Design Of Multi-channel Cortical Stimulator Based On FPGA

As the developing of medicine and engineering, artificial vision prosthesis based on functional electrical stimulation brings hopes to the blind. Artificial vision prosthesis technology implants a micro-electrode array in the visual pathway of certain nerve. An electrical stimulator is able to convert visual information into electronic pulses to stimulate the adjacent ganglion cells by micro-electrode array. Ganglion cells then pass the information the brain, so that patients can perceive the image. All visual prostheses have the same function: converting outside optical signal to certain current pulses and stimulating the relevant parts by the micro-electrode array to generated phosphenes. Electrical stimulator is to convert image information to current pulse in the vision prostheses.This paper designed a multi-channel electrical stimulator for the visual cortex prosthesis to convert image information to stimulation current. It uses FPGA as its main control chip. The FPGA comprises a spike generator, a serial-to-parallel converter and some multiplexers. The spike generator can generate various forms of pulse signal by counting the system clock, and put them into the signal input end of all multiplexers. The 32 bits serial-to-parallel converter is realized by increasing the trigger progression of macro function 74164. Serial-to-parallel converter can read the coded image signals and convert them to parallel signals to control 16 multiplexers simultaneously. Then the FPGA converts image signals to electrical pulse. In order to convert the output voltage signals to current signals, we designed peripheral conversion circuit. The peripheral conversion circuit contains some voltage-controlled bidirectional constant current source units. These voltage / current conversion circuits show good linearity and constant-current characteristics of conversion in testing. We made an initial prototype of 16-channel electrical stimulator based on the EP1C6 kernel board. And a system initial prototype of visual cortex prosthesis is made by the initial prototype of 16-channel electrical stimulator and an image process system. Result of testing shows that the system initial prototype can process image information correspondingly and convert image information to corresponding current pulse. And the prototype can make timely responses to the transformation of the image information. We performed the experimental evaluation by the initial prototype of 16-channel electrical stimulator. Put two electrode chips out of the left and right dura of cortex of a cat. Electrical stimulation was connected to the left one. Record the visual electrically evoked potential (EEP) by the right one. The results of experiment on cat's cortex show that the cortex can respond to the current stimulation, and the initial prototype is feasible.