Focal activation of the feline retina via a suprachoroidal electrode array

Abstract

This paper presents the results of the first investigations into the use of bipolar electrical stimulation of the retina with a suprachoroidal vision prosthesis, and the effects of different electrode configurations on localization of responses on the primary visual cortex. Cats were implanted with electrodes in the suprachoroidal space, and electrically evoked potentials were recorded on the visual cortex. Responses were elicited to bipolar and monopolar stimuli, with each stimulating electrode coupled with either six-return electrodes, two-return electrodes, or a single-return electrode. The average charge threshold to elicit a response with bipolar stimulation and six-return electrodes was 76.47+/-8.76 nC. Bipolar stimulation using six-return electrodes evoked responses half the magnitude of those elicited with a single or two-return electrodes. Monopolar stimulation evoked a greater magnitude, and area of cortical activation than bipolar stimulation. This study showed that suprachoroidal, bipolar stimulation can elicit localized activity in the primary visual cortex, with the extent of localization and magnitude of response dependent on the electrode configuration.

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.…”

“…This proved effective at charge containment and confirmed findings from studies in auditory prosthetics have shown that using different return path configurations can result in greater spatial selectivity. [31][32][33][34] Thus, this strategy also may be beneficial for retinal implants.…”

“…Electrodes were positioned in a hexagonal configuration, allowing 10 of the 24 stimulating electrodes to be surrounded by a full ring of return electrodes, and, thus, allowing the authors to use the hexapolar current-steering configuration. 31 Figure 1 illustrates the electrical connection established between the two stimulators' digital to analog converters (DACs) and the electrodes on the stimulating array. The matching push-pull configuration of the current sources and sinks used in the stimulator results in an equal amount of current injected by the source of any one DAC being drawn by the matching sink for that DAC.…”

Variation in Performance of Platinum Electrodes with Size and Surface Roughness

“…Stimulations were applied at 250 µA with 200 µs phase widths, which correlated with the charge injection threshold required to measure an evoked response in the visual cortex in previous in vivo experiments. (11,17) Electrodes were placed in an incubator at 37°C and 100% humidity, and the maximum voltage excursion across two electrode interfaces (bipolar stimulation) was measured daily for 50 days. Stimulation was constant at 250 Hz and the medium was replaced every 2 days to preserve the activity of the proteins.…”

“…Additionally, stimulation of vision using a more distant placement of devices, such as suprachoroidal and sclera placements, have been shown to require a higher charge injection per phase in animal models. (12,17) It is clear that sufficient stimulation for useful vision across a wide range of patients will require a careful balance between electrode size and the charge injection capacity of the chosen material. While coating technologies show considerable promise in improving safe charge injection, PEDOT is not yet approved by regulatory bodies, and activated IrOx for vision prostheses is still under efficacy testing.…”