| Visual Prosthesis is a newly appeared technology to create artificial vision, which delivers electrical stimulation onto visual system and generates phosphenes in brain, in purpose of restoring partial vision for the blind people. The retinal prosthesis has been studied for years, and optic nerve prosthesis with penetrating electrode array was first brought up and developed by C-Sight group in China. There're some basic questions in visual prosthesis need to be answered: How's the availability and feasibility of electrical stimulation on the visual system? How're the temporal and spatial properties of electrical stimulation? How do the various visual neurons response to electrical stimulation with different modes? As an important tool to study the nervous system, electrophysiology can be applied to investigate the electrical responses of visual system stimulated by visual prosthesis. In this project, action potentials of retina ganglion cells were recorded with electrical stimulation on retina, and cortical potentials were recorded with electrical stimulation on optic nerve. By studying the response properties, the basic theory of prosthetic vision and the parameter design of prosthetic device would be improved.In the study of electrical stimulation on retina, patch clamp was used to record the responses of a single ganglion cell, in order to study the response properties of different types of ganglion cells by various modes of stimulations. High frequency electrical stimulation was delivered (100Hz~700Hz). Significant heterogeneity was found in the ability of different types to follow pulse trains. There was additional heterogeneity in the response patterns across those types that could not follow high-rate trains. Interestingly, in the types that could not follow high-rate trains we found a second type of response: many pulses of the train elicited a small-amplitude biphasic waveform with a latency that was slightly faster than that of standard action potentials. Several possible origins of the small waveform and their implications for effective retinal stimulation are discussed. When the stimulating frequency goes up to 2000Hz, the response modes of these cells changed dramatically with different stimulating amplitude, and different types of ganglion cells had different modes. Further analysis indicated that the various responding modes under different stimulating amplitudes just came from ganglion cell activities, without involving retina network activities. But the mechanism of these phenomena is yet to be studied.In the study of optic nerve response of penetrating electrode array, rabbits were used in vivo for multi-site stimulations on optic nerve and multi-channel recordings on visual cortex. The stimulating electrodes were configured as triangularly or linearly to insert into optic nerve. The recording electrode array was home-made one with silver-ball tips, which can record multi-channel electrical evoked potentials (EEP) on visual cortex. The stimulating waveforms were charge-balanced current pulses with cathodel phase first or anodal phase first. The results showed visual cortex can be effectively evoked by optic nerve stimulation with penetrating electrodes. The threshold of current and charge density to elicit EEPs was 20.3±7.5μA and 37.8±13.9μC/cm~2, respectively. Current stimuli with cathode-first pulses elicited larger cortical responses than that with anode-first pulses. The intensity and extend of EEPs increased as the stimulating amplitude increased. The spatial distributions of multi-channel EEPs demonstrated distinctively different properties with different orientations of the stimulating electrodes. This study provided basic supports and implications for the design of optic nerve prosthesis. |
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