Activation of ganglion cell axons following epiretinal electrical stimulation with hexagonal electrodes

A hexagonal electrode configuration has been proposed as an advantageous alternative to conventional electrode arrangements used in retinal prosthesis design. In the present study, the aim was to characterize retinal ganglion cell axonal responses to epiretinal electrical stimulation. 50 and 125 microm disk electrodes, arranged in a hexagonal configuration, were tested using in vitro rabbit retinal preparations. 100 micros/phase anodic-first biphasic current pulses were applied to the inner retinal surface, and ganglion cell responses were recorded differentially with extracellular microelectrodes. Axonal activation thresholds were 4.7 ± 2.5 microA for 50 microm, and 9.3 ± 4.0 microA for 125 microm electrodes. With anodic monophasic pulses there was a 3.3 ± 0.8 times increase in threshold, compared to anodic-first biphasic stimulation. Thresholds increased up to 20 times when stimulating electrodes were lifted 100 microm above the retinal surface. Overall, axonal activation thresholds were within the safe charge injection limits for platinum electrodes, given that these electrodes were positioned in close proximity to the retinal surface.