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H. Benav, R. Wilke, A. Stett, E. Zrenner; Modeling Advantages of Subretinal Microphotodiode-Arrays Utilizing Sequential Electrode Activation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4593.
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© ARVO (1962-2015); The Authors (2016-present)
Subretinal microphotodiode-arrays (MPDAs) are designed to stimulate bipolar cells by converting visual information into topically ordered patterns of electrical stimulation. In general the output of stimulation electrodes is paced with a fixed frequency leading to simultaneous activation of all electrodes. Here we investigate differences between simultaneous and novel modes of sequential activation of stimulation electrodes, modeled at the level of retinal ganglion cell (RGC) output.
A previous model, describing light dependent activation of stimulation electrodes of the MPDA and the activity distribution in the retinal network elicited by single electrodes, was extended to incorporate temporal features of RGC activity following subretinal stimulation. RGC output was modeled for simultaneous activation of all MPDA-electrodes and compared to an implementation of a new sequential activation algorithm. An array of 30x30 electrodes was divided into 25 areas with 6x6 electrodes (grid size=6, other grid sizes also were investigated). In one area only one electrode was active at a time; the remaining electrodes were successively activated using a pseudo-random sampling algorithm, designed to both maximize and keep homogeneous the distances between two activations. The model was used to simulate the processing of animated images by the MPDA and the respective RGC output in dependence of the stimulation strategy. In either mode the RGC underwent frequency dependent adaptation.
RGC activity profiles generated in sequential activation mode conveyed a sufficient image quality while electrodes were activated with low frequencies (1-3 Hz), thus RGC activity was not weakened by adaptation. Grid size was found to be optimal at 5±1. Using simultaneous activation, frequencies > appr. 10 Hz were required for temporally continuous perceptions, thus RGC adapted within 1-2 s. This is in general agreement with results from human subjects carrying subretinal implants.
Future MPDAs utilizing sequential activation may provide significantly improved image quality and visual perceptions which do not fade due to RGC adaption.
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