Relationship between channel interaction and spectral-ripple discrimination in cochlear implant users

Cochlear-implant simulated spectral degradation attenuates emotional responses to environmental sounds

OBJECTIVE

Cochlear implants (CI) provide users with a spectrally degraded acoustic signal that could impact their auditory emotional experiences. This study evaluated the effects of CI-simulated spectral degradation on emotional valence and arousal elicited by environmental sounds.

DESIGN

Thirty emotionally evocative sounds were filtered through a noise-band vocoder. Participants rated the perceived valence and arousal elicited by each of the full-spectrum and vocoded stimuli. These ratings were compared across acoustic conditions (full-spectrum, vocoded) and as a function of stimulus type (unpleasant, neutral, pleasant).

STUDY SAMPLE

Twenty-five young adults (age 19 to 34 years) with normal hearing.

RESULTS

Emotional responses were less extreme for spectrally degraded (i.e., vocoded) sounds than for full-spectrum sounds. Specifically, spectrally degraded stimuli were perceived as more negative and less arousing than full-spectrum stimuli.

CONCLUSION

By meticulously replicating CI spectral degradation while controlling for variables that are confounded within CI users, these findings indicate that CI spectral degradation can compress the range of sound-induced emotion independent of hearing loss and other idiosyncratic device- or person-level variables. Future work will characterize emotional reactions to sound in CI users via objective, psychoacoustic, and subjective measures.

Clinical Applications for Spread of Excitation Functions Obtained Via Electrically Evoked Compound Action Potentials (eCAP)

OBJECTIVES

To assess the clinical utility of spread of excitation (SOE) functions obtained via electrically evoked compound action potentials (eCAP) to 1) identify electrode array tip fold-over, 2) predict electrode placement factors confirmed via postoperative computed tomography (CT) imaging, and 3) predict postoperative speech recognition through the first year post-activation in a large clinical sample.

STUDY DESIGN

Retrospective case review.

SETTING

Cochlear implant (CI) program at a tertiary medical center.

PATIENTS

Two hundred seventy-two ears (238 patients) with Cochlear Ltd. CIs (mean age = 46 yr, range = 9 mo-93 yr, 50% female) implanted between August 2014 and December 2022 were included.

MAIN OUTCOME MEASURES

eCAP SOE widths (mm) (probe electrodes 5, 11, and 17), incidence of electrode tip fold-over, CT imaging data (electrode-to-modiolus distance, angular insertion depth, scalar location), and speech recognition outcomes (consonant-nucleus-consonant [CNC], AzBio quiet, and +5 dB SNR) through the first year after CI activation.

RESULTS

1) eCAP SOE demonstrated a sensitivity of 85.7% for identifying tip fold-over instances that were confirmed by CT imaging. In the current dataset, the tip fold-over incidence rate was 3.1% (7 patients), with all instances involving a precurved electrode array. 2) There was a significant positive relationship between eCAP SOE and mean electrode-to-modiolus distance for precurved arrays, and a significant positive relationship between eCAP SOE and angular insertion depth for straight arrays. No relationships between eCAP SOE and scalar location or cochlea diameter were found in this sample. 3) There were no significant relationships between eCAP SOE and speech recognition outcomes for any measure or time point, except for a weak negative correlation between average eCAP SOE widths and CNC word scores at 6 months post-activation for precurved arrays.

CONCLUSIONS

In the absence of intraoperative CT or fluoroscopic imaging, eCAP SOE is a reasonable alternative method for identifying electrode array tip fold-over and should be routinely measured intraoperatively, especially for precurved electrode arrays with a sheath.

Spectral Grouping of Electrically Encoded Sound Predicts Speech-in-Noise Performance in Cochlear Implantees

OBJECTIVES

Cochlear implant (CI) users exhibit large variability in understanding speech in noise. Past work in CI users found that spectral and temporal resolution correlates with speech-in-noise ability, but a large portion of variance remains unexplained. Recent work on normal-hearing listeners showed that the ability to group temporally and spectrally coherent tones in a complex auditory scene predicts speech-in-noise ability independently of the audiogram, highlighting a central mechanism for auditory scene analysis that contributes to speech-in-noise. The current study examined whether the auditory grouping ability also contributes to speech-in-noise understanding in CI users.

