Monkeys were premedicated with an intramuscular injection of ketamine (10 mg/kg). The intraocular pressure (IOP) in both eyes was measured using a calibrated tonometer (Tono-Pen XL; Mentor Ophthalmics, Inc., Norwell, MA). To avoid manual pressure on the globe, the lids were opened with a Barraquer-Colibri speculum. Anesthesia was induced with intravenous (IV) propofol, and the monkeys were intubated and maintained on an inhalation mixture of N2O and O2, supplemented with a continuous IV drip of propofol at a rate of 5 mg/kg per hour throughout the recording session. Eye movement was prevented by periodical IV boluses of a nondepolarizing muscle relaxant (0.1 mg/kg vecuronium or 0.45 mg/kg rocuronium). Expired CO2, heart rate, respiratory rate, core temperature, blood oxygen level, and blood pressure were monitored continuously and maintained within normal range. Pupils were dilated with topical tropicamide and atropine. Hypertension-induced damage was assessed by retinal tomography (Heidelberg Engineering, Heidelberg, Germany). Optical correction of the eyes was evaluated with the tomograph and manually with a retinoscope, and the eyes were optically corrected accordingly.
Jet contact lens electrodes (Metrovision, Perenchies, France) were placed on the cornea of each eye and served as an active or a reference electrode interchangeably. Hydroxyethylcellulose gel was applied for corneal protection and for good adhesion of the Jet lenses. A gold cup electrode attached to the shaved skin rostral to the ear served as a ground electrode. The eyes were recorded in random order.
A sequence of brief positioning recordings was conducted for each eye before commencing the experimental recordings. The eye’s position was adjusted so that the macula was in the center of the stimulated field, and the optic disc was included in the retinal area stimulated.