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M. R. Behrend, A. K. Ahuja, M. S. Humayun, R. H. Chow, J. D. Weiland; Electrical Stimulation of the Retina in a 3-Dimensional Interface. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4569.
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© ARVO (1962-2015); The Authors (2016-present)
Using calcium imaging we measured stimulation thresholds in three dimensions a retinal prosthesis interface. We investigated how the retina responds to stimulation from a distance when a saline gel similar to the vitreous is present.
Wholemount tiger salamander retina was stimulated with a planar microelectrode array composed of transparent indium-tin-oxide electrodes. Thresholds were measured from each cell by fluorescence imaging of ganglion cells stained retrogradely via the optic nerve stump with Oregon Green BAPTA-1 dextran. Saline gels, made from 1% agarose in the superfusate solution, were cast directly onto the microelectrode array before mounting the retina. Gel thickness was measured to +/- 1 µm precision on the microscope by the distance between focal planes of the electrode and the RGC layer.
The stimulation threshold for 200-µm electrodes in contact with the retina with 400-us pulses was 3.1 +/- 0.35 µA (mean +/- SEM), but rose to 5.3 +/- 0.42 µA (mean +/- SEM) upon separating the retina from the array by 30 µm (p < 0.01), and to 12 +/- 0.9 µA (mean +/- SEM) with 50 µm of separation (p < 0.05). In all cases, the region of somata with lowest threshold was located 40 µm to the periphery of the stimulating electrode. The diameter of the responding region was 400 um, defined as the boundary where threshold is 50% higher than the minimum threshold. Changing electrode size from 200 µm to 60 µm reduced the diameter of the responding region to 170 µm, with a peripheral displacement of 44 µm. Our findings are consistent with the initial segment hypothesis for the site of action potential initiation. Smaller electrodes stimulate fewer cells, and positioning the electrodes as close to the retina as possible is advantageous for minimizing the stimulus amplitude. However, a 12-µA, 0.4-ms pulse applied to a 200-µm electrode has a charge density of 15 µC/cm2, well below published limits for platinum.
We have quantified the relationship between stimulus threshold for ganglion cells and stimulation electrode to retina proximity. Although distant stimulation leads to increased threshold, the charge requirements are still within safe limits when 200-um electrodes are used.
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