DESIGN

Forty-seven post-lingually deafened CI users were tested with psychophysical measures of spectral and temporal resolution, a stochastic figure-ground task that depends on the detection of a figure by grouping multiple fixed frequency elements against a random background, and a sentence-in-noise measure. Multiple linear regression was used to predict sentence-in-noise performance from the other tasks.

RESULTS

No co-linearity was found between any predictor variables. All three predictors (spectral and temporal resolution plus the figure-ground task) exhibited significant contribution in the multiple linear regression model, indicating that the auditory grouping ability in a complex auditory scene explains a further proportion of variance in CI users' speech-in-noise performance that was not explained by spectral and temporal resolution.

CONCLUSION

Measures of cross-frequency grouping reflect an auditory cognitive mechanism that determines speech-in-noise understanding independently of cochlear function. Such measures are easily implemented clinically as predictors of CI success and suggest potential strategies for rehabilitation based on training with non-speech stimuli.

Effects of spectral smearing on speech understanding and masking release in simulated bilateral cochlear implants

Differences in spectro-temporal degradation may explain some variability in cochlear implant users' speech outcomes. The present study employs vocoder simulations on listeners with typical hearing to evaluate how differences in degree of channel interaction across ears affects spatial speech recognition. Speech recognition thresholds and spatial release from masking were measured in 16 normal-hearing subjects listening to simulated bilateral cochlear implants. 16-channel sine-vocoded speech simulated limited, broad, or mixed channel interaction, in dichotic and diotic target-masker conditions, across ears. Thresholds were highest with broad channel interaction in both ears but improved when interaction decreased in one ear and again in both ears. Masking release was apparent across conditions. Results from this simulation study on listeners with typical hearing show that channel interaction may impact speech recognition more than masking release, and may have implications for the effects of channel interaction on cochlear implant users' speech recognition outcomes.

The Impact of Daily Processor Use on Adult Cochlear Implant Outcomes: Reexamining the Roles of Duration of Deafness and Age at Implantation

OBJECTIVE

To quantify the roles and relationships between age at implantation, duration of deafness (DoD), and daily processor use via data logging on speech recognition outcomes for postlingually deafened adults with cochlear implants.

STUDY DESIGN

Retrospective case review.

SETTING

Cochlear implant (CI) program at a tertiary medical center.

PATIENTS

Six-hundred fourteen postlingually deafened adult ears with CIs (mean age, 63 yr; 44% female) were included.

MAIN OUTCOME MEASURES

A stepwise multiple regression analysis was completed to investigate the combined effects of age, DoD, and daily processor use on CI-aided speech recognition (Consonant-Nucleus-Consonant monosyllables and AzBio sentences).

RESULTS

Results indicated that only daily processor use was significantly related to Consonant-Nucleus-Consonant word scores ( R2 = 0.194, p < 0.001) and AzBio in quiet scores ( R2 = 0.198, p < 0.001), whereas neither age nor DoD was significantly related. In addition, there was no significant relationship between daily processor use, age at implantation, or DoD and AzBio sentences in noise ( R2 = 0.026, p = 0.005).

CONCLUSIONS

Considering the clinical factors of age at implantation, DoD, and daily processor use, only daily processor use significantly predicted the ~20% of variance in postoperative outcomes (CI-aided speech recognition) accounted for by these clinical factors.

Effects of the Number of Channels and Channel Stimulation Rate on Speech Recognition and Sound Quality Using Precurved Electrode Arrays

PURPOSE

This study investigated the relationship between the number of active electrodes, channel stimulation rate, and their interaction on speech recognition and sound quality measures while controlling for electrode placement. Cochlear implant (CI) recipients with precurved electrode arrays placed entirely within scala tympani and closer to the modiolus were hypothesized to be able to utilize more channels and possibly higher stimulation rates to achieve better speech recognition performance and sound quality ratings than recipients in previous studies.

METHOD

Participants included seven postlingually deafened adult CI recipients with Advanced Bionics Mid-Scala electrode arrays confirmed to be entirely within scala tympani using postoperative computerized tomography. Twelve conditions were tested using four, eight, 12, and 16 electrodes and channel stimulation rates of 600 pulse per second (pps), 1,200 pps, and each participant's maximum allowable rate (1,245-4,800 pps). Measures of speech recognition and sound quality were acutely assessed.

RESULTS

For the effect of channels, results showed no significant improvements beyond eight channels for all measures. For the effect of channel stimulation rate, results showed no significant improvements with higher rates, suggesting that 600 pps was sufficient for maximum speech recognition performance and sound quality ratings. However, across all conditions, there was a significant relationship between mean electrode-to-modiolus distance and all measures, suggesting that a lower mean electrode-to-modiolus distance was correlated with higher speech recognition scores and sound quality ratings.

