The Relationship Between Insertion Angles, Default Frequency Allocations, and Spiral Ganglion Place Pitch in Cochlear Implants

Effect of stimulation parameters on sequential current-steered stimuli in cochlear implants

Manipulation of cochlear implant (CI) place pitch was carried out with current steering by stimulating two CI electrodes sequentially. The objective was to investigate whether shifts in activated neural populations could be achieved to produce salient pitch differences and to determine which stimulation parameters would be more effective in steering of current. These were the pulse rate and pulse width of electrical stimuli and the distance between the two current-steering electrodes. Nine CI users participated, and ten ears were tested. The pattern of pitch changes was not consistent across listeners, but the data suggest that individualized selection of stimulation parameters may be used to effect place pitch changes with sequential current steering. Individual analyses showed that pulse width generally had little influence on the effectiveness of current steering with sequential stimuli, while more salient place pitch shifts were often achieved at wider electrode spacing or when the stimulation pulse rate was the same as that indicated on the clinical MAP (the set of stimulation parameters) of the listener. Results imply that current steering may be used in CIs that allow only sequential stimulation to achieve place pitch manipulation.

Computed-Tomography Estimates of Interaural Mismatch in Insertion Depth and Scalar Location in Bilateral Cochlear-Implant Users

HYPOTHESIS

Bilateral cochlear-implant (BI-CI) users will have a range of interaural insertion-depth mismatch because of different array placement or characteristics. Mismatch will be larger for electrodes located near the apex or outside scala tympani, or for arrays that are a mix of precurved and straight types.

BACKGROUND

Brainstem superior olivary-complex neurons are exquisitely sensitive to interaural-difference cues for sound localization. Because these neurons rely on interaurally place-of-stimulation-matched inputs, interaural insertion-depth or scalar-location differences for BI-CI users could cause interaural place-of-stimulation mismatch that impairs binaural abilities.

METHODS

Insertion depths and scalar locations were calculated from temporal-bone computed-tomography scans for 107 BI-CI users (27 Advanced Bionics, 62 Cochlear, 18 MED-EL).

RESULTS

Median interaural insertion-depth mismatch was 23.4 degrees or 1.3 mm. Mismatch in the estimated clinically relevant range expected to impair binaural processing (>75 degrees or 3 mm) occurred for 13 to 19% of electrode pairs overall, and for at least three electrode pairs for 23 to 37% of subjects. There was a significant three-way interaction between insertion depth, scalar location, and array type. Interaural insertion-depth mismatch was largest for apical electrodes, for electrode pairs in two different scala, and for arrays that were both-precurved.

CONCLUSION

Average BI-CI interaural insertion-depth mismatch was small; however, large interaural insertion-depth mismatch-with the potential to degrade spatial hearing-occurred frequently enough to warrant attention. For new BICI users, improved surgical techniques to avoid interaural insertion-depth and scalar mismatch are recommended. For existing BI-CI users with interaural insertion-depth mismatch, interaural alignment of clinical frequency tables might reduce negative spatial-hearing consequences.

Systematic Literature Review of Hearing Preservation Rates in Cochlear Implantation Associated With Medium- and Longer-Length Flexible Lateral Wall Electrode Arrays

Background

The last two decades have demonstrated that preoperative functional acoustic hearing (residual hearing) can be preserved during cochlear implant (CI) surgery. However, the relationship between the electrode array length and postoperative hearing preservation (HP) with lateral wall flexible electrode variants is still under debate.

Aims/Objectives

This is a systematic literature review that aims to analyze the HP rates of patients with residual hearing for medium-length and longer-length lateral wall electrodes.

Method

A systematic literature review methodology was applied following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) recommendations to evaluate the HP rates of medium-length and longer-length lateral wall electrodes from one CI manufacturer (medium length FLEX 24, longer length FLEX 28 and FLEX SOFT, MED-EL, Innsbruck, Austria). A search using search engine PubMed (https://www.ncbi.nlm.nih.gov/pubmed/) was performed using the search terms “hearing preservation” or “residual hearing” and “cochlear implant” in “All fields.” Articles published only in English between January 01, 2009 and December 31, 2020 were included in the search.

Results

The HP rate was similar between medium-length (93.4%–93.5%) and longer (92.1%–86.8%) electrodes at 4 months (p = 0.689) and 12 months (p = 0.219). In the medium-length electrode group, patients under the age of 45 years had better HP than patients above the age of 45 years.

Conclusions

Both medium-length and longer electrode arrays showed high hearing preservation rates. Considering the hearing deterioration over time, implanting a longer electrode at primary surgery should be considered, thus preventing the need for future reimplantation.

Effect of Place-Based Versus Default Mapping Procedures on Masked Speech Recognition: Simulations of Cochlear Implant Alone and Electric-Acoustic Stimulation

PURPOSE

Cochlear implant (CI) recipients demonstrate variable speech recognition when listening with a CI-alone or electric-acoustic stimulation (EAS) device, which may be due in part to electric frequency-to-place mismatches created by the default mapping procedures. Performance may be improved if the filter frequencies are aligned with the cochlear place frequencies, known as place-based mapping. Performance with default maps versus an experimental place-based map was compared for participants with normal hearing when listening to CI-alone or EAS simulations to observe potential outcomes prior to initiating an investigation with CI recipients.

METHOD

A noise vocoder simulated CI-alone and EAS devices, mapped with default or place-based procedures. The simulations were based on an actual 24-mm electrode array recipient, whose insertion angles for each electrode contact were used to estimate the respective cochlear place frequency. The default maps used the filter frequencies assigned by the clinical software. The filter frequencies for the place-based maps aligned with the cochlear place frequencies for individual contacts in the low- to mid-frequency cochlear region. For the EAS simulations, low-frequency acoustic information was filtered to simulate aided low-frequency audibility. Performance was evaluated for the AzBio sentences presented in a 10-talker masker at +5 dB signal-to-noise ratio (SNR), +10 dB SNR, and asymptote.

RESULTS

Performance was better with the place-based maps as compared with the default maps for both CI-alone and EAS simulations. For instance, median performance at +10 dB SNR for the CI-alone simulation was 57% correct for the place-based map and 20% for the default map. For the EAS simulation, those values were 59% and 37% correct. Adding acoustic low-frequency information resulted in a similar benefit for both maps.

CONCLUSIONS

Reducing frequency-to-place mismatches, such as with the experimental place-based mapping procedure, produces a greater benefit in speech recognition than maximizing bandwidth for CI-alone and EAS simulations. Ongoing work is evaluating the initial and long-term performance benefits in CI-alone and EAS users.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.19529053.

Conversations in Cochlear Implantation: The Inner Ear Therapy of Today

As biomolecular approaches for hearing restoration in profound sensorineural hearing loss evolve, they will be applied in conjunction with or instead of cochlear implants. An understanding of the current state-of-the-art of this technology, including its advantages, disadvantages, and its potential for delivering and interacting with biomolecular hearing restoration approaches, is helpful for designing modern hearing-restoration strategies. Cochlear implants (CI) have evolved over the last four decades to restore hearing more effectively, in more people, with diverse indications. This evolution has been driven by advances in technology, surgery, and healthcare delivery. Here, we offer a practical treatise on the state of cochlear implantation directed towards developing the next generation of inner ear therapeutics. We aim to capture and distill conversations ongoing in CI research, development, and clinical management. In this review, we discuss successes and physiological constraints of hearing with an implant, common surgical approaches and electrode arrays, new indications and outcome measures for implantation, and barriers to CI utilization. Additionally, we compare cochlear implantation with biomolecular and pharmacological approaches, consider strategies to combine these approaches, and identify unmet medical needs with cochlear implants. The strengths and weaknesses of modern implantation highlighted here can mark opportunities for continued progress or improvement in the design and delivery of the next generation of inner ear therapeutics.

Sound Source Localization by Cochlear Implant Recipients with Normal Hearing in the Contralateral Ear: Effects of Spectral Content and Duration of Listening Experience

INTRODUCTION

Cochlear implant (CI) recipients with normal hearing (NH) in the contralateral ear experience a significant improvement in sound source localization when listening with the CI in combination with their NH-ear (CI + NH) as compared to with the NH-ear alone. The improvement in localization is primarily due to sensitivity to interaural level differences (ILDs). Sensitivity to interaural timing differences (ITDs) may be limited by auditory aging, frequency-to-place mismatches, the signal coding strategy, and duration of CI use. The present report assessed the sensitivity of ILD and ITD cues in CI + NH listeners who were recipients of long electrode arrays that provide minimal frequency-to-place mismatches and were mapped with a coding strategy that presents fine structure cues on apical channels.

METHODS

Sensitivity to ILDs and ITDs for localization was assessed using broadband noise (BBN), as well as high-pass (HP) and low-pass (LP) filtered noise for adult CI + NH listeners. Stimuli were 200-ms noise bursts presented from 11 speakers spaced evenly over an 180° arc. Performance was quantified in root-mean-squared error and response patterns were analyzed to evaluate the consistency, accuracy, and side bias of the responses. Fifteen listeners completed the task at the 2-year post-activation visit; seven listeners repeated the task at a later annual visit.

RESULTS

Performance at the 2-year visit was best with the BBN and HP stimuli and poorer with the LP stimulus. Responses to the BBN and HP stimuli were significantly correlated, consistent with the idea that CI + NH listeners primarily use ILD cues for localization. For the LP stimulus, some listeners responded consistently and accurately and with limited side bias, which may indicate sensitivity to ITD cues. Two of the 7 listeners who repeated the task at a later annual visit experienced a significant improvement in performance with the LP stimulus, which may indicate that sensitivity to ITD cues may improve with long-term CI use.

