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Devyani Nanduri, Jessy D. Dorn, Mark S. Humayun, Robert J. Greenberg, James D. Weiland; Percept Properties of Single Electrode Stimulation in Retinal Prosthesis Subjects. Invest. Ophthalmol. Vis. Sci. 2011;52(14):442.
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In an effort to restore functional form vision, more than 30 human subjects with advanced retinitis pigmentosa (RP) have been implanted with epiretinal electrode arrays to directly stimulate remaining retinal circuitry. Our group’s objective is to create a two-dimensional grid of phosphenes in the visual space that can be pieced together to form a representation of the world. The first stage is to investigate properties of single electrode phosphenes. We report here data that establishes single electrode phosphene shape consistency and explores changes in phosphene shape characteristics for different electrode locations on the retina, across subjects.
Experiments were performed on one 16-channel (ArgusTM I) and three 60-channel (ArgusTM II) prosthesis subjects with severe RP. We performed a control experiment with tactile targets to assess drawing error and a stimulation experiment using a set of single electrodes from the array. Stimuli were biphasic 0.45 ms pulse width, 20 Hz pulse trains at dim amplitude levels (1.25-2x threshold). We quantified the induced phosphene length, size and orientation by analyzing subjects tracings of their percepts on a touchscreen. All electrode stimulations were repeated 5 times randomized amongst other stimuli. Each test electrode’s location relative to the optic disc and approximated fovea locations were anatomically mapped using the subject’s most recent fundus photograph.
Variability in phosphene shape for any given electrode stimulus was comparable to drawing error values calculated from the tactile control experiments indicating phosphene appearance was consistent across trials for all 4 subjects. Individual electrode percepts were drawn as curved and straight lines, wedges, or relatively round contours. Phosphene length tended to increase with stimulating electrode distance to the fovea in 3 of 4 subjects.
Consistency of phosphene appearance implies multi-electrode stimulus patterns will likely generate reproducible percepts, which are necessary for the stable functioning of retinal prostheses. Shorter phosphenes in the vicinity of the fovea may help increase spatial resolution in the central vision of prosthesis subjects.
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