Occipital Versus Frontal Electrophysiological Signal and Macroscopic Cerebrospinal Fluid Flow in Early Sleep

The glymphatic hypothesis suggests that cerebrospinal fluid (CSF) bulk movement across the brain plays a major role in brain waste clearance. Previous studies using simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) indicate that CSF flow and EEG delta activity (0.5–4 Hz) are coupled, especially during sleep. In our study we investigated this relationship focusing on spatial distinction between the occipital and frontal brain region as well as slow (0.5–2 Hz) and fast (2–4 Hz) delta sub-bands. For this, we used a public EEG-fMRI dataset from 29 healthy young adults during wakefulness and early sleep stages. Our results show only slight changes in the correlation between EEG delta activity and CSF flow as individuals transitioned into light sleep. However, we observed differences within regions and frequency bands during sleep: Occipital delta activity exhibits a stronger correlation with CSF flow than frontal delta activity, and slow delta correlates more strongly than the fast delta sub-band. These results align with power spectral density (PSD) analyses, which reveal higher relative power in lower frequencies in occipital regions, while frontal regions display greater power in higher frequencies.