Electrical Effects and Perceptual Performance Using a Chronically Implanted 16–Channel Epiretinal Prosthesis in Blind Subjects

Abstract

Purpose: : To evaluate the electrical effects and perceptual performance of long–term epiretinal stimulation in subjects with outer retinal degeneration.

Methods: : A 16 channel microelectronic implant made by Second Sight Medical Products was implanted monocularly in 6 subjects at the Doheny Eye Institute. Preoperatively, all subjects had vision of light perception or worse and non–recordable electroretinograms. After a vitrectomy, using a pars plana incision, a 5 x 6 mm silicone–platinum electrode array was introduced into the eye and tacked onto the epiretinal surface. The first subject was implanted in February, 2002 and the sixth subject in June, 2004.

Results: : Both threshold and impedance values showed an inverse correlation with the distance of the array from the retina. Across all subjects, 75% of the electrodes had stimulation thresholds below the safe charge injection limit for platinum electrodes. The last 2 subjects had thresholds well below this safety limit; ranging between 12 –72 microAmps for 250 and 500 micron electrodes using a 1 ms per phase biphasic pulse. Tests of spatial vision (e.g. maps of the perceived locations of phosphenes, 2–point discrimination, and direction of motion discrimination) showed performance significantly above chance, indicating that subjects can spatially resolve each electrode within the array. We also found that when input is provided via a head mounted camera, subjects can use head scanning to localize objects (e.g. 5 degree wide square) and determine the orientation of test patterns.

Conclusions: : The 16–channel epiretinal array can produce visual percepts at charge density levels within the safety limits for platinum and tissue damage. Despite the low–resolution of the 4X4 array, blind subjects can reliably perform spatial and motion tasks using this retinal prosthesis in a test environment.

Keywords: retina • low vision • retinal degenerations: hereditary