P295 How baseline activity determines neural entrainment by transcranial alternating current stimulation (tACS) in recurrent inhibitory-excitatory networks
1Zapata-Briceño Institute of Neuroscience, Madrid, Spain 2Physics Department, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan, Iran
*Email: saeed.taghavi.v@gmail.com
Introduction Neuronal oscillations play a key role in cognition and can be modulated by transcranial alternating current stimulation (tACS). However, the mechanisms underlying network-level entrainment remain unclear. We investigate how a balanced excitatory-inhibitory network of adaptive exponential integrate-and-fire neurons responds to sinusoidal stimulation. We analyze phase-locking to determine how external rhythmic inputs influence neural synchronization at different baseline network states. Methods We simulate a recurrent EI network that receives Poisson-distributed background input. Three baseline synchronization levels are studied, reflecting the degree of natural synchronization in neuronal activity within the network before any external stimulation is applied. Additionally, tACS-like stimulation is applied at frequencies ranging from 5 to 60 Hz with five different amplitudes (3, 4, 6, 8, and 10). Each condition is repeated over nine trials to ensure reliability. To quantify network entrainment, we compute the phase-locking value between the population activity and the stimulation. Furthermore, we calculate the spike-field coherency of individual neurons and measure changes in SFC with and without stimulation to assess how neuronal firing aligns with the external signal. Results Our results show that baseline network synchrony strongly influences entrainment. Networks with higher intrinsic synchrony exhibit stronger phase locking with the stimulation. When the stimulation frequency is close to the endogenous frequency, PLV increases with stimulation amplitude, suggesting that stronger inputs enhance entrainment only when the stimulation frequency matches the endogenous frequency. Frequency-dependent effects emerge, with the most robust responses occurring near the network’s intrinsic oscillation frequency. Individual neurons display varying phase coherence, with some aligning strongly to the stimulation while others remain weakly affected. Discussion
We discovered that tACS-induced neural entrainment behaves in a way that challenges conventional expectations. While you might assume higher baseline synchrony leads to broader entrainment, we found the opposite. Networks with low baseline synchrony actually exhibit broader locking across a wider range of external frequencies. Conversely, highly synchronized networks show stronger locking, but it's tightly confined to the baseline frequency's vicinity. This counterintuitive result underscores the delicate balance between baseline synchrony and tACS effectiveness, highlighting the need for nuanced approaches in cognitive and therapeutic applications.
Figure 1. (a) Entrainment of population activity to tACS varies with network synchrony and stimulation strength. Higher synchrony or amplitude increases peak PLV but narrows the high-PLV region. (b) Stimulation does not significantly alter firing rates but enhances phase coherence. (c) Spike phase coherence changes shows a peak when stimulation matches network frequency. Acknowledgements
