An Interface for Nerve Recording and Stimulation with Cuff Electrodes

A nerve cuff electrode interface capable of both stimulating and recording from a nerve is described. The interface also rejects the EMG contamination in the recordings using reactive components without adding noise to the ENG signal. A transformer is added to the design for noise matching and the signal-to-noise ratio improvement is evaluated for a specific amplifier INTRODUCTION Electroneurogram (ENG) signals recorded from peripheral nerves using cuff electrodes are very small, sometimes in the order of a few micro volts, and the electromyogram (EMG) contamination from the surrounding muscles is a few order of magnitude larger. A monopolar cuff design[ 11 and the use of a potentiometer for balancing the contact impedances [ 21 have been suggested to reduce the EMG contamination. In this paper, we present a transformer interface that reduces the EMG contamination and also allows simultaneous stimulation of the nerve while recording its activity. We compare the new design with the standard tripolar configuration for EMG rejection and the SNR. Another transformer is added to the design to improve the signal-tonoise ratio (SNR) of the ENG recordings by noise matching. DESIGN CONSIDERATIONS The new interface design is shown in Figure 1. The resistances R1 through R3 represent the impedances measured between the contacts of the cuff electrode and a distant reference. The voltage sources simulate the potential differences that are induced at each contact against the reference electrode by the EMG and ENG activities. The EMG signals are assumed to be symmetrical with respect to the reference and the ENG activity is assumed to induce a potential only at the central contact. Transformer TR1 is used for both rejection of EMG signals and stimulation of the nerve through the end contacts (RI and R3). Transformer TR2 is used to match the source resistance to the noise characteristics of the input stage of the amplifier (AMP-01) to maximize the SNR. In the standard tripolar configuration (Figure 2), the EMG signal is entirely canceled only if the signal amplitudes induced on each half of the cuff by the EMG activity are the same and the impedances of the end contacts (RI and R3) are identical. A small imbalance in the contact impedances can cause a large EMG contamination in the ENG signal. Adding the tightly coupled coils L1 and L2 (nl=n2) with large reactive impedances in series to the end contacts (Figure 1) can help eliminate the imbalance problem without adding another source of thermal noise. The total source impedance ( A M P 0 1 ) . is not increased by L1 and L2 (which would otherwise degrade the SNR due to the input current noise (iN) of the amplifier, see figure 3) since the transformer TR2 ideally presents no impedance to the ENG currents. That is, unlike the EMG currents, the ENG currents flowing through L1 and L2 generate magnetic fluxes in opposite directions in TR1 and they cancel each other out. This transformer is also used to deliver currents to the nerve through the end contacts for the activation of the nerve. Ideally, the two identical but opposite polarity currents ( 1 ~ 1 and 1 ~ 2 ) induced inside L1 and L2 by the stimulus current applied to L3 cancel each other out and do not develop any potential across R2 and TR2, which would otherwise be observed as a stimulation artifact at the output. This should be true even though the contactherve impedances are not equal. The fast-recovery diode (1N914) is used to provide a path for I L ~ after the stim iulus is turned off.