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E. Greenbaum, C. A. Sanders, C. J. Cela, G. Lazzi, D. M. Zhou, R. Greenberg, J. D. Weiland, M. S. Humayun; Physical Analysis of the Spatial Distribution of Pulsed Electric Potentials Above Retinal Prosthesis Arrays. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3041.
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Visual prosthesis electrode arrays activated by an electrical stimulus can evoke a central neural interpretation of light by depolarizing retinal neural cells. A systematic study of the spatial distribution of pulsed electric potentials will define peak values over closely packed multielectrode arrays and influence the pattern of neuronal activation and image resolution in the cortex. Apart from actually implanting electrodes in a patient’s eye, the best way to assess the physical presence and symmetry patterns of electric potentials generated by prosthesis electrodes is to construct fine resolution potential maps using recording electrodes combined with numerical simulations that correlate with and validate the measurements. Numerical simulations also provide a theoretical approach for understanding the influence of the recording electrode.
We have developed instrumentation for mapping the spatial distribution of electric potentials surrounding individual electrodes in prosthesis arrays in synthetic vitreous humor. The microelectrode array was a retinal prosthesis prototype fabricated by Second Sight Medical Products. The recording electrode was positioned over a single electrode in the array, 20 µm above the surface, using an xyz translation stage with micron resolution. Electric potentials were measured with a Tektronix digital oscilloscope over the target array electrode while sequentially stimulating it and its neighboring disks to determine the "spillover" of electric potential.
Transient voltage potentials were generally inversely proportional to the distance (µm) between the recording and stimulated electrodes. The placement of the reference electrode out-of-plane from the array affected the overall amplitude of the recorded potentials. Its position also broke the symmetry of the field patterns that would be predicted based on simple geometric considerations.
Electrical potentials in the vitreous at the surfaces of active electrodes will propagate through the electrolyte with measurable spatial patterns. The potential field map over the array is related to the size and pitch of array electrodes and the placement of the return electrode. Anomalies in charge distribution, not related to the distance between array electrodes, may be related to differences in electrode impedance, traces and position of the return electrode.
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