Investigating dynamic functional connectivity during NREM sleep using combined EEG-fMRI

Brain activity recoded with EEG during sleep is shown to be dynamic. However the relationship between this dynamic activity and the brain’s functional architecture is not well understood. The aim is to investigate the sensitivity of functional magnetic resonance imaging (fMRI) functional connectivity (FC) in the assessment of dynamic changes in brain function with sleep stage, focussing on the FC of intrinsically connected networks (ICNs) underlying functionality. Firstly the dynamic capability of resting-state fMRI signal fluctuations to calculate FC is investigated, highlighting the benefits of dynamic FC analysis and the validity of measuring FC using thirty-second epochs. Simultaneous EEG-fMRI was then used to investigate the effects of sleep on ICN FC, showing ICN re-organisation with sleep stage, which is modulated by prior wakefulness and sleep history. Furthermore, the network reorganisation was specific to certain ICNs, providing evidence for their association with sleep-wake behaviour. We also show FC variability between epochs of the same sleep stage, suggesting additional dynamic FC changes across the sleep cycle, a promising area of future investigation. Overall, this work demonstrates that dynamic FC is a useful measure of brain activity during sleep and regionally specific functional brain reorganisation can be used as a sensitive marker of the sleeping brain