| Research Abstract |
1. Introduction
We recently developed an electrical stimulation method for retinal prosthesis named Suprachoroidal-Transretinal Stimulation (STS), and demonstrated that a monophasic pulse of STS focally applied between an anode on the fenestrated sclera and an intravitreal cathode (inward STS) could evoke well-localized field potentials in the superior colliculus (SC) of both normal and retinal dystrophic rats (Kanda et al, 2004). To define effective pulse parameters in STS-based artificial retinas, we here investigated relations of pulse duration (0.1, 0.2, 0.5, 1.0ms), pulse amplitude, and polarities of STS to the collicular responses in rats. We also examined the SC response to biphasic STS comparing with that to the monophasic STS, because biphasic pulses should be used in artificial retina to avoid electrochemical damage to the retina from charge-imbalances.
2. Materials and Methods
16 Long Evans rats and 1 retinal dystrophic rat (Royal College of Surgeons; RCS) were anesthetized and
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immobilized. A single monophasic or biphasic pulse of constant current was delivered as STS to the left eye between an epi-scleral electrode and an intravitreal electrode. Evoked potentials (EP) to STS were recorded from the superficial gray layer of the right SC. Various pulse duration (0.1, 0.2, 0.5, 1.0ms), and pulse amplitude was tested.
3. Results and Discussion
A monophasic inward STS elicited EP that was composed of early and late components with latencies of 5-11 ms and >25ms, respectively. The former consisted of a small positive wave (P1) followed by a large negative wave (N1). The P1-N1 was identified not only in the normal rats but also in the RCS rat. It tended to become larger in amplitude as the pulse duration is shorter at the same quantity of electric charge of STS. To the contrary, the late components were oscillatory, and the amplitude and latency were not affected by the duration of pulse at the same electric charge in the normal rats. Furthermore, the late components were never recognized in the RCS rats. Inverting the polarity of the two stimulation electrodes for a monophasic outward STS, only slow responses were evoked in the normal rats, but not in the RCS rat. The amplitude of the slow responses depended on the electric charge, but not on the duration of the stimulus pulse. On the basis of these results, the P1-N1 component seems to be derived from the direct activation of the retinal ganglion cells by the inward STS. To elicit this component in the STS-based artificial retina, the shorter duration of pulse is more efficient.
A biphasic pulse by a combination of the inward and outward STS elicited the P1-N1, which was comparable in waveform, latency and threshold to that to a monophasic pulse of inward STS. However, the amplitude to the biphasic STS is smaller than that to the inward monophasic STS in most cases. The late oscillatory components were substantially suppressed, especially with short pulses. Such suppression may provide us for high temporal resolution of STS-based artificial retina.
In conclusion, short pulse duration (0.1ms or 0.2ms) of biphasic STS seems to be adequate for an artificial retina. Less
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