Low-Frequency Amplitude-Modulated Ultrasound Stimulation Drives Hippocampal Oscillations and Enhances Memory in Mice

Although transcranial ultrasound stimulation (TUS) can noninvasively target the hippocampal oscillatory network, which serves as a critical interface between cellular functions and cognition, its modulatory efficiency is constrained by the low-pass filtering properties of neuronal membranes. To overcome this limitation, we developed a low-frequency amplitude-modulated (AM) TUS paradigm to enhance neuromodulatory efficiency by improving resonance within the hippocampal oscillatory network. We applied 5, 40, and 80 Hz sinusoidal AM-TUS and 5 Hz pulsed AM-TUS to the mouse hippocampus, and analyzed local field potentials before and after stimulation. Results showed that 5 Hz sinusoidal AM-TUS significantly enhanced the phase locked value (PLV) (increment: $0.054\pm 0.017$ ) and coherence (increment: $0.040\pm 0.016$ ) in the theta band, and the phase-amplitude coupling (PAC) in theta-low gamma (increment, $1.521\pm 0.249$ ) and theta-high gamma (increment, $0.821\pm 0.299$ ) bands. In contrast, the 5 Hz pulsed AM-TUS showed negligible effects. While the 40 Hz sinusoidal AM-TUS enhanced PLV and coherence in the theta band and PAC in theta-low gamma bands, the 80 Hz sinusoidal AM-TUS only enhanced PLV in the theta band. Thus, the 5 Hz sinusoidal AM-TUS demonstrated superior neuromodulatory efficiency over all other paradigms. The outstanding modulatory efficiency of 5 Hz sinusoidal AM-TUS on the oscillatory network was further confirmed in the 14-day TUS, demonstrating enhancements in spatial learning and memory. The 5 Hz sinusoidal AM-TUS presents a novel and efficient approach to precisely modulating hippocampal oscillatory network through entrainment.