Design and evaluation of a cochlear implant strategy based on a "Phantom" channel

Comparison of Performance for Cochlear-Implant Listeners Using Audio Processing Strategies Based on Short-Time Fast Fourier Transform or Spectral Feature Extraction

OBJECTIVES

We compared sound quality and performance for a conventional cochlear-implant (CI) audio processing strategy based on short-time fast-Fourier transform (Crystalis) and an experimental strategy based on spectral feature extraction (SFE). In the latter, the more salient spectral features (acoustic events) were extracted and mapped into the CI stimulation electrodes. We hypothesized that (1) SFE would be superior to Crystalis because it can encode acoustic spectral features without the constraints imposed by the short-time fast-Fourier transform bin width, and (2) the potential benefit of SFE would be greater for CI users who have less neural cross-channel interactions.

DESIGN

To examine the first hypothesis, 6 users of Oticon Medical Digisonic SP CIs were tested in a double-blind design with the SFE and Crystalis strategies on various aspects: word recognition in quiet, speech-in-noise reception threshold (SRT), consonant discrimination in quiet, listening effort, melody contour identification (MCI), and subjective sound quality. Word recognition and SRTs were measured on the first and last day of testing (4 to 5 days apart) to assess potential learning and/or acclimatization effects. Other tests were run once between the first and last testing day. Listening effort was assessed by measuring pupil dilation. MCI involved identifying a five-tone contour among five possible contours. Sound quality was assessed subjectively using the multiple stimulus with hidden reference and anchor (MUSHRA) paradigm for sentences, music, and ambient sounds. To examine the second hypothesis, cross-channel interaction was assessed behaviorally using forward masking.

RESULTS

Word recognition was similar for the two strategies on the first day of testing and improved for both strategies on the last day of testing, with Crystalis improving significantly more. SRTs were worse with SFE than Crystalis on the first day of testing but became comparable on the last day of testing. Consonant discrimination scores were higher for Crystalis than for the SFE strategy. MCI scores and listening effort were not substantially different across strategies. Subjective sound quality scores were lower for the SFE than for the Crystalis strategy. The difference in performance with SFE and Crystalis was greater for CI users with higher channel interaction.

CONCLUSIONS

CI-user performance was similar with the SFE and Crystalis strategies. Longer acclimatization times may be required to reveal the full potential of the SFE strategy.

Optimization of Sound Coding Strategies to Make Singing Music More Accessible for Cochlear Implant Users

Cochlear implants (CIs) are implantable medical devices that can partially restore hearing to people suffering from profound sensorineural hearing loss. While these devices provide good speech understanding in quiet, many CI users face difficulties when listening to music. Reasons include poor spatial specificity of electric stimulation, limited transmission of spectral and temporal fine structure of acoustic signals, and restrictions in the dynamic range that can be conveyed via electric stimulation of the auditory nerve. The coding strategies currently used in CIs are typically designed for speech rather than music. This work investigates the optimization of CI coding strategies to make singing music more accessible to CI users. The aim is to reduce the spectral complexity of music by selecting fewer bands for stimulation, attenuating the background instruments by strengthening a noise reduction algorithm, and optimizing the electric dynamic range through a back-end compressor. The optimizations were evaluated through both objective and perceptual measures of speech understanding and melody identification of singing voice with and without background instruments, as well as music appreciation questionnaires. Consistent with the objective measures, results gathered from the perceptual evaluations indicated that reducing the number of selected bands and optimizing the electric dynamic range significantly improved speech understanding in music. Moreover, results obtained from questionnaires show that the new music back-end compressor significantly improved music enjoyment. These results have potential as a new CI program for improved singing music perception.

Stimulating the Cochlear Apex Without Longer Electrodes: Preliminary Results With a New Approach

OBJECTIVE

To investigate a new surgical and signal processing technique that provides apical stimulation of the cochlea using a cochlear implant without extending the length of the electrode array.

PATIENTS

Three adult patients who underwent cochlear implantation using this new technique.

INTERVENTIONS

The patients received a cochlear implant. The surgery differed from the standard approach in that a ground electrode was placed in the cochlear helicotrema via an apical cochleostomy rather than in its typical location underneath the temporalis muscle. Clinical fitting was modified such that low frequencies were represented using the apically placed electrode as a ground.

MAIN OUTCOME MEASURES

Pitch scaling and speech recognition.

