High-frequency visual stimulation can increase medial temporal lobe ripple oscillation density

Background: Flickering visual stimulation can evoke neural oscillations, which can influence ongoing brain activity. Electrophysiological recordings of neural oscillations in the ripple band (80-180 Hz) showed that these high-frequency oscillations occur in the neocortex and the hippocampus, that they phase-synchronize across long distances, and that ripple oscillations in the neocortex often precede those in the hippocampus during wakefulness. It is therefore possible that the neocortical ripple oscillations propagate beyond sensory areas to the hippocampus, inducing ripple oscillations. Methods: To test this hypothesis, we conducted an experiment (N = 8) in humans, using ultra-high frequency visual stimulation (UHV-stimulation) to evoke ripple oscillations recorded through electrodes implanted in or near the hippocampus. Although hippocampal ripple oscillations, so-called sharp-wave-ripples (SWRs), mostly occur during quiet rest or slow-wave-sleep, we aimed to increase their abundance using visual stimulation during wakefulness. We hypothesized that UHV-stimulation would increase the number of SWRs relative to an eyes-open resting-state baseline. Results: In line with this hypothesis, we observed significantly more SWRs per second during periods of stimulation compared to a resting-state baseline before and after the stimulation. Conclusion: This indicates that UHV-stimulation can be used as a safe noninvasive tool to influence SWRs, which offers the potential to improve memory.