Transcranial focused ultrasound (tFUS) is a new noninvasive way to stimulate the brain in an awake and alert person. We do not yet have an easily observable way to know whether we are in the right brain location with the correct dose for that person. We wonder if we can produce a transient change in someone's visual field, called a scotoma, and whether we can use that to determine the minimum tFUS dose for that person.