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Citation information: Grumet, Andrew E. Electric Stimulation Parameters for an Epi-Retinal Prosthesis. Ph.D. Thesis, Massachusetts Institute of Technology, 1999.
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