Characteristics of the Adaptation Recovery Function of the Auditory Nerve and Its Association With Advanced Age in Postlingually Deafened Adult Cochlear Implant Users

Abstract

Objective: This study aimed to (1) characterize the amount and the speed of recovery from neural adaptation at the auditory nerve (AN) and(2) assess their associations with advanced age in postlingually deafened adult cochlear implant users.Design: Study participants included 25 postlingually deafened adult, Cochlear Nucleus device users, ranging in age between 24.83 and 83.21 years at the time of testing. The stimulus was a 100-ms pulse train presented at four pulse rates: 500, 900, 1800, and 2400 pulses per second (pps). The pulse trains were presented at the maximum comfortable level measured for the 2400-pps pulse train. The electrically evoked compound action potential (eCAP) evoked by the last pulse of the pulse train (i.e., the probe pulse) was recorded. The remaining pulses of the pulse train served as the pulse-train masker. The time interval between the probe pulse and the last pulse of the pulse-train masker [i.e., masker-probe-interval (MPI)] systematically increased from 0.359 ms up to 256 ms. The adaptation recovery function (ARF) was obtained by plotting normalized eCAP amplitudes (re: the eCAP amplitude measured at the MPI of 256 ms) as a function of MPIs. The adaptation recovery ratio (ARR) was defined as the ratio between the eCAP amplitude measured at the MPI of 256 ms and that measured for the single-pulse stimulus presented at the same stimulation level. The time constants of the ARF were estimated using a mathematical model with an exponential function with up to three components. Generalized Linear Mixed effects Models were used to compare ARRs and time constants measured at different electrode locations and pulse rates, as well as to assess the effect of advanced age on these dependent variables.Results: There were three ARF types observed in this study. The ARF type observed in the same study participant could be different at different electrode locations and/or pulse rates. Substantial variations in both the amount and the speed of neural adaptation recovery among study participants were observed. The ARR was significantly affected by pulse rate but was not affected by electrode location. The effect of electrode location on the time constants of the ARF was not statistically significant. Pulse rate had a statistically significant effect on τ 1, but not on τ 2 or τ 3 . There was no statistically significant effect of age on the ARR or the time constants of the ARF. Conclusions:Neural adaptation recovery processes at the AN demonstrate substantial variations among human cochlear implant users. The recovery pattern can be nonmonotonic with up to three phases. While the amount of neural adaptation recovery decreases as pulse rate increases, only the speed of the first phase of neural adaptation recovery is affected by pulse rate. Electrode location or advanced age has no robust effect on neural adaptation recovery processes at the level of the AN for a 100-ms pulse-train masker with pulse rates of 500 to 2400 pps. The lack of sufficient participants in this study who were 40 years of age or ...

Postlingually Deafened Adult Cochlear Implant Users With Prolonged Recovery From Neural Adaptation at the Level of the Auditory Nerve Tend to Have Poorer Speech Perception Performance

“…Approaches for measuring eCAPs elicited by individual pulses of a constant-amplitude pulse train with a fixed or varied MPI have been reported in detail in our previous studies (He et al 2021a,b). Briefly, eCAPs in response to individual pulses of a pulse train were measured using a modified forward-masking technique (He et al 2017).…”

“…It has been shown that adaptation during both phases is faster at higher pulse rates and varies substantially among patients (He et al 2021a). To evaluate adaptation recovery at the AN, the eCAP evoked by a probe pulse with a preceding pulse train masker at different MPIs is recorded (Dhuldhoya 2013; Adel et al 2017; Mussoi & Brown 2019; He et al 2021b). The amount of adaptation recovery can be quantified by comparing the eCAP amplitude measured at the longest MPI tested to the eCAP amplitude measured for the single-pulse stimulus that is presented at the same stimulation level (He et al 2021b).…”

“…To evaluate adaptation recovery at the AN, the eCAP evoked by a probe pulse with a preceding pulse train masker at different MPIs is recorded (Dhuldhoya 2013; Adel et al 2017; Mussoi & Brown 2019; He et al 2021b). The amount of adaptation recovery can be quantified by comparing the eCAP amplitude measured at the longest MPI tested to the eCAP amplitude measured for the single-pulse stimulus that is presented at the same stimulation level (He et al 2021b). The speed of adaptation recovery can be estimated using a mathematical model with up to three exponential components based on the amount of increase in the eCAP amplitude as the MPI increases (Dhuldhoya 2013; He et al 2021b).…”

“…The amount of adaptation recovery can be quantified by comparing the eCAP amplitude measured at the longest MPI tested to the eCAP amplitude measured for the single-pulse stimulus that is presented at the same stimulation level (He et al 2021b). The speed of adaptation recovery can be estimated using a mathematical model with up to three exponential components based on the amount of increase in the eCAP amplitude as the MPI increases (Dhuldhoya 2013; He et al 2021b). Overall, three types of neural adaptation recovery functions (ARFs), determined based on the number of components included in the function, have been recorded in human CI users.…”

“…Overall, three types of neural adaptation recovery functions (ARFs), determined based on the number of components included in the function, have been recorded in human CI users. Characteristics of these function types have been reported in detail in He et al (2021b). The amount of adaptation recovery varies with both function type and pulse rate.…”