Subretinal Placement of the Microelectrode Array is Associated With a Low Threshold for Electrical Stimualtion

Abstract

Abstract: : Purpose: To determine the retinal stimulation threshold of the microelectrode array (MEA) when placed at different locations in the subretinal space of the rabbit eye. Methods: Electrical evoked potential (EEP) electrodes were placed on the dura mater above the visual cortex and forehead in six rabbits. The EEP was recorded following electrical stimulation of the MEA placed beneath the visual streak, as well as adjacent to the optic disc in the subretinal space of the rabbit using the ab externo approach. Liquid perfluordecaline (0.4ml) was placed above the neurosensory retina to flatten it on the MEA after implantation. The retinal threshold of the EEP at each location was determined with biphasic electrical stimulation in three consecutive trials with 100 averaged measurements per trial. The MEA had 25 monopolar electrodes (4x10^–4cm²) with each electrode measuring 40µm x 40µm. The enucleated eyes were processed histologically with H&E staining. Results: The mean electrical threshold to generate an EEP was 9±7nC (0.023±0.016mC/cm²) beneath the visual streak (n=5) and 6±2nC (0.015±0.005mC/cm²) adjacent to the optic disc (n=5). No statistically significant difference was detectable between each location (p=0.34). The minimum threshold charge density to generate an EEP was 0.005mC/cm² beneath the visual streak and 0.013mC/cm² adjacent to the optic disc. Histology showed destruction of the outer segments of the photoreceptor layer from surgical manipulation. Conclusions: Placement of the MEA in the subretinal space of the rabbit resulted in a lower retinal threshold for stimulation than that reported for epiretinal or suprachoroidal placement. However, the location of the MEA beneath the visual streak or adjacent to the optic nerve did not influence the electrical stimulation threshold.

Keywords: retina • electrophysiology: non-clinical