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

Abstract

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.

Current Steering in Retinal Stimulation via a Quasimonopolar Stimulation Paradigm

“…35 In vivo and in vitro studies have lent support to this hypothesis, 31,36 wherein an increase in hexapolar activation threshold is observed to be up to three times that of monopolar.…”

“…36 Threshold values of P50 were calculated for hexapolar, monopolar, and quasimonopolar, and the results compared to determine the effect of combining hexapolar and monopolar stimulation on the resulting threshold level.…”

Techniques for Processing Eyes Implanted With a Retinal Prosthesis for Localized Histopathological Analysis

“…Therefore a method for localizing the tissue directly adjacent to the individual active electrodes would greatly increase the . It has been used with thin-film polyamide arrays 7,31 and also thin-film silicone/Pt arrays 32 . Some retinal prosthesis studies employed "bullet shaped" electrodes with intrascleral implantations 33 .…”

Electronic restoration of vision in those with photoreceptor degenerations

“…Indeed, recently, the response of the visual cortex to suprachoroidal stimulation from electrodes with different returns was compared in sighted cats. 81 Monopolar stimulation was found to elicit a cortical response with the lowest level of current. However, a multipolar configuration where the surrounding electrodes acted as ground, improved spatial resolution.…”

“…However, a multipolar configuration where the surrounding electrodes acted as ground, improved spatial resolution. 81 By choosing different return electrodes, multipolar stimulation also offers the ability to steer the current to particular locations in the retina. This could be useful for implants with a minimal number of electrodes with large electrode spacing, as the current could be directed into locations where an electrode is not present but neural activation is necessary to form a useful image.…”