The purpose of the study is to find out how a stimulation device worn on the ear works. This device is thought to stimulate nerves in the area around the ear to change the signals in the brain. The device has been shown to reduce pain and to reduce the symptoms of withdrawal. The investigator will also investigate changes in the way each participant perceive sensations of pressure and heat. The participant will be asked to reduce the amount of pain medication that they take. Then, the participant will spend several days and nights in the Clinical Research Center at UTMB (University of Texas Medical Branch) in Galveston. During that time, the participant will be monitored for withdrawal symptoms and will receive either active (e.g., real) brain stimulation or sham (e.g., fake) brain stimulation for two days (four hours each day). At two times over the course of the study (before and after ear stimulation treatment), the participant will complete questionnaires about their pain score and how they are feeling, sensory testing, and will undergo magnetic resonance imaging (MRI) of their brain. The investigator will collect the following information from the participant's medical record: age, gender, medication history, medical diagnoses, recent vital signs, past doctor visits or hospital stays, and results of urine drug tests. Participation in this study will last approximately four days, and the participant will stay in the Clinical Research Center.