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Daniel V Palanker, Henri Lorach, Georges A Goetz, Yossi Mandel, Richard Smith, David Boinagrov, Xin Lei, Ted Kamins, Keith Mathieson, Alexander Sher; Photovoltaic Restoration of Sight in Rats with Retinal Degeneration: Assessment of Spatial Resolution and Visual Functions. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5964. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize interactions between the prosthesis and the retina and to assess visual functions restored by a photovoltaic subretinal prosthesis in rats blinded by retinal degeneration.
Selectivity of the network-mediated retinal stimulation was assessed with patch-clamp recordings from retinal ganglion cells (RGCs). Modulation of the retinal response by irradiance and pulse duration, as well as electrical receptive fields (eRFs) of RGCs were mapped on 512-electrode array in-vitro. Stimulation thresholds, dynamic range and contrast sensitivity in-vivo were measured by electrically-induced visually evoked potentials (eVEPs) in response to full-field flashes in rats with retinal degeneration (RCS, P45-P60) and in wild-type animals following subretinal implantation. Visual acuity was assessed by eVEP response to alternating gratings (2Hz) using 915nm illumination at 40Hz flicker. Similarly, visual acuity was measured with visible light (532nm) in normally-sighted controls.
Network mediated RGC responses could be modulated by pulse width (1-20 ms) and peak irradiance (0.5-10 mW/mm2). Up to 7 spikes could be elicited by a single stimulation pulse in the most responsive neurons. With 70μm pixels, the eRF diameters of RGCs in normal retina were 248±59μm - similar to their visible light RF size: 249±44μm. In RCS retina the eRFs were slightly smaller: 205±69μm. With 140μm pixels the average eRF width in normal retina was twice as large: 535±153μm. In-vivo stimulation thresholds in WT and RCS rats with 10 ms pulses were 0.23±0.06 mW/mm2 for 140μm pixels and 0.47±0.10 mW/mm2 for 70μm pixels. RCS rats implanted with 70μm pixel arrays could resolve patterns with 100μm stripes, but did not resolve 50μm stripes. Normally-sighted animals could resolve patterns with 50μm stripes projected by visible light, but did not resolve stripes of 25μm.
Grating visual acuity in blind rats with a subretinal prosthesis is similar to the pitch of the electrode array (70μm pixels, 5μm gaps), which is only twice lower than the normal acuity in rats. Patients with similar implants might achieve even better visual performance utilizing eye scanning and training. Low stimulation thresholds, high resolution and ease of implantation of the wireless arrays support the photovoltaic approach to restoration of sight in patients blinded by retinal degeneration.
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