Grating Acuity of Prosthetic Vision in Blind Rats Matches the Pixel Pitch of Photovoltaic Subretinal Arrays Below 50µm

Abstract

Purpose : Currently, the best visual acuity demonstrated in clinical trials with retinal prostheses does not exceed 20/550. For functional restoration of sight in patients who lost central vision due to Age-related Macular Degeneration (AMD), prosthetic acuity should surpass 20/200, which corresponds to a pixel pitch of approximately 50µm. Here, we evaluate the grating visual acuity in rats implanted with hexagonal photovoltaic arrays having pixel sizes of 55µm and 40µm.

Methods : Photovoltaic arrays with (a) 55 and 40µm pixels having flat geometry, and (b) pixels with their active electrodes on 10µm tall pillars, were subretinally implanted into rats with retinal degeneration (Royal College of Surgeons, RCS). Visually evoked potentials (VEP) were recorded via screw electrodes placed onto the skull above visual cortices on both hemispheres. Collimated NIR laser (915nm) beam was spatially modulated by a Digital Micro-mirror Device (DMD), creating a pattern projected onto the implant. Stimulation thresholds were measured using full-field flashes of 10ms in duration at 2Hz. For measurement of the visual acuity, grating patterns with varying stripe widths were alternated at 2Hz frame rate, while being illuminated with 4ms pulses of 8mW/mm² irradiance at 40Hz.

Results : Stimulation thresholds with 55 and 40µm pixels at 10ms pulse duration ranged between 1 and 2mW/mm². Flat implants with 55µm pixels elicited VEP responses to alternating gratings, with the minimum stripe width equivalent to separation between the rows in a hexagonal array: 48µm. With 40µm pixels, VEP amplitude was lower, and its extrapolated minimum stripe width corresponded to 37µm. Shape of the VEP signals generated by the implants with elevated active electrode differed from that of the flat pixels by exhibiting a longer latency positive peak (~70ms) instead of a short latency (~15ms) negative peak.

Conclusions : Demonstrated grating visual acuity with spatial resolution better than 50µm provides hope that photovoltaic subretinal arrays could enable prosthetic vision with acuity exceeding the limit of legal blindness in US: 20/200. Three-dimensional electrode geometry may affect not only the stimulation threshold due to improved proximity to cells, but also the structure of the network-mediated responses.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.