Applying Neuromodulation to Boost the Efficacy of Motivational Interviewing

Motivational interviewing is a widely used counseling approach for substance use disorders, including cannabis use disorder. A central proposed mechanism of motivational interviewing is that clients resolve ambivalence by hearing themselves express reasons for change and reasons for maintaining current behavior. Prior empirical and meta-analytic research has linked these within-session speech patterns with subsequent substance use outcomes.

This proof-of-concept randomized, double-blind, sham-controlled trial tests whether transcranial alternating current stimulation (tACS) can modulate this mechanism during a motivational interviewing session for adults with cannabis use disorder. tACS is a noninvasive form of brain stimulation that delivers weak sinusoidal electrical current through scalp electrodes. In this study, stimulation is designed to target beta-band activity in left inferior temporal cortex, a region and frequency range implicated in speech acts and commitment-related language. Pilot work suggests that beta-band activity may be stronger during statements favoring behavior change than during statements favoring continued substance use, and that tACS during motivational interviewing is feasible.

Participants are assigned to motivational interviewing with active tACS or motivational interviewing with sham stimulation. Both groups receive a single motivational interviewing session focused on cannabis-use ambivalence while the stimulation device and scalp electrodes are in place. The active condition includes approximately 30 minutes of 20-Hz beta-frequency stimulation focused on left inferior temporal cortex at 2.0 mA total combined current across electrodes. The sham condition uses the same setup with brief ramping procedures to emulate the sensations of active stimulation but without active stimulation during the intervention period.

The study evaluates whether active tACS, compared with sham stimulation, affects client speech during the motivational interviewing session, post-session brain activity, and cannabis use during the month after the session. Session audio is analyzed to quantify speech that favors change and speech that favors continued cannabis use. Neuroimaging measures include magnetoencephalography (MEG) and magnetic resonance imaging (MRI) after the intervention session. Post-stimulation resting MEG is co-registered to structural MRI to estimate, with particular interest in beta-band activity in left temporal cortex in MEG. Near-term cannabis use is assessed with daily self-report check-ins and a follow-up assessment approximately one month after baseline.

The trial is intended to provide mechanistic evidence about whether noninvasive neuromodulation can alter a candidate process of behavior change during psychotherapy. The study is not designed to provide ongoing substance use disorder treatment, but to test whether pairing tACS with a brief motivational interviewing session changes speech, brain activity, and short-term cannabis-use outcomes relative to a sham-stimulation comparison.