Retinotopy of Cortical Activation by Epi- and Sub-retinal Visual Prostheses

¹Eysel U., ¹Kisvárday Z. F., ²Walter P., ³Sachs H., ⁴Gekeler F., ³Gabel V.-P., ⁵Zrenner E.,
¹Ruhr-University Bochum, Department of Neurophysiology (Bochum)
²Universität zu Köln, Zentrum für Augenheilkunde (Köln)
³Universität Regensburg, Klinik und Poliklinik für Augenheilkunde (Regensburg)
⁴Eberhard-Karls-Universität Tübingen, Universitäts-Augenklinik, Abt. II (Tübingen)
⁵Eberhard-Karls-Universität Tübingen, Universitäts-Augenklinik, Abt. II, Pathophysiologie des Sehens u. Neuroophthalmologie (Tübingen)

Purpose: Sub- and epiretinally implanted miniaturized microelectrode arrays as developed by the German Epiret- and Subret-consortia allow localized retinal stimulation. One crucial question was whether stimulation with different electrodes in the array generates reproducible patterns of cortical activation with a sufficient spatial resolution.
Methods: Retinal implant surgery (epi- and subretinal) was acutely performed in adult cats. After retinal surgery the cats were immediately prepared for cortical recording. Single electrodes were electrically stimulated (biphasic, 0.25 ms, 2-100 µA), and electric field potentials were recorded in the optic tract to ensure successful activation of the afferent visual pathway. Optical imaging of intrinsic signals (Imager 2001, Optical Imaging, USA) was performed to obtain 2-dimensional patterns of visual cortical activation.
Results: Stimulation of all single electrodes in the epi- and subretinal electrode arrays with currents between 4 and 78 µA successfully evoked optic tract field potentials. The activation maps obtained from the visual cortex yielded a clear cortical retinotopy in response to stimulation by adjacent electrodes. The activity peaks were well separable with an average half width of 0.9 mm at half height, corresponding to a visual resolution of 1.25° at 2° retinal eccentricity.
Conclusions: The quantitative evaluation of the regions with high intrinsic activity underestimates the realistic spatial resolution, because intrinsic signals equally visualize supra- and sub-threshold activity. A significantly better spatial resolution can be expected in human perception due to intracortical processing (e.g. lateral inhibition and threshold mechanisms). We conclude that the tested microelectrode arrays when used as visual prostheses can generate visual percepts with a useful spatial resolution in the range of 1° of visual angle.
(Supported by BMBF No.01KP0007)

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