Task repetition and acute tDCS effects on emotion–attention processing: Evidence from ERP measures in a within-subject crossover study

Transcranial direct current stimulation (tDCS) is widely used to probe cognitive-affective processes, yet evidence for reliable single-session effects in healthy adults remains inconsistent. We examined whether anodal tDCS was associated with behavioral and event-related potential (ERP) changes during emotional distraction, using a within-subject, double-blind crossover design to distinguish stimulation-order effects from changes across repeated task blocks. Fifty-three healthy adults performed a color-detection task with emotional face distractors during EEG recording. Each session included baseline (T0), post-first stimulation (T1), and post-second stimulation (T2) blocks. Participants received both 20-min anodal tDCS (1.5 mA; C3-right supraorbital montage) and sham stimulation in counterbalanced order. Reaction times and P1, N170, P2, and LPP amplitudes were analyzed across all three blocks, with stimulation order as a between-subject factor. Reaction times showed a modest overall time effect without significant pairwise differences. Posterior P1 amplitudes increased from T0 to T1 and remained elevated at T2. Posterior P2 amplitudes increased from T0 to T1 in the broad-region analysis, but this effect was attenuated in the component-specific occipital sensitivity analysis. N170 amplitudes varied with facial emotion and showed a modest overall time effect without significant pairwise differences, whereas the predefined 500-800 ms LPP showed no significant time effect. No primary behavioral or ERP outcome showed a significant TIME × ORDER interaction. Repeated task assessment was therefore associated most clearly with early posterior ERP changes, whereas reliable order-dependent effects of acute tDCS were not detected under the present protocol.