Sensing of Stimulus Artifact Suppressed Signals From Electrode Interfaces

Stimulus artifacts inhibit reliable acquisition of biological evoked potentials for several milliseconds if an electrode contact is utilized for both electrical stimulation and recording purposes. This hinders the measurement of evoked short-latency biological responses, which is otherwise elicited by stimulation in implantable prosthetic devices. We present an improved stimulus artifact suppression scheme using two electrode simultaneous stimulation and differential readout using high-gain amplifiers. Substantial reduction of artifact duration has been shown possible through the common-mode rejection property of an instrumentation amplifier for electrode interfaces. The performance of this method depends on good matching of electrode-electrolyte interface properties of the chosen electrode pair. A novel calibration algorithm has been developed that helps in artificial matching of impedance and thereby achieves the required performance in artifact suppression. Stimulus artifact duration has been reduced down to 50 μs from the stimulation-cum-recording electrodes, which is ~6× improvement over the present state of the art. The system is characterized with emulated resistor-capacitor loads and a variety of in-vitro metal electrodes dipped in saline environment. The proposed method is going to be useful for closed-loop electrical stimulation and recording studies, such as bidirectional neural prosthesis of retina, cochlea, brain, and spinal cord.

I. Introduction

Electrical stimulation and recording of excitable biological entities, such as brain, retina or cochlea, are generally performed using separate electrodes for minimizing stimulus artifact contamination. Stimulus artifact is a synchronous noise that originates from electrode-electrolyte interface during and after application of a coulombic charge injecting stimulation pulse. Artifact signals can saturate high gain recording amplifiers for up to several milli-seconds [1], if recording is performed through the stimulation or the adjacent electrode contacts. Fig. 1(a) shows a typical stimulus artifact contaminated signal as obtained from a high gain neural amplifier. The stimulus artifact or the amplifier saturation duration is quite long and prevents reliable acquisition of short-latency evoked responses (bio-potentials) from living tissues that are being stimulated. Since stimulus artifact overlap neural responses substantially in both time and frequency domains [2], as shown in Fig. 1(b), it is challenging to eliminate this type of noise by means of filtering. Fig. 1.

(a) A typical stimulus artifact contaminated signal obtained from a neural amplifier. (b) Power spectral density of neural and stimulation artifact signal [2].