Neural changes to transcranial alternating current stimulation in the gamma range over the left frontoparietal network: a preliminary eLORETA EEG study

Applying transcranial alternating current stimulation (tACS) at 40 Hz to the frontal and parietal regions can improve cognitive dysfunctions. This study aimed to explore the neural changes following transcranial electrical stimulation treatment. Electroencephalography (EEG) recordings were obtained from a cohort of 34 participants with various cognitive impairments before and after 12 sessions of tACS treatment. Alternating currents at 2.0 mA were administered to the electrode positions F3 and P3 for 20 minutes of each session, following the 10-20 EEG convention. Using eLORETA, scalp-recorded signals were reconstructed into cortical current source density (CSD). We then assessed the differences in power and connectivity strength across multiple spectra (delta to gamma). We observed a consistent trend of decreased CSD at the stimulating sites across different spectra, most prominent at beta and gamma bands (P < 0.01). On the contrary, the right hemisphere showed a trend of increased CSD, which was likely mediated by inter-hemispheric rivalry. In addition, the connectivity strength between the left frontal and parietal regions increased significantly (P = 0.017). Artificial injections of tACS would de-synchronize regional oscillation and enhance inter-regional crosstalk. The pattern of neural changes was concordant with our previous tACS reports (5-Hz), suggesting common neural mechanisms driving the neurophysiological effects of tACS.