CONCLUSION

These findings suggest that even well-placed precurved electrode array recipients may not be able to take advantage of more than eight channels or higher channel stimulation rates (> 600 pps), but that closer electrode array placement to the modiolus correlates with better outcomes for these recipients.

Association between lateral wall electrode array insertion parameters and audiological outcomes in bilateral cochlear implantation

Purpose

The aims of this study were to compare speech recognition at different postoperative times for both ears in bilaterally implanted patients and to assess the influence of the time of deafness, frequency-to-place mismatch, angular insertion depth (AID) and angular separation between neighbouring electrode contacts on audiometric outcomes.

Methods

This study was performed at an academic tertiary referral centre. A total of 19 adult patients (6 men, 13 women), who received sequential bilateral implantation with lateral wall electrode arrays, were analysed in retrospective. Statistical analysis was performed using two-sided t test, Wilcoxon test, median test, and Spearman’s correlation.

Results

Postlingually deafened patients (deafness after the age of 10) had a significantly better speech perception (WRS65[CI]) than the perilingually deafened subjects (deafness at the age of 1–10 years) (p < 0.001). Comparison of cochlear duct length between peri- and postlingually deafened subjects showed a slightly significantly smaller cochleae in perilingual patients (p = 0.045). No association between frequency-to-place mismatch as well as angular separation and speech perception could be detected. There was even no significant difference between the both ears in the intraindividual comparison, even if insertion parameters differed.

Conclusion

The exact electrode position seems to have less influence on the speech comprehension of CI patients than already established parameters as preoperative speech recognition or duration of deafness.

Speech recognition as a function of the number of channels for Mid-Scala electrode array recipients

This study investigated the number of channels needed for maximum speech understanding and sound quality in 15 adult cochlear implant (CI) recipients with Advanced Bionics (AB) Mid-Scala electrode arrays completely within scala tympani. In experiment I, CI programs used a continuous interleaved sampling (CIS)-based strategy and 4-16 active electrodes. In experiment II, CI programs used an n-of-m strategy featuring 16 active electrodes with either 8- or 12-maxima. Speech understanding and sound quality measures were assessed. For CIS programs, participants demonstrated performance gains using up to 4-10 electrodes on speech measures and sound quality ratings. For n-of-m programs, there was no significant effect of maxima, suggesting 8-maxima is sufficient for this sample's maximum performance and sound quality. These results are largely consistent with previous studies using straight electrode arrays [e.g., Fishman, Shannon, and Slattery (1997). J. Speech Lang. Hear. Res. 40, 1201-1215; Friesen, Shannon, Baskent, and Wang (2001). J. Acoust. Soc. Am. 110, 1150-1163; Shannon, Cruz, and Galvin (2011). Audiol. Neurotol. 16, 113-123; Berg, Noble, Dawant, Dwyer, Labadie, and Gifford (2020). J. Acoust. Soc. Am. 147, 3646-3656] and in contrast with recent studies looking at cochlear precurved electrode arrays [e.g., Croghan, Duran, and Smith (2017). J. Acoust. Soc. Am. 142, EL537-EL543; Berg, Noble, Dawant, Dwuer, Labadie, and Gifford (2019b). J. Acoust. Soc. Am. 145, 1556-1564], which found continuous improvements up to 16 independent channels. These findings suggest that Mid-Scala electrode array recipients demonstrate similar channel independence to straight electrode arrays rather than other manufacturer's precurved electrode arrays.

Spectral Resolution Development in Children With Normal Hearing and With Cochlear Implants: A Review of Behavioral Studies

PURPOSE

This review article provides a theoretical overview of the development of spectral resolution in children with normal hearing (cNH) and in those who use cochlear implants (CIs), with an emphasis on methodological considerations. The aim was to identify key directions for future research on spectral resolution development in children with CIs.

METHOD

A comprehensive literature review was conducted to summarize and synthesize previously published behavioral research on spectral resolution development in normal and impaired auditory systems.

CONCLUSIONS

In cNH, performance on spectral resolution tasks continues to improve through the teenage years and is likely driven by gradual maturation of across-channel intensity resolution. A small but growing body of evidence from children with CIs suggests a more complex relationship between spectral resolution development, patient demographics, and the quality of the CI electrode-neuron interface. Future research should aim to distinguish between the effects of patient-specific variables and the underlying physiology on spectral resolution abilities in children of all ages who are hard of hearing and use auditory prostheses.

