Patients with neurological disorders will be excluded. In particular, persons with a personal or family history of seizures or epilepsy; structural cerebral disorders (e.g. hemiplegia or other abnormalities of the corticospinal tracts). We will also exclude from our study any persons who have a personal or family history of sensorineural loss. Person with abnormally decreased pigmentation (albinism) will also be excluded from the study.