The third visit would take place at the Center for Magnetic Resonance Research (CMRR) and would include a brain MRI. The MRI scan is noninvasive and takes about 2 hours. Participants will be asked to provide a urine sample and complete a urine toxicology screen (drug test) and pregnancy test (if female) prior to the MRI scan. Participants may not participate if drug or pregnancy testing is positive. The MRI involves taking pictures of the brain, from which measures of the integrity of certain brain tissues can be derived. For the scan, participants will be asked to lie down quietly on a scanner bed. Once they are inside the scanner, it will start to take pictures. While in the scanner, participants will be doing things like resting, or listening to music.