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.

Relationship Between Speech Recognition in Quiet and Noise and Fitting Parameters, Impedances and ECAP Thresholds in Adult Cochlear Implant Users

OBJECTIVES

The objective of this study was to identify parameters which are related to speech recognition in quiet and in noise of cochlear implant (CI) users. These parameters may be important to improve current fitting practices.

DESIGN

Adult CI users who visited the Amsterdam UMC, location VUmc, for their annual follow-up between January 2015 and December 2017 were retrospectively identified. After applying inclusion criteria, the final study population consisted of 138 postlingually deaf adult Cochlear CI users. Prediction models were built with speech recognition in quiet and in noise as the outcome measures, and aided sound field thresholds, and parameters related to fitting (i.e., T and C levels, dynamic range [DR]), evoked compound action potential thresholds and impedances as the independent variables. A total of 33 parameters were considered. Separate analyses were performed for postlingually deafened CI users with late onset (LO) and CI users with early onset (EO) of severe hearing impairment.

RESULTS

Speech recognition in quiet was not significantly different between the LO and EO groups. Speech recognition in noise was better for the LO group compared with the EO group. For CI users in the LO group, mean aided thresholds, mean electrical DR, and measures to express the impedance profile across the electrode array were identified as predictors of speech recognition in quiet and in noise. For CI users in the EO group, the mean T level appeared to be a significant predictor in the models for speech recognition in quiet and in noise, such that CI users with elevated T levels had worse speech recognition in quiet and in noise.

CONCLUSIONS

Significant parameters related to speech recognition in quiet and in noise were identified: aided thresholds, electrical DR, T levels, and impedance profiles. The results of this study are consistent with previous study findings and may guide audiologists in their fitting practices to improve the performance of CI users. The best performance was found for CI users with aided thresholds around the target level of 25 dB HL, and an electrical DR between 40 and 60 CL. However, adjustments of T and/or C levels to obtain aided thresholds around the target level and the preferred DR may not always be acceptable for individual CI users. Finally, clinicians should pay attention to profiles of impedances other than a flat profile with mild variations.

Impact of Cochlear Implant Array Placement on Speech Perception

PURPOSE

To assess the role of flat panel computed tomography (FPCT) in the evaluation of cochlear implant (CI) electrode position and its relation to speech perception.

METHODS

From March 2015 to March 2019, we retrospectively enrolled deaf subjects ≥ 18 years who underwent unilateral CI by one surgeon, imaged with FPCT and assessed with disyllabic words score before CI and at 6 months of follow-up. We calculated the disyllabic score difference before CI and after CI (ΔSDS) and divided the subjects in favorable and unfavorable outcome groups using the median ΔSDS as a cutoff. We compared the demographic, clinical, electrode characteristics, and the CI positioning variables scalar position, surgical insertion depth (SID), linear insertion depth (LID), angular insertion depth (AID) and wrapping factor (WF).

RESULTS

We studied 50 subjects (F/M = 27/23; median age = 60.5 years, IQR: 50-70 years). The median ΔSDS was 80% (interquartile range [IQR]: 60-100%) in quiet and 80% (IQR: 47.5-100%) in noise. Of the subjects 23 demonstrated a favorable outcome and had earlier age at CI (median 52 years; IQR 45-67 years versus median 62 years; IQR: 56-71 years p = 0.032) and a significantly higher SID (median: 4.02 mm IQR: 3.00-5.35 mm versus median: 2.94 mm IQR: 2.06-3.90 mm; p = 0.029). No difference was found for LID (p = 0.977), AID (p = 0.302), and WF (p = 0.224). A logistic regression model built with the age at CI, number of CI electrodes, and the SID was significant χ2 ((df = 3, N = 50) = 14.517, p = 0.002). The model explained 33.7% (Nagelkerke R2) of ΔSDS variance and correctly classified 76% of the cases.

CONCLUSION

The SID measured by FPCT predicts the ΔSDS at 6 months follow-up, alongside with age at implantation and number of CI electrodes.

