Performance of photovoltaic arrays in-vivo and characteristics of prosthetic vision in animals with retinal degeneration

Abstract

Summary Loss of photoreceptors during retinal degeneration leads to blindness, but information can be reintroduced into the visual system using electrical stimulation of the remaining retinal neurons. Subretinal photovoltaic arrays convert pulsed illumination into pulsed electric current to stimulate the inner retinal neurons. Since required irradiance exceeds the natural luminance levels, an invisible near-infrared (915nm) light is used to avoid photophobic effects. We characterized the thresholds and dynamic range of cortical responses to prosthetic stimulation with arrays of various pixel sizes and with different number of photodiodes. Stimulation thresholds for devices with 140µm pixels were approximately half those of 70µm pixels, and with both pixel sizes, thresholds were lower with 2 diodes than with 3 diodes per pixel. In all cases these thresholds were more than two orders of magnitude below the ocular safety limit. At high stimulation frequencies (>20Hz), the cortical response exhibited flicker fusion. Over one order of magnitude of dynamic range could be achieved by varying either pulse duration or irradiance. However, contrast sensitivity was very limited. Cortical responses could be detected even with only a few illuminated pixels. Finally, we demonstrate that recording of the corneal electric potential in response to patterned illumination of the subretinal arrays allows monitoring the current produced by each pixel, and thereby assessing the changes in the implant performance over time.

Temporal properties of network-mediated responses to repetitive stimuli are dependent upon retinal ganglion cell type

“…However, it is not known whether those subjects have any remaining outer retinal cell bodies. Subretinal implantation has been shown to damage photoreceptors although photoreceptor nuclei can be observed up to 5 weeks after implantation (Lorach et al 2014), suggesting there may be some variability in the sensitivity to implantation. Further testing will be necessary to determine whether different shapes or coatings of implanted electrodes (Butterwick et al 2009) can minimize the deterioration of the outer retina.…”

Electronic approaches to restoration of sight

“…For subretinal stimulation of RGCs, anodic-first pulses have lower stimulation thresholds for the same reason [105]. Similarly, for subretinal stimulation of the inner nuclear layer, anodic pulses have a lower threshold [11, 106] because bipolar cells have a higher concentration of Ca ion channels towards the epiretinal side. Stimulation thresholds can be 2 to 7.5 times lower with anodic-first pulses than with the opposite polarity [105, 11].…”

“…For a photovoltaic prosthesis, this translates into light intensity thresholds on the order of 1 mW/mm 2 [46, 97, 106]. Thresholds depend on pixel size and on configuration of the return electrodes.…”

“…Healthy retinas respond faster to subretinal stimulation than degenerate ones [106]. This decrease in amplitude of the retinal response has been described both as “desensitization” [117] of the retina, as well as more recently “adaptation” to constant stimulation [48].…”

Interactions of Prosthetic and Natural Vision in Animals With Local Retinal Degeneration

“…Near-infrared (915 nm) illumination was used to activate the photovoltaic pixels while avoiding any visual response in rats. 15,19 The subretinal implantation triggered the loss of photoreceptor outer segments above the implant within a month and a subsequent loss of the outer nuclear layer after 3 months 20 ( Fig. 2A).…”