An improved method of reducing stimulus artifact in the electrically evoked whole-nerve potential

Recording a compound action potential in response to electrical stimulation requires attention to minimize contamination due to electrical stimulus artifact. In patients implanted with the Nucleus 24 device, the electrically evoked whole-nerve potential (EAP) is recorded using a neural response telemetry (NRT) system. This system employs a forward-masking technique that greatly reduces stimulus artifact. However, theoretical considerations and experimental animal data suggest that the technique may distort the acquired EAP waveform under some situations. We proposed and evaluated a modification to the forward-masking technique that addresses this concern, particularly during collection of refractory recovery data.

We first examined neural responses of the electrically stimulated auditory nerve using cat preparations. Through single-fiber recordings from cats, we demonstrated underlying physiological limitations likely encountered with the "standard" forward masking technique. We then recorded feline EAP waveforms using both the standard technique and our proposed, modified, technique. Finally, we collected EAP data from human cochlear implant patients using both artifact reduction methods. These comparisons allowed us to evaluate the effectiveness of our modification.

The cat EAP data demonstrated that the standard forward-masking technique currently in use in the Nucleus NRT system can distort the EAP waveform when the nerve is partially refractory. In the cat, this distortion resulted in forward-masking recovery curves with artifactually prolonged recovery times and inaccurate latency trends. Similar effects were observed in the comparison of human recovery curves obtained using both the standard and modified techniques. In some cases, the modified technique produced EAP waveforms with more clearly defined peaks than were obtainable with the standard method.

Consideration should be given to implementing our modified forward-masking artifact reduction scheme, because it introduces less distortion of the EAP waveform and accordingly provides for more accurate assessment of the refractory properties of the electrically stimulated nerve.