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D. Nanduri, M. J. McMahon, J. D. Dorn, M. S. Humayun, R. J. Greenberg, J. D. Weiland; Spatial Summation of Phosphenes With Electrode Pair Retinal Stimulation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4582.
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In an effort to restore functional vision, subjects with severe retinitis pigmentosa have been implanted with epiretinal electrode arrays to elicit percepts using electrical stimulation (Humayun et al., 1999). We report here the ability of subjects to see patterns generated by individual phosphenes by measuring both the perceived shape of single electrode phosphenes and the phosphenes generated by stimulating pairs of these same electrodes.
The study protocol was approved by the University of Southern California IRB. The experiments were performed on a single subject with severe retinitis pigmentosa who was implanted with a 16-channel epiretinal prosthesis in 2004. Stimuli on each electrode were monopolar, 500 ms balanced biphasic 20 Hz pulse trains with a pulse width of 0.45 ms at 2x threshold amplitude. Shape data was collected for 9 pairs of electrodes, 1600µm or 1790µm separation, by capturing and analyzing video recordings of the subject tracing the perceived phosphenes on a grid screen placed in front of them. Each experiment consisted of 4 stimulus conditions: each of the two electrodes individually activated, both activated synchronously, and both activated asynchronously. Each condition was presented 5 times in random order.
Individual electrode percepts were drawn as curved lines, wedges, or relatively round contours. There were 6 experiments in which individual electrodes produced different percept shapes; in these cases, the phosphene generated by both electrodes together consisted of two distinct, spatially separated percepts, each of which resembled the individual electrode percepts. In the 3 other cases where individual electrodes produced similar percept shapes, stimulating both electrodes together produced only a single percept that resembled that of the individual electrodes. There was no qualitative difference in the percepts generated by synchronous vs. asynchronous pulse stimulation of electrode pairs.
In most cases, stimulating pairs of electrodes generated a percept that was the direct combination of the percepts produced by the individual electrodes. These findings demonstrate that it is possible to produce form vision with a retinal prosthesis through predictable summation of spatially distinct percepts.
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