RESULTS

All surgeries were successful with no complications. Pitch scaling demonstrated that use of the apically placed electrode as a ground lowered the perceived pitch of electric stimulation relative to monopolar stimulation. Speech understanding was improved compared with preoperative scores.

CONCLUSIONS

The new surgical approach and clinical fitting are feasible. A lower pitch is perceived when using the apically placed electrode as a ground relative to stimulation using an extracochlear ground (i.e., monopolar mode), suggesting that stimulation can be provided more apically without the use of a longer electrode array. Further work is required to determine potential improvements in outcomes and optimal signal processing for the new approach.

Cochlear Implant Compression Optimization for Musical Sound Quality in MED-EL Users

OBJECTIVES

Variations in loudness are a fundamental component of the music listening experience. Cochlear implant (CI) processing, including amplitude compression, and a degraded auditory system may further degrade these loudness cues and decrease the enjoyment of music listening. This study aimed to identify optimal CI sound processor compression settings to improve music sound quality for CI users.

DESIGN

Fourteen adult MED-EL CI recipients participated (Experiment No. 1: n = 17 ears; Experiment No. 2: n = 11 ears) in the study. A software application using a modified comparison category rating (CCR) test method allowed participants to compare and rate the sound quality of various CI compression settings while listening to 25 real-world music clips. The two compression settings studied were (1) Maplaw, which informs audibility and compression of soft level sounds, and (2) automatic gain control (AGC), which applies compression to loud sounds. For each experiment, one compression setting (Maplaw or AGC) was held at the default, while the other was varied according to the values available in the clinical CI programming software. Experiment No. 1 compared Maplaw settings of 500, 1000 (default), and 2000. Experiment No. 2 compared AGC settings of 2.5:1, 3:1 (default), and 3.5:1.

RESULTS

In Experiment No. 1, the group preferred a higher Maplaw setting of 2000 over the default Maplaw setting of 1000 (p = 0.003) for music listening. There was no significant difference in music sound quality between the Maplaw setting of 500 and the default setting (p = 0.278). In Experiment No. 2, a main effect of AGC setting was found; however, no significant difference in sound quality ratings for pairwise comparisons were found between the experimental settings and the default setting (2.5:1 versus 3:1 at p = 0.546; 3.5:1 versus 3:1 at p = 0.059).

CONCLUSIONS

CI users reported improvements in music sound quality with higher than default Maplaw or AGC settings. Thus, participants preferred slightly higher compression for music listening, with results having clinical implications for improving music perception in CI users.

Phantom Stimulation for Cochlear Implant Users With Residual Low-Frequency Hearing

In cochlear implants (CIs), phantom stimulation can be used to extend the pitch range toward apical regions of the cochlea. Phantom stimulation consists of partial bipolar stimulation, in which current is distributed across two intracochlear electrodes and one extracochlear electrode as defined by the compensation coefficient σ. The aim of this study was, (1) to evaluate the benefit of conveying low-frequency information through phantom stimulation for cochlear implant (CI) subjects with low-frequency residual hearing using electric stimulation alone, (2) to compare the speech reception thresholds obtained from electric-acoustic stimulation (EAS) and electric stimulation in combination with phantom stimulation (EPS), and (3) to investigate the effect of spectrally overlapped bandwidth of speech conveyed via simultaneous acoustic and phantom stimulation on speech reception thresholds.

Interaction Between Electric and Acoustic Stimulation Influences Speech Perception in Ipsilateral EAS Users

OBJECTIVES

The aim of this study was to determine electric-acoustic masking in cochlear implant users with ipsilateral residual hearing and different electrode insertion depths and to investigate the influence on speech reception. The effects of different fitting strategies-meet, overlap, and a newly developed masking adjusted fitting (UNMASKfit)-on speech reception are compared. If electric-acoustic masking has a detrimental effect on speech reception, the individualized UNMASKfit map might be able to reduce masking and thereby enhance speech reception.

DESIGN

Fifteen experienced MED-EL Flex electrode recipients with ipsilateral residual hearing participated in a crosssover design study using three fitting strategies for 4 weeks each. The following strategies were compared: (1) a meet fitting, dividing the frequency range between electric and acoustic stimulation, (2) an overlap fitting, delivering part of the frequency range both acoustically and electrically, and (3) the UNMASKfit, reducing the electric stimulation according to the individual electric-on-acoustic masking strength. A psychoacoustic masking procedure was used to measure the changes in acoustic thresholds due to the presence of electric maskers. Speech reception was measured in noise with the Oldenburg Matrix Sentence test.

