Multifocal Visual Evoked Potentials in the Anesthetized Non–Human Primate

Abstract

Abstract: : Purpose: To establish the feasibility of recording multifocal visually evoked potentials (mfVEPs) from the scalp in monkeys anesthetized with pentobarbital. Methods: One rhesus and one cynomolgus monkey with unilateral optic nerve transection (ONT) were tested longitudinally. Five cynomolgus and four rhesus monkeys (all ophthalmologically normal) were also tested. All animals were tested while deeply anesthetized by intravenous administration of sodium pentobarbital (15 mg/kg). This anesthetic regimen suppresses eye and body movements and has been used in our laboratory to reliably record multifocal electroretinograms (mfERGs) to a wide range of stimulus sizes. We used a large, seven–element hexagonal stimulus array that elicited reliable VEPs from corresponding portions of the central 88°. The central element of the stimulus array subtended ∼22° and the peripheral elements subtended ∼30°. Electroencephalographic activity from visual cortex was recorded from scalp electrodes inserted 1 cm superior to the occipital ridge and 1 cm lateral to the midline. Electroretinographic activity was recorded simultaneously from corneal contact lens electrodes. The fovea was aligned with the center of the display using a reversible direct ophthalmoscope. The stimulation sequence was a standard 13.33 ms base rate with an m–sequence length of 2¹⁵–1. VERIS Science^TM 4.9 software was used to extract the first–order kernel (K1) mfERG and second–order kernel, first slice (K2.1) mfVEP responses from the seven stimulus locations. Results: Stimulation of the non–transected eye in the ONT animals elicited robust K2.1 mfVEPs and K1 mfERGs from all areas of the visual field. Testing of the transected eye yielded no mfVEP but normal mfERGs from all retinal locations. Longitudinal recordings over one year of testing showed consistent signal/noise ratios of intact/ONT eye mfVEP RMS values. The nine ophthalmologically normal animals had large mfVEPs and mfERGs from all of the stimulated retinal regions. In all intact eyes tested, mfVEP responses were most pronounced in the center of the visual field. Conclusions: This study demonstrates that mfVEPs, uncontaminated by electroretinographic activity, can be reliably recorded from non–human primates under pentobarbital anesthesia on a long–term basis. mfVEPs from the non–human primate thus are a practical, non–invasive measure of retino–geniculo–striate pathway function following manipulations, including chronic experimental ocular hypertension, which differentially affects portions of the visual field.

Keywords: electrophysiology: non-clinical • retina: proximal (bipolar, amacrine, and ganglion cells) • lesion study