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N. J. Desai, J. F. Rizzo, S. I. Fried, C. Cai; Evaluation of Elicited Action Potential Kinetics in Response to Electric Stimulation of the Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3045.
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© ARVO (1962-2015); The Authors (2016-present)
This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. Electric stimulation of the retina of blind patients generates visual percepts called phosphenes. Eliciting complex visual detail has proven difficult however and we are studying the response of retinal neurons to electric stimulation as a step towards improving clinical outcomes. Here, we explored the latency, threshold, and dynamic range of elicited action potentials when a single ganglion cell was stimulated in various locations: soma, sodium channel (SOC) band, and distal axon.
We used light responses to classify ganglion cells into known types. Identified ganglion cells were stimulated with a small-tipped Pt-Ir electrode (far-field return) with short duration (200 µs) cathodic pulses of variable intensity. Threshold levels for spike initiation, dynamic range of threshold, and action potential latency were measured as a function of stimulating electrode position.
Action potential latencies ranged from 0.23 to 0.82ms. There was no statistical difference in latency with regards to location of stimulation, or between cell types. However, latency decreased with increasing stimulus intensity. Threshold was consistently found to be lowest in the region of the SOC - average of 10.9uA in the SOC band and 22.1 and 38.9uA in the soma and axon respectively. Dynamic range of threshold was found to be lowest over the SOC band when compared to other locations.
Latencies were short and consistent suggesting that precise temporal control over spike timing is possible. Threshold and dynamic range differences in various parts of a single cell suggest multiple processes underlie the ganglion cell response to electrical stimulation. A better understanding of the mechanisms that underlie these different responses will lead to improvements in the quality of elicited percepts during future clinical trials.
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