect was specific to ripples and did not occur after other hippocampal events or in other resting-state networks. The findings link circuit-level neural dynamics—ripples—to network-level fluctuations in the DMN, providing mechanistic support for the DMN's role in memory consolidation.
Brain-wide signal levels can reliably distinguish sleep and anesthesia from the awake state in human and monkey fMRI resting state data. A whole-brain computational model reproduces changes in global synchronization, functional connectivity, structure-function relationship, integration, and segregation across vigilance states. The awake brain operates near a Hopf bifurcation, which coincides with globally correlated fMRI signals. Simulated lesions of connectivity hubs in the posterior brain and subcortical nuclei disrupt the model's awake state, matching predictions from graph-theoretical analyses of structural data.
Spontaneous, naturally occurring thoughts have distinct brain signatures that can be detected with combined fMRI and EEG recordings. Using machine learning on 240 samples from eight participants, internally versus externally oriented experiences were distinguished with 65.4% accuracy by fMRI and 62.5% by EEG. Externally oriented states involved greater activity in salience, auditory, and visuospatial brain networks and lower occipital alpha power, while internally oriented states showed the opposite pattern, extending prior accounts focused on the default mode network. Across modalities, alpha power correlated negatively with BOLD fluctuations in parietal and occipital regions, indicating that coordinated large-scale network dynamics and alpha oscillations track the natural alternation between inward and outward focus.
Alpha oscillations (8–13 Hz) are the dominant electrophysiological rhythms during resting state and correlate more strongly than other frequency bands with blood oxygen-level dependent (BOLD) signals in default mode network (DMN) regions. Using simultaneously collected high-density EEG and fMRI data from 20 healthy volunteers at rest, the authors detected the DMN from source-reconstructed EEG data across multiple frequency bands and mapped correlations between EEG-derived DMN activity and fMRI-BOLD signals voxel by voxel. The correlation map for alpha oscillations overlapped with DMN regions more than maps for other bands did, supporting a primary role of alpha oscillations in DMN functioning. Simultaneous EEG-fMRI is suggested as a tool for investigating human brain networks.