A custom 512-electrode system (Hottowy et al., 2008 (link), 2012 (link), Grosberg et al., 2017 (link)) was used to stimulate and record from hundreds of RGCs in isolated rhesus macaque monkey (Macaca mulatta) retina. Retinas were obtained from terminally anesthetized animals euthanized in the course of experiments by other laboratories. Briefly, eyes were hemisected in room light following enucleation. The vitreous was then removed and the posterior portion of the eye containing the retina was kept in darkness in warm, oxygenated, bicarbonate buffered Ames’ solution (Sigma). Patches of retina ~3 mm on a side were isolated under infrared light, placed RGC side down on the multielectrode array, and superfused with Ames solution at 33 °C. Electrodes were 8–10 μm in diameter and arranged in a 16 × 32 isosceles triangular lattice with 60 μm spacing between adjacent electrodes (Litke et al., 2004 (link)). Electrodes were electroplated with platinum. Voltage recordings were band-pass filtered between 43 and 5,000 Hz and sampled at 20 kHz.