"Is tDCS-induced Cognitive Improvement Associated with Better Treatment Outcome in Addiction? "
We previously reported that tDCS combined with cognitive training may increase functional connectivity between the left dorsolateral prefrontal cortex (LDLPFC) and the negative emotionality network, reducing relapse risk in substance use disorder (SUD) (Camchong et al., 2023). The present study further investigated potential mechanisms underlying this intervention, with a focus on cognitive flexibility, a core executive function involved in adaptive decision-making, behavioral control, and resistance to habitual substance-use patterns. Specifically, we examined whether cognitive flexibility improved over the course of the intervention and whether cognitive change differed by treatment outcome. 32 participants were recruited from a residential treatment program, and complete data were available for 19 individuals in recovery from SUD. Participants were randomly assigned to active tDCS or sham and completed five days of cognitive flexibility training. Transfer of cognitive flexibility was assessed at pre- and post-intervention using switch cost on the Number-Letter Switching Task. Treatment outcomes, including relapse status and days abstinent, were assessed at 1- and 2-month follow-up after discharge. Repeated-measures ANOVA showed that switch cost decreased from pre- to post-intervention across participants, indicating improved cognitive flexibility following the intervention. However, cognitive change scores were not significantly associated with days abstinent at either follow-up, and abstainers and relapsers did not differ significantly in the magnitude of cognitive improvement. At 1-month follow-up, relapsers showed greater switch cost reduction than abstainers, suggesting that cognitive improvement alone may be insufficient to predict treatment outcome. These findings suggest that tDCS combined with cognitive training may improve cognitive flexibility, but its relationship to abstinence remains unclear. Future research should examine whether baseline factors, such as impulsivity, moderate treatment response in larger samples.