• Fenne M Smits, Elbert Geuze, Guido J de Kort, Karlijn Kouwer, Lisa Geerlings, Jack van Honk, and Dennis J L G Schutter.
• Brain Research & Innovation Centre, Ministry of Defence, Utrecht, The Netherlands; Department of Psychiatry, UMC Utrecht Brain Center, University Medical Center Utrecht, Utrecht, The Netherlands. Electronic address: F.M.Smits-2@umcutrecht.nl.
• Neuromodulation. 2023 Jun 1; 26 (4): 817828817-828.

ObjectivesTop-down stress regulation, important for military operational performance and mental health, involves emotional working memory and the dorsolateral prefrontal cortex (DLPFC). Multisession transcranial direct current stimulation (tDCS) applied over the DLPFC during working memory training has been shown to improve working memory performance. This study tested the hypothesis that combined tDCS with working memory training also improves top-down stress regulation. However, tDCS response differs between individuals. Resting-state electrophysiological brain activity was post hoc explored as a possible predictor of tDCS response. The predictive value of the ratio between slow-wave theta oscillations and fast-wave beta oscillations (theta/beta ratio) was examined, together with the previously identified tDCS response predictors age, education, and baseline working memory performance.Materials And MethodsHealthy military service members (n = 79) underwent three sessions of real or sham tDCS over the right DLPFC (anode: F4, cathode: behind C2) at 2 mA for 20 minutes during emotional working memory training (N-back task). At baseline and within a week after the tDCS training sessions, stress regulation was assessed by fear-potentiated startle responses and subjective fear in a threat-of-shock paradigm with instructed emotional downregulation. Results were analyzed in generalized linear mixed-effects models.ResultsThreat-of-shock responses and emotional working memory performance showed no significant group-level effects of the real vs sham tDCS training intervention (p > 0.07). In contrast, when considering baseline theta/beta ratios or the other tDCS response predictors, exploratory results showed a trait-dependent beneficial effect of tDCS on emotional working memory training performance during the first session (p < 0.01).ConclusionsNo evidence was found for effectivity of the tDCS training intervention to improve stress regulation in healthy military personnel. The emotional working memory training results emphasize the importance of studying the effects of tDCS in relation to individual differences.Clinical Trial RegistrationThis study was preregistered on September 16, 2019, at the Netherlands Trial Register (www.trialregister.nl) with ID: NL8028.Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.

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