Browsing by Author "Allen, Penny"

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    Evaluation of stimulus parameters and electrode geometry for an effective suprachoroidal retinal prosthesis
    (IOP Publishing Ltd, 2010) Shivdasani, Mohit; Luu, Chi; Cicione, Rosemary; Fallon, James; Allen, Penny; Leuenberger, James; Suaning, Gregg; Lovell, Nigel; Shepherd, Robert; Williams, Chris
    Several approaches have been proposed for placement of retinal prostheses; epiretinal, sub-retinal, and suprachoroidal. We aimed to systematically evaluate the effectiveness of varying a range of stimulus parameters and electrode geometry for a suprachoroidal electrode array, using cortical evoked responses to monopolar electrical stimulation in cats. Our results indicate that charge thresholds were not dependent on electrode size, pulse widths or position of the return electrode tested, but were dependent on the number of sites stimulated in parallel. Further, we found that the combination of monopolar stimulation with large diameter electrodes, wide pulse widths and parallel stimulation minimized the voltage requirements for stimulation. These results provide useful insights for the design specifications of a low voltage suprachoroidal stimulator.
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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.
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    Visual cortex responses to suprachoroidal electrical stimulation of the retina: effects of electrode return configuration
    (IOP Publishing Ltd, 2012) Cicione, Rosemary; Shivdasani, Mohit; Fallon, James; Luu, Chi; Allen, Penny; Rathbone, Graeme; Shepherd, Robert; Williams, Chris
    A clinically effective retinal prosthesis must evoke localized phosphenes in a retinotopic manner in response to stimulation of each of the retinal electrodes, evoke brightness cues over a wide dynamic range and function within safe stimulus limits. The effects of varying return configuration for retinal stimulation are currently unknown. To investigate this, we implanted a flexible, 7x12 electrode array into the suprachoroidal space of normally-sighted, anesthetized cats. Multi-unit activity in the primary visual cortex was recorded in response to electrical stimulation using various return configurations: monopolar vitreous (MPV), common ground (CG), hexagonal (HX), monopolar remote (MPR) and bipolar (BP_N). MPV stimulation was found to be the most charge efficient and was most likely to induce cortical activity within safe charge limits. HX and CG stimulation were found to exhibit greater retinal selectivity compared to the MPV return at the expense of lower cortical yield and higher P50 charge levels, while cortical selectivity was unaffected by choice of return. Responses using MPR and widely spaced BP_N configurations were similar to those using the MPV return. These results suggest that choice of return configuration for a retinal prosthesis will be balanced between resolution and stimulation within safe charge limits.