Speech recognition as a function of the number of channels for an array with large inter-electrode distances

This study investigated the number of channels available to cochlear implant (CI) recipients for maximum speech understanding and sound quality for lateral wall electrode arrays-which result in large electrode-to-modiolus distances-featuring the greatest inter-electrode distances (2.1-2.4 mm), the longest active lengths (23.1-26.4 mm), and the fewest number of electrodes commercially available. Participants included ten post-lingually deafened adult CI recipients with MED-EL electrode arrays (FLEX28 and STANDARD) entirely within scala tympani. Electrode placement and scalar location were determined using computerized tomography. The number of channels was varied from 4 to 12 with equal spatial distribution across the array. A continuous interleaved sampling-based strategy was used. Speech recognition, sound quality ratings, and a closed-set vowel recognition task were measured acutely for each electrode condition. Participants did not demonstrate statistically significant differences beyond eight channels at the group level for almost all measures. However, several listeners showed considerable improvements from 8 to 12 channels for speech and sound quality measures. These results suggest that channel interaction caused by the greater electrode-to-modiolus distances of straight electrode arrays could be partially compensated for by a large inter-electrode distance or spacing.

Recovery from forward masking in cochlear implant listeners: Effects of age and the electrode-neuron interface

Older adults exhibit deficits in auditory temporal processing relative to younger listeners. These age-related temporal processing difficulties may be further exacerbated in older adults with cochlear implant (CIs) when CI electrodes poorly interface with their target auditory neurons. The aim of this study was to evaluate the potential interaction between chronological age and the estimated quality of the electrode-neuron interface (ENI) on psychophysical forward masking recovery, a measure that reflects single-channel temporal processing abilities. Fourteen CI listeners (age 15 to 88 years) with Advanced Bionics devices participated. Forward masking recovery was assessed on two channels in each ear (i.e., the channels with the lowest and highest signal detection thresholds). Results indicated that the rate of forward masking recovery declined with advancing age, and that the effect of age was more pronounced on channels estimated to interface poorly with the auditory nerve. These findings indicate that the quality of the ENI can influence the time course of forward masking recovery for older CI listeners. Channel-to-channel variability in the ENI likely interacts with central temporal processing deficits secondary to auditory aging, warranting further study of programming and rehabilitative approaches tailored to older listeners.

Evaluation of auditory spectral resolution abilities in children with benign epilepsy with centrotemporal spikes using spectral temporally modulated ripple test

PURPOSE

Spectral resolution is imperative for complex listening tasks such as understanding speech in the presence of background noise and has a significant role in children, particularly classroom learning. The present study evaluated the auditory spectral resolution abilities of children with Benign epilepsy with centrotemporal spikes (BECTS).

METHOD

This cross-sectional study conducted from August 2017 to March 2020 recruited 23 children with clinical and electrographic features consistent with BECTS as cases. Fifteen age and sex matched typically developing children (TDC) were taken as controls. Spectral resolution abilities were evaluated using the recently developed Spectral temporally modulated Ripple test (SMRT).

RESULTS

The mean age of the cases was 10.63 ± 1.91 years with a slight male preponderance (69%). The mean (±SD) SMRT thresholds in the cases and controls were 5.90 (±1.91) and 7.21 (±1.03) respectively. The auditory spectral resolution threshold measured by SMRT in children with BECTS was observed to be significantly lower when compared to the controls (p of 0.021).

CONCLUSION

Children with BECTS have a lower spectral resolution threshold by SMRT.

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.

Auditory performance of post-lingually deafened adult cochlear implant recipients using electrode deactivation based on postoperative cone beam CT images

PURPOSE

The use of image processing techniques to estimate the position of intra-cochlear electrodes has enabled the creation of personalized maps to meet the individual stimulation needs of cochlear implant (CI) recipients. The aim of this study was to evaluate a novel technique of electrode deactivation based on postoperative cone beam computed tomography (CBCT) images in post-lingually deafened adult CI recipients.

METHODS

Based on postoperative CBCT images, the positioning of the electrodes was estimated in relation to the modiolus in 14 ears of 13 post-lingually deafened adult CI recipients. The electrodes sub-optimally positioned or involved in kinking and tip fold-over were deactivated. Speech perception scores in silence and in noise were obtained from subjects using the standard map and were followed up 4 weeks after image-based electrode deactivation reprogramming technique (IBEDRT). The participants selected their preferred map after 4 weeks of IBEDRT use.

RESULTS

There were statistically significant improvements in the speech recognition tests in silence and noise when comparing IBEDRT performance to the standard map. All participants elected the IBEDRT as their new preferred map.

