EEG Changes in Schizophrenia Following tDCS: A Systematic Scoping Review

Introduction Transcranial direct current stimulation (tDCS) is investigated as an adjunct treatment in schizophrenia, but electroencephalographic (EEG) studies have produced inconsistent findings. Objective To review the literature and elucidate the effects of tDCS on EEG variables in schizophrenia. Method: This is a systematic scoping review according to PRISMA guidelines, consulting four databases: PubMed (MEDLINE), Cochrane Library, Web of Science and ScienceDirect. It was structured following PIO framework (Population, Intervention, Outcome): P: schizophrenia; I: tDCS; O: any EEG variable. For data synthesis, each time a variable was investigated, it was counted as an occurrence. Results A total of twenty-five papers were included, totaling forty-two occurrences: twenty-five were event-related potentials and seventeen were based on spectral/power, connectivity or coherence variables. Most papers applied 20 min of 2 mA stimulation (76%), in a bicephalic montage. The most investigated variable was the MMN, followed by N100, P300, EEG coherence, gamma activity, beta and alpha power. N100 was the variable that responded most to tDCS stimulation, with 80% response rate. Gamma activity had 67% response, MMN showed 60%, coherence, alpha and beta power 50%. All papers investigating P300 reported no significant results. Other EEG parameters were investigated only once. Conclusion EEG changes induced by tDCS in schizophrenia predominantly affected the sensory-auditory potential N100, had a lesser impact on pre-attentive potential MMN, and showed no observable effect on higher-order cognitive potentials, such as P300. The modulatory effects of tDCS on cognition are still unclear. This review was registered at the Open Science Framework (osf.io/7yzrj).