CONCLUSIONS

CI recipients with a NH-ear primarily use ILD cues for sound source localization, though some may use ITD cues as well. Sensitivity to ITD cues may improve with long-term CI listening experience.

Distortion of Spectral Ripples Through Cochlear Implants Has Major Implications for Interpreting Performance Scores

The spectral ripple discrimination task is a psychophysical measure that has been found to correlate with speech recognition in listeners with cochlear implants (CIs). However, at ripple densities above a critical value (around 2 RPO, but device-specific), the sparse spectral sampling of CI processors results in stimulus distortions resulting in aliasing and unintended changes in modulation depth. As a result, spectral ripple thresholds above a certain number are not ordered monotonically along the RPO dimension and thus cannot be considered better or worse spectral resolution than each other, thus undermining correlation measurements. These stimulus distortions are not remediated by changing stimulus phase, indicating these issues cannot be solved by spectrotemporally modulated stimuli. Speech generally has very low-density spectral modulations, leading to questions about the mechanism of correlation between high ripple thresholds and speech recognition. Existing data showing correlations between ripple discrimination and speech recognition include many observations above the aliasing limit. These scores should be treated with caution, and experimenters could benefit by prospectively considering the limitations of the spectral ripple test.

No Benefit of Deriving Cochlear-Implant Maps From Binaural Temporal-Envelope Sensitivity for Speech Perception or Spatial Hearing Under Single-Sided Deafness

OBJECTIVES

This study tested whether speech perception and spatial acuity improved in people with single-sided deafness and a cochlear implant (SSD+CI) when the frequency allocation table (FAT) of the CI was adjusted to optimize frequency-dependent sensitivity to binaural disparities.

DESIGN

Nine SSD+CI listeners with at least 6 months of CI listening experience participated. Individual experimental FATs were created to best match the frequency-to-place mapping across ears using either sensitivity to binaural temporal-envelope disparities or estimated insertion depth. Spatial localization ability was measured, along with speech perception in spatially collocated or separated noise, first with the clinical FATs and then with the experimental FATs acutely and at 2-month intervals for 6 months. Listeners then returned to the clinical FATs and were retested acutely and after 1 month to control for long-term learning effects.

RESULTS

The experimental FAT varied between listeners, differing by an average of 0.15 octaves from the clinical FAT. No significant differences in performance were observed in any of the measures between the experimental FAT after 6 months and the clinical FAT one month later, and no clear relationship was found between the size of the frequency-allocation shift and perceptual changes.

CONCLUSION

Adjusting the FAT to optimize sensitivity to interaural temporal-envelope disparities did not improve localization or speech perception. The clinical frequency-to-place alignment may already be sufficient, given the inherently poor spectral resolution of CIs. Alternatively, other factors, such as temporal misalignment between the two ears, may need to be addressed before any benefits of spectral alignment can be observed.

Upward Shifts in the Internal Representation of Frequency Can Persist Over a 3-Year Period for Cochlear Implant Patients Fit With a Relatively Short Electrode Array

Patients fit with cochlear implants (CIs) commonly indicate at the time of device fitting and for some time after, that the speech signal sounds abnormal. A high pitch or timbre is one component of the abnormal percept. In this project, our aim was to determine whether a number of years of CI use reduced perceived upshifts in frequency spectrum and/or voice fundamental frequency. The participants were five individuals who were deaf in one ear and who had normal hearing in the other ear. The deafened ears had been implanted with a 18.5 mm electrode array which resulted in signal input frequencies being directed to locations in the spiral ganglion (SG) that were between one and two octaves higher than the input frequencies. The patients judged the similarity of a clean signal (a male-voice sentence) presented to their implanted ear and candidate, implant-like, signals presented to their normal-hearing (NH) ear. Matches to implant sound quality were obtained, on average, at 8 months after device activation (see section “Time 1”) and at 35 months after activation (see section “Time 2”). At Time 1, the matches to CI sound quality were characterized, most generally, by upshifts in the frequency spectrum and in voice pitch. At Time 2, for four of the five patients, frequency spectrum values remained elevated. For all five patients F0 values remained elevated. Overall, the data offer little support for the proposition that, for patients fit with shorter electrode arrays, cortical plasticity nudges the cortical representation of the CI voice toward more normal, or less upshifted, frequency values between 8 and 35 months after device activation. Cortical plasticity may be limited when there are large differences between frequencies in the input signal and the locations in the SG stimulated by those frequencies.

The effect of simulated insertion depth differences on the vocal pitches of cochlear implant users

Cochlear implant (CI) users often produce different vocal pitches when using their left versus right CI. One possible explanation for this is that insertion depth differs across the two CIs. The goal of this study was to investigate the role of electrode insertion depth in the production of vocal pitch. Eleven individuals with bilateral CIs used maps simulating differences in insertion depth. Participants produced a sustained vowel and sang Happy Birthday. Approximately half the participants significantly shifted the pitch of their voice in response to different simulated insertion depths. The results suggest insertion depth differences can alter produced vocal pitch.

Predicting Cochlear Implant Electrode Placement Using Monopolar, Three-Point and Four-Point Impedance Measurements

OBJECTIVE

This study aimed to investigate the relationship between cochlear implant (CI) electrode distances to the cochleas inner wall (the modiolus) and electrical impedance measurements made at the CIs electrode contacts. We introduced a protocol for three-point impedances in which we recorded bipolar impedances in response to monopolar stimulation at a neighboring electrode. We aimed to assess the usability of three-point impedances and two existing CI impedance measurement methods (monopolar and four-point impedances) for predicting electrode positioning during CI insertion.

METHODS

Impedances were recorded during stepwise CI electrode array insertions in cadaveric human temporal bones. The positioning of the electrodes with respect to the modiolus was assessed at each step using cone beam computed tomography. Linear mixed regression analysis was performed to assess the relationship between the impedances and electrode-modiolar distances. The experimental results were compared to clinical impedance data and to an existing lumped-element model of an implanted CI.

RESULTS

Three-point and four-point impedances strongly correlated with electrode-modiolar distance. In contrast, monopolar impedances were only minimally affected by changes in electrode positioning with respect to the modiolus. An overall model specificity of 62% was achieved when incorporating all impedance parameters. This specificity could be increased beyond 73% when prior expectations of electrode positioning were incorporated in the model.

CONCLUSION

Three-point and four-point impedances are promising measures to predict electrode-modiolar distance in real-time during CI insertion.

SIGNIFICANCE

This work shows how electrical impedance measurements can be used to predict the CIs electrode positioning in a biologically realistic model.

Insertion Depth and Cochlear Implant Speech Recognition Outcomes: A Comparative Study of 28- and 31.5-mm Lateral Wall Arrays

OBJECTIVES

1) To compare speech recognition outcomes between cochlear implant (CI) recipients of 28- and 31.5-mm lateral wall electrode arrays, and 2) to characterize the relationship between angular insertion depth (AID) and speech recognition.

STUDY DESIGN

Retrospective review.

SETTING

Tertiary academic referral center.

PATIENTS

Seventy-five adult CI recipients of fully inserted 28-mm (n = 28) or 31.5-mm (n = 47) lateral wall arrays listening with a CI-alone device.

INTERVENTIONS

Cochlear implantation with postoperative computed tomography.

MAIN OUTCOME MEASURES

Consonant-nucleus-consonant (CNC) word recognition assessed with the CI-alone at 12 months postactivation.

RESULTS

The mean AID of the most apical electrode contact for the 31.5-mm array recipients was significantly deeper than the 28-mm array recipients (628° vs 571°, p < 0.001). Following 12 months of listening experience, mean CNC word scores were significantly better for recipients of 31.5-mm arrays compared with those implanted with 28-mm arrays (59.5% vs 48.3%, p = 0.004; Cohen's d = 0.70; 95% CI [0.22, 1.18]). There was a significant positive correlation between AID and CNC word scores (r = 0.372, p = 0.001), with a plateau in performance observed around 600°.

CONCLUSIONS

Cochlear implant recipients implanted with a 31.5-mm array experienced better speech recognition than those with a 28-mm array at 12 months postactivation. Deeper insertion of a lateral wall array appears to confer speech recognition benefit up to ∼600°, with a plateau in performance observed thereafter. These data provide preliminary evidence of the insertion depth necessary to optimize speech recognition outcomes for lateral wall electrode arrays among CI-alone users.

Correlation Between Cochlear Length, Insertion Angle, and Tonotopic Mismatch for MED-EL FLEX28 Electrode Arrays

OBJECTIVE

To investigate the relationship between cochlear length, insertion angle, and tonotopic mismatch and to compare the tonotopic mismatches with respect to the spiral ganglion and the organ of Corti.

STUDY DESIGN

Retrospective.

SETTING

Tertiary referral center with cochlear implant program.

PATIENTS

Analyses of patients' computed tomography images after cochlear implant surgery.

INTERVENTION

Cochlear implantation with 28-mm-long straight lateral wall electrode arrays.

MAIN OUTCOME MEASURE

Cochlear length, insertion angle, and insertion depth were assessed using the OTOPLAN software. Tonotopic mismatch for each electrode contact was estimated using the Greenwood (organ of Corti) and the Stakhovskaya (spiral ganglion) maps and compared.

RESULTS

106 cochleae were analyzed. 99% of the electrode arrays were located in the tympanic ramp. The insertion was complete in 96% of cases. The mean cochlear length was 34.5 mm and the mean insertion angle of the apical electrode was 545°. Cochlear length was negatively correlated with the insertion angle of the contacts E1 to E9 (all p < 0.004). The tonotopic mismatch was greater at the organ of Corti than at the spiral ganglion. It was also greater at the organ of Corti in larger cochleae (correlation with mismatch for E1 r = 0.421, p < 0.0001) and in the apical than in the middle and basal regions of the cochlea.

