Current distribution during parallel stimulation: implications for an epiretinal neuroprosthesis

Progress in artificial vision through suprachoroidal retinal implants

Retinal implants have proven their ability to restore visual sensation to people with degenerative retinopathy, characterized by photoreceptor cell death and the retina's inability to sense light. Retinal bionics operate by electrically stimulating the surviving neurons in the retina, thus triggering the transfer of visual sensory information to the brain. Suprachoroidal implants were first investigated in Australia in the 1950s. In this approach, the neuromodulation hardware is positioned between the sclera and the choroid, thus providing significant surgical and safety benefits for patients, with the potential to maintain residual vision combined with the artificial input from the device. Here we review the latest advances and state of the art devices for suprachoroidal prostheses, highlight future technologies and discuss challenges and perspectives towards improved rehabilitation of vision.

Quasi-monopolar stimulation: a novel electrode design configuration for performance optimization of a retinal neuroprosthesis

In retinal neuroprostheses, spatial interaction between electric fields from various electrodes – electric crosstalk – may occur in multielectrode arrays during simultaneous stimulation of the retina. Depending on the electrode design and placement, this crosstalk can either enhance or degrade the functional characteristics of a visual prosthesis. To optimize the device performance, a balance must be satisfied between the constructive interference of crosstalk on dynamic range and power consumption and its negative effect on artificial visual acuity. In the present computational modeling study, we have examined the trade-off in these positive and negative effects using a range of currently available electrode array configurations, compared to a recently proposed stimulation strategy – the quasi monopolar (QMP) configuration – in which the return current is shared between local bipolar guards and a distant monopolar electrode. We evaluate the performance of the QMP configuration with respect to the implantation site and electrode geometry parameters. Our simulation results demonstrate that the beneficial effects of QMP are only significant at electrode-to-cell distances greater than the electrode dimensions. Possessing a relatively lower activation threshold, QMP was found to be superior to the bipolar configuration in terms of providing a relatively higher visual acuity. However, the threshold for QMP was more sensitive to the topological location of the electrode in the array, which may need to be considered when programming the manner in which electrode are simultaneously activated. This drawback can be offset with a wider dynamic range and lower power consumption of QMP. Furthermore, the ratio of monopolar return current to total return can be used to adjust the functional performance of QMP for a given implantation site and electrode parameters. We conclude that the QMP configuration can be used to improve visual information-to-stimulation mapping in a visual prosthesis, while maintaining low power consumption.

Efficacy of the hexpolar configuration in localizing the activation of retinal ganglion cells under electrical stimulation

Retinal visual prostheses provide hope of restoring sight to patients suffering from retinal degeneration such as retinitis pigmentosa and age-related macular degeneration. Retinal prostheses are used to electrically stimulate residual neurons that are spared in these diseases, namely the retinal ganglion cells (RGCs), eliciting percepts of light termed 'phosphenes'. The elicitation of multiple phosphenes via an electrode array allows patterns to be produced, resulting in a rudimentary form of vision. For such patterns to be produced effectively, the prosthesis must generate well-defined phosphenes. To this end, the hexpolar configuration has been proposed as an alternative to the traditional monopolar or bipolar configurations. It utilizes six electrodes surrounding the stimulating electrode to serve as a combined return, or 'hex guard', purportedly localizing the activation to cells located within them. In this study, the efficacy of the hexpolar configuration in localizing activity was investigated by using patch-clamp electrophysiology to measure the activation thresholds of RGCs to electrical stimulation in isolated rabbit retina. Cells located outside the hex guard were found to have significantly higher relative hexpolar thresholds (>2 fold) as compared to cells located within the hex guard. This confirms the efficacy of the hexpolar configuration in localizing activity to within the hex guard. Furthermore, the effect of using cathodic-first versus anodic-first stimulation on hexpolar threshold and localization was investigated. No significant difference was observed between the two groups, in terms of lowering thresholds or improving localization.

