Frequency and amplitude modulation have different effects on the percepts elicited by retinal stimulation

Cortical responses elicited by photovoltaic subretinal prostheses exhibit similarities to visually evoked potentials

We have previously developed a wireless photovoltaic retinal prosthesis, in which camera-captured images are projected onto the retina using pulsed near-IR light. Each pixel in the subretinal implant directly converts pulsed light into local electric current to stimulate the nearby inner retinal neurons. Here we report that implants having pixel sizes of 280, 140 and 70 μm implanted in the subretinal space in rats with normal and degenerate retina elicit robust cortical responses upon stimulation with pulsed near-IR light. Implant-induced eVEP has shorter latency than visible light-induced VEP, its amplitude increases with peak irradiance and pulse duration, and decreases with frequency in the range of 2-20 Hz, similar to the visible light response. Modular design of the arrays allows scalability to a large number of pixels, and combined with the ease of implantation, offers a promising approach to restoration of sight in patients blinded by retinal degenerative diseases.

Visual cortex responses to suprachoroidal electrical stimulation of the retina: effects of electrode return configuration

A clinically effective retinal prosthesis must evoke localized phosphenes in a retinotopic manner in response to stimulation of each of the retinal electrodes, evoke brightness cues over a wide dynamic range and function within safe stimulus limits. The effects of varying return configuration for retinal stimulation are currently unknown. To investigate this, we implanted a flexible, 7 × 12 electrode array into the suprachoroidal space of normally-sighted, anesthetized cats. Multi-unit activity in the primary visual cortex was recorded in response to electrical stimulation using various return configurations: monopolar vitreous (MPV), common ground (CG), hexagonal (HX), monopolar remote (MPR) and bipolar (BP_N). MPV stimulation was found to be the most charge efficient and was most likely to induce cortical activity within safe charge limits. HX and CG stimulation were found to exhibit greater retinal selectivity compared to the MPV return at the expense of lower cortical yield and higher P50 charge levels, while cortical selectivity was unaffected by choice of return. Responses using MPR and widely spaced BP_N configurations were similar to those using the MPV return. These results suggest that choice of return configuration for a retinal prosthesis will be balanced between resolution and stimulation within safe charge limits.