RESULTS

Behavioral thresholds of acoustic probe tones were significantly elevated in the presence of electric maskers. A maximum of masking was observed when the difference in location between the electric and acoustic stimulation was around one octave in place frequency. Speech reception scores and strength of masking showed a dependency on residual hearing, and speech reception was significantly reduced in the overlap fitting strategy. Electric- acoustic stimulation significantly improved speech reception over electric stimulation alone, with a tendency toward a larger benefit with the UNMASKfit map. In addition, masking was significantly inversely correlated to the speech reception performance difference between the overlap and the meet fitting.

CONCLUSIONS

(1) This study confirmed the interaction between ipsilateral electric and acoustic stimulation in a psychoacoustic masking experiment. (2) The overlap fitting yielded poorer speech reception performance in stationary noise especially in subjects with strong masking. (3) The newly developed UNMASKfit strategy yielded similar speech reception thresholds with an enhanced acoustic benefit, while at the same time reducing the electric stimulation. This could be beneficial in the long-term if applied as a standard fitting, as hair cells are exposed to less possibly adverse electric stimulation. In this study, the UNMASKfit allowed the participants a better use of their natural hearing even after 1 month of adaptation. It might be feasible to transfer these results to the clinic, by fitting patients with the UNMASKfit upon their first fitting appointment, so that longer adaptation times can further improve speech reception.

Evaluating hearing performance with cochlear implants within the same patient using daily randomization and imaging-based fitting - The ELEPHANT study

Background

Prospective research in the field of cochlear implants is hampered by methodological issues and small sample sizes. The ELEPHANT study presents an alternative clinical trial design with a daily randomized approach evaluating individualized tonotopical fitting of a cochlear implant (CI).

Methods

A single-blinded, daily-randomized clinical trial will be implemented to evaluate a new imaging-based CI mapping strategy. A minimum of 20 participants will be included from the start of the rehabilitation process with a 1-year follow-up period. Based on a post-operative cone beam CT scan (CBCT), mapping of electrical input will be aligned to natural place-pitch arrangement in the individual cochlea. The CI’s frequency allocation table will be adjusted to match the electrical stimulation of frequencies as closely as possible to corresponding acoustic locations in the cochlea. A randomization scheme will be implemented whereby the participant, blinded to the intervention allocation, crosses over between the experimental and standard fitting program on a daily basis, and thus effectively acts as his own control, followed by a period of free choice between both maps to incorporate patient preference. With this new approach the occurrence of a first-order carryover effect and a limited sample size is addressed.

Discussion

The experimental fitting strategy is thought to give rise to a steeper learning curve, result in better performance in challenging listening situations, improve sound quality, better complement residual acoustic hearing in the contralateral ear and be preferred by recipients of a CI. Concurrently, the suitability of the novel trial design will be considered in investigating these hypotheses.

Trial registration

ClinicalTrials.gov: NCT03892941. Registered 27 March 2019.

Formant frequency discrimination with a fine structure sound coding strategy for cochlear implants

Recent sound coding strategies for cochlear implants (CI) have focused on the transmission of temporal fine structure to the CI recipient. To date, knowledge about the effects of fine structure coding in electrical hearing is poorly charactarized. The aim of this study was to examine whether the presence of temporal fine structure coding affects how the CI recipient perceives sound. This was done by comparing two sound coding strategies with different temporal fine structure coverage in a longitudinal cross-over setting. The more recent FS4 coding strategy provides fine structure coding on typically four apical stimulation channels compared to FSP with usually one or two fine structure channels. 34 adult CI patients with a minimum CI experience of one year were included. All subjects were fitted according to clinical routine and used both coding strategies for three months in a randomized sequence. Formant frequency discrimination thresholds (FFDT) were measured to assess the ability to resolve timbre information. Further outcome measures included a monosyllables test in quiet and the speech reception threshold of an adaptive matrix sentence test in noise (Oldenburger sentence test). In addition, the subjective sound quality was assessed using visual analogue scales and a sound quality questionnaire after each three months period. The extended fine structure range of FS4 yields FFDT similar to FSP for formants occurring in the frequency range only covered by FS4. There is a significant interaction (p = 0.048) between the extent of fine structure coverage in FSP and the improvement in FFDT in favour of FS4 for these stimuli. FS4 Speech perception in noise and quiet was similar with both coding strategies. Sound quality was rated heterogeneously showing that both strategies represent valuable options for CI fitting to allow for best possible individual optimization.

