In this natural history study, participants will be followed for at least five years. Because three years may be required to enroll 25 participants, this study will last up to eight years. All participants will undergo a standardized medical/ophthalmic history and a complete baseline eye examination, including non-invasive electrophysiology (e.g., electroretinography), psychophysiology (e.g., microperimetry, static perimetry), and diagnostic imaging examinations (e.g., optical coherence tomography). In addition, participants will undergo a detailed neurology exam, neuroimaging (MRI, including special sequences) and neuropsychologic assessment.