Faculty Bibliography

OBJECTIVES/OBJECTIVE:This study aims to validate vocoders as acoustic models of cochlear implants by determining whether they capture both perceptual sound quality and speech performance. We hypothesize that valid acoustic models of cochlear implants require listener-specific amounts of frequency mismatch between input filters and output tones or noise bands. DESIGN/METHODS:Forty-four adult single-sided deaf cochlear implant users were tested 1 to 5 times for a total of 73 sessions at different time points after initial stimulation. Participants had a cochlear implant in one ear and normal or near-normal hearing in the contralateral ear, allowing within-subject comparisons. In Experiment 1, participants used a method-of-adjustment procedure to select acoustic models most similar to their cochlear implant by adjusting three parameters: low- and high-frequency cutoffs of the acoustic output, and channel interaction (overlap among output sound carriers). In Experiment 2, participants rated the perceptual similarity of five acoustic model types using questionnaires assessing overall similarity and four acoustic dimensions (intelligibility, pleasantness, harshness, and loudness). The five model types included the self-selected model, two all-channel frequency-matched vocoders (tone and noise), and two six-channel frequency-matched vocoders (tone and noise). Frequency-matched acoustic models had output noise bands or tones that were frequency-matched to the analysis filters. In Experiment 3, speech perception was evaluated under six conditions: with the cochlear implant alone and with each of the five acoustic model types presented to the normal hearing ear. RESULTS:Nearly all participants (69 of 73 sessions) selected acoustic models with frequency ranges different from their clinical frequency allocation tables. The low-frequency edge of self-selected models was significantly higher than clinical allocations (456 Hz for Cochlear Ltd. and 266 Hz for MED-EL). Over 80% of selections used minimal channel interaction (tones, nonoverlapping noise bands, or adjacent noise bands). Self-selected models received significantly higher similarity ratings (mean of 6.11, where 6 means "somewhat similar" and 7 "very similar") compared with all frequency-matched models, which were rated around 3 ("not very similar"). Self-selected models were rated most similar to the cochlear implant across multiple acoustic dimensions and were the only models not rated significantly different from the cochlear implant on any dimension. For speech perception, all-channel frequency-matched models significantly overestimated performance compared with the cochlear implant for both words and sentences. Self-selected models provided speech scores closest to cochlear implant performance. Joint analysis of similarity ratings and speech perception scores demonstrated that self-selected models were the only acoustic models achieving both perceptual similarity to the cochlear implant (rating of 6.11 with 7 being "very similar") and comparable speech perception scores (within five percentage points). CONCLUSIONS:Frequency-matched acoustic models fail to replicate the sound of a cochlear implant and all-channel frequency-matched acoustic models also significantly overestimate speech perception. In contrast, self-selected acoustic models incorporating listener-specific perceptual frequency mismatch provide substantially better matches in both subjective sound quality and speech intelligibility. However, variability in similarity ratings across individuals may suggest that additional perceptual components remain unaccounted for in the current parameter set. These findings question the validity of frequency-matched acoustic models of cochlear implants in research applications.

OBJECTIVES/UNASSIGNED:To better understand the implications of tumor size on auditory brainstem implant (ABI) categories of auditory performance (CAP) score to facilitate more precise patient counselling. DESIGN/UNASSIGNED:Single-center retrospective chart review. SETTING/UNASSIGNED:Tertiary referral center. PARTICIPANTS/UNASSIGNED:Patients > 18 years old with neurofibromatosis type 2 who underwent ABI placement between the years 2009 and 2023 were included. Patients with prior surgical resection were excluded to reduce the potential of previous cochlear nucleus trauma confounding results. MAIN OUTCOME MEASURES/UNASSIGNED:Ipsilateral tumor volume was measured from the preoperative MRI, and the primary endpoint was the 1-year CAP score. RESULTS/UNASSIGNED: = 0.010). CONCLUSION/UNASSIGNED:Increased tumor size has a negative correlation with ultimate ABI performance. There were patients in this study with larger tumor sizes who still benefited from ABI placement. While larger tumor size should not be a contraindication for ABI placement, understanding the role tumor size can have on performance variation can serve to improve tumor management strategies and preoperative counselling.

