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R. J. Jensen, J. F. Rizzo, III; Responses of Rabbit Retinal Ganglion Cells to Repetitive Electrical Stimulation. Invest. Ophthalmol. Vis. Sci. 2007;48(13):649.
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Retinal ganglion cells (RGCs) can be activated electrically either directly or indirectly (via the retinal neural network). Previous studies have shown that RGCs can follow high stimulus rates (≥ 200 pulses/s) when directly activated. In the present study, we investigated how well RGCs can follow repetitive stimulation of the neural network.
The retina from a rabbit was isolated and placed photoreceptor-side down on a platform which was embedded with a 400-µm diameter platinum wire. This wire served as the stimulating electrode. Extracellular recordings were made from RGCs, whose receptive field centers were located either above or within 200 µm of the stimulating electrode. Electrical stimuli consisted of symmetric, biphasic pulses (1.0 ms per phase). Anodal pulses always preceded cathodal pulses; the interpulse separation between anodal and cathodal pulses was 0.5 ms. Pulses were delivered through two constant-current stimulus isolation units attached to a Grass-Telefactor S88 stimulator.
We studied the responses (spike activity) of RGCs to stimulation with paired pulses applied at different interpulse intervals and trains of pulses applied at different frequencies. We found that the response amplitude of a RGC to a current pulse applied soon (< 400 ms) after a preceding current pulse was diminished. This depression in response amplitude became greater as the interval between pulses became shorter. At an interpulse interval of 15 ms (shortest tested), the response amplitude to the 2nd current pulse was reduced on average 94%. When a train of 10 stimulus pulses were applied, further depression was observed, particularly at high stimulation frequencies. The depression with each successive pulse was relatively moderate compared to the depression to the 2nd pulse.
The results of this study have implications for the design of electrical stimulation strategies in a retinal prosthesis, and in particular the repetition rate of transmitting electronic images to the retina.
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