Prefrontal TDCS does not improve working memory performance in individuals with chronic alcohol and tobacco use

Working memory (WM) deficits are common in psychiatric disorders that are associated with decreased prefrontal cortex activity. As WM is essential for cognitive functions, deficits interfere with daily life and treatment. The lateralization of WM components remains unclear, but stimuli matching, often assessed using the n-back task, has been associated with right-hemispheric dominance. This study examines whether anodal transcranial direct current stimulation (tDCS) targeting the right dorsolateral prefrontal cortex (DLPFC) could enhance WM performance in alcohol-dependent patients (AD), tobacco users (TU) and healthy controls (HC). In a double-blind, sham-controlled study, tDCS was applied to upregulate right DLPFC activity. A total of 46 participants received anodal tDCS with a current intensity of 1.5 mA for 20 min or sham stimulation. In addition to the 7x5cm anode, a large reference electrode (10x10cm) was situated over the contralateral supraorbital area. While being stimulated, participants performed the n-back task as a measure of WM performance. Results revealed no significant differences in WM performance between active and sham stimulated participants, nor between groups, and no significant interaction between stimulation condition and group. Bayesian analysis supported the null effects. These findings do not provide evidence that single-session right DLPFC stimulation reliably enhances working memory across all stimulus types. The outcomes may have been influenced by task-stimulation mismatch, sample heterogeneity, small sample size, and stimulation parameters, which could limit the ability to detect subtle tDCS effects in both clinical and healthy populations.