Closed-loop auditory stimulation in phase with slow waves during sleep enhances cerebrospinal fluid flow in humans

Cerebrospinal fluid (CSF) is a key component of healthy brain function, constantly circulating to maintain brain homeostasis. Large waves of CSF flow appear in nonrapid eye movement (NREM) sleep, and these CSF flow waves are associated with neural slow waves in the electroencephalogram (EEG). Whether neural slow waves are causally linked to CSF flow, and whether this flow can be enhanced, is not yet established. We developed a technique for performing closed-loop auditory stimulation of sleep slow waves with simultaneous magnetic resonance imaging (MRI) using real-time denoising and a neural network-based strategy for stimulus targeting. We first established that our technique could increase EEG slow waves during sleep inside the MRI scanner in healthy adults. We then found that closed-loop auditory stimulation during sleep caused increased CSF flow waves. Furthermore, this CSF flow effect was phase dependent and only apparent when auditory stimuli were aligned with slow-wave peaks. Widespread hemodynamic waves were also elicited by the stimulus, suggesting a brain-wide modulation contributing to this CSF flow effect. This work demonstrates that closed-loop slow-wave neurofeedback causes waves of CSF flow and provides a technique for simultaneous modulation and imaging of CSF flow during sleep that can next be explored as a potential translational tool in clinical populations.