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David Boinagrov, Keith Matheison, Georges Goetz, Ludwig Galambos, Daniel Palanker; Characteristics of Retinal Stimulation with Monopolar and Bipolar Subretinal Microelectrode Array. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5544.
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© ARVO (1962-2015); The Authors (2016-present)
When multiple monopolar electrodes in subretinal array are activated simultaneously their electric fields add-up thereby reducing resolution. To minimize this cross-talk effect we use bipolar electrodes with local return in each pixel. This study compared direct and network-mediated stimulation thresholds of the retinal ganglion cells (RGC) with subretinal electrodes of different sizes (20-80 μm in diameter), configuration (monopolar vs. bipolar), and distance from the cell.
The whole-cell patch-clamp recordings were obtained from RGCs of Long Evans rat retina in vitro. Stimulation with cathodic-first biphasic symmetric current pulses of 4 ms per phase was applied at 1 Hz repetition rate via subretinal SIROF microelectrode array. Concentric ring return electrodes in bipolar configuration were separated from the active disk electrodes by one diameter.
Thresholds of direct response (< 3 ms latencies) were at least twice higher than the network-mediated (latencies from 8 to 25 ms). With increased stimulus the number of spikes in the network-mediated burst increased from 1 up to 6, depending on individual cell. With monopolar 80 μm electrodes the stimulation threshold was 6.2 ± 4.9 μA, with 40 μm: 3.9 ± 1.9 μA, with 20 μm: 2.3 ± 0.4 μA. Threshold with bipolar 40 μm electrodes was 4.7 ± 2.1 μA, with 20 μm: 4.8 ± 2.2 μA. Stimulation threshold changed by less than 30% when two neighboring electrodes on both sides of the central electrode separated by 250 μm were activated with the same current. However, when the distance to the neighboring electrodes decreased to 125 μm the threshold decreased 2.5 times, compared to that of a single electrode. For 20 μm monopolar electrode 65 μm aside from the cell the threshold was 1.7 ± 0.5 times higher than for electrode right under the stimulated cell. For 20 μm bipolar electrode it was 7.6 ± 3.0 times higher.
Effect of the monopolar neighboring electrodes was rather minor when electrodes separation was 250 μm, but became very significant at separation of 125 μm and smaller. Stimulation threshold by the neighboring electrode was significantly increased with bipolar electrodes, compared to monopolar. On the other hand, stimulation threshold with bipolar electrode of 40 μm central disk right under the cell was only 20% higher than with monopolar, while for 20 μm it was twice higher.
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