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H. Sawai, M. Kuroda, T. Fujikado, Y. Tano; Pulse Parameters Affecting Rat Collicular Evoked Potentials to Suprachoroidal-Transretinal Electrical Stimulation for Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2007;48(13):652.
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To find evidence for defining effective pulse parameters of Suprachoroidal-Transretinal Stimulation (STS), which we have been developing as a method for less-invasive retinal prosthesis.
In normal and retinal degenerated rats (Royal College of Surgeons; RCS) under anesthesia, a single rectangular pulse of STS was focally applied to the left eyeball between an anode on the sclera and cathode in the vitreous body (inward STS), and Evoked Potentials (EPs) were recorded from the contralateral superior colliculus (SC). We evaluated the effect of pulse duration (0.1~1 milliseconds), amplitude (50~800 micro amps), and polarities (inward and outward) of STS to the EPs. The response to charge-balanced biphasic STS (with inward phase and outward phase) was also examined comparing with that to the monophasic STS, because biphasic pulses will be used in artificial retina to avoid electrochemical retinal damage. In some cases the duration of outward phase was extended with the charge balance kept (asymmetrical biphasic STS).
A monophasic inward STS elicited localized EPs composed of early and late components with latencies of 5-11 ms and >25 ms, respectively. The former consisted of a positive wave (P1) followed by a negative wave (N1) and they were identified in both normal and RCS rats. The P1-N1 amplitude became larger as the pulse duration set shorter at the same quantity of electric charge. In the late component, the amplitude and latency tend to be not affected by the duration of pulse at the same electric charge in normal rats. This component was not observed in RCS rats. A biphasic pulse elicited the P1-N1, which was comparable in waveform and latency to that to inward monophasic STS. The amplitude of P1-N1 to the biphasic STS was smaller than that to the inward monophasic STS. Asymmetrical biphasic STS has a possibility to be more effective when the duration of cathodic phase set longer.
The present results suggest that retinal excitation and its transmission to the visual centers can be more efficiently induced by shorter stimulus duration of the biphasic pulse in STS-based retinal prosthesis. Further investigation is required for establishing optimal pulse parameters from the viewpoint of tissue and electrode damage, threshold of pain sensation, energy consumption, and so on.
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