This study will last 6 weeks. Participants will have study visits each week for the duration of the study. During each visit, participants will be randomly assigned to receive either DMXB-A or placebo. An electrocardiogram (EKG) will measure the heart function of participants and participants' blood pressure will be measured. After the first dose of either DMXB-A or placebo, participants will receive a second dose 2 hours later. An evoked potential test, which measures the brain's response to stimuli, will be performed after both doses. Neuropsychological tests, such as verbal reasoning and visual retention, will be performed following the second dose of either DMXB-A or placebo.