Electric stimulation parameters for an epi-retinal prosthesis

Abstract

This work was undertaken to contribute to the development of an epi-retinal prosthesis which may someday restore vision to patients blinded by outer retinal degenerations like retinitis pigmentosa. By stimulating surviving cells in tens or hundreds of distinct regions across the retinal surface, the prosthesis might convey the visual scene in the same way that images are represented on a computer screen. The anatomical and functional arrangement of retinal neurons, however, poses a potential obstacle to the success of this approach. Stimulation of ganglion cell axons-which lie in the optic nerve fiber layer between stimulating electrodes and their intended targets, and which originate from a relatively diffuse peripheral region-would probably convey the perception of a peripheral blur, detracting from the usefulness of the imagery. Inspired by related findings in brain and peripheral nerve stimulation, experiments were performed in the isolated rabbit retina to determine if excitation thresholds for ganglion cell axons could be raised by orienting the stimulating electric field perpendicularly to the axons' path. Using a custom-designed apparatus, axon (and possibly dendrite) thresholds were measured for stimulation through a micro-fabricated array of disk electrodes each having a diameter of ten microns. The electrodes were driven singly versus a distant return (monopolar stimulation) and in pairs (bipolar stimulation) oriented along fibers (longitudinal orientation) or across fibers (transverse orientation). Transverse thresholds were measured for a range of fiber displacements between the two poles of the bipolar electrode pair, and compared in each case with the monopolar threshold for the closer pole. Transverse/ monopolar threshold ratios were near unity when one of the poles was directly over the fiber, but rose rapidly with improved centering of the bipolar pair. Longitudinal/monopolar threshold ratios were near unity over the same range of displacements. As in previous work by others, thresholds were highest for perpendicular stimulating fields. Practical application of this result will require electrode designs which minimize longitudinal fringing fields.