Acoustically Evoked Compound Action Potentials Recorded From Cochlear Implant Users With Preserved Acoustic Hearing

Abstract

Objectives: Less traumatic intracochlear electrode design and the introduction of the soft surgery technique allow for the preservation of low-frequency acoustic hearing in many cochlear implant (CI) users. Recently, new electrophysiologic methods have also been developed that allow acoustically evoked peripheral responses to be measured in vivo from an intracochlear electrode. These recordings provide clues to the status of peripheral auditory structures. Unfortunately, responses generated from the auditory nerve (auditory nerve neurophonic [ANN]) are somewhat difficult to record because they are smaller than the hair cell responses (cochlear microphonic). Additionally, it is difficult to completely segregate the ANN from the cochlear microphonic, complicating the interpretation and limiting clinical applications. The compound action potential (CAP) is a synchronous response of multiple auditory nerve fibers and may provide an alternative to ANN where the status of the auditory nerve is of primary interest. This study is a within-subject comparison of CAPs recorded using traditional stimuli (clicks and 500 Hz tone bursts) and a new stimulus (CAP chirp). We hypothesized that the chirp stimulus might result in a more robust CAP than that recorded using traditional stimuli, allowing for a more accurate assessment of the status of the auditory nerve.

Design: Nineteen adult Nucleus L24 Hybrid CI users with residual low-frequency hearing participated in this study. CAP responses were recorded from the most apical intracochlear electrode using a 100 μs click, 500 Hz tone bursts, and chirp stimuli presented via the insert phone to the implanted ear. The chirp stimulus used in this study was CAP chirp generated using parameters from human-derived band CAPs ( Chertoff et al. 2010 ). Additionally, nine custom chirps were created by systematically varying the frequency sweep rate of the power function used to construct the standard CAP chirp stimulus. CAPs were recorded using all acoustic stimuli, allowing for within-subject comparisons of the CAP amplitude, threshold, percentage of measurable CAP responses, and waveform morphology.

Results: Considerable variation in response morphology was apparent across stimuli and stimulation levels. Clicks and CAP chirp significantly evoked identifiable CAP response more compared to 500 Hz tone bursts. At relatively high stimulation levels, the chirp-evoked CAPs were significantly larger in amplitude and less ambiguous in morphology than the click-evoked CAPs. The status of residual acoustic hearing at high frequencies influenced the likelihood that a CAP could be reliably recorded. Subjects with better preserved hearing at high frequencies had significantly larger CAP amplitudes when CAP chirp was used. Customizing the chirp stimulus by varying the frequency sweep rates significantly affected the CAP amplitudes; however, pairwise comparisons did not show significant differences between chirps.

Conclusions: CAPs can be measured more effectively using broadband acoustic stimuli than 500 Hz tone bursts in CI users with residual low-frequency acoustic hearing. The advantage of using CAP chirp stimulus relative to standard clicks is dependent on the extent of preserved acoustic hearing at high frequencies and the stimulus level. The chirp stimulus may present an attractive alternative to standard clicks or tone bursts for this CI population when the goal is to record robust CAP responses.

Figure 1.

Postoperative unaided audiometric thresholds of all study participants at the time of testing.

Figure 2.

Stimulation and recording setup to measure CAP responses from CI users with residual acoustic hearing.

Figure 3.

Time waveforms of clicks (panel A), 500 Hz tone bursts (TB 500Hz, panel B), and CAP chirp (panel C).

Figure 4.

Time waveforms of CAP chirp and clicks (panel A) and frequency spectra of an output of CAP chirp and clicks from an insert phone (panel B).

Figure 5.

Time waveforms of nine custom chirps (chirp N_k = 9 to N_k = 1) aligned to an onset of standard clicks (dotted line).

Figure 6.

Frequency spectra of an output of nine custom chirps (chirp N_k = 9 to N_k = 1) from an insert phone.

Figure 7.

Grand mean CAP waveforms evoked using clicks (panel A), CAP chirp (panel B), and 500 Hz tone bursts (Panel C) extracted from the pilot data. Blue shaded areas indicate the time window selected for the peak picking analysis.

Figure 8.

Examples of waveform morphology that were classified as good (panels A1-A3), fair (panels B1-B3), poor (panels C1-C3), and no response (panels D1-D3). Waveforms recorded using clicks (panels A1-D1), CAP chirp (panels A2-D2), and 500 Hz tone bursts (panels A3-D3) across stimulation levels. Red dots indicate the N1-P1 peaks selected in the peak picking analysis.

Figure 9.

Percentage of measurable CAPs averaged across all stimulation levels (panel A), at lower stimulation levels (averaged at 60, 65, 70 dB nHL, panel B left) and higher stimulation levels (averaged at 75, 80, 85 dB nHL, panel B right) across clicks, CAP chirp, and 500 Hz tone bursts. Error bar indicates ± 1SD. Asterisk (*) indicates statistical significance at α = 0.05.

Figure 10.

Grand mean CAP waveforms evoked using clicks (panels A1-A3), CAP chirp (panels B1-B3), and 500 Hz tone bursts (panels C1-C3) measured at three stimulation levels.

Figure 11.

Amplitude growth functions obtained from individuals with identifiable CAP responses across clicks (panel A), CAP chirp (panel B), and 500 Hz tone bursts (panel C), with mean amplitude growth functions across three main stimuli (panel D). A solid pink line indicates the noise floor (0.5235 μV).

Figure 12.

Comparison of CAP amplitudes measured at 85 dB nHL across clicks, CAP chirp, and 500 Hz tone bursts. Each symbol indicates individual CAP amplitude. The box plot shows the mean (bold line) and median (light line) values. Asterisk (*) indicates statistical significance at α= 0.05.

Figure 13.

Correlations between CAP suprathreshold amplitudes evoked using clicks and CAP chirp and averaged audiometric thresholds at high frequencies (1 – 4 kHz, panel A for clicks and panel B for CAP chirp) and low frequencies (125 – 500 Hz, panel C for clicks and panel D for CAP chirp).

Figure 14.

Grand mean CAP waveforms recorded from eight subjects using nine custom chirps (chirp N_k = 9 to N_k = 1, panels A - I ) at 85 dB nHL.

Figure 15.

Average CAP amplitudes recorded from eight subjects that were plotted across nine custom chirps (chirp N_k = 9 to N_k = 1) at three stimulation levels (85, 80, and 75 dB nHL). CAP amplitudes recorded using clicks were inserted as a reference.