 The study consists of one outpatient screening session and one experimental session. Informed consent and a history and neurological examination will be carried out at the screening visit. Vital signs pulse oximeter, heart rate, respiratory rate and skin temperature will be measured immediately before and after the experimental scan session, and pulse oximeter, heart rate and respiration will be monitored when the subject is in the scanner. Subjects will remain in the scanner for approximately 1 hour. During the experimental session, while the subject is in the scanner, they will be asked to carry out a brief test of attention at 10-15 minute intervals, up to 3 times during the session. At the end of the experimental session, subjects will fill out the MRI exit questionnaire to rate subjective sensations such as nausea, vertigo, and lightheadedness, on a Likert scale and complete a computerized neuro-cognitive test battery. 