Safety was defined as the prevalence of Serious Adverse Events for the study. Subjects were monitored for Serious Adverse Events from date of first intervention session until the final follow-up visit (2 months post-intevention). Subjects were monitored using the Symptom Rating Questionnaire (SRQ), Medication/Medical Update Interview, and medical chart review. Serious Adverse Events were defined as: Death, life threatening incidents, hospitalizations (initial or prolonged), disability or permanent damage, congenital anomaly or birth defect, or an event that required intervention to prevent permanent impairment or damage. Mean and standard deviation of Serious Adverse Events was recorded across groups. A lower number indicates fewer Serious Adverse Events.

Brain activation change from pre-intervention to post-intervention was planned to be compard between active tDCS and sham groups. We hypothesized that the active tDCS group will have a larger increase in brain circuit engagement than the sham group and, thus, a better outcome.

Due to extreme motion artifacts preventing data acquisition, we were unable to collect minimum quantity of data necessary for the imaging analysis software to generate descriptive statistics.

Cognitive performance change was compared between active tDCS and sham groups. Cognitive performance change was defined as improvement on the WAIS-IV Digit Span (DS). Score was calculating by subtracting the DS scaled score at baseline from the DS scaled score at 2-Month Follow Up. We hypothesized that the active tDCS group will have a larger improvement in cognitive performance than the sham group. A higher number indicates a higher impact of cognitive training and, thus, a better outcome.

The DS Scaled Score has a range between 1 (min.) and 10 (max). Therefore, the computed difference between two DS Scaled Scores has a range of -9 (min.) to 9 (max.)

Small numbers of subjects due to COVID-19 pandemic, insufficient N for interpreting data.

Relapse was defined as any illiciit drug use (whether reported by the patient or reported as a positive drug screen in study or medical records) that occurred at some point between study intervention and the final follow-up visit (2 months post-intervention). Relapse was measured with the Timeline Follow Back questionnaire, saliva drug screen at the study visit, and chart review of urine drug screens. Relapse was coded 0 (did not relapse during the study) or 1 (relapsed during the study). We hypothesized that the active tDCS group will have a lower relapse rate than the sham group. A higher number indicates a higher count of participants with a relapse.

Four participants who were randomized but were lost to follow-up prior to the 2 month follow up were not included in these analyses.

"Durability of cognitive training was defined as improvement on the WAIS-IV Digit Scale Symbol/Coding (CD) test. The improvement period was measured between baseline and the study completion (2 months post-intervention). The score was calculated by subtracting the CD scaled score at baseline from the CD Scaled Score at 2-months post-intervention. A higher number indicates a higher impact on cognitive abilities, and a better outcome.

The CD Scaled Score has a range between 1 (min.) and 10 (max). Therefore, the computed difference between two CD Scaled Scores has a range of -9 (min.) to 9 (max.)"