Abstract:

Transcranial alternating current stimulation (tACS) is a non-invasive electrical neuromodulation technique that delivers periodic microcurrents of specific frequencies via electrodes placed on the scalp, thereby modulating neural oscillations in targeted brain regions. By entraining these oscillations, tACS can alter specific cognitive functions or alleviate clinical symptoms. A recent development, temporal interference (TI) stimulation, extends the capability of tACS by superimposing two or more high-frequency currents with slightly different frequencies to generate low-frequency envelope signals in deep brain areas, such as the hippocampus, enabling targeted modulation beyond cortical regions. Since its introduction into psychological research in 2008, tACS has been widely used to elucidate causal links between neural oscillations across distinct frequency bands and various cognitive processes, offering a powerful approach to probe the functional roles of brain rhythms.

The primary mechanism of tACS is the synchronization of neural electrical activity. By delivering alternating currents at specific frequencies, tACS can align neural oscillations in the target brain region to align in phase. This phase-dependent modulation induces rhythmic fluctuations in presynaptic membrane potentials, thereby facilitating synaptic plasticity and dynamically regulating cortical excitability. Specifically, the mechanism can be described at three levels: Regulation of local neural oscillations, optimization of brain network connectivity, and enhancement of neural plasticity. At the local oscillation level, tACS induces synchronization between endogenous oscillations and exogenous currents by applying stimulation that matches the intrinsic rhythm of the target region. At the network level, tACS modifies synchronization and coupling properties of endogenous oscillations, including frequency-specific synchronization and cross-frequency phase-amplitude coupling, by delivering alternating currents with adjustable phase differences across brain regions. At the plasticity level, the modulatory effects of tACS rely on the activation of synaptic plasticity, which can persist for minutes to hours after stimulation ends.

tACS modulates neural oscillations at specific frequencies to regulate the synchronization and coupling states of brain functional networks, thereby influencing a wide range of cognitive functions. This frequency-specific modulation provides a novel perspective for elucidating the neural basis of cognitive processes and advancing interventions for mental and neurological disorders. Evidence from previous studies suggests that tACS at different frequencies exerts relatively distinct functional effects: Alpha band primarily influences sensory processing and spatial attention; beta band motor control; and theta- and gamma- band play critical roles in learning, memory, and emotion regulation. Furthermore, in the field of interpersonal interaction research, tACS has been shown to enhance neural synchrony between brains, thereby promoting collaboration, intention understanding, and social learning.

Currently, the field of tACS remains in an early stage of development, with its mechanisms of neural activity, practical application methods, and stimulation parameters yet to be fully elucidated. Therefore, we propose that future research should focus on three key aspects. First, precise control of the phase relationship between the applied alternating current and the brain’ s spontaneous EEG rhythms is essential. Most existing studies have neglected this phase difference, and misalignment may weaken the entrainment effect of tACS on spontaneous oscillations, reducing intervention efficacy. Achieving accurate neural modulation requires real-time monitoring of spontaneous EEG phase before and during stimulation. Novel algorithms such as stimulation artifact source separation (SASS) can support this process by enabling real-time phase adjustment to maintain optimal phase alignment, thereby maximizing neural entrainment. Furthermore, the development of closed-loop modulation systems employing adaptive tACS protocols is needed to dynamically regulate EEG rhythms. Second, individual differences must be taken into account for personalized tACS protocols. Some studies highlight the benefits of personalized stimulation frequencies, while optimizing stimulation targets based on individual anatomy has been shown to enhance regulatory effects. In addition, incorporating control frequencies in tACS research is crucial for accounting for non-specific effects and validating the specificity of particular stimulation frequencies, thereby strengthening causal inferences. Third, systematic evaluation of treatment protocols and the persistence of therapeutic effects in clinical applications is needed. Although some studies have investigated the lasting efficacy of tACS in patients with depression, Parkinson’ s disease, and other disorders, follow-up durations have been relatively short, insufficient to translate immediate effects into long-term benefits.

Overall, tACS has become a powerful tool for investigating the causal links between neural oscillations and cognitive processes in the human brain, and future research should focus on achieving precise neural modulation and conducting systematic clinical evaluation to better elucidate underlying mechanisms and guide therapeutic applications.

Key words: transcranial electrical stimulation, transcranial alternating current stimulation, brain oscillations, cognitive function, emotion regulation, clinical treatment