CONCLUSION

Small cochlea size corresponded to higher insertion angle and reduction of tonotopic mismatch on a 28-mm-long straight lateral wall electrode array. Tonotopic mismatch could be minimized preoperatively by choosing electrode arrays according to the individual cochlear morphology and postoperatively by appropriate frequency fitting.

Effects of tonotopic matching and spatial cues on segregation of competing speech in simulations of bilateral cochlear implants

In the clinical fitting of cochlear implants (CIs), the lowest input acoustic frequency is typically much lower than the characteristic frequency associated with the most apical electrode position, due to the limited electrode insertion depth. For bilateral CI users, electrode positions may differ across ears. However, the same acoustic-to-electrode frequency allocation table (FAT) is typically assigned to both ears. As such, bilateral CI users may experience both intra-aural frequency mismatch within each ear and inter-aural mismatch across ears. This inter-aural mismatch may limit the ability of bilateral CI users to take advantage of spatial cues when attempting to segregate competing speech. Adjusting the FAT to tonotopically match the electrode position in each ear (i.e., increasing the low acoustic input frequency) is theorized to reduce this inter-aural mismatch. Unfortunately, this approach may also introduce the loss of acoustic information below the modified input acoustic frequency. The present study explored the trade-off between reduced inter-aural frequency mismatch and low-frequency information loss for segregation of competing speech. Normal-hearing participants were tested while listening to acoustic simulations of bilateral CIs. Speech reception thresholds (SRTs) were measured for target sentences produced by a male talker in the presence of two different male talkers. Masker speech was either co-located with or spatially separated from the target speech. The bilateral CI simulations were produced by 16-channel sinewave vocoders; the simulated insertion depth was fixed in one ear and varied in the other ear, resulting in an inter-aural mismatch of 0, 2, or 6 mm in terms of cochlear place. Two FAT conditions were compared: 1) clinical (200-8000 Hz in both ears), or 2) matched to the simulated insertion depth in each ear. Results showed that SRTs were significantly lower with the matched than with the clinical FAT, regardless of the insertion depth or spatial configuration of the masker speech. The largest improvement in SRTs with the matched FAT was observed when the inter-aural mismatch was largest (6 mm). These results suggest that minimizing inter-aural mismatch with tonotopically matched FATs may benefit bilateral CI users' ability to segregate competing speech despite substantial low-frequency information loss in ears with shallow insertion depths.

Beamforming and Single-Microphone Noise Reduction: Effects on Signal-to-Noise Ratio and Speech Recognition of Bimodal Cochlear Implant Users

We have investigated the effectiveness of three noise-reduction algorithms, namely an adaptive monaural beamformer (MB), a fixed binaural beamformer (BB), and a single-microphone stationary-noise reduction algorithm (SNRA) by assessing the speech reception threshold (SRT) in a group of 15 bimodal cochlear implant users. Speech was presented frontally towards the listener and background noise was established as a homogeneous field of long-term speech-spectrum-shaped (LTSS) noise or 8-talker babble. We pursued four research questions, namely: whether the benefits of beamforming on the SRT differ between LTSS noise and 8-talker babble; whether BB is more effective than MB; whether SNRA improves the SRT in LTSS noise; and whether the SRT benefits of MB and BB are comparable to their improvement of the signal-to-noise ratio (SNR). The results showed that MB and BB significantly improved SRTs by an average of 2.6 dB and 2.9 dB, respectively. These benefits did not statistically differ between noise types or between the two beamformers. By contrast, physical SNR improvements obtained with a manikin revealed substantially greater benefits of BB (6.6 dB) than MB (3.3 dB). SNRA did not significantly affect SRTs per se in omnidirectional microphone settings, nor in combination with MB and BB. We conclude that in the group of bimodal listeners tested, BB had no additional benefits on speech recognition over MB in homogeneous noise, despite the finding that BB had a substantial larger benefit on the SNR than MB. SNRA did not improve speech recognition.

Considerations for Fitting Cochlear Implants Bimodally and to the Single-Sided Deaf

When listening with a cochlear implant through one ear and acoustically through the other, binaural benefits and spatial hearing abilities are generally poorer than in other bilaterally stimulated configurations. With the working hypothesis that binaural neurons require interaurally matched inputs, we review causes for mismatch, their perceptual consequences, and experimental methods for mismatch measurements. The focus is on the three primary interaural dimensions of latency, frequency, and level. Often, the mismatch is not constant, but rather highly stimulus-dependent. We report on mismatch compensation strategies, taking into consideration the specific needs of the respective patient groups. Practical challenges typically faced by audiologists in the proposed fitting procedure are discussed. While improvement in certain areas (e.g., speaker localization) is definitely achievable, a more comprehensive mismatch compensation is a very ambitious endeavor. Even in the hypothetical ideal fitting case, performance is not expected to exceed that of a good bilateral cochlear implant user.

The Perception of Ramped Pulse Shapes in Cochlear Implant Users

The electric stimulation provided by current cochlear implants (CI) is not power efficient. One underlying problem is the poor efficiency by which information from electric pulses is transformed into auditory nerve responses. A novel stimulation paradigm using ramped pulse shapes has recently been proposed to remedy this inefficiency. The primary motivation is a better biophysical fit to spiral ganglion neurons with ramped pulses compared to the rectangular pulses used in most contemporary CIs. Here, we tested the hypotheses that ramped pulses provide more efficient stimulation compared to rectangular pulses and that a rising ramp is more efficient than a declining ramp. Rectangular, rising ramped and declining ramped pulse shapes were compared in terms of charge efficiency and discriminability, and threshold variability in seven CI listeners. The tasks included single-channel threshold detection, loudness-balancing, discrimination of pulse shapes, and threshold measurement across the electrode array. Results showed that reduced charge, but increased peak current amplitudes, was required at threshold and most comfortable levels with ramped pulses relative to rectangular pulses. Furthermore, only one subject could reliably discriminate between equally-loud ramped and rectangular pulses, suggesting variations in neural activation patterns between pulse shapes in that participant. No significant difference was found between rising and declining ramped pulses across all tests. In summary, the present findings show some benefits of charge efficiency with ramped pulses relative to rectangular pulses, that the direction of a ramped slope is of less importance, and that most participants could not perceive a difference between pulse shapes.

Sound quality perception of cochlear implant recipients: low-frequency information and foreign-language effect

OBJECTIVE

This study aimed to adapt a method used in sound quality measurements named CI-MUSHRA (the multiple stimuli with hidden reference and anchor for cochlear implant users) to the Turkish language. The effect of low-frequency information and non-native musical stimuli on sound quality perception was investigated.

DESIGN

Subjects completed the Turkish version of the MUSHRA test, called TR-MUSHRA, and the original CI-MUSHRA test. Participants also completed the Turkish monosyllabic word recognition test and the spectral temporal modulated ripple test (SMRT).

STUDY SAMPLE

19 cochlear implant (CI) users and 16 normal-hearing (NH) adults were included.

RESULTS

CI users demonstrated a lack of ability to detect the sound quality differences between original stimuli and stimuli with omitted low-frequency information up to 600 Hz in both tests. There was no significant main effect of the test version on sound quality ratings for the two groups. No significant correlation was found between mean sound quality scores, SMRT, and speech recognition in quiet and noise conditions.

CONCLUSIONS

Our study suggests that CI users perform poorly in discriminating high-pass filtered musical sounds regardless of the language of the musical stimuli. The TR-MUSHRA can be used as a reliable research tool to evaluate the perceived sound quality.

Interaural Place-of-Stimulation Mismatch Estimates Using CT Scans and Binaural Perception, But Not Pitch, Are Consistent in Cochlear-Implant Users

Bilateral cochlear implants (BI-CIs) or a CI for single-sided deafness (SSD-CI; one normally functioning acoustic ear) can partially restore spatial-hearing abilities, including sound localization and speech understanding in noise. For these populations, however, interaural place-of-stimulation mismatch can occur and thus diminish binaural sensitivity that relies on interaurally frequency-matched neurons. This study examined whether plasticity-reorganization of central neural pathways over time-can compensate for peripheral interaural place mismatch. We hypothesized differential plasticity across two systems: none for binaural processing but adaptation for pitch perception toward frequencies delivered by the specific electrodes. Interaural place mismatch was evaluated in 19 BI-CI and 23 SSD-CI human subjects (both sexes) using binaural processing (interaural-time-difference discrimination with simultaneous bilateral stimulation), pitch perception (pitch ranking for single electrodes or acoustic tones with sequential bilateral stimulation), and physical electrode-location estimates from computed-tomography (CT) scans. On average, CT scans revealed relatively little BI-CI interaural place mismatch (26° insertion-angle mismatch) but a relatively large SSD-CI mismatch, particularly at low frequencies (166° for an electrode tuned to 300 Hz, decreasing to 14° at 7000 Hz). For BI-CI subjects, the three metrics were in agreement because there was little mismatch. For SSD-CI subjects, binaural and CT measurements were in agreement, suggesting little binaural-system plasticity induced by mismatch. The pitch measurements disagreed with binaural and CT measurements, suggesting place-pitch plasticity or a procedural bias. These results suggest that reducing interaural place mismatch and potentially improving binaural processing by reprogramming the CI frequency allocation would be better done using CT-scan than pitch information.SIGNIFICANCE STATEMENT Electrode-array placement for cochlear implants (bionic prostheses that partially restore hearing) does not explicitly align neural representations of frequency information. The resulting interaural place-of-stimulation mismatch can diminish spatial-hearing abilities. In this study, adults with two cochlear implants showed reasonable interaural alignment, whereas those with one cochlear implant but normal hearing in the other ear often showed mismatch. In cases of mismatch, binaural sensitivity was best when the same cochlear locations were stimulated in both ears, suggesting that binaural brainstem pathways do not experience plasticity to compensate for mismatch. In contrast, interaurally pitch-matched electrodes deviated from cochlear-location estimates and did not optimize binaural sensitivity. Clinical correction of interaural place mismatch using binaural or computed-tomography (but not pitch) information may improve spatial-hearing benefits.