Dynamic current steering with phantom electrode in cochlear implants

Phantom electrode (PE) stimulation can extend the lower limit of pitch perception with cochlear implants (CIs) by using simultaneous out-of-phase stimulation of the most apical primary electrode and the adjacent basal compensating electrode. The total electrical field may push the excitation pattern beyond the most apical electrode to elicit a lower pitch, depending on the ratio of current between the compensating and primary electrodes (i.e., the compensation coefficient σ). This study tested the hypothesis that dynamic current steering of PE stimuli can be implemented by varying σ over time to encode spectral details in low frequencies. To determine the range of σ for current steering and the corresponding current levels, Experiment 1 tested CI users' loudness balance and pitch ranking of static PE stimuli with σ from 0 to 0.6 in steps of 0.2. It was found that the equal-loudness most comfortable level significantly increased with σ and can be modeled by a piecewise linear function of σ. Consistent with the previous findings, higher σ elicited either lower or similar pitches without salient pitch reversals than lower σ. Based on the results of Experiment 1, Experiment 2 created flat, rising, and falling pitch contours of 300-1000 ms using dynamic PE stimuli with time-varying σ from 0 to 0.6 and equal-loudness current levels. In a pitch contour identification (PCI) task, CI users scored 80% and above on average. Increasing the stimulus duration from 300 to 1000 ms slightly but did not significantly improve the PCI scores. Across subjects, the 1000-ms PCI scores in Experiment 2 were significantly correlated with the cumulative pitch-ranking sensitivity in Experiment 1. It is thus feasible to use dynamic current steering with PE to encode low-frequency pitch cues for CI users.

Effectiveness of Phantom Stimulation in Shifting the Pitch Percept in Cochlear Implant Users

OBJECTIVES

Phantom electrode stimulation was developed for cochlear implant (CI) systems to provide a lower pitch percept by stimulating more apical regions of the cochlea, without inserting the electrode array deeper into the cochlea. Phantom stimulation involves simultaneously stimulating a primary and a compensating electrode with opposite polarity, thereby shifting the electrical field toward the apex and eliciting a lower pitch percept. The current study compared the effect sizes (in shifts of place of excitation) of multiple phantom configurations by matching the perceived pitch with phantom stimulation to that perceived with monopolar stimulation. Additionally, the effects of electrode location, type of electrode array, and stimulus level on the perceived pitch were investigated.

DESIGN

Fifteen adult advanced bionics CI users participated in this study, which included four experiments to eventually measure the shifts in place of excitation with five different phantom configurations. The proportions of current delivered to the compensating electrode, expressed as σ, were 0.5, 0.6, 0.7, and 0.8 for the symmetrical biphasic pulses (SBC0.5, SBC0.6, SBC0.7, and SBC0.8) and 0.75 for the pseudomonophasic pulse shape (PSA0.75). A pitch discrimination experiment was first completed to determine which basal and apical electrode contacts should be used for the subsequent experiments. An extensive loudness balancing experiment followed where both the threshold level (T-level) and most comfortable level (M-level) were determined to enable testing at multiple levels of the dynamic range. A pitch matching experiment was then performed to estimate the shift in place of excitation at the chosen electrode contacts. These rough shifts were then used in the subsequent experiment, where the shifts in place of excitation were determined more accurately.

RESULTS

Reliable data were obtained from 20 electrode contacts. The average shifts were 0.39, 0.53, 0.64, 0.76, and 0.53 electrode contacts toward the apex for SBC0.5, SBC0.6, SBC0.7, SBC0.8, and PSA0.75, respectively. When only the best configurations per electrode contact were included, the average shift in place of excitation was 0.92 electrode contacts (range: 0.25 to 2.0). While PSA0.75 leads to equal results as the SBC configurations in the apex, it did not result in a significant shift at the base. The shift in place of excitation was significantly larger at the apex and with lateral wall electrode contacts. The stimulus level did not affect the shift.

CONCLUSIONS

Phantom stimulation results in significant shifts in place of excitation, especially at the apical part of the electrode array. The phantom configuration that leads to the largest shift in place of excitation differs between subjects. Therefore, the settings of the phantom electrode should be individualized so that the phantom stimulation is optimized for each CI user. The real added value to the sound quality needs to be established in a take-home trial.

