Model Based Investigation of Virtual Electrodes of the Multi-Electrode Array for Epiretinal Prostheses
Visual prostheses are an emerging technology to restore vision in blind
individuals. The level of vision currently attainable with these prostheses is crude and
far from the level of normal vision though. Epiretinal prostheses work by using a multi-
electrode array implanted within the eye on the inner layer of the retina to electrically
stimulate the neural elements beneath the electrodes and produce punctate visual
percepts of light called phosphenes. Stimulation by serially delivering a cathodic
monopolar pulse of current with each electrode in the array would require the least
power to construct pixilated images of the visual scene. There is the possibility of
complex stimulation schemes that may be able to preferentially stimulate the neural
elements between the electrodes of the multi-electrode array by utilizing multiple
electrodes of the array at once though. Although this would require more power, this
would effectively increase the resolution capabilities of the epiretinal prosthesis without
the need to increase the number of electrodes on the multi-electrode array. To
investigate the possibility of such a stimulation scheme, a computational model of the
inner layers of the human retina including the nerve fiber layer and ganglion cells was
constructed. The model response was validated against studies of biological ganglion
cells, and under comparable conditions reproduced features of epiretinal stimulation
seen clinically. The response of the computational model of the inner retinal layers to
stimulation by up to two electrodes at once in the multi-electrode array was then
determined to evaluate the possibility of producing phosphenes between the electrodes.
The investigation found that disk electrodes using rectangular pulses of equal
magnitude could not produce a distinct phosphene between the electrodes of the model.

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