Adults with cochlear implants can use prosody to determine the clausal structure of spoken sentences

Speech prosody, including pitch contour, word stress, pauses, and vowel lengthening, can aid the detection of the clausal structure of a multi-clause sentence and this, in turn, can help listeners determine the meaning. However, for cochlear implant (CI) users, the reduced acoustic richness of the signal raises the question of whether CI users may have difficulty using sentence prosody to detect syntactic clause boundaries within sentences or whether this ability is rescued by the redundancy of the prosodic features that normally co-occur at clause boundaries. Twenty-two CI users, ranging in age from 19 to 77 years old, recalled three types of sentences: sentences in which the prosodic pattern was appropriate to the location of a clause boundary within the sentence (congruent prosody), sentences with reduced prosodic information, or sentences in which the location of the clause boundary and the prosodic marking of a clause boundary were placed in conflict. The results showed the presence of congruent prosody to be associated with superior sentence recall and a reduced processing effort as indexed by the pupil dilation. The individual differences in a standard test of word recognition (consonant-nucleus-consonant score) were related to the recall accuracy as well as the processing effort. The outcomes are discussed in terms of the redundancy of the prosodic features, which normally accompany a clause boundary and processing effort.

Valid Acoustic Models of Cochlear Implants: One Size Does Not Fit All

HYPOTHESIS

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

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.

Reducing interaural tonotopic mismatch preserves binaural unmasking in cochlear implant simulations of single-sided deafness

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.

On the Accuracy of Clinical Insertion Angle Predictions With a Surgical Planning Platform for Cochlear Implantation

HYPOTHESIS

Various studies over the last few decades have shown that the cochlea is not a uniform structure, but that its size and shape may vary quite substantially in between subjects. The surgical planning platform enables the user to quickly approximate the size of a cochlea within clinical imaging data by measuring the basal cochlear diameters A and B. It also allows for contact specific insertion angle predictions for MED-EL cochlear implant electrode arrays based on this individual anatomy approximation. The proposed, retrospective study was performed to evaluate the accuracy of these predictions.

METHODS

Preoperative CBCT scans of N = 91 MED-EL cochlear implant patients with different types of FLEX electrode arrays (flexible, thin, and straight arrays) were evaluated using a planning module. Both the initial version (based on an equation proposed by Escudé et al.) as well as a novel, recently proposed approach (called elliptic-circular approximation) was employed. All predictions were then compared to the actual insertion angles which were derived from postoperative CBCT images of the same patient.

RESULTS

Most prediction deviations of the investigated cases stayed below 45deg for all electrode arrays and both prediction methods. In general, prediction deviations increased from base to apex were found to be larger for longer electrode arrays. Hardly any significant differences between the two prediction methods were observed. However, particularly large deviations were found for the Escudé method and could be substantially deceased with the updated elliptic-circular approximation approach.

CONCLUSIONS

The new platform version with its updated prediction module allows to reliably predict insertion angles even for cochlear anatomies with slightly unusual features and shapes.

Precise evaluation of the postoperative cochlear duct length by flat-panel volume computed tomography - Application of secondary reconstructions

OBJECTIVE

There is still a lack in precise postoperative evaluation of the cochlea because of strong artifacts. This study aimed to improve accuracy of postoperative two-turn (2TL) and cochlear duct length (CDL) measurements by applying flat-panel volume computed tomography (fpVCT), secondary reconstruction (fpVCT_SECO) and three-dimensional curved multiplanar reconstruction.

METHODS

First, 10 temporal bone specimens with or without electrode were measured in multi-slice computed tomography (MSCT), fpVCT and fpVCT_SECO and compared to high-resolution micro-CT scans. Later, pre- and postoperative scans of 10 patients were analyzed in a clinical setting.

RESULTS

Concerning 2TL, no statistically significant difference was observed between implanted fpVCT_SECO and nonimplanted micro-CT in 10 temporal bone specimens. In contrast, there was a significant discrepancy for CDL (difference: -0.7 mm, P = 0.004). Nevertheless, there were no clinically unacceptable errors (±1.5 mm). These results could be confirmed in a clinical setting. Using fpVCT_SECO, CDL was slightly underestimated postoperatively (difference: -0.5 mm, P = 0.002) but without any clinically unacceptable errors.

CONCLUSION

fpVCT_SECO can be successfully applied for a precise measurement of the cochlear lengths pre- and postoperatively. However, users must be aware of a slight systematic underestimation of CDL postoperatively. These results may help to refine electrode selection and frequency mapping.

The Use of Clinically Measurable Cochlear Parameters in Cochlear Implant Surgery as Indicators for Size, Shape, and Orientation of the Scala Tympani

OBJECTIVES

(1) To assess variations of the human intracochlear anatomy and quantify factors which might be relevant for cochlear implantation (CI) regarding surgical technique and electrode design. (2) Search for correlations of these factors with clinically assessable measurements.

DESIGN

Human temporal bone study with micro computed tomography (μCT) data and analysis of intracochlear geometrical variations: μCT data of 15 fresh human temporal bones was generated, and the intracochlear lumina scala tympani (ST) and scala vestibuli were manually segmented using custom software specifically designed for accurate cochlear segmentation. The corresponding datasets were processed yielding 15 detailed, three-dimensional cochlear models which were investigated in terms of the scalae height, cross-sectional size, and rotation as well as the interrelation of these factors and correlations to others.

RESULTS

The greatest anatomical variability was observed within the round window region of the cochlea (basal 45°), especially regarding the cross-sectional size of the ST and its orientation relative to the scala vestibuli, which were found to be correlated (p < 0.001). The cross-sectional height of the ST changes substantially for both increasing cochlear angles and lateral wall distances. Even small cochleae were found to contain enough space for all commercially available CI arrays. Significant correlations of individual intracochlear parameters to clinically assessable ones were found despite the small sample size.

CONCLUSION

While there is generally enough space within the ST for CI, strong intracochlear anatomical variations could be observed highlighting the relevance of both soft surgical technique as well as a highly flexible and self-adapting cochlear implant electrode array design. Cochlear dimensions (especially at the round window) could potentially be used to indicate surgically challenging anatomies.

Binaural Pitch Fusion: Binaural Pitch Averaging in Cochlear Implant Users With Broad Binaural Fusion

OBJECTIVES

Individuals who use hearing aids (HAs) or cochlear implants (CIs) can experience broad binaural pitch fusion, such that sounds differing in pitch by as much as 3 to 4 octaves are perceptually integrated across ears. Previously, it was shown in HA users that the fused pitch is a weighted average of the two monaural pitches, ranging from equal weighting to dominance by the lower pitch. The goal of this study was to systematically measure the fused pitches in adult CI users, and determine whether CI users experience similar pitch averaging effects as observed in HA users.

DESIGN

Twelve adult CI users (Cochlear Ltd, Sydney, Australia) participated in this study: six bimodal CI users, who wear a CI with a contralateral HA, and six bilateral CI users. Stimuli to HA ears were acoustic pure tones, and stimuli to CI ears were biphasic pulse trains delivered to individual electrodes. Fusion ranges, the ranges of frequencies/electrodes in the comparison ear that were fused with a single electrode (electrode 22, 18, 12, or 6) in the reference ear, were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus. Once the fusion ranges were measured, the fused binaural pitch of a reference-pair stimulus combination was measured by finding a pitch match to monaural comparison stimuli presented to the paired stimulus ear.

RESULTS

Fusion pitch weighting in CI users varied depending on the pitch difference of the reference-pair stimulus combination, with equal pitch averaging occurring for stimuli closer in pitch and lower pitch dominance occurring for stimuli farther apart in pitch. The averaging region was typically 0.5 to 2.3 octaves around the reference for bimodal CI users and 0.4 to 1.5 octaves for bilateral CI users. In some cases, a bias in the averaging region was observed toward the ear with greater stimulus variability.

CONCLUSIONS

Fusion pitch weighting effects in CI users were similar to those observed previously in HA users. However, CI users showed greater inter-subject variability in both pitch averaging ranges and bias effects. These findings suggest that binaural pitch averaging could be a common underlying mechanism in hearing-impaired listeners.

Enhancement of interaural level differences for bilateral cochlear implant users

Bilateral cochlear implant (BiCI) users do not localize sounds as well as normal hearing (NH) listeners do. NH listeners rely on two binaural cues to localize sounds in the horizontal plane, namely interaural level differences (ILDs) and interaural time differences. BiCI systems, however, convey these cues poorly. In this work, we investigated two methods to improve the coding of ILDs in BiCIs. The first method enhances ILDs by applying an artificial current-versus-angle function to the clinical levels delivered by the basal electrodes of the CI contralateral to the target sound. The second method enhances ILDs by using bilaterally linked N-of-M band selection. Results indicate that the participants were able to discriminate the location of the sound more accurately at narrow azimuths when the ILD enhancement was applied, compared to when they were using natural ILDs. Also, the results show that linking the band selection had a positive effect on left/right discrimination accuracy at larger azimuths for three out of the 10 tested participants, when compared to unlinked band selection. Based on these results, we conclude that ILD enhancement besides linked N-of-M band selection can help some BiCI participants to discriminate sound sources on the frontal horizontal plane.

