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Y. T. Wong, M. McCombe, J.-M. Seo, Y. A. Kerdraon, S. C. Chen, N. B. Dommel, J. W. Morley, H. Chung, N. H. Lovell, G. J. Suaning; Electrically Evoked Potentials Elicited Through Bipolar Supra-Choroidal Electrical Stimulation. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3016.
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Electronic retinal prostheses aim to elicit visual sensations to those blinded by diseases such as retinitis pigmentosa. A complementary metal oxide semiconductor (CMOS) stimulator has been developed that can electrically stimulate multiple sites simultaneously in a bipolar fashion. The stimulator has a unique internal switching arrangement that allows it to drive electrodes organized in a hexagons (to provide focused charge injection). We are testing the efficacy of this device by stimulating the retina of rabbits and examining the responses in the visual cortex.
New Zealand white rabbits were anaesthetized and a 14-channel array of platinum stimulating electrodes on a silicone carrier (thickness 75 µm, width 4 mm) was inserted into the supra-choroidal space. Stimulating electrodes were planar with 200 µm diameter circular contacts, organized into two hexagons with centre-to-centre spacing of 600 µm. Charge balanced bipolar current waveforms (varied from 72 µA to 436 µA, and 100 µs to 500 µs) were applied to the retina by the CMOS stimulator. Fourteen 700 µm diameter planar platinum recording electrodes were placed on the visual cortex after the dura mater was removed. Visually and electrically evoked potentials (EEP) were recorded.
EEP's were measured on the visual cortex due to bipolar supra-choroidal stimulation using greater than 28 nC of charge per phase. As the injected charge was increased the magnitude of response increased across all electrodes. At 28 nC charge injection, the most lateral/posterior electrode recorded an EEP with a magnitude of 12.3 µV. An electrode 1.875 mm further towards the midline recorded an EEP with a magnitude of 10.5 µV, and an electrode 4.75 mm anterior to this measured no EEP. At 163 nC charge injection, the magnitude of the EEP’s increased to 56.7 µV, 124.3 µV, and 18.1 µV respectively.
Thresholds for bipolar stimulation of supra-choroidal electrode arrays in rabbits were shown to be similar to that of trans-retinal stimulation. It was also shown that as charge injection was increased, the spread of activity across the cortex was increased.
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