Imaging the retina ’ s response to electrical stimulation with genetically 1 encoded calcium indicators 2 3

34 Epiretinal implants for the blind are designed to stimulate surviving retinal neurons, thus 35 bypassing the diseased photoreceptor layer. Single-unit or multielectrode recordings from 36 isolated animal retina are commonly used to inform the design of these implants. However, such 37 electrical recordings provide limited information about the spatial patterns of retinal activation. 38 Calcium imaging overcomes this limitation, as imaging enables high spatial resolution mapping 39 of retinal ganglion cell (RGC) activity as well as simultaneous recording from hundreds of 40 RGCs. Prior experiments in amphibian retina have demonstrated proof of principle, yet 41 experiments in mammalian retina have been hindered by the inability to load calcium indicators 42 into mature mammalian RGCs. Here we report a method for labeling the majority of ganglion 43 cells in adult rat retina with genetically encoded calcium indicators, specifically GCaMP3 and 44 GCaMP5G. Intravitreal injection of an adeno-associated viral vector targets roughly 85% of 45 ganglion cells with high specificity. Due to the large fluorescence signals provided by the 46 GCaMP sensors, we can now for the first time visualize the retina’s response to electrical 47 stimulation in real time. Imaging transduced retinas mounted on multielectrode arrays reveals 48 how stimulus pulse shape can dramatically affect the spatial extent of RGC activation, which has 49 clear implications in prosthetic applications. Our method can be easily adapted to work with 50 other fluorescent indicator proteins in both wild-type and transgenic mammals. 51 52