Research On Properties Of Visual Cortical Response Elicited By Penetrative Optic Nerve And Noninvasive Retinal Electrical Stimulation

Targeting at the residual functional neural tissue in the visual pathway,the visual prostheses electrically activate the neural tissue using implanted electrodes to produce visual perception and restore vision.Penetrative optic nerve visual prosthesis has its unique advantages including covering a wider visual field,lowering the threshold and increasing the spatial resolution.However,the properties of visual cortical responses evoked by penetrative optic nerve electrical stimulation with different modes have not been systematically investigated.Besides,as a screening tool to identify suitable recipients for the visual prostheses noninvasive transcorneal electrical stimulation(TcES)method has been widely used.So an objective and quantitative study of the cortical response evoked by TcES is needed.The present thesis mainly focused on these two aspects to carry out the research work.For the penetrative optic nerve stimulation with different modes,in vivo electrophysiological experiment and computer simulation were used to investigate the visual cortical response properties and optic nerve fibres activation respectively.First we recorded the electrically evoked cortical potentials using silver ball electrode array under different electrical stimulation modes on rabbits.The stimulating thresholds,response amplitudes and spatial spread extents of response were studied.The results show that the monopolar stimulation mode had a similar stimulating threshold with the bipolar stimulation mode.When using suprathreshold stimulation,the response to monopolar stimulation spread narrower than bipolar stimulation,which indicates that monopolar stimulation mode could provide more localized stimulation.Then we predict the optic nerve fibres activation under different electrical stimulation modes by an integrated computational model based on geometrical and electrical parameters of rabbit optic nerve.The results show that the electrode pair with a larger distance and a larger radial shift and the electrode pair with a smaller spacing and a smaller radial shift could explain the phenomena found in the animal experiments.Bipolar stimulation with larger spacing electrode pair activated bigger activation area which spread wider than monopolar stimulation.The area elicited by bipolar stimulation with smaller spacing electrode pair was similar with that by monopolar stimulation but spread more dispersedly.The simulation results support the conclusion that monopolar stimulation was more localized.Besides,the simulation results indicate that if much finer electrodes could be fabricated and the influence of scatter of the optic nerve fibres could be neglected,the bipolar stimulation mode may improve the stimulation selectivity,which provides a rational way of advance the spatial resolution of penetrative optic nerve stimulation.In the study of cortical response evoked by TcES,electrophysiological recording and OIS imaging method were used to investigate the cortical response elicited by ERG-jet electrode.The OIS imaging method showed that the response regions mainly located in cortical areas representing peripheral visual field after TcES,where also showed strong evoked field potentials according to electrophysiological results.In the investigation of different TcES parameters the results indicated that the largest optical responses were acquired with the stimulus frequencies between 10 and 20 Hz and “long duration with small intensity current” was more efficient when using fixed injected charge.Besides,the TcES evoked response recorded by OIS imaging presented an antagonistic pattern.Normal hemodynamic response(positive OIS response,POR)was found in the cortical areas representing peripheral visual field,and an inverted hemodynamic response(negative OIS response,NOR)was found in the cortical region posterior to the POR region.By UEA and LMA we recorded the local field potentials and multiunit activities in the POR and NOR regions.The results showed that increased neuronal activity and decreased neuronal activity coexisted in the NOR region.Our work provides evidence that the NOR may be caused by changes in the balance between neuronal inhibition and excitation.And the response pattern in our study is comparable with the antagonistic response pattern elicited by optical stimulation in other researcher's study,which can help to comprehend the response mechanism of visual system to electrical stimulation.