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V. Chowdhury, J.W. Morley, M.T. Coroneo; The Neurophysiologic Response to Surface Stimulation of the Visual Cortex in the Cat by a Bionic Eye Device . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5068.
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Purpose: To evaluate surface stimulation of the visual cortex by a multi-electrode array, in order to evaluate components of a visual cortex prosthesis to restore basic visual perception to blind patients. To develop an in-vivo method of assessing visual cortex stimulation in the cat, by measuring the transcallosal evoked response in the contralateral visual cortex. Methods: In four anaesthetised cats, a craniotomy was performed to expose the visual cortex and the dura was removed. A 21-electrode array containing 3 rows of 0.7mm diameter platinum electrodes in a silicone base was placed on the surface of the visual cortex of one hemisphere. An identical 21-electrode array was placed over the surface of a homologous area of the contralateral visual cortex. Electrical stimulation was performed on the electrodes in one arrray and the transcallosal responses to this stimulation were recorded on the contralateral array. Results: Monopolar stimulation of the electrodes with monophasic pulses from a bench stimulator resulted in a transcallosal response in the contralateral hemisphere which consisted of a initial negative peak at approximately 4ms followed by a positive peak at approximately 6ms. Threshold currents to elicit a transcallosal response from surface (bench) stimulation of the visual cortex in the cat were calculated for each electrode. Anodal monopolar stimulation displayed thresholds of approximately 1.7mA. Cathodal monopolar stimulation displayed thresholds of approximately 3mA. Stimulation of the electrodes was also performed with biphasic pulses from an implantable neurostimulator and corresponding response thresholds were calculated. Conclusions: The transcallosal evoked response is a useful method to evaluate the parameters of surface stimulation of the visual cortex for a bionic eye device. It allows thresholds for stimulation to be calculated, which may correlate with thresholds for visual cortex stimulation in human subjects. Higher currents are necessary when using cathodal monopolar stimulation than are needed with anodal monopolar stimulation. We evaluated an implantable transcutaneously controlled neurostimulator and subdural (surface) electrode grid which form the components of our prototype visual prosthesis. The prototype device resulted in effective stimulation of the visual cortex in this acute cat model. This device would offer considerable advantages over percutaneously controlled visual cortex prostheses.
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