Effects of Spectral Resolution and Frequency Mismatch on Speech Understanding and Spatial Release From Masking in Simulated Bilateral Cochlear Implants

OBJECTIVES

Due to interaural frequency mismatch, bilateral cochlear-implant (CI) users may be less able to take advantage of binaural cues that normal-hearing (NH) listeners use for spatial hearing, such as interaural time differences and interaural level differences. As such, bilateral CI users have difficulty segregating competing speech even when the target and competing talkers are spatially separated. The goal of this study was to evaluate the effects of spectral resolution, tonotopic mismatch (the frequency mismatch between the acoustic center frequency assigned to CI electrode within an implanted ear relative to the expected spiral ganglion characteristic frequency), and interaural mismatch (differences in the degree of tonotopic mismatch in each ear) on speech understanding and spatial release from masking (SRM) in the presence of competing talkers in NH subjects listening to bilateral vocoder simulations.

DESIGN

During testing, both target and masker speech were presented in five-word sentences that had the same syntax but were not necessarily meaningful. The sentences were composed of five categories in fixed order (Name, Verb, Number, Color, and Clothes), each of which had 10 items, such that multiple sentences could be generated by randomly selecting a word from each category. Speech reception thresholds (SRTs) for the target sentence presented in competing speech maskers were measured. The target speech was delivered to both ears and the two speech maskers were delivered to (1) both ears (diotic masker), or (2) different ears (dichotic masker: one delivered to the left ear and the other delivered to the right ear). Stimuli included the unprocessed speech and four 16-channel sine-vocoder simulations with different interaural mismatch (0, 1, and 2 mm). SRM was calculated as the difference between the diotic and dichotic listening conditions.

RESULTS

With unprocessed speech, SRTs were 0.3 and -18.0 dB for the diotic and dichotic maskers, respectively. For the spectrally degraded speech with mild tonotopic mismatch and no interaural mismatch, SRTs were 5.6 and -2.0 dB for the diotic and dichotic maskers, respectively. When the tonotopic mismatch increased in both ears, SRTs worsened to 8.9 and 2.4 dB for the diotic and dichotic maskers, respectively. When the two ears had different tonotopic mismatch (e.g., there was interaural mismatch), the performance drop in SRTs was much larger for the dichotic than for the diotic masker. The largest SRM was observed with unprocessed speech (18.3 dB). With the CI simulations, SRM was significantly reduced to 7.6 dB even with mild tonotopic mismatch but no interaural mismatch; SRM was further reduced with increasing interaural mismatch.

CONCLUSIONS

The results demonstrate that frequency resolution, tonotopic mismatch, and interaural mismatch have differential effects on speech understanding and SRM in simulation of bilateral CIs. Minimizing interaural mismatch may be critical to optimize binaural benefits and improve CI performance for competing speech, a typical listening environment. SRM (the difference in SRTs between diotic and dichotic maskers) may be a useful clinical tool to assess interaural frequency mismatch in bilateral CI users and to evaluate the benefits of optimization methods that minimize interaural mismatch.

Frequency-to-Place Mismatch: Characterizing Variability and the Influence on Speech Perception Outcomes in Cochlear Implant Recipients

OBJECTIVES

The spatial position of a cochlear implant (CI) electrode array affects the spectral cues provided to the recipient. Differences in cochlear size and array length lead to substantial variability in angular insertion depth (AID) across and within array types. For CI-alone users, the variability in AID results in varying degrees of frequency-to-place mismatch between the default electric frequency filters and cochlear place of stimulation. For electric-acoustic stimulation (EAS) users, default electric frequency filters also vary as a function of residual acoustic hearing in the implanted ear. The present study aimed to (1) investigate variability in AID associated with lateral wall arrays, (2) determine the subsequent frequency-to-place mismatch for CI-alone and EAS users mapped with default frequency filters, and (3) examine the relationship between early speech perception for CI-alone users and two aspects of electrode position: frequency-to-place mismatch and angular separation between neighboring contacts, a metric associated with spectral selectivity at the periphery.

DESIGN

One hundred one adult CI recipients (111 ears) with MED-EL Flex24 (24 mm), Flex28 (28 mm), and FlexSOFT/Standard (31.5 mm) arrays underwent postoperative computed tomography to determine AID. A subsequent comparison was made between AID, predicted spiral ganglion place frequencies, and the default frequency filters for CI-alone (n = 84) and EAS users (n = 27). For CI-alone users with complete insertions who listened with maps fit with the default frequency filters (n = 48), frequency-to-place mismatch was quantified at 1500 Hz and angular separation between neighboring contacts was determined for electrodes in the 1 to 2 kHz region. Multiple linear regression was used to examine how frequency-to-place mismatch and angular separation of contacts influence consonant-nucleus-consonant (CNC) scores through 6 months postactivation.

RESULTS

For CI recipients with complete insertions (n = 106, 95.5%), the AID (mean ± standard deviation) of the most apical contact was 428° ± 34.3° for Flex24 (n = 11), 558° ± 65.4° for Flex28 (n = 48), and 636° ± 42.9° for FlexSOFT/Standard (n = 47) arrays. For CI-alone users, default frequency filters aligned closely with the spiral ganglion map for deeply inserted lateral wall arrays. For EAS users, default frequency filters produced a range of mismatches; absolute deviations of ≤ 6 semitones occurred in only 37% of cases. Participants with shallow insertions and minimal or no residual hearing experienced the greatest mismatch. For CI-alone users, both smaller frequency-to-place mismatch and greater angular separation between contacts were associated with better CNC scores during the initial 6 months of device use.

CONCLUSIONS

There is significant variability in frequency-to-place mismatch among CI-alone and EAS users with default frequency filters, even between individuals implanted with the same array. When using default frequency filters, mismatch can be minimized with longer lateral wall arrays and insertion depths that meet the edge frequency associated with residual hearing for CI-alone and EAS users, respectively. Smaller degrees of frequency-to-place mismatch and decreased peripheral masking due to more widely spaced contacts may independently support better speech perception with longer lateral wall arrays in CI-alone users.

Sound Localization Bias and Error in Bimodal Listeners Improve Instantaneously When the Device Delay Mismatch Is Reduced

Users of a cochlear implant (CI) in one ear, who are provided with a hearing aid (HA) in the contralateral ear, so-called bimodal listeners, are typically affected by a constant and relatively large interaural time delay offset due to differences in signal processing and differences in stimulation. For HA stimulation, the cochlear travelling wave delay is added to the processing delay, while for CI stimulation, the auditory nerve fibers are stimulated directly. In case of MED-EL CI systems in combination with different HA types, the CI stimulation precedes the acoustic HA stimulation by 3 to 10 ms. A self-designed, battery-powered, portable, and programmable delay line was applied to the CI to reduce the device delay mismatch in nine bimodal listeners. We used an A-B-B-A test design and determined if sound source localization improves when the device delay mismatch is reduced by delaying the CI stimulation by the HA processing delay (τ_HA). Results revealed that every subject in our group of nine bimodal listeners benefited from the approach. The root-mean-square error of sound localization improved significantly from 52.6° to 37.9°. The signed bias also improved significantly from 25.2° to 10.5°, with positive values indicating a bias toward the CI. Furthermore, two other delay values (τ_HA –1 ms and τ_HA +1 ms) were applied, and with the latter value, the signed bias was further reduced in some test subjects. We conclude that sound source localization accuracy in bimodal listeners improves instantaneously and sustainably when the device delay mismatch is reduced.

Binaural Optimization of Cochlear Implants: Discarding Frequency Content Without Sacrificing Head-Shadow Benefit

OBJECTIVES

Single-sided deafness cochlear-implant (SSD-CI) listeners and bilateral cochlear-implant (BI-CI) listeners gain near-normal levels of head-shadow benefit but limited binaural benefits. One possible reason for these limited binaural benefits is that cochlear places of stimulation tend to be mismatched between the ears. SSD-CI and BI-CI patients might benefit from a binaural fitting that reallocates frequencies to reduce interaural place mismatch. However, this approach could reduce monaural speech recognition and head-shadow benefit by excluding low- or high-frequency information from one ear. This study examined how much frequency information can be excluded from a CI signal in the poorer-hearing ear without reducing head-shadow benefits and how these outcomes are influenced by interaural asymmetry in monaural speech recognition.

DESIGN

Speech-recognition thresholds for sentences in speech-shaped noise were measured for 6 adult SSD-CI listeners, 12 BI-CI listeners, and 9 normal-hearing listeners presented with vocoder simulations. Stimuli were presented using nonindividualized in-the-ear or behind-the-ear head-related impulse-response simulations with speech presented from a 70° azimuth (poorer-hearing side) and noise from 70° (better-hearing side), thereby yielding a better signal-to-noise ratio (SNR) at the poorer-hearing ear. Head-shadow benefit was computed as the improvement in bilateral speech-recognition thresholds gained from enabling the CI in the poorer-hearing, better-SNR ear. High- or low-pass filtering was systematically applied to the head-related impulse-response-filtered stimuli presented to the poorer-hearing ear. For the SSD-CI listeners and SSD-vocoder simulations, only high-pass filtering was applied, because the CI frequency allocation would never need to be adjusted downward to frequency-match the ears. For the BI-CI listeners and BI-vocoder simulations, both low and high pass filtering were applied. The normal-hearing listeners were tested with two levels of performance to examine the effect of interaural asymmetry in monaural speech recognition (vocoder synthesis-filter slopes: 5 or 20 dB/octave).

RESULTS

Mean head-shadow benefit was smaller for the SSD-CI listeners (~7 dB) than for the BI-CI listeners (~14 dB). For SSD-CI listeners, frequencies <1236 Hz could be excluded; for BI-CI listeners, frequencies <886 or >3814 Hz could be excluded from the poorer-hearing ear without reducing head-shadow benefit. Bilateral performance showed greater immunity to filtering than monaural performance, with gradual changes in performance as a function of filter cutoff. Real and vocoder-simulated CI users with larger interaural asymmetry in monaural performance had less head-shadow benefit.