Computational model of electrical stimulation of a retinal ganglion cell with hexagonally arranged electrodes

In retinal prosthetic devices an electrode array is used for electrical stimulation of retinal neurons to induce phosphene perception. The shape and size of the evoked phosphenes are in part dependent on the spatial patterns of retinal activation. In this study, a computational model of a cat beta retinal ganglion cell (RGC) excitation following simulated electrical stimulation was investigated. Seven epiretinal disk electrodes with hexagonal configuration (Hex electrodes) were used. 100 µs/phase anodic-first biphasic pulses were injected at the center electrode and one sixth of the total current was returned at each surround electrode. The aim was to obtain a spatial threshold map of the RGC excitation. We found that the spatial threshold pattern was highly dominated by axonal excitation. With 50 µm Hex electrodes, relative thresholds for activation of the distal axon was almost the same as that for excitation of the axonal trigger segment (high sodium channel density region), causing an elongated activation pattern. The model presented in this study can be used to investigate the extent to which spatial RGC activation patterns are influenced by cell and stimulus parameters.

Charge recovery during concurrent stimulation for a vision prosthesis

Parallel or concurrent stimulation in an epiretinal neuroprosthesis is likely necessary in order to deliver sufficient phosphenes for effective vision. Important issues with concurrent stimulation are the effect of current distribution which introduces current leakage or 'cross talk' between adjacent electrodes and charge recovery which determines balanced charge being delivered/recovered at each electrode from the previous phase. In this paper, we present the effect of concurrent stimulation of two hexagonally arranged platinum electrode arrays on charge recovery. Balanced and imbalanced (unequal) currents were delivered to the hexagonal arrays when they were immersed in physiological saline. Both simulation and experimental results revealed that charge was not recovered at individual electrodes, particularly when imbalanced currents were delivered. However, total charge injected to both hexagonal arrays was recovered.

Microelectronic retinal prosthesis: I. A neurostimulator for the concurrent activation of multiple electrodes

An application specific integrated circuit (ASIC) capable of delivering charge to multiple electrodes in unison has been developed. The ASIC is designed to function as part of an epi-retinal prosthesis driving an array of electrodes in a hexagonal mosaic. This unique organization of electrodes and the use of current sources and sinks is implemented to reduce the electrical cross-talk that occurs when many electrodes are activated in unison. Due to the large numbers of electrodes needed to provide useful vision to implantees, the interleaving strategies used in cochlear implants will not suffice for a vision prosthesis, where the "frame rate" is important for acceptable perceptual outcomes. This paper describes the design, and architectural approaches and performance testing of the developed ASIC.

The design and testing of an epi-retinal vision prosthesis neurostimulator capable of concurrent parallel stimulation

An application specific integrated circuit (ASIC) neurostimulator capable of stimulating multiple electrodes in unison has been designed and tested. The ASIC utilizes multiple matched current sinks and sources to provide localized stimulation and is designed to drive electrodes organized in a hexagonal mosaic. This organization allows each stimulating electrode to be surrounded by up to six return electrodes, effectively isolating each stimulation site. The ASIC was manufactured using a high-voltage complementary metal-oxide-semiconductor process, which allows up to 20 V to be applied across the circuitry. This provides the greatest versatility for testing with electrodes and tissues of varying impedances in-situ and allows the device to be used in other neurostimulation applications such as functional electrical stimulation. The design has been thoroughly tested and meets all the design specifications.

Retinal neurostimulator for a multifocal vision prosthesis

A neurostimulator application-specific integrated circuit (ASIC) with scalable circuitry that can stimulate 14 channels, has been developed for an epi-retinal vision prosthesis. This ASIC was designed to allow seven identical units to be connected to control up to 98 channels, with the ability to stimulate 14 electrodes simultaneously. The neurostimulator forms part of a vision prosthesis, designed to restore vision to patients who have lost their sight due to retinal diseases such as retinitis pigmentosa and macular degeneration. For charge balance, the neurostimulator was designed to stimulate with current sources and sinks operating together, and with the ability to drive a hexagonal mosaic of electrodes to reduce the electrical crosstalk that occurs when multiple bipolar stimulation sites are active simultaneously. A hexagonal mosaic of electrodes surrounds each stimulation site and has been shown to effectively isolate each site, increasing the ability to inject localized independent charge into multiple regions simultaneously.