Integration of Recording channel for the Evoked Compound Action Potential in an Implantable Neurostimulator

Implanted neurostimulators are commonly used to activate peripheral nerves or brain cells through one or more electrodes. Because of the variable conditions, the stimulation parameters required to obtain the seeked effect vary from subject to subject and from time to time in the same subject. Sensing the evoked compound action potential is therefore an important aspect of neural stimulation. However, physiological signals are typically in the mu V to mV range and generated at a short distance from stimulating pulses with magnitudes 3 to 6 orders larger. Anatomical and surgical constraints typically do not allow the stimulation and recording location to be separated by large distances. There is a pressing demand to combine both features in a single electrode. Avoiding a prolonged amplifier saturation induced by the huge stimulation artefact becomes a major challenge. Because of the geometrical proximity between the electrode contacts, the response to be recorded is often expected to emerge within no more than one ms after the stimulus, putting even more emphasis on the required fast amplifier recovery. A last challenge is the fact that the stimulator and recording electronics must be integrated to reduce the size of an implant. The galvanic insulation usually exploited to further reduce stimulation artefacts cannot be applied here. In this paper, we describe a recording circuit sharing common power supplies with a neuro-stimulator. The circuit features an specific artefact suppression system based on highpass filter switching and ac coupling. It is further completed with digital filtering and noise rejection algorithms including the possibility to average responses to repeated stimuli in order to improve signal to noise ratio when necessary. Satisfactory results have been obtained in vivo on rat vagus nerves using a single cuff electrode with adequate organization of the stimulation and recording contacts. The recordings allow neural response thresholds measurements to be performed in vivo.