Counter-empathic pleasantness as a potential mediator of single-session HD-tDCS effects on laboratory retaliatory aggression: A randomized sham-controlled trial in healthy young adults.

Retaliatory aggression may be influenced not only by cognitive-control processes but also by counter-empathic pleasantness in response to an offender's suffering. In this randomized, double-blind, sham-controlled study, 42 healthy young adults were randomized to real or sham Fpz-centered high-definition transcranial direct current stimulation (HD-tDCS). One participant per group withdrew, leaving 20 per group for analysis. Real stimulation was delivered at 2 mA for 25 min, whereas sham current was applied only during the first 60 s. Before and after stimulation, participants rated the pleasantness of seeing fair or unfair targets receive electric shocks and selected the final shock intensity ostensibly delivered to each target. Compared with sham stimulation, real HD-tDCS produced larger reductions in selected shock intensity for both fair and unfair targets. Counter-empathic pleasantness also decreased overall; at lower initial shock intensities, the reduction was not clearly condition-specific, whereas at higher initial intensities it was mainly observed for unfair targets. Mediation analyses showed a significant indirect effect through pleasantness overall and in the insufficient-punishment condition, but not in the expected-punishment condition. Reductions in frontal N2 and parietal LPP event-related potential (ERP) amplitudes following real HD-tDCS did not differ significantly between fair and unfair contexts. Behavioral, ERP, and mediation analyses were hypothesis-driven, whereas phase transfer entropy was exploratory. Together, these findings indicate that single-session Fpz-centered HD-tDCS can alter immediate laboratory shock-selection behavior in healthy young adults and that changes in counter-empathic pleasantness may partly account for this effect, providing behavioral and electrophysiological evidence linking Fpz-centered neuromodulation to retaliatory punishment.