All procedures were performed with animals under general anesthesia. In all cases, anesthesia was induced with 15 mg/kg intramuscular (IM) ketamine (Fort Dodge Animal Health, Fort Dodge, IA) and 1.5 mg/kg IM xylazine (Phoenix Scientific Inc., St. Joseph, MO), along with a single subcutaneous injection of atropine sulfate (0.05 mg/kg; Phoenix Scientific Inc.). Animals were intubated and breathed 100% oxygen. Body temperature was maintained with a warm-water heating pad set at 37°C. Pulse rate and oximetry were monitored (Propaq Encore unit, model 206EL; Protocol Systems, Inc., Beaverton, OR) and maintained between 85 to 125 per minute and 98% to 100%, respectively. Pupils were fully dilated (≥7 mm) with 1.0% tropicamide and 2.5% phenylephrine (Alcon Laboratories Inc., Fort Worth, TX). For all ERG sessions, anesthesia was maintained with ketamine (5 mg/kg per hour intravenously) and xylazine (0.8 mg/kg per hour IM). Topical corneal anesthesia was provided with 0.5% topical proparacaine (Alcon Laboratories Inc.) and an ocular lubricating agent (Celluvisc; Allergan, Irvine, CA) was periodically applied. Recording sessions lasted approximately 2 hours. During all other sessions (e.g., laser photocoagulation, stereo optic disc photography, fluorescein angiography), anesthesia was maintained with 2% to 3% isoflurane (Baxter, Deerfield, IL).