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R. J. Jensen, J. F. Rizzo, III, L. Johnson; Responses of Retinal Ganglion Cells to Stimulation With a 3200 Microelectrode Array. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4573.
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This work is related to the efforts of the Boston Retinal Implant Project to develop a subretinal prosthesis to restore vision to the blind. Here, we examined the responses of retinal ganglion cells to a ‘moving’ electrical field, to simulate a moving light stimulus.
A retina, isolated from a deeply anesthetized rabbit, was placed photoreceptor side down on a 3200 microelectrode array, which was developed at the Naval Research Laboratory and is described in detail in a recent publication (Scribner et al 2007, IEEE Trans. Biomed. Circuits & Systems). In brief, the device consists of 80 x 40 individual electrodes in an area of 4 x 1.4 mm. Each electrode is 40 x 26 um. Spacing between electrodes is 10 um. The test stimulus was a 5 x 40 matrix of electrodes, in the shape of a rectangular bar. This ‘bar’ was stepped in 50 um increments across the retina. With each increment a brief biphasic current pulse was delivered to the retina. To mimic different speeds of a moving light stimulus, the time interval of each step was set at 63, 125, 250, 500 or 1000 ms. Extracellular spike activity from retinal ganglion cells was analyzed.
Recordings were made from 17 ON and OFF brisk ganglion cells. The cells responded best when the electrical stimulus was stepped at 1000 ms intervals and responded worst when the electrical stimulus was stepped at 63 ms step intervals.The responses of retinal ganglion cells to 500 ms interval steps were generally only slightly less than the responses to 1000 ms interval steps. However, with each decrease in step interval, the total number of spikes elicited by the retinal ganglion cells progressively decreased. In the end, the total number of spikes elicited by the retinal ganglion cells to 63 ms step intervals was on average 75% lower than the total number of spikes elicited by the retinal ganglion cells to 1000 ms step intervals.
The findings from this study suggest that simple translation of object motion to sequential stimulation of neighboring electrodes may not provide useful motion percepts for a patient with a retinal prosthesis.
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