Animals were dark adapted for 12 to 15 hours, first overnight and then, on the morning of the recording, until they were prepared for the recording session. Animals were anesthetized by intraperitoneal injection of ketamine and xylazine (60 and 7 mg/kg, respectively; Parnell Manufacturing Pty. Ltd., Alexandria, NSW, Australia) and then prepared for recording under dim red illumination. Mydriasis was achieved with topical application of atropine sulphate 1.0% (Bausch & Lomb Australia Pty. Ltd., Macquarie Park, NSW, Australia). Proxymetacaine (0.5%; Alcon Laboratories Pty. Ltd., French Forrest, NSW, Australia) was applied topically for corneal anesthesia. The eyeball was drawn forward with a loosely tied thread behind its equator to minimize contamination of recordings by lid activity. Corneal hydration was maintained during recordings with Carbomer (polyacrylic acid) (2 mg/g; Novartis Pharmaceuticals, North Ryde, NSW, Australia). The animal's body temperature was reliably maintained at 37°C to 37.5°C as monitored with a rectal temperature probe (Harvard Apparatus, Holliston, MA, USA).