1. From The Howard Hughes Medical Institute, and the Departments of 2Ophthalmology and Visual Sciences, and
2. the Departments of Ophthalmology and Visual Sciences, and 3Pediatrics, University of Iowa, Iowa City, Iowa;
3. the Departments of Ophthalmology and Visual Sciences, and
4. the Departments of Ophthalmology and Visual Sciences, and 4Veterans Administration, Iowa City, Iowa; and
5. Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois.