Participation in this study includes four main visits and an optional follow-up visit. The first visit is a baseline interview session to assess participant eligibility using a structured interview. The second visit is an initial fMRI scan at the fMRI center. The third visit is the anesthetic infusion at the hospital; management of any adverse events during the infusion will be monitored by the Post Anesthesia Care Unit ( PACU) personnel. The fourth visit is a follow-up fMRI scan at the fMRI center. There is an optional one week follow-up visit at the fMRI center for a final fMRI scan.