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Thresholds for Activation of Rabbit Retinal Ganglion Cells with Relatively Large, Extracellular Microelectrodes

¹From The Center for Innovative Visual Rehabilitation, VA Boston Healthcare System, Boston, Massachusetts; the ²Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts; and the ³Department of Ophthalmology, Harvard Medical School and the Massachusetts Eye and Ear Infirmary, Boston, Massachusetts.

PURPOSE. To investigate the responses of retinal ganglion cells (RGCs) to electrical stimulation, using electrodes comparable in size to those used in human studies investigating the feasibility of an electronic retinal prosthesis.

METHODS. Rabbit retinas were stimulated in vitro with current pulses applied to the inner surface with 125- and 500-µm diameter electrodes while the responses of RGCs were recorded extracellularly.

RESULTS. Both short-latency (SL; 3–5 ms) and long-latency (LL; 9 ms) responses were observed after electrical stimulation within the receptive field of an RGC. With short, 0.1-ms current pulses, the threshold current for the SL cell response was significantly lower than that for the LL cell response. With long (10- to 20-ms) pulses, the threshold currents for the SL and LL cell responses were very similar. The threshold current for the SL cell response increased more steeply than did the LL cell response when the electrode was displaced from the point of lowest electrical threshold, either above or along the surface of the retina. Stimulation of an RGC axon outside of the cell’s receptive field produced only an SL response. For 0.1-ms duration pulses, the threshold current for the axonal response was significantly higher than the threshold current for the SL cell response. At pulse durations > 1 ms, the thresholds were very similar.

CONCLUSIONS. RGC responses to electrical stimulation depend on the current pulse duration and location of the stimulating electrode. For an epiretinal prosthesis, short-duration current pulses may be preferable since they result in a more localized activation of the retina.

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