Activation of retinal ganglion cells following epiretinal electrical stimulation with hexagonally arranged bipolar electrodes

We investigated retinal ganglion cell (RGC) responses to epiretinal electrical stimulation delivered by hexagonally arranged bipolar (Hex) electrodes, in order to assess the feasibility of this electrode arrangement for future retinal implant devices. In vitro experiments were performed using rabbit retinal preparations, with results compared to a computational model of axonal stimulation. Single-unit RGC responses to electrical stimulation were recorded with extracellular microelectrodes. With 100 mu s/phase biphasic pulses, the threshold charge densities were 24.0 +/- 11.2 and 7.7 +/- 3.2 mu C cm(-2) for 50 and 125 mu m diameter Hex electrodes, respectively. Threshold profiles and response characteristics strongly suggested that RGC axons were the neural activation site. Both the model and in vitro data indicated that localized tissue stimulation is achieved with Hex electrodes.