Dissertation
Acoustically evoked compound action potentials (CAPs) recorded from cochlear implant users with preserved acoustic hearing
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2020
DOI: 10.17077/etd.005647
Abstract
Introduction: Individuals with good low frequency hearing but substantial high frequency hearing loss who often receive limited benefits from hearing aids are now candidates for cochlear implantation (CI) (Turner, 2006). Less traumatic intracochlear electrode design and the introduction of the “soft surgery” techniques allow for preservation of low frequency acoustic hearing in the implanted ear. Recently, noninvasive recording methods have been developed that allow acoustically evoked peripheral responses to be measured in vivo from an intracochlear electrode (Abbas et al., 2017). These recordings provide clues to the status of the peripheral auditory structures. Unfortunately, responses generated from the auditory nerve (auditory nerve neurophonic, ANN) are smaller than hair cell responses (cochlear microphonic, CM) and complete segregation of the ANN from the CM has proven difficult, complicating interpretation and limiting clinical applications (Forgues et al., 2014). The compound action potential (CAP) is a synchronous response of multiple auditory nerve fibers and may provide an alternative to the ANN where the status of the auditory nerve is of primary interest. In this study, CAPs recorded using traditional stimuli (clicks, tone bursts) and new stimuli (chirps) were compared within subject. We hypothesized that this novel stimulus might enhance CAPs compared to traditional stimuli, improving the accuracy of assessing the status of the auditory nerve. Methods: Nineteen adult Nucleus L24 Hybrid CI users with residual low frequency hearing participated. CAP growth functions were recorded from the most apical intracochlear electrode using a 100 μs click, 500 Hz tone bursts, and a series of chirps presented via insert phone to the implanted ear. The CAP chirp stimulus based on human CAP recording obtained using derived-band techniques was used (Chertoff et al., 2010; Eggermont, 1979). Nine custom chirps with systemically varied frequency sweep rates were also generated by modifying a fitting constant of the power function in the chirp equation (Morimoto et al., 2019). CAPs were compared based on peak amplitude, latency, threshold, and general morphology across stimuli.
Results: Click- and chirp-evoked CAPs were more reliably measurable than tone burst evoked CAPs. Chirp-evoked CAPs were slightly larger in amplitudes and less ambiguous in morphology at higher stimulation levels compared to those with click-evoked CAPs. However, chirp-evoked CAPs were less persistent at lower stimulation levels with elevated thresholds compared to clicks. Tone burst evoked CAPs were less frequently recordable. The status of residual acoustic hearing at high frequencies had an influence on CAPs and this was more apparent on response amplitudes when chirps were used. The chirp stimuli tended to significantly enhance CAP amplitudes in subjects who had more hearing preserved at high frequencies. Preliminary results suggested that customizing chirps by varying the frequency sweeping rates might have an impact on CAP waveform morphology in a subset of subjects. When chirps with a faster frequency sweeping rate (e.g. chirp Nk = 4) was used, CAP responses were measurable in all subjects. Enhanced CAP amplitudes and sharper peak morphology were obtained in few subjects when chirps with a faster rate of frequency change was used.
Conclusions: Results of this study suggest that CAPs can be measured more effectively using either clicks or chirps compared to low frequency tone bursts in CI users with residual acoustic hearing. The advantage of using chirps relative to standard clicks appeared to be dependent upon the extent of preserved acoustic hearing at high frequencies. Chirps may present an attractive alternative to standard clicks and tone bursts in terms of recording CAPs reliably in this CI population. Further improvement in response morphology might be realized by customizing chirps on an individual basis.
Details
- Title: Subtitle
- Acoustically evoked compound action potentials (CAPs) recorded from cochlear implant users with preserved acoustic hearing
- Creators
- Jeong-Seo Kim
- Contributors
- Carolyn Brown (Advisor)Paul Abbas (Committee Member)Shawn Goodman (Committee Member)Inyong Choi (Committee Member)Camille Dunn (Committee Member)Marlan Hansen (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Speech and Hearing Science
- Date degree season
- Autumn 2020
- DOI
- 10.17077/etd.005647
- Publisher
- University of Iowa
- Number of pages
- xiv, 119 pages
- Copyright
- Copyright 2020 Jeong-Seo Kim
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 104-119).
- Public Abstract (ETD)
Cochlear implant (CI) candidacy criteria have been significantly relaxed to individuals with good low frequency hearing but substantial high frequency hearing loss. Less traumatic CI electrode design and the use of the soft surgical techniques allow for preservation of low frequency acoustic hearing in an implanted ear. Recently, noninvasive recording methods was developed that allow responses from the hair cells and auditory nerve to be measured from an electrode inside the cochlea for these CI users. In this study, we focused on the compound action potential (CAP) as the status of auditory nerve was of primary interest. CAPs recorded using traditional stimuli (clicks, tone bursts) and new stimuli (chirps) were compared to optimize the method to assess the status of the auditory nerve.
Nineteen adult CI users who had residual acoustic hearing participated. CAPs were measured from a CI electrode while subjects were listening to three stimuli (clicks, chirps, tone bursts) through an insert phone in their implanted ear. Nine custom chirps were also generated by changing the design to determine the feasibility of customizing chirps for this CI population.
Results suggested that clicks and chirps worked more efficiently to measure CAPs reliably compared to tone bursts. CAPs evoked using chirps had slightly larger amplitudes and enhanced morphology at higher signal levels than clicks. Chirps tended to enhance CAP amplitudes in subjects who had more residual hearing at high frequencies. Customizing chirps by changing the design might have an impact on enhancing CAP response morphology in a subset of subjects.
- Academic Unit
- Communication Sciences and Disorders
- Record Identifier
- 9984035694902771
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