Personalized Theta Transcranial Alternating Current Stimulation and Gamma Transcranial Alternating Current Stimulation Bring Differential Neuromodulatory Effects on the Resting Electroencephalogram: Characterizing the Temporal, Spatial, and Spectral Dimensions of Transcranial Alternating Current Stimulation

Abstract

Objectives: The neuromodulatory effects of transcranial alternating current stimulation (tACS) on electroencephalogram (EEG) dynamics are quite heterogenous. The primary objective of the study is to comprehensively characterize the effects of two tACS protocols on resting-state EEG.

Materials and methods: A total of 36 healthy participants were recruited and were randomized into three groups. Two groups received either personalized theta (4-8 Hz) or gamma (40 Hz) stimulation bilaterally in the frontal regions for 20 minutes (4 minutes ON, 1 minute OFF, four cycles). The third group performed relaxed breath watching for 20 minutes. Artifact-free, 1-minute EEG segments from the baseline, during tACS, and after stimulation resting EEG were characterized to see the effects of tACS. Threshold-free cluster enhanced permutation tests (for spectral measures) and two-way mixed analysis of variance (for aperiodic slope) were used for statistical inferences.

Results: Current modeling simulation using ROAST with preset parameters (800 μA, AF3 AF4 locations) showed that induced electric fields can activate frontal cortical regions. During the stimulation period, personalized theta tACS entrained theta band power in the centro-parietal areas. There was a compensatory power decrease in the beta and gamma bands after theta tACS. No entrainment effects were observed for gamma tACS during stimulation, but a significant entrainment was observed in the theta and beta bands in the parieto-occipital regions after stimulation. The delta band power decreased in the central regions. No spectral modulations were seen after breath watching. The spectral slope, which measures aperiodic activity, was not affected by either breath watching or tACS.

Conclusions: Characterizing the effects of multiple tACS protocols is critical to effectively target specific neural oscillatory patterns and to personalize the protocols. The study can be extended to target specific oscillatory patterns associated with cognitive deficits in neuro-psychiatric conditions.

Keywords: Current modeling; EEG frequency bands; neuromodulation; personalized protocols; tACS.