OBJECTIVES/UNASSIGNED:To develop and study a novel means of imaging auditory brainstem implant (ABI) paddle placement utilizing postoperative MRI images to determine correlation between ABI performance and ideal paddle placement properties. DESIGN/UNASSIGNED:Single-center retrospective review and image analysis. SETTING/UNASSIGNED:Tertiary referral center. PARTICIPANTS/UNASSIGNED:Patients >18 years old who received an ABI from 2009 to 2023 and had subsequent MRI and auditory performance testing completed. MAIN OUTCOME MEASURES/UNASSIGNED:ABI paddle angulation and insertion depth measure, in addition to categories of auditory performance (CAP) score. RESULTS/UNASSIGNED: = 0.049) once patients with large tumor burden were excluded. Insertion depth and axial angulation showed a negative correlation, in which higher insertion depth was associated with a decrease in axial angulation value, although not statistically significant. CONCLUSION/UNASSIGNED:We present a novel means of measuring ABI paddle placement utilizing MRI to improve the objectivity of paddle placements assessments. Multicenter collaboration to increase the number of patients studied with this new measurement scheme will be required before stronger measurement associations can be determined.

Developed by the Center for Robust Speech Systems at the University of Texas at Dallas, in collaboration with New York University and the University of Wisconsin-Madison, CCi-MOBILE addresses a critical challenge in optimizing cochlear implant (CI) fitting and enhancing sound coding strategies. Existing clinical CI processors and research tools often lack either the necessary computational power and flexibility or the portability required for real-world testing. CCi-MOBILE bridges this gap by enabling the implementation and evaluation of diverse real-time sound coding algorithms in both laboratory and real-world settings, including those requiring synchronized bilateral stimulation. Building upon previous publications, this paper provides new detailed discussion on parameter setting for stimulus generation with CCi-MOBILE and serves as a comprehensive resource for scientists and engineers developing novel real-time sound coding and signal processing strategies with this platform. As part of an ongoing development effort, future generations of CCi-MOBILE may offer additional functionalities beyond those described here.

OBJECTIVES/UNASSIGNED:This study aims to synthesize current knowledge and outcomes related to pediatric auditory brainstem implantation (ABI) in children with severe inner ear malformations (IEMs). It highlights the clinical management practices, challenges, and potential future directions for consensus development in this field. METHODS/UNASSIGNED:A systematic review of findings presented at the Third International Pediatric ABI Symposium organized by the Hacettepe Cochlear Implant team between 3 and 5 September 2020 was conducted, incorporating data from 41 departments across 19 countries. Relevant clinical outcomes, imaging techniques, surgical approaches, and rehabilitation strategies were analyzed to identify key trends and variability in practices. RESULTS/UNASSIGNED:The review indicates that children receiving ABIs exhibit diverse auditory outcomes influenced by individual anatomical variations and developmental factors. Early implantation, particularly before the age of three, positively correlates with better auditory and language development. Multicenter experiences underscore the necessity of tailored decision-making, which considers both surgical candidacy and comprehensive rehabilitation resources. DISCUSSION:/UNASSIGNED:The variability in outcomes emphasizes the need for improved consensus and guidelines regarding eligibility, surgical techniques, and multidisciplinary rehabilitation approaches. Notable complications and the necessity for thorough imaging assessments were also identified as critical components affecting clinical decisions. CONCLUSION/UNASSIGNED:A formal consensus statement is warranted to standardize best practices in ABI management. This will not only enhance patient outcomes but also guide future research efforts to address the remaining challenges in the treatment of children with severe IEMs. Enhanced collaboration among team members will be pivotal in achieving these objectives.

Acoustic vocoders play a key role in simulating the speech information available to cochlear implant (CI) users. Traditionally, the intelligibility of vocoder CI simulations is assessed through speech recognition experiments with normally-hearing subjects, a process that can be time-consuming, costly, and subject to individual variability. As an alternative approach, we utilized an advanced deep learning speech recognition model to investigate the intelligibility of CI simulations. We evaluated model's performance on vocoder-processed words and sentences with varying vocoder parameters. The number of vocoder bands, frequency range, and envelope dynamic range were adjusted to simulate sound processing settings in CI devices. Additionally, we manipulated the low-cutoff frequency and intensity quantization of vocoder envelopes to simulate psychophysical temporal and intensity resolutions in CI patients. The results were evaluated within the context of the audio analysis performed in the model. Interestingly, the deep learning model, despite not being originally designed to mimic human speech processing, exhibited a human-like response to alterations in vocoder parameters, resembling existing human subject results. This approach offers significant time and cost savings compared to testing human subjects, and eliminates learning and fatigue effects during testing. Our findings demonstrate the potential of speech recognition models in facilitating auditory research.