Systematic Review of Auditory Training in Pediatric Cochlear Implant Recipients

Objective The purpose of this systematic review is to evaluate the published research in auditory training (AT) for pediatric cochlear implant (CI) recipients. This review investigates whether AT in children with CIs leads to improvements in speech and language development, cognition, and/or quality of life and whether improvements, if any, remain over time post AT intervention. Method A systematic search of 7 databases identified 96 review articles published up until January 2017, 9 of which met the inclusion criteria. Data were extracted and independently assessed for risk of bias and quality of study against a PICOS (participants, intervention, control, outcomes, and study) framework. Results All studies reported improvements in trained AT tasks, including speech discrimination/identification and working memory. Retention of improvements over time was found whenever it was assessed. Transfer of learning was measured in 4 of 6 studies, which assessed generalization. Quality of life was not assessed. Overall, evidence for the included studies was deemed to be of low quality. Conclusion Benefits of AT were illustrated through the improvement in trained tasks, and this was observed in all reviewed studies. Transfer of improvement to other domains and also retention of benefits post AT were evident when assessed, although rarely done. However, higher quality evidence to further examine outcomes of AT in pediatric CI recipients is needed.

Effects of Phantom Electrode Stimulation on Vocal Production in Cochlear Implant Users

OBJECTIVES

Cochlear implant (CI) users suffer from a range of speech impairments, such as stuttering and vocal control of pitch and intensity. Though little research has focused on the role of auditory feedback in the speech of CI users, these speech impairments could be due in part to limited access to low-frequency cues inherent in CI-mediated listening. Phantom electrode stimulation (PES) represents a novel application of current steering that extends access to low frequencies for CI recipients. It is important to note that PES transmits frequencies below 300 Hz, whereas Baseline does not. The objective of this study was to explore the effects of PES on multiple frequency-related characteristics of voice production.

DESIGN

Eight postlingually deafened, adult Advanced Bionics CI users underwent a series of vocal production tests including Tone Repetition, Vowel Sound Production, Passage Reading, and Picture Description. Participants completed all of these tests twice: once with PES and once using their program used for everyday listening (Baseline). An additional test, Automatic Modulation, was included to measure acute effects of PES and was completed only once. This test involved switching between PES and Baseline at specific time intervals in real time as participants read a series of short sentences. Finally, a subjective Vocal Effort measurement was also included.

RESULTS

In Tone Repetition, the fundamental frequencies (F0) of tones produced using PES and the size of musical intervals produced using PES were significantly more accurate (closer to the target) compared with Baseline in specific gender, target tone range, and target tone type testing conditions. In the Vowel Sound Production task, vowel formant profiles produced using PES were closer to that of the general population compared with those produced using Baseline. The Passage Reading and Picture Description task results suggest that PES reduces measures of pitch variability (F0 standard deviation and range) in natural speech production. No significant results were found in comparisons of PES and Baseline in the Automatic Modulation task nor in the Vocal Effort task.

CONCLUSIONS

The findings of this study suggest that usage of PES increases accuracy of pitch matching in repeated sung tones and frequency intervals, possibly due to more accurate F0 representation. The results also suggest that PES partially normalizes the vowel formant profiles of select vowel sounds. PES seems to decrease pitch variability of natural speech and appears to have limited acute effects on natural speech production, though this finding may be due in part to paradigm limitations. On average, subjective ratings of vocal effort were unaffected by the usage of PES versus Baseline.

Electrical stimulation of the midbrain excites the auditory cortex asymmetrically

BACKGROUND

Auditory midbrain implant users cannot achieve open speech perception and have limited frequency resolution. It remains unclear whether the spread of excitation contributes to this issue and how much it can be compensated by current-focusing, which is an effective approach in cochlear implants.

OBJECTIVE

The present study examined the spread of excitation in the cortex elicited by electric midbrain stimulation. We further tested whether current-focusing via bipolar and tripolar stimulation is effective with electric midbrain stimulation and whether these modes hold any advantage over monopolar stimulation also in conditions when the stimulation electrodes are in direct contact with the target tissue.

METHODS

Using penetrating multielectrode arrays, we recorded cortical population responses to single pulse electric midbrain stimulation in 10 ketamine/xylazine anesthetized mice. We compared monopolar, bipolar, and tripolar stimulation configurations with regard to the spread of excitation and the characteristic frequency difference between the stimulation/recording electrodes.

