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D.S. Hahn, F. Darvas, D. Thyerlei, M.J. McMahon, J.D. Weiland, R.M. Leahy, M.S. Humayun; Spatiotemporal Analysis of Visually Evoked Potentials in the Cortex of a Blind Subject With a Chronic Intraocular Retinal Prosthesis . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1496.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:To localize cortical activity induced by stimulation of microelectrodes in a blind subject with a chronic intraocular retinal prosthesis through electroencephalography (EEG) and source localization. Methods: After patching the subject’s left eye (prosthesis in right eye), a 64 channel electrode skullcap was placed on her head in a standard 10–20 setup. Using a 500Hz sampling rate, a gain of 250, and a continuous data acquisition mode, all 16 electrodes of the prosthesis were stimulated with biphasic pulses (3ms). Data was recorded for 1140s repeating the stimuli every second. For reconstruction from EEG data, a 3 shell sphere model and a warped generic realistic head model (Montreal brain phantom) were used to compute the bioelectric forward model. The warped brain phantom was also used to display results in an anatomical atlas. Point and distributed sources in the interval from 50–450ms were reconstructed using RAP–MUSIC and cortical constrained minimum norms. Results: Data from 6 faulty channels (FP2, C2, F6, FT8, T4, TP10) resulting from eye movement or interference from electric fields of the stimulation device were removed. 416 epochs were selected manually and averaged to reject artifact. Significant activity was noted in the parietal/occipital channels from 200–300ms as compared to the pre–stimulus data. Minimum norms showed activity on the cortical white matter of V1 at 144ms, 204ms, and 292ms with a shift in activation at these three time points from the right to the left and back to the right hemispheres respectively. RAP–MUSIC found 3 sources, 2 close to the visual cortex and 1 corresponding to the stimulation device which was localized to the right side of the subject. Subspace correlation for this stimulation source was 0.98 (1=best). From activity in V1, one source was found deep in the cortex sitting between the hemispheres with a subspace correlation of 0.99. The second source close to the visual cortex was found at a shallower position relative to the first source and its subspace correlation was 0.95. Conclusions: Visual evoked potentials as seen on EEG were used with source localization methods to positively identify two regions of visual cortex which were activated as a result of artificial electrical stimulation of the retina.
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