Cortical Responses to Subretinal Photovoltaic Stimulation can be modulated Similarly to Light-Induced Visual Evoked Potentials

Abstract

Purpose: In the photovoltaic approach to retinal prosthesis the camera-captured images are projected onto the retina using pulsed infra-red (NIR) light. Each pixel in the subretinal implant converts pulsed light into local electric current to stimulate the nearby inner retinal neurons. The study characterized the cortical responses to photovoltaic stimulation and compared them with responses elicited by visible light.

Methods: Subretinal photodiode array with pixels 70, 140 and 280μm in width were implanted in the subretinal space of wild type (WT)(n=12) and Royal College of Surgeon (RCS) rats with degenerated retina (n=15). Cortical responses were recorded over a 6 months follow-up period using three skull electrodes. The NIR (915nm) and visible (635nm)laser beams weer projected onto the retina via a slit lamp. A VEP response threshold was defined as the signal 3 standard deviation above the noise level. Stimuli were modulated by pulse duration, peak power and repetition rate.

Results: The implant induced cortical responses (eVEP) at irradiance levels two orders of magnitude below the ocular safetly limit for 915nm radiation. Average thresholds of eVEP were 0.43, 1.0 and 2.1 mW/mm2 for pixel sizes of 280, 140 and 70μm, respectively, at 10ms pulse duration. Latency of the visible light-induced VEP decreased wiht increasing irradiance from 70 to 30ms, unlike the significantly shorter latency of eVEP, which did not vary with stimuli irradiance. In both rats type the eVEP amplitude increased with peak irradiance and pulse duration, and decreased with increasing frequency in the range of 2-20Hz, similar to the visible light response. However, from 20 to 40Hz the VEP continued to deceased while the eVEP did not change as much.

Conclusions: Robust cortical responses to photovoltaic subretinal stimulation and similarity in the eVEP modulation by NIR irradiance, pulse duration and frequency to VEP modulation by visible light suggest similarity in visual cortex processing of the retinal responses elicited by both types of stimuli. The small size and lack of wires makes photovoltaic arrays easy to implant and well tolerated in the subretinal space. Photovoltaic retinal prosthesis offers a promising approach to restoration of sight in patients blinded by retinal degenerative diseases.

Keywords: 688 retina • 494 degenerations/dystrophies