RESULTS

The cortical responses were distributed asymmetrically around the characteristic frequency of the stimulated midbrain region with a strong activation in regions tuned up to one octave higher. We found no significant differences between monopolar, bipolar, and tripolar stimulation in threshold, evoked firing rate, or dynamic range.

CONCLUSION

The cortical responses to electric midbrain stimulation are biased towards higher tonotopic frequencies. Current-focusing is not effective in direct contact electrical stimulation. Electrode maps should account for the asymmetrical spread of excitation when fitting auditory midbrain implants by shifting the frequency-bands downward and stimulating as dorsally as possible.

Perceptual changes with monopolar and phantom electrode stimulation

Phantom electrode (PE) stimulation is achieved by simultaneously stimulating out-of-phase from two adjacent intra-cochlear electrodes with different amplitudes. If the basal electrode stimulates with a smaller amplitude than the apical electrode of the pair, the resulting electrical field is pushed away from the basal electrode producing a lower pitch. There is great interest in using PE stimulation in a processing strategy as it can be used to provide stimulation to regions of the cochlea located more apically than the most apical contact on the electrode array. The result is that even lower pitch sensations can be provided without additional risk of a deeper insertion. However, it is unknown if there are perceptual differences between monopolar (MP) and PE stimulation other than a shift in place pitch. Furthermore, it is unknown if the effect and magnitude of changing from MP to PE stimulation is dependent on electrode location. This study investigates the perceptual differences (including pitch and other sound quality differences) at multiple electrode positions using MP and PE stimulation using both a multidimensional scaling procedure (MDS) and a traditional scaling procedure. 10 Advanced Bionics users reported the perceptual distances between 5 single electrode (typically 1, 3, 5, 7, and 9) stimuli in either MP or PE (σ = 0.5) mode. Subjects were asked to report how perceptually different each pair of stimuli were using any perceived differences except loudness. Subsequently, each stimulus was presented in isolation and subjects scaled how "high" or how "clean" each sounded. Results from the MDS task suggest that perceptual differences between MP and PE stimulation can be explained by a single dimension. The traditional scaling suggests that the single dimension is place pitch. PE stimulation elicits lower pitch perceptions in all cochlear regions. Analysis of Cone Beam Computer Tomography (CBCT) data suggests that PE stimulation may be more effective at the apical part of the cochlea. PE stimulation can be used for new sound coding strategies in order to extend the pitch range for cochlear implant (CI) users without perceptual side effects.

Pure-Tone Masking Patterns for Monopolar and Phantom Electrical Stimulation in Cochlear Implants

OBJECTIVES

Monopolar stimulation of the most apical electrode produces the lowest pitch sensation in cochlear implants clinically. A phantom electrode that uses out-of-phase electrical stimulation between the most apical and the neighboring basal electrode can produce a lower pitch sensation than that associated with the most apical electrode. However, because of the absence of contacts beyond the apical tip of the array, the ability to assess the spread of electrical excitation associated with phantom stimulation is limited in the typical cochlear implant subject with no residual hearing. In the present study, the spread of electrical excitation associated with monopolar and phantom stimulation of the most apical electrode was assessed using electrical masking of acoustic thresholds in cochlear implant subjects with residual, low-frequency, acoustic hearing.

DESIGN

Eight subjects with an Advanced Bionics cochlear implant and residual hearing in the implanted ear participated in this study (nine ears in total). Unmasked and masked thresholds for acoustic pure tones were measured at 125, 250, 500, 750, 1000, and 2000 Hz in the presence of monopolar and phantom electrode stimulation presented at the apical-most end of the array. The current compensation for phantom electrode stimulation was fixed at 50%. The two electrical maskers were loudness balanced. Differences between the unmasked and masked acoustic thresholds can be attributed to (1) the electrical stimulus-induced interference in the transduction/conduction of the acoustic signal through cochlear periphery and the auditory nerve and/or (2) masking at the level of the central auditory system.

RESULTS

The results show a significant elevation in pure-tone thresholds in the presence of the monopolar and phantom electrical maskers. The unmasked thresholds were subtracted from the masked thresholds to derive masking patterns as a function of the acoustic probe frequency. The masking patterns show that phantom stimulation was able to produce more masking than that associated with the monopolar stimulation of the most apical electrode.

CONCLUSION

These results suggest that for some cochlear implant subjects, phantom electrode stimulation can shift the neural stimulation pattern more apically in the cochlea, which is consistent with reports that phantom electrode stimulation produces lower pitch sensations than those associated with monopolar stimulation of the most apical electrode alone.

