Development of Stimulation/Recording System for Evaluation of Retinal Implants

Abstract

Purpose:: This work is related to the efforts of the Boston Retinal Implant Project to develop a sub-retinal prosthesis to restore vision to the blind. In order to test retinal implants it is necessary to have a flexible external system capable of stimulating the implant and recording biological responses. This system needs to have an easy and flexible graphical user interface, as well as providing both wireless and wired connections to the implant and the subject under test.

Methods:: We built a software program written entirely in Labview that runs on a National Instrument PXI system that includes various special purpose I/O plug-ins, such as D/A boards, A/D boards and Relay Multiplexers. This system was connected to custom electronic circuitry including a voltage to current driver, a multi-channel nerve amplifier with stimulation artifact blanking, and a trigger circuit for an external ERG strobe. The stimulation software generates both RF encoded pulses for stimulating wireless implants and direct biphasic pulses which drive the voltage to current driver for acute wired experiments. Both types of outputs are generated from a common operator panel that specifies the biphasic pulse amplitudes on an electrode-by-electrode basis as well as the cycle timing. The recorded signals are presented on the visual interface, including averaging of multiple responses, and are stored in a data base for later retrieving and processing. Sample rate and record length are under operator control. All recording operations, as well as any optical stimulation are synchronized to a single time base from the PXI system.

Results:: As the system evolved it has been in fairly constant experimental use, averaging about 2 or 3 surgeries with animals per month. The stimulations have been shown to cover a wide range of possible stimulation patterns. The recording system has been shown to obtain reproducible results with noise levels in the microvolt or sub-microvolt range.

Conclusions:: The system is unique in its ability to combine multichannel low level recording from an implanted array with simultaneous stimulation of the subject from different electrodes on the same array. The system has worked effectively in recording biological signals but the experimental program needs to be continually expanded to include more complex stimulation and recording patterns.

Keywords: retina • electrophysiology: non-clinical • perception