Non-invasive electrical stimulation technologies can modulate brain activity and are now widely used; however, the detailed mechanisms by which electric fields interact with endogenous neural network activity patterns remain incompletely understood. In this work, we investigated how AC electric fields applied at different amplitudes and frequencies interact with ongoing cortical slow oscillations / slow waves. Slow waves (< 2 Hz) characterize states of deep sleep and deep anesthesia; increasing evidence indicates that they also emerge during wakefulness in association with pathological conditions such as stroke, epilepsy, and Parkinson’s disease. Accordingly, modulating slow waves has a therapeutic potential.
Our study revealed that the effects of AC fields on slow waves depend on the stimulation frequency and amplitude relative to the endogenous slow-wave rhythm. When the stimulation frequency is close to the intrinsic slow-wave frequency, the two signals synchronize—a phenomenon known as entrainment. Entrainment occurred within a triangular region of high synchrony (the Arnold tongue) centered on the spontaneous slow-wave frequency. In contrast, when stimulation frequencies lay just above the Arnold tongue range, the effect reversed: stimulation reduced synchronization, shifting cortical activity from slow waves toward a desynchronized, “awake-like” state. This disruption of synchrony is relevant for treating neuropsychiatric disorders characterized by pathological hypersynchronous oscillations.
We further propose that efficacy could be enhanced by combining AC stimulation with negative or positive DC offsets, which facilitated desynchronization and entrainment, respectively. All experimental observations were quantitatively reproduced in a computational model of spiking neurons, suggesting that interactions among nonlinear oscillators can predict network responses to AC fields. Together, these results deepen our understanding of cortical dynamics and their modulation and provide a foundation for developing precise, targeted brain stimulation therapies.
