Browsing by Author "John, Sam"

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    An automated system for rapid evaluation of high-density electrode arrays in neural prostheses
    (Institute of Physics, 2011-06) John, Sam; Shivdasani, Mohit; Leuenberger, James; Fallon, James; Shepherd, Robert; Millard, Rodney; Rathbone, Graeme; Williams, Chris
    The success of high density electrode arrays for use in neural prostheses depends on efficient impedance monitoring and fault detection. Conventional methods of impedance testing and fault detection are time consuming and are not always suited for in-vivo assessment of high density electrode arrays. Additionally the ability to evaluate impedances and faults such as open and short circuits both, in-vitro and in-vivo are important to ensure safe and effective stimulation. In the present work we describe an automated system for the rapid evaluation of high density electrode arrays. The system uses a current pulse similar to that used to stimulate neural tissue and measures the voltage across the electrode in order to calculate the impedance. The switching of the system was validated by emulating a high density electrode array using light emitting diodes and a resistor- capacitor network. The system was tested in-vitro and in-vivo using a range of commercially available and in-house developed electrode arrays. The system accurately identified faults on an 84 electrode array in less than 20 seconds and reliably measured impedances up to 110 kn using a 200µA, 250 µs per phase current pulse. This system has direct application for screening high density electrode arrays in both a clinical and experimental setting.
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    Minimally invasive endovascular stent-electrode array for high-fidelity, chronic recordings of cortical neural activity
    (Nature Publishing Group, 2016-02-08) Oxley, Thomas; Opie, Nicholas; John, Sam; Rindl, Gil; Ronayne, Stephen; Wheeler, Tracey; Judy, Jack; McDonald, Alan; Dornom, Anthony; Lovell, Timothy; Steward, Christopher; Garrett, David; Moffat, Bradford; Lui, Elaine; Yassi, Nawaf; Campbell, Bruce; Wong, Yan; Fox, Kate; Nurse, Ewan; Bennett, Iwan; Bauquier, Sebastien; Lyanage, Kishan; van de Nagel, Nicole; Perucca, Piero; Ahnood, Arman; Gill, Katherine; Yan, Bernard; Churilov, Leonid; French, Christopher; Desmond, Patricia; Horne, Malcolm; Kiers, Lynette; Prawer, Steven; Davis, Stephen; Burkitt, Anthony; Mitchell, Peter; Grayden, David; May, Clive; O'Brien, Terence
    High-fidelity intracranial electrode arrays for recording and stimulating brain activity have facilitated major advances in the treatment of neurological conditions over the past decade. Traditional arrays require direct implantation into the brain via open craniotomy, which can lead to inflammatory tissue responses, necessitating development of minimally invasive approaches that avoid brain trauma. Here we demonstrate the feasibility of chronically recording brain activity from within a vein using a passive stent-electrode recording array (stentrode). We achieved implantation into a superficial cortical vein overlying the motor cortex via catheter angiography and demonstrate neural recordings in freely moving sheep for up to 190 d. Spectral content and bandwidth of vascular electrocorticography were comparable to those of recordings from epidural surface arrays. Venous internal lumen patency was maintained for the duration of implantation. Stentrodes may have wide ranging applications as a neural interface for treatment of a range of neurological conditions.
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    Suprachoroidal electrical stimulation: Effects of stimulus pulse parameters on visual cortical responses
    (IOPscience, 2013-10) John, Sam; Shivdasani, Mohit; Williams, Chris; Morley, John; Shepherd, Robert; Rathbone, Graeme; Fallon, James
    Objective. Neural responses to biphasic constant current pulses depend on stimulus pulse parameters such as polarity, duration, amplitude and interphase gap. The objective of this study was to systematically evaluate and optimize stimulus pulse parameters for a suprachoroidal retinal prosthesis. Approach. Normally sighted cats were acutely implanted with platinum electrode arrays in the suprachoroidal space. Monopolar stimulation comprised of monophasic and biphasic constant current pulses with varying polarity, pulse duration and interphase gap. Multiunit responses to electrical stimulation were recorded in the visual cortex. Main results. Anodal stimulation elicited cortical responses with shorter latencies and required lower charge per phase than cathodal stimulation. Clinically relevant retinal stimulation required relatively larger charge per phase compared with other neural prostheses. Increasing the interphase gap of biphasic pulses reduced the threshold of activation; however, the benefits of using an interphase gap need to be considered in light of the pulse duration and polarity used and other stimulation constraints. Based on our results, anodal first biphasic pulses between 300–1200 µs are recommended for suprachoroidal retinal stimulation. Significance. These results provide insights into the efficacy of different pulse parameters for suprachoroidal retinal stimulation and have implications for the design of safe and clinically relevant stimulators for retinal prostheses.