CONCLUSIONS

The "exclusion frequency" ranges that could be removed without diminishing head-shadow benefit are interpreted in terms of low importance in the speech intelligibility index and a small head-shadow magnitude at low frequencies. Although groups and individuals with greater performance asymmetry gained less head-shadow benefit, the magnitudes of these factors did not predict the exclusion frequency range. Overall, these data suggest that for many SSD-CI and BI-CI listeners, the frequency allocation for the poorer-ear CI can be shifted substantially without sacrificing head-shadow benefit, at least for energetic maskers. Considering the two ears together as a single system may allow greater flexibility in discarding redundant frequency content from a CI in one ear when considering bilateral programming solutions aimed at reducing interaural frequency mismatch.

The smaller the frequency-to-place mismatch the better the hearing outcomes in cochlear implant recipients?

OBJECTIVE

To investigate the effect of frequency-to-place mismatch, i.e. the mismatch between the tonotopic frequency map in the cochlea and the frequency band that is assigned to an electrode contact of a cochlear implant (CI) at the same cochlear location on speech perception outcomes, using postoperative CT images.

STUDY DESIGN

Retrospective observational single-centre study.

METHODS

Retrospective pre- and postoperative clinical CT data of 39 CI recipients with normal cochlear anatomy were analysed in an otological surgical planning software. The tonotopic frequency at each electrode position was estimated using the Greenwood function. For each patient, frequency-to-place mismatch between the tonotopic frequency and the fitted centre frequency for each electrode contact was calculated. The influence of frequency-to-place mismatch on speech perception in noise at 6 and 12 months after CI activation was studied.

RESULTS

A significant linear correlation was found between the frequency-to-place mismatch and speech perception in noise 6 months after cochlear implantation (p < 0.05). The smaller the frequency-to-place mismatch, the better the initial speech perception in noise results of the CI recipients. The significant effect disappeared after 12 months CI experience.

CONCLUSION

The study findings support the idea of minimizing the frequency-to-place mismatch in CI recipients in order to pursue better initial speech perception in noise. Further research is needed to investigate the prospect of tonotopic fitting strategies based upon postoperative CT images of the exact locations of the electrode contacts.

Comparison of Speech Recognition With an Organ of Corti Versus Spiral Ganglion Frequency-to-Place Function in Place-Based Mapping of Cochlear Implant and Electric-Acoustic Stimulation Devices

OBJECTIVE

To compare acute speech recognition with a cochlear implant (CI) alone or electric-acoustic stimulation (EAS) device for place-based maps calculated with an organ of Corti (OC) versus a spiral ganglion (SG) frequency-to-place function.

PATIENTS

Eleven adult CI recipients of a lateral wall electrode array.

INTERVENTION

Postoperative imaging was used to derive place-based maps calculated with an OC versus SG function.

MAIN OUTCOME MEASURE

Phoneme recognition was evaluated at initial activation with consonant-nucleus-consonant (CNC) words presented using an OC versus a SG place-based map.

RESULTS

For the 9 CI-alone users, there was a nonsignificant trend for better acute phoneme recognition with the SG map (mean 18 RAUs) than the OC map (mean 9 RAUs; p = 0.071, 95% CI [≤-1.2]). When including the 2 EAS users in the analysis, performance was significantly better with the SG map (mean 21 RAUs) than the OC map (mean 7 RAUs; p = 0.019, 95% CI [≤-6.2]).

CONCLUSIONS

Better phoneme recognition with the SG frequency-to-place function could indicate more natural tonotopic alignment of information compared with the OC place-based map.A prospective, randomized investigation is currently underway to assess longitudinal outcomes with place-based mapping in CI-alone and EAS devices using the SG frequency-to-place function.

An Approach for Individualized Cochlear Frequency Mapping Determined from 3D Synchrotron Radiation Phase-Contrast Imaging

OBJECTIVE

Cochlear implants are traditionally programmed to stimulate according to a generalized frequency map, where individual anatomic variability is not considered when selecting the centre frequency of stimulation of each implant electrode. However, high variability in cochlear size and spatial frequency distributions exist among individuals. Generalized cochlear implant frequency maps can result in large pitch perception errors and reduced hearing outcomes for cochlear implant recipients. The objective of this work was to develop an individualized frequency mapping technique for the human cochlea to allow for patient-specific cochlear implant stimulation.

METHODS

Ten cadaveric human cochleae were scanned using synchrotron radiation phase-contrast imaging (SR-PCI) combined with computed tomography (CT). For each cochlea, ground truth angle-frequency measurements were obtained in three-dimensions using the SR-PCI CT data. Using an approach designed to minimize perceptual error in frequency estimation, an individualized frequency function was determined to relate angular depth to frequency within the cochlea.

RESULTS

The individualized frequency mapping function significantly reduced pitch errors in comparison to the current gold standard generalized approach.

CONCLUSION AND SIGNIFICANCE

This paper presents for the first time a cochlear frequency map which can be individualized using only the angular length of cochleae. This approach can be applied in the clinical setting and has the potential to revolutionize cochlear implant programming for patients worldwide.

Comparing Cochlear Duct Lengths Between CT and MR Images Using an Otological Surgical Planning Software

OBJECTIVE

We sought to examine the intra- and interobserver variability in measuring the cochlear duct length (CDL) from magnetic resonance imaging (MRI) images versus computed tomography (CT) images using an otological surgical planning software that uses measurements of the basal turn diameter and cochlear width to estimate the CDL.

PATIENTS

Twenty-one adult cochlear implant patients with preoperative MRI and CT images.

INTERVENTION

Three fellowship-trained neurotologists served as the raters in the study. One rater measured the CDL using preoperative CT scans to serve as the benchmark. Two of the raters measured the CDL on preoperative MRI scans. One rater also remeasured the scans using MRI images after a period of 1 week to assess intraobserver variability.

MAIN OUTCOME MEASURE

Intraclass correlational coefficients were calculated to assess for intra- and interobserver agreement.

RESULTS

The mean CDL measured from the CT scans was 32.7 ± 2.0 mm (range 29.4 - 37.6 mm). The mean difference between the raters when measuring the CDL using MRI scans was -0.15 ± 2.1 mm (range -3.2 to 4.3 mm). The intraclass correlational coefficients for inter-rater reliability of CDL determination using MRI scans was judged as fair to excellent (0.68; 95% CI 0.41-0.84). The intrarater reliability of CDL determination using MRI scans was judged at fair to excellent (0.73; 95% CI 0.491-0.866).

CONCLUSION

We demonstrate that a validated otological surgical planning software for estimating the CDL preoperatively had comparable performance using MRI scans versus the gold-standard CT scans.

Audiologic Outcomes of Cochlear Implantation in Cochlear Malformations: A Comparative Analysis of Lateral Wall and Perimodiolar Electrode Arrays

OBJECTIVE

Cochlear implantation in children with inner ear malformations has been shown to be beneficial. The aims of this study are to evaluate open set word recognition outcomes among children with cochlear implants who have cochlear malformations, and to further assess if either the lateral wall (LW) or perimodiolar (PM) electrode arrays confer any performance outcome advantages.

STUDY DESIGN

Retrospective case series.

SETTING

Tertiary referral center.

PATIENTS

Pediatric cochlear implant recipients with cochlear malformations who were implanted at our institution within the last 10 years and had speech perception scores were eligible for inclusion in the study. Potential participants were excluded if they had less than 1 year of listening experience with the cochlear implant or suspected cochlear nerve deficiency.

INTERVENTION

None.

MAIN OUTCOME MEASURE

Most recent consonant-nucleus-consonant word score.

RESULTS

ANOVA analysis demonstrated that the type of cochlear malformation was significantly associated with speech perception outcome (p = 0.006). Those with IP2 malformations had significantly better word recognition outcomes than the remaining cochlear malformations. Array type (LW or PM) was not associated with better word recognition outcomes in long-term follow-up of patients with IP2 malformations (p = 0.13).

CONCLUSIONS

In children who have cochlear malformations, cochlear implantation results in varying word recognition outcomes based on the type of malformation. While the participants in this study demonstrated postoperative open set word recognition skills, those with IP2 malformations demonstrated the most benefit. Electrode type was not found to significantly impact outcomes in this cohort.

Individual Variability in Recalibrating to Spectrally Shifted Speech: Implications for Cochlear Implants

OBJECTIVES

Cochlear implant (CI) recipients are at a severe disadvantage compared with normal-hearing listeners in distinguishing consonants that differ by place of articulation because the key relevant spectral differences are degraded by the implant. One component of that degradation is the upward shifting of spectral energy that occurs with a shallow insertion depth of a CI. The present study aimed to systematically measure the effects of spectral shifting on word recognition and phoneme categorization by specifically controlling the amount of shifting and using stimuli whose identification specifically depends on perceiving frequency cues. We hypothesized that listeners would be biased toward perceiving phonemes that contain higher-frequency components because of the upward frequency shift and that intelligibility would decrease as spectral shifting increased.

DESIGN

Normal-hearing listeners (n = 15) heard sine wave-vocoded speech with simulated upward frequency shifts of 0, 2, 4, and 6 mm of cochlear space to simulate shallow CI insertion depth. Stimuli included monosyllabic words and /b/-/d/ and /∫/-/s/ continua that varied systematically by formant frequency transitions or frication noise spectral peaks, respectively. Recalibration to spectral shifting was operationally defined as shifting perceptual acoustic-phonetic mapping commensurate with the spectral shift. In other words, adjusting frequency expectations for both phonemes upward so that there is still a perceptual distinction, rather than hearing all upward-shifted phonemes as the higher-frequency member of the pair.

