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.