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Henri Lorach, Yossi Mandel, Georges A Goetz, Philip Huie, Xin Lei, Ted Kamins, Ludwig Galambos, Jim Harris, Daniel V Palanker; Visual Acuity of Rats with Photovoltaic Subretinal Prostheses. Invest. Ophthalmol. Vis. Sci. 2014;55(13):5965. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To measure in-vivo stimulation thresholds and visual acuity achieved with photovoltaic subretinal prostheses in animals with retinal degeneration
The 1mm diameter implants consist of 70um or 140um photovoltaic pixels separated by 5um trenches. Each pixel contains two to three photodiodes connected in series between the active and return electrodes, which convert pulses of near infrared light (NIR) into bi-phasic pulses of electric current. These chips were implanted in the subretinal space of rats with retinal degeneration (RCS), as well as in wild type rats (Long Evans). Implant location and retinal health was evaluated with optical coherence tomography and fluorescein angiography. Electrical current generated by the implant was recorded on the cornea. Visually evoked potentials (eVEPs) were recorded by 1mm diameter trans-cranial screw electrodes implanted above the visual cortex. Stimulation thresholds and dynamic range were measured with full-field illumination of the implants using 915nm light of 10ms in duration at 2Hz repetition rate. Visual acuity was measured using the cortical responses to alternating gratings with visible and NIR stimulation. Patterns alternating at 2Hz were projected using 40Hz flicker frequency, with grating stripes varying from 6um to 200um in width.
Long Evans rats implanted with the subretinal chip exhibited a loss of photoreceptors above the prosthesis, creating a model of local retinal degeneration. Cortical activation thresholds were as low as 0.23 (±0.06) mW/mm2 for implants with 140um pixels and 0.47 (±0.10) mW/mm2 for 70um pixels. There was no significant difference between the thresholds in normal and degenerate (RCS) animals. Amplitude of the eVEP was modulated by light intensity and by pulse duration over more than one order of magnitude. RCS rats implanted with 70um pixel arrays could resolve NIR patterns with 100um per stripes (0.33 cpd), whereas normal sighted animals could resolve stripes of 50um (0.66 cpd) in visible light.
Perceptual thresholds with photovoltaic stimulation are more than 2 orders of magnitude below the ocular safety limit for pulsed NIR illumination. To encode multiple grey levels, cortical responses can be modulated by either light intensity or pulse duration. Visual acuity achieved with the photovoltaic implants in rats with retinal degeneration was only twice lower than the native level.
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