Assessment and improvement of sound quality in cochlear implant users

Objectives

Cochlear implants (CIs) have successfully provided speech perception to individuals with sensorineural hearing loss. Recent research has focused on more challenging acoustic stimuli such as music and voice emotion. The purpose of this review is to evaluate and describe sound quality in CI users with the purposes of summarizing novel findings and crucial information about how CI users experience complex sounds.

Data Sources

Here we review the existing literature on PubMed and Scopus to present what is known about perceptual sound quality in CI users, discuss existing measures of sound quality, explore how sound quality may be effectively studied, and examine potential strategies of improving sound quality in the CI population.

Results

Sound quality, defined here as the perceived richness of an auditory stimulus, is an attribute of implant‐mediated listening that remains poorly studied. Sound quality is distinct from appraisal, which is generally defined as the subjective likability or pleasantness of a sound. Existing studies suggest that sound quality perception in the CI population is limited by a range of factors, most notably pitch distortion and dynamic range compression. Although there are currently very few objective measures of sound quality, the CI‐MUSHRA has been used as a means of evaluating sound quality. There exist a number of promising strategies to improve sound quality perception in the CI population including apical cochlear stimulation, pitch tuning, and noise reduction processing strategies.

Conclusions

In the published literature, sound quality perception is severely limited among CI users. Future research should focus on developing systematic, objective, and quantitative sound quality metrics and designing therapies to mitigate poor sound quality perception in CI users.

Level of Evidence

NA

Validation of a Cochlear Implant Patient-Specific Model of the Voltage Distribution in a Clinical Setting

Cochlear Implants (CIs) are medical implantable devices that can restore the sense of hearing in people with profound hearing loss. Clinical trials assessing speech intelligibility in CI users have found large intersubject variability. One possibility to explain the variability is the individual differences in the interface created between electrodes of the CI and the auditory nerve. In order to understand the variability, models of the voltage distribution of the electrically stimulated cochlea may be useful. With this purpose in mind, we developed a parametric model that can be adapted to each CI user based on landmarks from individual cone beam computed tomography (CBCT) scans of the cochlea before and after implantation. The conductivity values of each cochlea compartment as well as the weighting factors of different grounding modes have also been parameterized. Simulations were performed modeling the cochlea and electrode positions of 12 CI users. Three models were compared with different levels of detail: a homogeneous model (HM), a non-patient-specific model (NPSM), and a patient-specific model (PSM). The model simulations were compared with voltage distribution measurements obtained from the backward telemetry of the 12 CI users. Results show that the PSM produces the lowest error when predicting individual voltage distributions. Given a patient-specific geometry and electrode positions, we show an example on how to optimize the parameters of the model and how to couple it to an auditory nerve model. The model here presented may help to understand speech performance variability and support the development of new sound coding strategies for CIs.

Use of the Phantom Electrode strategy to improve bass frequency perception for music listening in cochlear implant users

OBJECTIVES

The Phantom Electrode strategy makes use of partial bipolar stimulation on the two most apical electrodes in an effort to extend the frequency range available to cochlear implant (CI) users. This study aimed to quantify the effect of the Phantom Electrode strategy on bass frequency perception in music listening in CI users.

METHODS

Eleven adult Advanced Bionics users with the Fidelity 120 processing strategy and 16 adult normal hearing (NH) individuals participated in the study. All subjects completed the CI-multiple stimulus with hidden reference and anchor (MUSHRA), a test of an individual's ability to make discriminations in sound quality following the removal of bass frequency information. NH participants completed the CI-MUSHRA once, whereas CI users completed the task twice - once with their baseline clinical program and once with the Phantom Electrode strategy, in random order. CI users' performance was assessed in comparison with NH performance.

RESULTS

The Phantom Electrode strategy improved CI users performance on the CI-MUSHRA compared with Fidelity 120.

DISCUSSION

Creation of a Phantom Electrode percept through partial bipolar stimulation of the two most apical electrodes appears to improve CI users' perception of bass frequency information in music, contributing to greater accuracy in the ability to detect alterations in musical sound quality.

CONCLUSION

The Phantom Electrode processing strategy may enhance the experience of listening to music and thus acoustic stimuli more broadly by improving perception of bass frequencies, through direction of current towards the apical portion of the cochlea beyond the termination of the electrode.