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.