The Investigation Of Spatiotemporal Properties Of Cortical Responses Elicited By Penetrative Optic Nerve Stimulation And The Visuotopic Correspondence Study

Vision is essential for human to get information from the world. Blindnesscaused by retina disease has become a disability that affects the quality of peoples’daily life. Visual prosthesis is a newly appeared technology to restore functionalvision. After processing the aquired image, the encoded pulses were transferredthrough electrode array to stimulate the visual system, and create artificial vision forblind patients. Visual prosthesis based on penentrative optic nerve stimulation is anew approach to restore visual function. But there still remain several basic questionsto be answered, such as to explore the suitable stimulating modes and strategy foroptic nerve stimulation, the visuotopic correspondence and organization of penetrativeoptic nerve prosthesis implanted into optic nerve just postier to eyeball, and theachieved spatial resolusion and localization of optic nerve stimulation. In thisdissertation, the electrically evoked cortical responses to penetrative optic nervestimulation were recorded to investigate the spatio-temporal properties of the activity.The results will give experimental support for the basic theory, parameters design andthe estimated achievements of this prosthetic device.In visual prosthesis research, all experiments involving electrical stimulation andelectrophysiological recording suffer to some degree from stimulus-induced electricaldistortion caused by instantaneous electrotonic current spread through the brain tissueand could not be removed by the traditional methods. These stimulus artifactscontaminate the EEP waveform and often make subsequent analysis of the underlyingneural responses difficult.This is particularly serious when investigating responses toelectrical stimulation with high strength, long duration, multi-pulses and complicatedmodes. We applied independent component analysis to remove these electricalstimulation-induced artifacts and evaluate the performance by recording the acturalartifacts after optic nerve block by lidocaine injection. The results show that, theelectrical-induced artifacts could be removed by this method under stimulation withvarious current strength, pulse waveforms and stimulating modes, withoutalgorithm-induced distortion to the responses. There is a performance dependence onthe number of recording channels when applying the algorithm. This technique haspotential applications in the following electrophysiological studies on optic nervevisual prostheses. In penetrative visual prosthesis research, we implanted platinum–iridium intooptic nerve and recorded electrically evoked cortical potentials by silver-ball electrodearray on visual cortex of albino rabbits. The spatio-temporal properties of the corticalactivities were analyzed to explore the suitable stimulating modes, parameters,electrode size, inter electrode distance and electrode distribution for optic nerveprosthesis to realize low threshold, localized and discriminatable stimulation. Theresults showed that, the threshold and latency of different components in theelectrically evoked multi-peaked potential varied. And the P1, which has lowerthreshold and latency, has a well spatial correspondence to the stimulating sites,measured by histological and electrophysiological recording. The cathodic first pulsesunder monopolar mode could elicit more localized cortical responses with lowerthreshold. The effects on localized stimulation by electrodes with different size werecompared. We also demonstrated that two electrodes with inter-distance of150μmalong the optic nerve axis could elicit discriminatable cortical responses. This studywill give benefits to design the stimulating parameters, modes, penetrative electrodesizes and distributions for the penetrative optic nerve prosthesis.Then we established a method to locate the visual field position of penetrativeoptic nerve electrode on cats. The visuotopic organization, spatial resolusion andlocalization of optic nerve stimulation were studied by analyzing the spatial propertiesof electrically evoked and visually evoked cortical potentials. The results showed thatthe charge density threshold of optic nerve prosthesis with small electrode is high.The sparse noise method was used to locate the visual field position of penetrativeoptic nerve electrode, and to obtain the visuotopic map of each cat’s visual cortex. Itwas also demonstrated that the visuotopic stimulation could be achieved by implant awell-refined electrode array into optic nerve just posterier the eyeball. The penetrativeoptic nerve prosthesis could achieve1–2°localized stimulation with spatialresolusion of2–3°. This study will give experimental evidence for designing thepenetrative optic nerve electrode array.