Thresholds for Activation of Rabbit Retinal Ganglion Cells with a Subretinal Electrode

The ultimate success of a retinal prosthesis to create vision will likely depend upon developing a base of knowledge of how best to electrically stimulate the retina. Previously, we studied the responses of rabbit retinal ganglion cells (RGCs) to current pulses applied with an electrode placed on the epiretinal surface. In the present study, we examined the responses of rabbit RGCs to current pulses applied with a subretinal electrode. Single-unit extracellular recordings were made from OFF RGCs and ON RGCs in isolated retinas, which were stimulated with monophasic current pulses (0.1-50ms in duration), delivered through a 500-mum diameter electrode. All RGCs elicited one or more bursts of action potentials upon electrical stimulation of the retina. The timing of the bursts depended upon both the polarity of the electrical stimulus and the RGC type. With near-threshold current pulses, the response latencies of OFF RGCs to anodal stimulation were comparable to those of ON RGCs to cathodal stimulation, whereas the response latencies of OFF RGCs to cathodal stimulation were comparable to those of ON RGCs to anodal stimulation. Threshold currents for activation of RGCs decreased with increased pulse duration. For OFF RGCs, threshold currents for cathodal current pulses were, on average, 2-7.5 times higher (depending upon pulse duration) than the threshold currents for anodal current pulses. For ON RGCs, threshold currents for cathodal and anodal current pulses were, on average, nearly identical for all pulse durations and were equivalent to threshold currents of OFF RGCs to anodal stimulation. With respect to a subretinal prosthesis, our findings suggest the possibility that cathodal current pulses may bias activation of ON RGCs in blind patients.