RESULTS

For moderate amounts of spectral shifting, group data suggested a general "halfway" recalibration to spectral shifting, but individual data suggested a notably different conclusion: half of the listeners were able to recalibrate fully, while the other halves of the listeners were utterly unable to categorize shifted speech with any reliability. There were no participants who demonstrated a pattern intermediate to these two extremes. Intelligibility of words decreased with greater amounts of spectral shifting, also showing loose clusters of better- and poorer-performing listeners. Phonetic analysis of word errors revealed certain cues were more susceptible to being compromised due to a frequency shift (place and manner of articulation), while voicing was robust to spectral shifting.

CONCLUSIONS

Shifting the frequency spectrum of speech has systematic effects that are in line with known properties of speech acoustics, but the ensuing difficulties cannot be predicted based on tonotopic mismatch alone. Difficulties are subject to substantial individual differences in the capacity to adjust acoustic-phonetic mapping. These results help to explain why speech recognition in CI listeners cannot be fully predicted by peripheral factors like electrode placement and spectral resolution; even among listeners with functionally equivalent auditory input, there is an additional factor of simply being able or unable to flexibly adjust acoustic-phonetic mapping. This individual variability could motivate precise treatment approaches guided by an individual's relative reliance on wideband frequency representation (even if it is mismatched) or limited frequency coverage whose tonotopy is preserved.

Dichotic listening performance with cochlear-implant simulations of ear asymmetry is consistent with difficulty ignoring clearer speech

There are an increasing number of bilateral and single-sided-deafness cochlear-implant (CI) users who hope to achieve improved spatial-hearing abilities through access to sound in both ears. It is, however, unclear how speech is processed when inputs are functionally asymmetrical, which may have an impact on spatial-hearing abilities. Therefore, functionally asymmetrical hearing was controlled and parametrically manipulated using a channel vocoder as a CI simulation. In Experiment 1, normal-hearing (NH) listeners performed a dichotic listening task (i.e., selective attention to one ear, ignoring the other) using asymmetrical signal degradation. Spectral resolution varied independently in each ear (4, 8, 16 channels, and unprocessed control). Performance decreased with decreasing resolution in the target ear and increasing resolution in the interferer ear. In Experiment 2, these results were replicated using a divided attention task (attend to both ears, report one after sentence completion) in both NH and bilateral CI listeners, although overall performance was lower than in Experiment 1. In Experiment 3, frequency-to-place mismatch simulated shallow CI insertion depths (0, 3, 6-mm shifts, and unprocessed control). Performance mostly decreased with increasing shift in the target ear and decreasing shift in the interferer ear; however, performance nonmonotonicities occurred. The worst performance occurred when the shift matched across ears, suggesting that pitch similarity increases difficulty. The results show that it is more difficult to attend an ear that is relatively degraded or distorted, which may set spatial-hearing limitations for CI users when trying to attend to a target in complex auditory scenes.

Residual Hair Cell Responses in Electric-Acoustic Stimulation Cochlear Implant Users with Complete Loss of Acoustic Hearing After Implantation

Changes in cochlear implant (CI) design and surgical techniques have enabled the preservation of residual acoustic hearing in the implanted ear. While most Nucleus Hybrid L24 CI users retain significant acoustic hearing years after surgery, 6-17 % experience a complete loss of acoustic hearing (Roland et al. Laryngoscope. 126(1):175-81. (2016), Laryngoscope. 128(8):1939-1945 (2018); Scheperle et al. Hear Res. 350:45-57 (2017)). Electrocochleography (ECoG) enables non-invasive monitoring of peripheral auditory function and may provide insight into the pathophysiology of hearing loss. The ECoG response is evoked using an acoustic stimulus and includes contributions from the hair cells (cochlear microphonic-CM) as well as the auditory nerve (auditory nerve neurophonic-ANN). Seven Hybrid L24 CI users with complete loss of residual hearing months after surgery underwent ECoG measures before and after loss of hearing. While significant reductions in CMs were evident after hearing loss, all participants had measurable CMs despite having no measurable acoustic hearing. None retained measurable ANNs. Given histological data suggesting stable hair cell and neural counts after hearing loss (e.g., Quesnel et al. Hear Res. 333:225-234. (2016)), the loss of ECoG and audiometric hearing may reflect reduced synaptic input. This is consistent with the theory that residual CM responses coupled with little to no ANN responses reflect a "disconnect" between hair cells and auditory nerve fibers (Fontenot et al. Ear Hear. 40(3):577-591. 2019). This "disconnection" may prevent proper encoding of auditory stimulation at higher auditory pathways, leading to a lack of audiometric responses, even in the presence of viable cochlear hair cells.

Melodic interval perception with acoustic and electric hearing in bimodal and single-sided deaf cochlear implant listeners

Two notes sounded sequentially elicit melodic intervals and contours that form the basis of melody. Many previous studies have characterized pitch perception in cochlear implant (CI) users to be poor which may be due to the limited spectro-temporal resolution and/or spectral warping with electric hearing compared to acoustic hearing (AH). Poor pitch perception in CIs has been shown to distort melodic interval perception. To characterize this interval distortion, we recruited CI users with either normal (single sided deafness, SSD) or limited (bimodal) AH in the non-implanted ear. The contralateral AH allowed for a stable reference with which to compare melodic interval perception in the CI ear, within the same listener. Melodic interval perception was compared across acoustic and electric hearing in 9 CI listeners (4 bimodal and 5 SSD). Participants were asked to rank the size of a probe interval presented to the CI ear to a reference interval presented to the contralateral AH ear using a method of constant stimuli. Ipsilateral interval ranking was also measured within the AH ear to ensure that listeners understood the task and that interval ranking was stable and accurate within AH. Stimuli were delivered to the AH ear via headphones and to the CI ear via direct audio input (DAI) to participants' clinical processors. During testing, a reference and probe interval was presented and participants indicated which was larger. Ten comparisons for each reference-probe combination were presented. Psychometric functions were fit to the data to determine the probe interval size that matched the reference interval. Across all AH reference intervals, the mean matched CI interval was 1.74 times larger than the AH reference. However, there was great inter-subject variability. For some participants, CI interval distortion varied across different reference AH intervals; for others, CI interval distortion was constant. Within the AH ear, ipsilateral interval ranking was accurate, ensuring that participants understood the task. No significant differences in the patterns of results were observed between bimodal and SSD CI users. The present data show that much larger intervals were needed with the CI to match contralateral AH reference intervals. As such, input melodic patterns are likely to be perceived as frequency compressed and/or warped with electric hearing, with less variation among notes in the pattern. The high inter-subject variability in CI interval distortion suggests that CI signal processing should be optimized for individual CI users.

A cochlear scaling model for accurate anatomy evaluation and frequency allocation in cochlear implantation

The human cochlea has a highly individual microanatomy. Cochlear implantation therefore requires an evaluation of the individual cochlear anatomy to reduce surgical risk of implantation trauma. However, in-vivo cochlear imaging is limited in resolution. To overcome this issue, cochlear models based on exact anatomical data have been developed. These models can be fitted to the limited parameters available from clinical imaging to provide a prediction of the precise cochlear microanatomy. Recently, models have become available with improved precision that additionally allow predicting the 3D form of an individual cochlea. The present study has further improved the precision of modelling by incorporating microscopic details of a large set of 108 human cochleae from corrosion casts. The new model provides a more flexible geometric shape that can better predict local variations like vertical dips and jumps and provides an approximation of frequency allocation in the cochlea. The outcome of this and five other models have been quantified (validated) on an independent set of 20 µCTs of human cochleae. The new model outperformed previous models and is freely available for download and use.

Assessment of Frequency-Place Mismatch by Flat-Panel CT and Correlation With Cochlear Implant Performance

OBJECTIVE

To calculate the frequency allocation mismatch in a group of very selected cochlear implant (CI) recipients and to contrast it with the speech perception performances.

STUDY DESIGN

Cross-sectional observational prospective study.

SETTINGS

Tertiary Audiological Department, University hospital.

PATIENTS

Fifteen adults receiving the same CI array by the same surgeon through a posterior tympanotomy, round window approach.

MAIN OUTCOME MEASURES

1) High definition flat panel computed tomography (FPCT) control of the intracochlear position of each electrode contact, and computation of the relative frequency allocation mismatch; 2) analysis of speech perception outcomes in relation with the mismatch.

RESULTS

Despite a consistent and reproducible surgical procedure with the same intracochlear array, significant deviations from the frequency allocation tables (FAT) assigned by default by the manufacturer were observed in this study.Their influences on speech perception performances were negligible in the simple tasks of words or sentences recognition in quiet (and, to a lesser extent also in noise). The greatest effect of a significant mismatch was observed for the vocal-consonant-vocal (VCV) sequences recognition under noise masking, the emotional and the linguistic prosody recognition, and the phonemes discrimination of the Auditory Speech Sound Evaluation (A§E) test.

CONCLUSIONS

The greatest frequency-to-place occurred at the high frequencies. The effect was rather irrelevant on simple words and sentences recognition, while it negatively impacted on the more complex perceptual tasks.

Comparison of Speech Recognition and Localization Ability in Single-sided Deaf Patients Implanted With Different Cochlear Implant Electrode Array Designs

OBJECTIVE

Choice of electrode array (EA) design and differences in outcome are major concerns both to patients with single-sided deafness (SSD) and to surgeons before cochlear implant (CI) surgery. The present work investigates the effects of EA design on 1) insertion depths, and 2) audiological outcomes of SSD CI recipients.

STUDY DESIGN

Retrospective study.

SETTING

Tertiary academic center.

