EEG Microstates Reveal Differential Network Dynamics under Constant Current and Oscillatory Brain Stimulation

Background: EEG microstates are brief, quasi-stable scalp topographies that index large-scale network dynamics and may sensitively capture the net effects of transcranial electrical stimulation (tES). Objective: To evaluate differential effects of three types of tES - tDCS, tACS and oscillatory tDCS - (otDCS), on canonical EEG microstates (A-D) in healthy adults. Methods: In a randomized, sham-controlled, crossover study, 42 participants completed four sessions (tDCS, tACS, otDCS, sham). Stimulation (20 min) used a P3-cheek montage: tDCS +1.5 mA; tACS at individualized theta frequency (ITF, 4-8 Hz), ±1 mA; otDCS anodal with ±0.5 mA oscillation around +1.5 mA at ITF. A five-minute resting EEG (eyes closed then eyes open) was recorded pre- and post-intervention. Microstates were extracted (A to D), back-fitted, and assessed on duration, occurrence, contribution, and mean GFP using linear mixed-effects models with sham and pre/post adjustments. Results: Four canonical microstates explained ~80% variance with stable topographies across conditions. Modulation patterns were modality-specific. MS A (sensory/arousal) increased across all active protocols, strongest after otDCS. MS B (visual-autobiographical) was consistently suppressed, again most following otDCS. MS C (self-referential) decreased selectively after oscillatory stimulation (tACS, otDCS) only. MS D (executive/attention) diverged by waveform: enhanced by tACS and otDCS but reduced by tDCS. Across outcomes, otDCS produced the largest and most widespread effects, overlapping features of both tDCS (tonic/stabilizing) and tACS (oscillatory/entraining) influences. Conclusions: Resting-state EEG microstates provide a sensitive systems-level assay of tES aftereffects. Constant and oscillatory current waveforms reorganize network states in dissociable ways, with otDCS exerting the most robust, comprehensive modulation. These findings support microstates as practical biomarkers for differentiating, optimizing, and monitoring neuromodulation strategies.