Existing epiretinal implants for the blind are designed to electrically stimulate large groups of surviving
retinal neurons using a small number of electrodes with diameters of several hundred μm. To increase the spatial resolution of artificial
sight, electrodes much smaller than those currently in use are desirable. In this study we stimulated and recorded
ganglion cells in isolated pieces of rat,
guinea pig, and monkey
retina. We utilized micro-fabricated hexagonal arrays of 61
platinum disk electrodes with diameters between 6 and 25 μm, spaced 60 μm apart. Charge-balanced current pulses evoked one or two spikes at latencies as short as 0.2 ms, and typically only one or a few recorded
ganglion cells were stimulated. Application of several synaptic blockers did not abolish the evoked responses, implying direct activation of
ganglion cells. Threshold charge densities were typically below 0.1 mC / cm2 for a
pulse duration of 100 μs, corresponding to charge thresholds of less than 100 pC. Stimulation remained effective after several hours and at high frequencies. To demonstrate that closely spaced electrodes can elicit independent ganglion
cell responses, we utilized the multi-
electrode array to stimulate several nearby ganglion cells simultaneously. From these data we conclude that electrical stimulation of mammalian
retina with small-
diameter electrode arrays is achievable and can provide high temporal and spatial precision at low charge densities. We review previous epiretinal stimulation studies and discuss our results in the context of 32 other publications, comparing threshold parameters and safety limits.