CONCLUSIONS

IBEDRT is a promising technique for fitting CI recipients and minimizing channel interaction increased by the positioning of the electrodes sub-optimally placed, thereby improving their auditory performance. We propose a novel electrode deactivation technique based on postoperative CBCT imaging, with a limited number of deactivated electrodes and a low-dosing scanning which could be applied for clinical routine.

Spectral aliasing in an acoustic spectral ripple discrimination task

Spectral ripple discrimination tasks are commonly used to probe spectral resolution in cochlear implant (CI), normal-hearing (NH), and hearing-impaired individuals. In addition, these tasks have also been used to examine spectral resolution development in NH and CI children. In this work, stimulus sine-wave carrier density was identified as a critical variable in an example spectral ripple-based task, the Spectro-Temporally Modulated Ripple (SMR) Test, and it was demonstrated that previous uses of it in NH listeners sometimes used values insufficient to represent relevant ripple densities. Insufficient carry densities produced spectral under-sampling that both eliminated ripple cues at high ripple densities and introduced unintended structured interference between the carriers and intended ripples at particular ripple densities. It was found that this effect produced non-monotonic psychometric functions for NH listeners that would cause systematic underestimation of thresholds with adaptive techniques. Studies of spectral ripple detection in CI users probe a density regime below where this source of aliasing occurs, as CI signal processing limits dense ripple representation. While these analyses and experiments focused on the SMR Test, any task in which discrete pure-tone carriers spanning frequency space are modulated to approximate a desired pattern must be designed with the consideration of the described spectral aliasing effect.

Accommodation of gender-related phonetic differences by listeners with cochlear implants and in a variety of vocoder simulations

Speech perception requires accommodation of a wide range of acoustic variability across talkers. A classic example is the perception of "sh" and "s" fricative sounds, which are categorized according to spectral details of the consonant itself, and also by the context of the voice producing it. Because women's and men's voices occupy different frequency ranges, a listener is required to make a corresponding adjustment of acoustic-phonetic category space for these phonemes when hearing different talkers. This pattern is commonplace in everyday speech communication, and yet might not be captured in accuracy scores for whole words, especially when word lists are spoken by a single talker. Phonetic accommodation for fricatives "s" and "sh" was measured in 20 cochlear implant (CI) users and in a variety of vocoder simulations, including those with noise carriers with and without peak picking, simulated spread of excitation, and pulsatile carriers. CI listeners showed strong phonetic accommodation as a group. Each vocoder produced phonetic accommodation except the 8-channel noise vocoder, despite its historically good match with CI users in word intelligibility. Phonetic accommodation is largely independent of linguistic factors and thus might offer information complementary to speech intelligibility tests which are partially affected by language processing.

Dynamic current focusing for loudness encoding in cochlear implants: a take-home trial

Objective: This study aimed to evaluate a more energy-efficient dynamic current focussing (DCF) speech-processing strategy after long-term listening experience. In DCF, tripolar stimulation is used near the threshold and loudness is controlled by the compensation coefficient σ. A recent acute pilot study showed improved spectral-temporally modulated ripple test (SMRT) scores at low loudness levels, but battery life was reduced to 1.5-4 hours. Design: Within-subject comparisons were made for the clinical versus. DCF strategy after 5 weeks of at-home usage. Speech intelligibility in noise, spectral ripple discrimination, temporal modulation detection, loudness growth, and subjective ratings were assessed. Study sample: Twenty HiRes90K (Advanced Bionics, Valencia, USA) cochlear implant (CI) users. Results: Average battery life was 9 hours with the newly implemented DCF compared to 13.4 hours with the clinical strategy. Compared with measurements made at the beginning of the study, SMRT-scores and speech intelligibility in noise were significantly improved with DCF. However, both measures suffered from unexpected learning effects over time. The improvement disappeared and speech intelligibility in noise declined significantly relative to the final control measurement with the clinical strategy. Conclusion: Most CI users can adapt to the DCF strategy in a take-home setting. Although DCF has the potential to improve performance on the SMRT test, learning effects complicate the interpretation of the current results.

