Spatial Characteristics Of Cortical Responses Evoked By Optic Nerve And Suprachoroidal-retinal Stimulation And Virtual Electrode Strategy

Vision is the most important sensation about the surrounding world,its deprivation not only markedly affects the life of the blind people,but also gives a great burden to the society.A visual prosthesis is an electronic device that helps the blindness to regain visual function.The external visual information is captured and transferred to electrical pulses,and then via the microelectrodes implanted inside the body,the pulses are applied to the visual pathway to artificially restore visual perceptions.There still are many basic issues remains to be explored in the development of visual prosthesis.The spatial characteristics of the visual system responses to electrical stimulation and its underlying mechanisms are still lacking in-deep research.In present study,based on the penetrating optic nerve(ON)prosthesis and suprachoroidal-retinal prosthesis,we studied the above issue by using in vivo electrophysiological methods,details are as follows.One of the key issues for visual restoration by using penetrating ON prosthesis is to investigate the visuotopic correspondence and spatial characteristics of ON stimulation.We used the cat as the animal model,implanted refined penetrating electrodes into the ON,and made a recording array that covered both sides of the visual cortex.Using a sparse-noise method to localize the representative visual filed of the ON penetrating electrodes,we explored the spatial characteristics of the optic nerve stimulation.Our results showed that,by using refined penetrating ON electrodes with the exposure tip of several microns,the current threshold is relatively low but the charge density threshold is relatively high;spatially localized response could be evoked by electrical stimulating the ON with penetrating electrodes implanted 2 mm behind the eyeball;electrical stimulation elicited responses in visuotopopic corresponding areas of the cortex;and changing the electrode arrangement and depth in ON changed the locations of the cortical response which also followed the visuotopic correspondence.The representative location of the ON electrode is an important factor for the achievable spatial resolution,the highest spatial resolution is in the central visual field,with the location eccentricity increases the spatial resolution decreases.This study provides useful experimental data for the spatial resolution and influencing factors of the penetrating ON visual prosthesis,and guided the design of density,size and arrangement of the ON stimulating electrodes.To solve the problem of electrode density limitation in present visual prosthesis,we investigated the feasibility of current steering to create ?virtual electrodes? to improve the spatial resolution in visual prosthesis.Penetrating electrodes at different inter-electrode distances were implanted into the ON and the cortical electrical evoked potentials in V1 were recorded.Combining computational methods,the spatial features and influent factors of the distribution pattern of the evoked responses were studied.Results showed that threshold reduction was found with simultaneous stimulation of closely-spaced electrode pairs;spatially shifted cortical responses were achieved using current steering,and by changing the current distribution among the electrodes,several intermediate virtual electrodes could be produced which located between the real electrodes,with the amplitudes and spatial spreads of the responses similar to that elicited by a single real electrode.Computer simulation showed that the centroid of the ON fiber recruitment area could be modulated by current steering stimulation while the total recruitment area did not vary a lot.Our results indicate that current steering is useful strategy to enhance the spatial resolution of an ON prosthesis without increasing the number of physical electrodes.This study provides useful information for optimizing the design of stimulation strategies with a penetrating ON prosthesis.In the study of suprachoroidal-transretinal stimulation(STS),a 4 × 4 thin-film platinum microelectrode stimulating array(200 ?m electrode diameter and 400 ?m center-to-center distance)was fabricated by Micro-electro-mechanical systems(MEMS)techniques and implanted into the suprachoroidal space of albino rabbits.Results showed that the current threshold to elicit reliable cortical responses by a single electrode was within the reported safety limits,and combining stimulation of multiple electrodes could lead to a even lower threshold.Spatially differentiated cortical responses could be evoked by STS through different rows or columns of electrical stimulation;furthermore,shifts in the location of the maximum cortical activities were consistent with cortical visuotopic maps,and at least a spatial resolution of 2° was achievable with the STS.Increasing the number of simultaneously stimulating electrodes increased the response amplitudes and expanded the spatial spread as well.This study indicates that the STS approach by a MEMS based electrode array is a feasible alternative for visual restoration,and relatively high spatial discrimination may be achieved.