A Simulation Study On The Electrical Stimulation Of Visual Cortex Through Duramater

Blindness will not only bring a lot of inconvenience and distress upon the patients, but also involve the society in the enormous damage. There were about 45,000,000 blind people all over the world, of which 5,000,000 were in China, and the number increased at a rate of 450,000 per year. Blindness might be caused by the injury of any section in the visual pathway, electrical stimulation could excite the neurons in the injured section and eventually got some sort of visual information, which was called artificial vision rehabilitation. According to the different stimulation sites, visual prosthesis could be divided into retinal prosthesis, optic nerve prosthesis and visual cortex prosthesis. The visual cortical prosthesis was fit for many kinds of blind people, and had the ability to resume the most patients. To overcome the injury brought by the conventional cortical visual prosthesis, with the support of national'863'program (No. 2007AA04Z324), our group probed into the feasibility of the electrical stimulation on the dura mate above the visual context in order to repair the vision.As a basic study of this project, this paper analyzed the electric field distribution of nerve tissue under the electrical stimulation and the response mechanism of the neurons theoretically. A finite element model of cat's primary visual cortex was established, and the electric field distribution in the cortex within single or multi stimulus was calculated in COMSOL Multiphysics, then Fourier-FEM was used to analyze the variation of the tissue's electric field in one cycle. And the activating function was used as a measure of neuronal activity and got the tissue activation of electrode assigning the threshold of activating function. Next the effect of stimulus parameters'variations on the stimulating effects was discussed. Then a pyramidal cell model layed in layer III of cat visual cortex was established. Whether neuronal activition can be induced by the external electric stimulation at different position was identified throuh the changes of the neuronal potential by NEURON, and the scope role of electrode as well as effective stimulation parameters could be determined. Finally, an animal experimental platform was established and the surface array was used to stimulate, while the potential and electrical activity of neurons in the cortex were recorded by tungsten microelectrode.The simulation results indicated that when the electrical stimulation was applied on the dura, cerebrospinal fluid had great effect on the distribution of electric field in the cortex for its large conductivity than other tissues, which not only increased the action sphere of one electrode by the transverse diffusion of the induced current, but also attenuated the electric field. After the current flowing from the stimulating electrode to the cortex, the potential declined about 34 times, the current density would lower about 118 times and half of the current flow into the cortex, which suggested that visual prosthesis based upon epidural electrical stimulation was possible, just needed greater stimulus intensities than cortex stimulation. When the electrode array was used to stimulate, if the distance between near electrodes was less than 1 mm, the aliasing of the signals from adjacent electrodes would occur, therefore the visual resolution was reduced, which meant the resolution of the epidural prosthesis was limited comparing with the direct cortical stimulation prosthesis. In addition, the stimulation result indicated that within low-frequency stimulation the dielectric properties of tissue played an important role in the change of electrical field and should not be ignored simply.When the electric filed distribution in the tissue was known, the action sphere of one electrode could be calculated by assigning the threshold of activating function. Then the amplitude, frequency and duration of monophasic pulse was tuned separately, and it's found that the action sphere increased with the increasement of either parameters, and be more sensitive to the change of amplitude than the other two parameters. The next strategy was to study the neuron's activities which had the same distance to the surface of cortex, so as to determine the stimulus distance of one electrode. It's found that the stimulus distance was proportional to either amplitude, frequency or duration for monophasic pulse, and still more sensitive to the change of amplitude than the other two parameters. In our previous animal experiments, only electric potential and neuron's membrane voltage in the cortex were recorded by microelectrode, and some experimental results agreed well with the simulation results.This paper mainly investigated the mechanism of vision rehabilitation by epidural electrical stimulation theoretically, the present results could provide some theoretical guidance for the design of visual cortex prosthetics, and had established some theoretical basis for further research.