Browsing by Author "Morley, John"

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    Peripheral Nerve Activation Evokes Machine-Learnable Signals in the Dorsal Column Nuclei
    (Frontiers in Systems Neuroscience, 2019-04) Loutit, Alastair; Shivdasani, Mohit; Maddess, Ted; Redmond, Stephen; Morley, John; Stuart, Greg; Birznieks, Ingvars; Vickery, Richard; Potas, Jason
    The brainstem dorsal column nuclei (DCN) are essential to inform the brain of tactile and proprioceptive events experienced by the body. However, little is known about how ascending somatosensory information is represented in the DCN. Our objective was to investigate the usefulness of high-frequency (HF) and low-frequency (LF) DCN signal features (SFs) in predicting the nerve from which signals were evoked. We also aimed to explore the robustness of DCN SFs and map their relative information content across the brainstem surface. DCN surface potentials were recorded from urethane-anesthetized Wistar rats during sural and peroneal nerve electrical stimulation. Five salient SFs were extracted from each recording electrode of a seven-electrode array. We used a machine learning approach to quantify and rank information content contained within DCN surface-potential signals following peripheral nerve activation. Machine-learning of SF and electrode position combinations was quantified to determine a hierarchy of information importance for resolving the peripheral origin of nerve activation. A supervised back-propagation artificial neural network (ANN) could predict the nerve from which a response was evoked with up to 96.8 +/- 0.8% accuracy. Guided by feature-learnability, we maintained high prediction accuracy after reducing ANN algorithm inputs from 35 (5 SFs from 7 electrodes) to 6 (4 SFs from one electrode and 2 SFs from a second electrode). When the number of input features were reduced, the best performing input combinations included HF and LF features. Feature-learnability also revealed that signals recorded from the same midline electrode can be accurately classified when evoked from bilateral nerve pairs, suggesting DCN surface activity asymmetry. Here we demonstrate a novel method for mapping the information content of signal patterns across the DCN surface and show that DCN SFs are robust across a population. Finally, we also show that the DCN is functionally asymmetrically organized, which challenges our current understanding of somatotopic symmetry across the midline at sub-cortical levels.
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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.
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    Visual Cortex Responses to Single-and Simultaneous Multiple-Electrode Stimulation of the Retina: Implications for Retinal Prostheses
    (The Association for Research in Vision and Ophthalmology, 2012) Shivdasani, Mohit; Fallon, James; Luu, Chi; Cicione, Rosemary; Allen, Penny; Morley, John; Williams, Chris
    Purpose: The aim of this study was to compare simultaneous stimulation of multiple electrodes to single electrode stimulation in a retinal prosthesis. Methods: A platinum electrode array was implanted into the suprachoroidal space in six normally-sighted anesthetized cats. Multi-unit activity from the primary visual cortex in response to retinal stimulation was recorded. Cortical thresholds, yield of responses, dynamic ranges, and the spread of retinal activation were measured for three modes of stimulation; single electrode, half-row (6-electrode horizontal line) and column (7-electrode vertical line). Results: Stimulation of the best half-rows and columns was found to elicit activity with higher yield and lower charge thresholds per electrode compared to the best single electrodes. Dynamic ranges between the three modes were similar. As expected, peak voltages measured for columns and half-rows were lower than those measured for single electrodes. Spread of retinal activation, determined by the increase in threshold with distance in the retina from the best site was found to be similar between single and multiple electrode stimulation but dependent on orientation. Conclusions: The lower thresholds, higher yield, equivalent dynamic ranges and equivalent spread of retinal activation observed from simultaneous stimulation of multiple electrodes may be due to current and/or neural summation within the retina. Such stimulation techniques could be useful for the presentation of lines and edges of objects using a suprachoroidal retinal stimulator with low voltage compliance. Furthermore, the results suggest more complex visual processing strategies in addition to sequential stimulation of individual electrodes should be considered for retinal prostheses.