Participants undergo the tests and procedures required in the NEI study in which they previously enrolled. In addition, for the current study, they undergo metabolic mapping. For this procedure, the subject's eyes are dilated, and different amounts of low-level light are shone into the eye to see how different cells respond with changes in metabolism. Measurements are taken while the subject breathes room air and while he or she breathes medical grade oxygen for about 1 minute. The entire procedure takes about 15 minutes.