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) show 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 the hypothesis that neocortical ripple oscillations induced by visual stimulation induce hippocampal ripple oscillations, we conduct an exploratory experiment (N = 8) in humans, using ultra-high frequency visual stimulation to induce ripple oscillations recorded through electrodes implanted in or near the hippocampus. Although hippocampal ripple oscillations, so-called sharp-wave-ripples, mostly occur during quiet rest or slow-wave sleep, we aim to increase their abundance using visual stimulation during wakefulness in this exploratory study. We hypothesize that ultra-high frequency visual stimulation increases the number of sharp-wave-ripples relative to an eyes-open resting-state baseline. RESULTS: In this exploratory and preliminary study, we observe significantly more sharp-wave-ripples per second during periods of stimulation compared to a resting-state baseline before and after the stimulation. CONCLUSIONS: The increased number of sharp-wave ripples during stimulation suggests that ultra-high-frequency visual stimulation can be used as a safe noninvasive tool to influence sharp-wave ripples, which offers the potential to improve memory.