Assessing auditory nerve recovery function with a modified subtraction method: results and mathematical modeling.

Abstract

One of the main difficulties in electrical compound action potential (ECAP) recordings is to reduce the stimulus artifact due to electrical stimulation. The neural response telemetry (NRT) system of the Nucleus 24 cochlear implant extracts the ECAP response using a forward-masking (standard) subtraction technique. However, it has been shown that this subtraction technique may distort the ECAP responses in certain situations. In order to improve ECAP recordings, a modified forward-masking subtraction technique was recently proposed (Ear Hear. 21 (2000) 280). This modified subtraction technique can be applied to recovery function measurement. The objective of this study is to compare results obtained with the modified method to those obtained using the standard method. ECAP responses were recorded in 4 adult patients using a Nucleus 24 cochlear implant. Data were collected for the 20 active electrodes. For each electrode, measurements consisted of the recovery function recording using 16 different Masker-Probe intervals. The modified method was then applied and the results compared with the standard method. Comparison between the two methods revealed that results were different when using the standard or modified method. Using the modified method, more ECAP responses were obtained (61.8 vs. 44.2%), but the P1 peak was sometimes attenuated; when using the standard method, N1 peak was missing in many cases. A mathematical model has been established and the mathematical simulation confirms the results obtained in patients. The results suggest that both methods have limitations and advantages. The modified subtraction method seems to be better for analyzing ECAP recordings in recovery function measurement because of the higher number of responses obtained compared to the standard method.