Tuning the brain rhythms: How internal coherence influences network entrainment by tACS

Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain activity, but its effects depend critically on the ongoing dynamics of the targeted networks. Here, we use a biophysically plausible model of a recurrent excitatory–inhibitory neuronal network to investigate how the endogenous coherence of neural oscillations modulates network response to external periodic inputs. By systematically varying the level of background excitation, we manipulate the synchrony and the amplitude of emergent rhythms. We then apply sinusoidal inputs with varying frequency and intensity to probe neuronal and network-level entrainment. At the single-neuron level, we assess phase locking of the spiking of the individual neurons to the stimulation; at the network level, we analyze the coherence between the population activity and the stimulation signal. Our results reveal increased phase locking in a subset of neurons, which is more pronounced in inhibitory neurons. Crucially, we observe that the Arnold tongue, which refers to the range of frequencies and intensities over which the network entrains to the external drive, broadens significantly when the network’s endogenous coherence is low. These findings suggest that the initial state of brain oscillations plays a key role in determining tACS efficacy, with implications for individualized stimulation protocols.