Spatial resolution of retinal response to photovoltaic stimulation

Abstract

Purpose: To assess the spatial resolution of the retinal ganglion cells (RGCs) response to subretinal stimulation with photovoltaic arrays.

Methods: Photovoltaic arrays with pixel sizes of 70 and 140µm were placed on the photoreceptor side of normally-sighted (Long Evans, LE) and degenerate (Royal College of Surgeons, RCS) rat retinas, while network-mediated responses were recorded from the RGCs using a 512 channel multielectrode array. A sparse white noise pattern was projected on the prosthesis using 880nm light to measure the electrical receptive fields (eRFs) of individual neurons. A visible white noise stimulus was used to measure the natural receptive fields (RFs) of the same neurons. Spatial resolution was also evaluated by reversing the contrast of linear gratings of various spatial frequencies at 2Hz.

Results: With 140µm photodiode pixels, the mean eRF diameter in LE retinas was 535 ± 153µm. With 70µm pixels, approximately 90% of the RGCs had localized eRFs with a mean diameter of 248 ± 59µm, comparable to the mean RF of the same neurons, 249 ± 44µm. The remaining 10% of RGCs exhibited a center-surround response structure, with strong localized stimulation in the center of the eRF and lower amplitude, diffuse stimulation over a surround that could spread over a circular area of 490-560µm diameter . With 70µm pixels in RCS rats, the mean eRF diameter was 205 ± 69µm. For RCS rat retinas stimulated by 70µm pixels, individual RGCs responded to contrast reversal of the 140µm stripes and did not respond to 35µm grating stripes.

Conclusions: The average size of RGC eRFs obtained with a photovoltaic subretinal prosthesis, having 70µm bipolar pixels, is similar to the average size of natural RFs in the rat retina, and average eRF diameter doubles when pixel size increases from 70 to 140um. The average size of eRFs in degenerate retina (RCS) is similar to that in the WT retina. 70 µm bipolar pixels can sometimes elicit “center-surround” responses, with center and surround having different latencies. Initial results indicate that the 70µm implant can resolve gratings with stripes smaller than 140µm, and larger than 35µm .

Keywords: 702 retinitis • 508 electrophysiology: non-clinical