Tolerability and blinding efficacy of personalized theta-modulated transcranial electrical current stimulation

Abstract

Transcranial electrical stimulation (tES) is widely used in neuromodulation research. Numerous studies show that the conventional type of tES – transcranial direct current stimulation (tDCS) is a safe and well-tolerated technique. An increasingly popular line of research focuses on implementing alternative tES protocols aimed at the entrainment of brain oscillations. Thus, there is a need for a detailed assessment of the side effects profile and blinding efficacy of such protocols. Here we evaluate the tolerability and blinding efficacy of two personalized oscillatory tES protocols: Transcranial alternating current stimulation (tACS) and oscillatory tDCS (otDCS), applied at individual theta frequency for each participant (4-8Hz). Forty-two young, healthy individuals took part in a within-subject experiment in which they went through four stimulation conditions in a counterbalanced order: TACS (2mA peak-to-peak), otDCS (1.5mA±0.5mA), constant tDCS (1.5mA), and sham. The participants rated discomfort level at four timepoints during the 20-minute stimulation. Before and after each stimulation condition, they completed the standardized side effects questionnaire. The successfulness of blinding was assessed with the end-of-study guess of the sham condition. Overall, the reported discomfort level was low and comparable across all stimulation conditions, with otDCS being slightly more unpleasant than sham. Mild tingling and itching sensations were the most common side effects. No serious side effects were detected. Participant blinding was successful, and sham-guessing was not related to the reported discomfort in any of the stimulation conditions. The data has shown that personalized theta otDCS and tACS are well-tolerated and safe, with adequate blinding success.