The Sensitivity of the Electrically Stimulated Auditory Nerve to Amplitude Modulation Cues Declines With Advanced Age

Objectives: This study aimed to investigate effects of aging and duration of deafness on sensitivity of the auditory nerve (AN) to amplitude modulation (AM) cues delivered using trains of biphasic pulses in adult cochlear implant (CI) users. Design: There were 21 postlingually deaf adult CI users who participated in this study. All study participants used a Cochlear Nucleus device with a full electrode array insertion in the test ear. The stimulus was a 200-ms pulse train with a pulse rate of 2000 pulses per second. This carrier pulse train was sinusodially AM at four modulation rates (20, 40, 100, 200 Hz). The peak amplitude of the modulated pulse train was the maximum comfortable level (i.e., C level) measured for the carrier pulse train. The electrically evoked compound action potential (eCAP) to each of the 20 pulses selected over the last two AM cycles were measured. In addition, eCAPs to single pulses were measured with the probe levels corresponding to the levels of 20 selected pulses from each AM pulse train. There were seven electrodes across the array evaluated in 16 subjects (i.e., electrodes 3 or 4, 6, 9, 12, 15, 18, and 21). For the remaining five subjects, 4 to 5 electrodes were tested due to impedance issues or time constraints. The modulated response amplitude ratio (MRAR) was calculated as the ratio of the difference in the maximum and the minimum eCAP amplitude measured for the AM pulse train to that measured for the single pulse, and served as the dependent variable. Age at time of testing and duration of deafness measured/defined using three criteria served as the independent variables. Linear Mixed Models were used to assess the effects of age at testing and duration of deafness on the MRAR. Results: Age at testing had a strong, negative effect on the MRAR. For each subject, the duration of deafness varied substantially depending on how it was defined/measured, which demonstrates the difficulty of accurately measuring the duration of deafness in adult CI users. There was no clear or reliable trend showing a relationship between the MRAR measured at any AM rate and duration of deafness defined by any criteria. After controlling for the effect of age at testing, MRARs measured at 200 Hz and basal electrode locations (i.e., electrodes 3 and 6) were larger than those measured at any other AM rate and apical electrode locations (i.e., electrodes 18 and 21) Conclusions: The AN sensitivity to AM cues implemented in the pulse-train stimulation significantly declines with advanced age. Accurately measuring duration of deafness in adult CI users is challenging, which, at least partially, might have accounted for the inconclusive findings in the relationship between the duration of deafness and the AN sensitivity to AM cues in this study.