HYPOTHESIS/OBJECTIVE:This study tests the hypothesis that it is possible to find tone or noise vocoders that sound similar and result in similar speech perception scores to a cochlear implant (CI). This would validate the use of such vocoders as acoustic models of CIs. We further hypothesize that those valid acoustic models will require a personalized amount of frequency mismatch between input filters and output tones or noise bands. BACKGROUND:Noise or tone vocoders have been used as acoustic models of CIs in hundreds of publications but have never been convincingly validated. METHODS:Acoustic models were evaluated by single-sided deaf CI users who compared what they heard with the CI in one ear to what they heard with the acoustic model in the other ear. We evaluated frequency-matched models (both all-channel and 6-channel models, both tone and noise vocoders) as well as self-selected models that included an individualized level of frequency mismatch. RESULTS:Self-selected acoustic models resulted in similar levels of speech perception and similar perceptual quality as the CI. These models also matched the CI in terms of perceived intelligibility, harshness, and pleasantness. CONCLUSION/CONCLUSIONS:Valid acoustic models of CIs exist, but they are different from the models most widely used in the literature. Individual amounts of frequency mismatch may be required to optimize the validity of the model. This may be related to the basalward frequency mismatch experienced by postlingually deaf patients after cochlear implantation.

Binaural unmasking, a key feature of normal binaural hearing, can refer to the improved intelligibility of masked speech by adding masking that facilitates perceived separation of target and masker. A question relevant for cochlear implant users with single-sided deafness (SSD-CI) is whether binaural unmasking can still be achieved if the additional masking is spectrally degraded and shifted. CIs restore some aspects of binaural hearing to these listeners, although binaural unmasking remains limited. Notably, these listeners may experience a mismatch between the frequency information perceived through the CI and that perceived by their normal hearing ear. Employing acoustic simulations of SSD-CI with normal hearing listeners, the present study confirms a previous simulation study that binaural unmasking is severely limited when interaural frequency mismatch between the input frequency range and simulated place of stimulation exceeds 1-2 mm. The present study also shows that binaural unmasking is largely retained when the input frequency range is adjusted to match simulated place of stimulation, even at the expense of removing low-frequency information. This result bears implications for the mechanisms driving the type of binaural unmasking of the present study and for mapping the frequency range of the CI speech processor in SSD-CI users.

The reasons why clinical outcomes with auditory brainstem implants (ABIs) are generally poorer than with cochlear implants (CIs) are still somewhat elusive. Prior work has focused on differences in processing of spectral information due to possibly poorer tonotopic representation and higher channel interaction with ABIs than with CIs. In contrast, this study examines the hypothesis that a potential contributing reason for poor speech perception in ABI users may be the relative lack of temporal responsiveness of the primary neurons that are stimulated by the ABI. The cochlear nucleus, the site of ABI stimulation, consists of different neuron types, most of which have much more complex responses than the auditory nerve neurons stimulated by a CI. Temporal responsiveness of primary stimulated neurons was assessed in a group of ABI and CI users by measuring recovery of electrically evoked compound action potentials (ECAPs) from single-pulse forward masking. Slower ECAP recovery tended to be associated with poorer hearing outcomes in both groups. ABI subjects with the longest recovery time had no speech understanding or even no hearing sensation with their ABI device; speech perception for the one CI outlier with long ECAP recovery time was well below average. To the extent that ECAP recovery measures reveal temporal properties of the primary neurons that receive direct stimulation form neural prosthesis devices, they may provide a physiological underpinning for clinical outcomes of auditory implants. ECAP recovery measures may be used to determine which portions of the cochlear nucleus to stimulate, and possibly allow us to enhance the stimulation paradigms.

Stimulation pulse rate affects current amplitude discrimination by cochlear implant (CI) users, indicated by the evidence that the JND (just noticeable difference) in current amplitude delivered by a CI electrode becomes larger at higher pulse rates. However, it is not clearly understood whether pulse rate would affect discrimination of speech intensities presented acoustically to CI processors, or what the size of this effect might be. Intensity discrimination depends on two factors: the growth of loudness with increasing sound intensity and the loudness JND (or the just noticeable loudness increment). This study evaluated the hypothesis that stimulation pulse rate affects loudness JND. This was done by measuring current amplitude JNDs in an experiment design based on signal detection theory according to which loudness discrimination is related to internal noise (which is manifested by variability in loudness percept in response to repetitions of the same physical stimulus). Current amplitude JNDs were measured for equally loud pulse trains of 500 and 3000 pps (pulses per second) by increasing the current amplitude of the target pulse train until it was perceived just louder than a same-rate or different-rate reference pulse train. The JND measures were obtained at two presentation levels. At the louder level, the current amplitude JNDs were affected by the rate of the reference pulse train in a way that was consistent with greater noise or variability in loudness perception for the higher pulse rate. The results suggest that increasing pulse rate from 500 to 3000 pps can increase loudness JND by 60 % at the upper portion of the dynamic range. This is equivalent to a 38 % reduction in the number of discriminable steps for acoustic and speech intensities.