Phase-synchronized tACS-induced neural oscillations modulate cortico-cortical signaling efficacy

Introduction Synchronized oscillatory brain activity is considered a basis for flexible neuronal network communication. However, the causal role of inter-regional oscillatory phase-relations in modulating signaling efficacy in cortical networks has not been directly demonstrated in humans so far. Aim The current study addresses the causal role of tACS-induced oscillatory cross-network phase-relations in modulating signalling efficacy across human cortical networks. Methods To this end, concurrent transcranial alternating current stimulation (tACS), transcranial magnetic stimulation (TMS), and electroencephalography (EEG) was employed to measure the modulation of excitability and signaling efficacy across cortical networks during externally induced neural oscillations. Theta oscillatory activity was introduced through tACS in two nodes of the human frontoparietal network; the dorsolateral prefrontal cortex (DLPFC) and the posterior parietal cortex (PPC). 6 Hz tACS was applied to the DLPFC and PPC simultaneously in an in-phase or antiphase manner. Additionally, single-pulse TMS was administered over the DLPFC at four different phases of tACS and the propagation of TMS-evoked neuronal activity was measured with EEG. Results We show that tACS-induced theta oscillations modulate TMS-evoked potentials (TEPs) in a phase-dependent manner, and that the induced oscillatory phase-relation across the frontoparietal network affects the propagation of phase-dependent TEPs within as well as beyond the frontoparietal network. Conclusion We show that the effect of tACS-induced phase-relation across the frontoparietal network on signal transmission extends beyond the frontoparietal network. The results support a causal role of inter-nodal oscillatory phase-synchrony in routing cortico-cortical information flow.