Philip Preston

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.

An efficient multiplexing method for addressing large numbers of electrodes in a visual neuroprosthesis

Recent clinical trials using modified cochlear implants employ a small number of electrodes to stimulate surviving retinal neurons in blind patients and indicate that spatially-mapped phosphenes may indeed be elicited through these means. The next obvious step forward in the path toward achieving a useful visual prosthesis for the blind will be to increase the quantity of stimulation sites such that shapes, characters and rudimentary images may be conveyed. An important objective that must be obtained in the pursuit of this task is the ability to configure and deliver the stimulation with sufficient speed so as to avoid delays that are perceived by the patient as flicker within the visual scene. As the quantity of electrodes within the prosthesis increases, so too does the complexity of achieving this objective. This paper describes a means through which large numbers of electrode sites may be efficiently addressed in a neurostimulation circuit so as to increase the rate at which said circuit may be configured for the delivery of stimulation.

Bank note recognition for the vision impaired

Blind Australians find great difficulty in recognising bank notes. Each note has the same feel, with no Braille markings, irregular edges or other tangible features. In Australia, there is only one device available that can assist blind people recognise their notes. Internationally, there are devices available; however they are expensive, complex and have not been developed to cater for Australian currency. This paper discusses a new device, the MoneyTalker that takes advantage of the largely different colours and patterns on each Australian bank note and recognises the notes electronically, using the reflection and transmission properties of light. Different coloured lights are transmitted through the inserted note and the corresponding sensors detect distinct ranges of values depending on the colour of the note. Various classification algorithms were studied and the final algorithm was chosen based on accuracy and speed of recognition. The MoneyTalker has shown an accuracy of more than 99%. A blind subject has tested the device and believes that it is usable, compact and affordable. Based on the devices that are available currently in Australia, the MoneyTalker is an effective alternative in terms of accuracy and usability.

In-Vitro Testing of Simultaneous Charge Injection and Recovery in a Retinal Neuroprosthesis

In order to deliver sufficient phosphene quantities to convey effective vision in a prosthesis device, simultaneous stimuli is necessary. We present in vitro experimental results of the current distribution between stimulation sites during simultaneous stimulation of platinum electrodes immersed in physiological saline. Stimuli were delivered using circuitry that utilizes (a) current source only, (b) current sink only, and (c) the combination of a balanced current source and current sink, to deliver and recover balanced charge at each stimulation site. The results from these experiments support our decision to implement balanced combined current source and current sink circuitry in an application specific integrated circuit (ASIC)

Current Distribution During Parallel Stimulation: Implications for an Epiretinal Neuroprosthesis

A simplified mathematical model has been developed in order to better understand local current spread when multiple simultaneous current sources are used in an epiretinal neuroprosthesis. To test the model, pairs of platinum electrodes of 430 mum diameter and an intra-pair spacing of 1 mm between centers, were arranged either in-line or in parallel, in a bath of physiological saline. Each pair was separated by distances from 1 mm to 6 mm. The currents in each electrode in the bath were measured and compared with the computational model of the same arrangement. This approach allowed us to quantify return current interaction between parallel sources. As predicted, with parallel electrodes and matching currents in each electrode pair, there is no current cross-talk. However with imbalanced current sources, significant cross-talk is evident. The cross-talk decreases as a function of electrode pair separation. The implication of this work in the design of an epiretinal neuroprosthesis is discussed

A retinal neuroprosthesis design based on simultaneous current injection

We present an epiretinal neuroprosthesis design based on a hexagonally-latticed 98 electrode array and the capacity to multiplex up to 14 simultaneous current sources. The digital and analogue electronics required to perform this function and how this would be incorporated into an application specific integrated circuit (ASIC) are described. Simulation data and data from saline bath testing of a platinum/silicone electrode array (and associated driving electronics) are presented. Simulations were performed using a 2D computational model solved using a custom collocation method. The guarding affect of the hexagonal array is investigated and shown in simple simulations to be an approach worthy of further investigation.

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

Implant electronics for intraocular epiretinal neuro-stimulators

In this paper, we discuss system architectures, design challenges and circuit implementation principles for intraocular epiretinal neuro-stimulators to be used as part of vision prostheses in partially restoring vision to the blind. Our unique hexagonal electrode placement allows focused simultaneous stimulation which allow the electrode count to scale. A new dual-channel transcutaneous inductive link is used to transfer power and data efficiently to the implant, and a new dual-voltage power rectifier provides two implant supplies without the need for a DC-DC converter. Low-power designs of current stimulators and ECAP amplifiers are also outlined in the paper.

An FPGA-Based Vision Prosthesis Prototype: Implementing an Efficient Multiplexing Method for Addressing Electrodes

A prototype of an epi-retinal vision prosthesis based upon an efficient electrode addressing schema has been developed. This system has the ability to stimulate multiple electrode regions simultaneously, hence greatly improving the maximum rate of stimulation compared to many currently available neural stimulation devices based on serial stimulation protocols. To minimize the problem of cross talk between stimulating electrodes, a hexagon layout of electrodes was implemented. Basic tests were completed using a field programmable gate array logic system driving analogue circuitry to inject current into physiological saline via electrodes in hexagon arrangements and in a simple paired arrangement. The hexagon layout of electrodes was shown to clearly reduce the interaction between multiple current sources and hence cross talk

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.