PATIENTS

Forty patients with acquired SSD matched according to duration of deafness MAIN OUTCOME MEASURES:: Fourteen CI recipients were implanted with a perimodiolar electrode (cochlear perimodiolar [CPM]), 12 with a shorter lateral wall electrode (cochlear lateral wall [CLW]), and 14 with a longer lateral wall electrode array (medEl lateral wall [MLW]). Postoperative rotational tomography was evaluated to determine cochlear size and EA angle of insertion depth (AID). Binaural speech comprehension in noise (in three configuration presentations) and localization ability were assessed 12 months postoperatively with CI.

RESULTS

AID was significantly deeper in MLW (mean 527.94 degrees) compared with the CPM (mean 366.35 degrees) and CLW groups (mean 367.01 degrees). No significant difference in AID was seen between the CPM and CLW groups (difference 0.66 degrees). Cochlear sizes revealed no significant differences between any groups. All three groups showed significant improvement in head shadow effect (difference on average CPM: 6.3 dB SPL, CLW 5 dB SPL, and MLW 4.05 dB SPL) and localization ability at 12 months postoperatively (difference on average CPM: 19.72 degrees, CLW: 24 degrees, and MLW: 12.9 degrees). No significant difference in the extent of audiological benefit was observed between any groups.

CONCLUSION

No effect on binaural benefit was apparent from the selection of the three EA designs in SSD CI recipients. Further studies focusing on subjective results, sound quality, and music perception depending on EA design in SSD CI recipients are needed.

The Temporal Limits Encoder as a Sound Coding Strategy for Bilateral Cochlear Implants

The difference in binaural benefit between bilateral cochlear implant (CI) users and normal hearing (NH) listeners has typically been attributed to CI sound coding strategies not encoding the acoustic fine structure (FS) interaural time differences (ITD). The Temporal Limits Encoder (TLE) strategy is proposed as a potential way of improving binaural hearing benefits for CI users in noisy situations. TLE works by downward-transposition of mid-frequency band-limited channel information and can theoretically provide FS-ITD cues. In this work, the effect of choice of lower limit of the modulator in TLE was examined by measuring performance on a word recognition task and computing the magnitude of binaural benefit in bilateral CI users. Performance listening with the TLE strategy was compared with the commonly used Advanced Combinational Encoder (ACE) CI sound coding strategy. Results showed that setting the lower limit to ≥200 Hz maintained word recognition performance comparable to that of ACE. While most CI listeners exhibited a large binaural benefit (≥6 dB) in at least one of the conditions tested, there was no systematic relationship between the lower limit of the modulator and performance. These results indicate that the TLE strategy has potential to improve binaural hearing abilities in CI users but further work is needed to understand how binaural benefit can be maximized.

Cochlear Implantation in Cases of Asymmetric Hearing Loss: Subjective Benefit, Word Recognition, and Spatial Hearing

A prospective clinical trial evaluated the effectiveness of cochlear implantation in adults with asymmetric hearing loss (AHL). Twenty subjects with mild-to-moderate hearing loss in the better ear and moderate-to-profound hearing loss in the poorer ear underwent cochlear implantation of the poorer hearing ear. Subjects were evaluated preoperatively and at 1, 3, 6, 9, and 12 months post-activation. Preoperative performance was evaluated unaided, with traditional hearing aids (HAs) or with a bone-conduction HA. Post-activation performance was evaluated with the cochlear implant (CI) alone or in combination with a contralateral HA (bimodal). Test measures included subjective benefit, word recognition, and spatial hearing (i.e., localization and masked sentence recognition). Significant subjective benefit was reported as early as the 1-month interval, indicating better performance with the CI compared with the preferred preoperative condition. Aided word recognition with the CI alone was significantly improved at the 1-month interval compared with preoperative performance with an HA and continued to improve through the 12-month interval. Subjects demonstrated early, significant improvements in the bimodal condition on the spatial hearing tasks compared with baseline preoperative performance tested unaided. The magnitude of the benefit was reduced for subjects with AHL when compared with published data on CI users with normal hearing in the contralateral ear; this finding may reflect significant differences in age at implantation and hearing sensitivity across cohorts.

Place-Pitch Interval Perception With a Cochlear Implant

OBJECTIVES

Pitch is poorly perceived by cochlear implant (CI) users. However, as it is not well understood how pitch is encoded with electric stimulation, improving pitch representation with a CI is challenging. Changes in place of stimulation along the cochlea have been described as changes in pitch and can be accurately ranked by CI users. However, it remains unknown if place-pitch can be used to encode musical intervals, which are a necessary attribute of pitch. The objective of these experiments is to determine if place-pitch coding can be used to represent musical intervals with a CI.

DESIGN

In the first experiment, 10 CI users and 10 normal hearing (NH) controls were tested on their sensitivity to changes in the semitone spacing between each of the notes in the melody "Happy Birthday." The changes were implemented by uniformly expanding or compressing the frequency differences between each note in the melody. The participant's task was to scale how "out-of-tune" the melody was for various semitone spacing distortions. The notes were represented by pure-tones ≥440 Hz to minimize potential useful temporal information from the stimuli. A second experiment replicated the first experiment using single-sided deafened CI users allowing for a within-subject control. A third experiment verified that the CI users who participated in Experiment 1 were each able to determine pitch direction reliably.

RESULTS

Unlike NH listeners, CI listeners often ranked all distortions of interval spacing similarly in both the first and second experiment, and no effect of interval spacing was detected across CI users. Some participants found distorted interval spacings to be less out-of-tune than the nominally correct interval spacings. However, these patterns were inconsistent across listeners. Although performance was better for the NH listeners, the third experiment demonstrated that the CI listeners were able to reliably identify changes in pitch direction from place-pitch coding.

CONCLUSIONS

The data suggest that place-pitch intervals are not properly represented through a CI sound processor. Some limited support is found for place-pitch being useful for interval encoding as some participants demonstrated improved ratings for certain interval distortions. Presumably the interval representation for these participants could be improved by a change to the frequencies represented by each electrode. However, as these patterns vary across listeners, there is not a universal correction to frequency representation that will solve this issue. As results are similar for single-sided deafened CI users, the limitations in ratings are likely not limited by an eroded representation of the melody caused by an extended duration of deafness.

Long-Term Influence of Electrode Array Length on Speech Recognition in Cochlear Implant Users

OBJECTIVES/HYPOTHESIS

Results from a prospective trial demonstrated better speech recognition for cochlear implant (CI) recipients implanted with a long lateral wall electrode array compared to subjects with a short array after 1 year of listening experience. As short array recipients may require an extended adaptation period, this study investigated whether differences in speech recognition continued through 4 years of CI use.

STUDY DESIGN

Long-term follow-up of a prospective randomized trial.

METHODS

Subjects were randomized to receive a MED-EL medium (24 mm) or standard (31.5 mm) array. Linear mixed models compared speech recognition between cohorts with word recognition in quiet and sentence recognition in noise at 1, 3, 6, 12, 24, and 48 months postactivation. Postoperative imaging and electric frequency filters were reviewed to assess the influence of frequency-to-place mismatch and angular separation between neighboring contacts, a metric associated with peripheral spectral selectivity.

RESULTS

Long (31.5 mm) array recipients demonstrated superior speech recognition out to 4 years postactivation. There was a significant effect of angular separation between contacts, with more closely spaced contacts associated with poorer speech recognition. There was no significant effect of mismatch, yet this may have been obscured by changes in frequency filters over time.

CONCLUSIONS

Conventional MED-EL CI recipients implanted with 31.5-mm arrays experience better speech recognition than 24-mm array recipients, initially and with long-term listening experience. The benefit conferred by longer arrays in the present cohort can be partially attributed to more widely spaced electrode contacts, presumably a result of reduced channel interaction.

LEVEL OF EVIDENCE

2 Laryngoscope, 131:892-897, 2021.

Perceptual weighting of acoustic cues for accommodating gender-related talker differences heard by listeners with normal hearing and with cochlear implants

Listeners must accommodate acoustic differences between vocal tracts and speaking styles of conversation partners-a process called normalization or accommodation. This study explores what acoustic cues are used to make this perceptual adjustment by listeners with normal hearing or with cochlear implants, when the acoustic variability is related to the talker's gender. A continuum between /ʃ/ and /s/ was paired with naturally spoken vocalic contexts that were parametrically manipulated to vary by numerous cues for talker gender including fundamental frequency (F0), vocal tract length (formant spacing), and direct spectral contrast with the fricative. The goal was to examine relative contributions of these cues toward the tendency to have a lower-frequency acoustic boundary for fricatives spoken by men (found in numerous previous studies). Normal hearing listeners relied primarily on formant spacing and much less on F0. The CI listeners were individually variable, with the F0 cue emerging as the strongest cue on average.

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.

Speech Perception Changes in the Acoustically Aided, Nonimplanted Ear after Cochlear Implantation: A Multicenter Study

In recent years there has been an increasing percentage of cochlear implant (CI) users who have usable residual hearing in the contralateral, nonimplanted ear, typically aided by acoustic amplification. This raises the issue of the extent to which the signal presented through the cochlear implant may influence how listeners process information in the acoustically stimulated ear. This multicenter retrospective study examined pre- to postoperative changes in speech perception in the nonimplanted ear, the implanted ear, and both together. Results in the latter two conditions showed the expected increases, but speech perception in the nonimplanted ear showed a modest yet meaningful decrease that could not be completely explained by changes in unaided thresholds, hearing aid malfunction, or several other demographic variables. Decreases in speech perception in the nonimplanted ear were more likely in individuals who had better levels of speech perception in the implanted ear, and in those who had better speech perception in the implanted than in the nonimplanted ear. This raises the possibility that, in some cases, bimodal listeners may rely on the higher quality signal provided by the implant and may disregard or even neglect the input provided by the nonimplanted ear.