Alternating current stimulation entrains and connects cortical regions in a neural mass model

Transcranial alternating current stimulation (tACS) is a neuromodulatory technique that is widely used to investigate the functions of oscillations in the brain. Despite increasing usage in both research and clinical settings, the mechanisms of tACS are still not completely understood. To shed light on these mechanisms, we injected alternating current into a Jansen and Rit neural mass model. Two cortical columns were linked with long-range connections to examine how alternating current impacted cortical connectivity. Alternating current injected to both columns increased power and coherence at the stimulation frequency; however this effect was greatest at the model's resonant frequency. Varying the phase of stimulation impacted the time it took for entrainment to stabilize, an effect we believe is due to constructive and destructive inteference with endogenous membrane currents. The power output the model also depended on the phase of the stimulation between cortical columns. These results provide insight on the mechanisms of neurostimulation, by demonstrating that tACS increases both power and coherence at a neural network's resonant frequency, in a phase-dependent manner.