Design of a High-channel-count Current Source for Use in Retinal and Cortical Visual Prostheses

Abstract

Abstract: : Purpose: Retinal or cortical prostheses require large numbers of stimulation sites to achieve formed visual percepts. These devices generally employ electrical or neurotransmitter stimulus paradigms. The purpose of this study was to design an isolated, scalable, high-channel-count, extra-corporeal current source for use in electrical or neurotransmitter-based visual prosthesis development and testing. Methods: A bipolar linear voltage-to current converter was designed with a range of nanoamperes, for use in the iontophoretic delivery of neurotransmitters, to milliamperes for electrical current stimulation using electrodes. The 14-bit resolution, bipolar digital-to-analog (D/A) converter has a 1.1 megasample per second bandwidth over 32 channels. In order to lower the interface bandwidth requirements, a parallel-to-serial interface was developed using a complex programmable logic device (CPLD). The CPLD reduces the microprocessor-to-D/A converter bandwidth stream from 19 megabits/sec, serially, to 1.1 million 19-bit parallel outputs per second. This reduces utilization of the embedded FLASH microcontroller, allowing it to receive commands from a laptop computer using the universal serial bus (USB). Each of the 32-channel stimulus modules is mounted upon a scalable backplane. This backplane utilizes another CPLD to perform a serial-to-parallel conversion, allowing the USB data stream to control multiple 32-channel stimulus modules. An analog multiplexer coupled to an FET input buffer allows real-time monitoring of the voltage across all electrodes, simultaneously. This subsystem also allows the user to stimulate from a subset of channels while measuring buffered neuronal responses from others. Results: The above system architecture allows 1000 stimulus channels to be controlled via an 8 megabit/sec USB data stream. The bipolar current resolution is 14-bit with 32 ks/sec temporal resolution. Bit-mapped waveforms may be arbitrary. Electrode voltages and/or extracellular neuronal spikes may be measured during current stimulation. Conclusions: Current sources for use in visual prostheses should be specialized to operate under relatively low-bandwidth control signals while providing high channel counts and the ability to acquire voltage feedback information.

Keywords: electrophysiology: non-clinical • image processing • visual cortex