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Sam E. John, Mohit N. Shivdasani, James B. Fallon, Graeme Rathbone, Chris E. Williams; Cortical Responses to Repetitive Electrical Stimulation of the Retina using Suprachoroidal Visual Prostheses. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6949.
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Repetitive electrical stimulation can potentially replicate spike trains generated by the retinal ganglion cells. Such stimulation can improve quality of vision and has been proposed as a method of stimulation for visual prostheses. However, it is not clear how spikes generated by repetitive electrical stimulation are encoded in the higher visual centres. In the current work, we investigated the nature of responses in the visual cortex to repetitive electrical stimulation of the retina.
Platinum disc (84 discs- 420 µm indiameter) electrode arrays were acutely implanted in the suprachoroidal space of normally sighted feline eyes (n=10). 500ms bursts of cathodal first biphasic current pulses (500us/phase and 25us interphase gap) were presented at rates of 1- 20 Hz on individual electrodes (n=15). Responses to electrical stimulation were recorded using multichannel arrays in the primary visual cortex (V1). Multiunit responses (n=480) were quantified by the Repetition Rate Transfer Function (RRTF) and we evaluated the maximum firing rate, Best Repetition Rate (BRR) and cut-off rate (at 50%maximum firing rate).
Neurons in V1 were able to respond to stimulation rates up to 20Hz (50ms Inter Stimulus Interval-ISI). At the low frequency ≤ 2Hz (ISI- 500ms), the response to the second pulse was the same as the response to the first pulse. However, at higher frequencies there was considerable depression of responses to the subsequent pulses compared to that to the first pulse. The repetition rate transfer function showed a characteristic band pass filter shape with the best repetition rate at 9.27±0.95 (mean ± SD) Hz with a cut-off frequency at 11.67 ±0.91Hz.
The capacity of V1 neurons to respond to repetitive electrical stimulation up to 20 Hz reflects their ability to encode temporally complex signals. However, the depression in the spiking response at higher frequencies indicate that, the rate of repetitive stimulation should be kept below 12 Hz for efficient transfer of visual information. These results have implications for the development of efficient stimulation stratergies for retinal prostheses.
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