The action mechanisms of tACS are generally thought to be entrainment of endogenous rhythms at the frequency of stimulation 39, 40 and induction of synaptic changes via spike-timing dependent plasticity 41, 42, the latter of which may underlie offline tACS effects on cortical rhythms and behavior 41. Similarly, theories of PAC and phase synchronization propose that these coding schemes play a critical role in the support and promotion of synaptic plasticity, with some of the best-studied examples including theta-gamma PAC and theta phase synchronization within and between structures of the hippocampus and neocortex (e.g., prefrontal and temporal regions) during working memory, information encoding, and long-term memory retrieval 14. Given previous research suggesting a general function of PAC and phase synchronization in synaptic plasticity 14, 43, prior work showing that phase synchronization can change spike time-dependent plasticity 44, 45, and the present results demonstrating offline stimulation effects for longer than 50 minutes, we speculate that the external manipulation of theta-gamma PAC and theta phase synchronization that improved behavior may be due to neuroplastic changes in functional connectivity. That is, by noninvasively
