Applying Neuromodulation to Boost the Efficacy of Motivational Interviewing

The goal of this clinical trial is to learn how adding brain stimulation to a brief counseling session affects adults with cannabis use disorder. The counseling session uses motivational interviewing, which helps people talk through their reasons for changing or not changing cannabis use. The brain stimulation is called transcranial alternating current stimulation (tACS). tACS uses electrodes on the scalp to give a very weak electrical current for a short time. The main questions it aims to answer are: * Does active tACS affect what participants say during the counseling session? * Does active tACS change brain activity after the session? * Is active tACS related to less cannabis use during the month after the session? Researchers will compare motivational interviewing with active tACS to motivational interviewing with placebo stimulation. Placebo stimulation uses the same setup but does not provide the active 30-minute stimulation. Participants will be assigned by chance to one of these groups, and neither participants nor the study staff member running the session will know which type of stimulation is used. Participants will complete questionnaires, breath and urine tests, a recorded counseling session during active or placebo stimulation, MEG and MRI brain scans, daily text-message check-ins about cannabis use, and follow-up questionnaires about one month later.

Inclusion Criteria: * self-identification as having problematic cannabis use * age 21-65 years * SCID-5-RV module E indicating current cannabis use disorder Exclusion Criteria: * currently engaged in treatment for any substance use disorder (SUD) * receiving treatment for any SUD within the prior 30 days * history of brain injury or major neurological diagnosis, vascular/neurodegenerative disorder, or complex primary headache disorder * evidence of current psychosis * past-year substance use disorder other than cannabis, alcohol, or nicotine * positive urine drug screen for any illicit drug other than cannabis * current or recent (past-month) medically significant withdrawal * MRI/tACS contraindications, including pregnancy * impaired hearing (psychotherapy using American Sign Language is not possible within the scope of this study) * insufficient corrected visual acuity to complete the assessment instruments * unable to read/speak English fluently * unable to provide valid informed consent