The relapsing nature of alcoholism is a major obstacle to successful treatment. About 60% of those entering treatment will relapse within one year. To improve treatment outcome, new interventions targeting the underlying brain biomarkers of relapse vulnerability hold significant promise in reducing this critical public health problem. Using resting functional magnetic resonance imaging (fMRI) the investigators have identified brain biomarkers that support long-term abstinence. Cross-sectional and longitudinal findings provide evidence that higher functional connectivity (FC), particularly between nucleus accumbens (NAcc) and dorsolateral prefrontal cortex (DLPFC), is a potential brain biomarker that supports abstinence. Long-term abstinent alcoholics (7 years of abstinence) have higher resting FC between NAcc and DLPFC when compared to controls. Short-term abstinent alcoholics (11 weeks of abstinence) have intermediate FC (lower than long-term abstinent alcoholics and higher than controls). Further, lower FC between NAcc and DLPFC at 11 weeks of abstinence can be a predictor of subsequent relapse (with 74% accuracy). Moreover, in a pilot longitudinal FC study examining resting FC of NAcc at 5 and 13 weeks of abstinence in individuals with substance use disorder, the investigators found that FC between NAcc and DLPFC decreased from 5 to 13 weeks of abstinence in subsequent relapsers, while it increased in subsequent abstainers. Based on the above, the investigators believe that long-term abstinence is supported by a compensatory mechanism that mediates proper executive function over reward (mediated by PFC-NAcc FC), a potential brain biomarker that could be an intervention target. These findings provide a compelling case to explore whether this brain biomarker can be modulated to enhance patients' ability to remain abstinent. There is a need to investigate methods that can be used to increase FC between DLPFC and NAcc.

Cognitive flexibility, the ability to change maladaptive behavior, depends on DLPFC input to NAcc. DLPFC transmits reward representations to NAcc through glutamatergic projections that guide goal-directed behavior. If DLPFC fails to activate when required, a common observation in substance use disorder, target neurons in the NAcc core do not receive critical information needed to select the appropriate outcome, causing acquired maladaptive response patterns to persist (e.g. drug consumption). Higher FC between DLPFC and NAcc may be achieved by stimulating DLPFC while subjects perform a task that requires cognitive flexibility, the reversal learning task.

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate brain connectivity. DLPFC stimulation may increase input to NAcc to facilitate proper selection of goal-directed behavior and may also decrease craving in individuals with substance use disorder.

Genetics and treatment response: A source of treatment response variability could stem from differences between participants in baseline genetic profiles or epigenetic changes over the course of treatment. Genetic polymorphisms, especially in genes important for neuroplasticity, may also mediate neuroplastic changes underlying the effects of tDCS, as has been demonstrated with gene BDNF. In light of these genetic influences on key tenants of the study - i.e. treatment response in alcohol use disorder and psychological effects of tDCS - the investigators will collect genetic samples from participants to determine whether genetic or epigenetic variations may affect response to the cognitive training and tDCS intervention. As the secondary aim, participants will be given the choice in the consent form to opt in or out of the genetic sampling procedure.

In a double-blind randomized design, up to 100 abstinent (2 weeks abstinent) individuals with alcohol use disorder (AUD) recruited from Lodging Plus Program will receive 10 sessions (2 sessions per day for 5 days) of either (i) transcranial direct current stimulation (tDCS) to PFC or (ii) sham-tDCS. All subjects will perform the reversal learning task during tDCS intervention (active or sham) to prime the engagement of the NAcc-PFC brain circuit that mediates cognitive flexibility. Rest fMRI and craving measures will be collected before the first and after the last day of tDCS sessions. Monthly follow-up interviews will be conducted for 6 months after study completion to query relapse status. Dependent variables will be (i) change in NAcc-PFC FC between 2 and 3 weeks of abstinence, (ii) change in craving scores between 2 and 3 weeks of abstinence and (iii) relapse status \~8 months after study participation. Aim 1: To investigate whether NAcc-PFC FC can be modulated, the investigators will compare magnitude and durability of change in NAcc-PFC FC between active-tDCS and sham-tDCS groups. Aim 2: To determine if PFC stimulation has a short-term effect on behavior related to clinical outcome, the investigators will compare change in craving scores (difference in craving scores between 2 and 3 weeks of abstinence) between active-tDCS and sham-tDCS groups. Aim 3: To correlate neuromodulation intervention with long-term clinical outcome, the investigators will record craving and relapse status during the \~8 months following treatment discharge. The investigators will examine the relationship between change in NAcc-PFC FC between 2 and 3 weeks of abstinence and subsequent (i) monthly craving scores and (ii) relapse status. Aim 4: Examine relationship between genetic variants, epigenetic changes and treatment outcome (e.g. durability, functional connectivity).