There is evidence, for example, for specialized circuits that convey directional selectivity (DS) (Barlow et al., 1964), segregate object from background motion (Lettvin et al., 1959), and anticipate moving stimuli (Berry et al., 1999). In addition, viral vectors now allow expression of calcium indicators such as GCaMP6s (Yin et al., 2013; Godat et al., 2022), enabling recording from single RGCs in vivo. Finally, optogenetic activation of RGCs has been demonstrated in both macaque (McGregor et al., 2020) and human (Sahel et al., 2021) retina in vivo. These recent technical advances taken together now offer the possibility of causal experiments in which single RGCs are optogenetically stimulated in the living primate eye while the behavioral consequences are measured. Here, we demonstrate the first step toward establishing this capability: showing successful stimulation of single RGCs in the living primate eye.