Dynamic Current Focusing: A Novel Approach to Loudness Coding in Cochlear Implants

OBJECTIVES

In an attempt to improve spectral resolution and speech intelligibility, several current focusing methods have been proposed to increase spatial selectivity by decreasing intracochlear current spread. For example, tripolar stimulation administers current to a central electrode and uses the two flanking electrodes as the return pathway, creating a narrower intracochlear electrical field and hence increases spectral resolution when compared with monopolar (MP) stimulation. However, more current is required, and in some patients, specifically the ones with high electrode impedances, full loudness growth cannot be supported because of compliance limits. The present study describes and analyses a new loudness encoding approach that uses tripolar stimulation near threshold and gradually broadens the excitation (by decreasing compensation coefficient σ) to increase loudness without the need to increase overall current. It is hypothesized that this dynamic current focusing (DCF) strategy increases spatial selectivity, especially at lower loudness levels, while maintaining maximum selectivity at higher loudness levels, without reaching compliance limits.

DESIGN

Eleven adult cochlear implant recipients with postlingual hearing loss, with at least 9 months of experience with their HiRes90K implant, were selected to participate in this study. Baseline performance regarding speech intelligibility in noise (Dutch matrix sentence test), spectral ripple discrimination at 45 and 65 dB, and temporal modulation detection thresholds were assessed using their own clinical program, fitted on a Harmony processor. Subsequently, the DCF strategy was fitted on a research Harmony processor. Threshold levels were determined with σ = 0.8, which means 80% of current is returned to the flanking electrodes and the remaining 20% to the extracochlear ground electrode. Instead of increasing overall pulse magnitude, σ was decreased to determine most comfortable loudness. After 2 to 3 hr of adaptation to the research strategy, the same psychophysical measures were taken.

RESULTS

At 45 dB, average spectral ripple scores improved significantly from 2.4 ripples per octave with their clinical program to 3.74 ripples per octave with the DCF strategy (p = 0.016). Eight out of 11 participants had an improved spectral resolution at 65 dB. Nevertheless, no significant difference between DCF and MP was observed at higher presentation levels. Both speech-in-noise and temporal modulation detection thresholds were equal for MP and DCF strategies. Subjectively, 2 participants preferred the DCF strategy over their own clinical program, 2 preferred their own strategy, while the majority of the participants had no preference. Battery life was decreased and ranged from 1.5 to 4 hr.

CONCLUSIONS

The DCF strategy gives better spectral resolution, at lower loudness levels, but equal performance on speech tests. These outcomes warrant for a longer adaptation period to study long-term outcomes and evaluate if the outcomes in the ripple tests transfer to the speech scores. Further research, for example, with respect to fitting rules and reduction of power consumption, is necessary to make the DCF strategy suitable for routine clinical application.

A Dynamically Focusing Cochlear Implant Strategy Can Improve Vowel Identification in Noise

Supplemental Digital Content is available in the text.

Objectives:

The standard, monopolar (MP) electrode configuration used in commercially available cochlear implants (CI) creates a broad electrical field, which can lead to unwanted channel interactions. Use of more focused configurations, such as tripolar and phased array, has led to mixed results for improving speech understanding. The purpose of the present study was to assess the efficacy of a physiologically inspired configuration called dynamic focusing, using focused tripolar stimulation at low levels and less focused stimulation at high levels. Dynamic focusing may better mimic cochlear excitation patterns in normal acoustic hearing, while reducing the current levels necessary to achieve sufficient loudness at high levels.

Design:

Twenty postlingually deafened adult CI users participated in the study. Speech perception was assessed in quiet and in a four-talker babble background noise. Speech stimuli were closed-set spondees in noise, and medial vowels at 50 and 60 dB SPL in quiet and in noise. The signal to noise ratio was adjusted individually such that performance was between 40 and 60% correct with the MP strategy. Subjects were fitted with three experimental strategies matched for pulse duration, pulse rate, filter settings, and loudness on a channel-by-channel basis. The strategies included 14 channels programmed in MP, fixed partial tripolar (σ = 0.8), and dynamic partial tripolar (σ at 0.8 at threshold and 0.5 at the most comfortable level). Fifteen minutes of listening experience was provided with each strategy before testing. Sound quality ratings were also obtained.

Results:

Speech perception performance for vowel identification in quiet at 50 and 60 dB SPL and for spondees in noise was similar for the three tested strategies. However, performance on vowel identification in noise was significantly better for listeners using the dynamic focusing strategy. Sound quality ratings were similar for the three strategies. Some subjects obtained more benefit than others, with some individual differences explained by the relation between loudness growth and the rate of change from focused to broader stimulation.

Conclusions:

These initial results suggest that further exploration of dynamic focusing is warranted. Specifically, optimizing such strategies on an individual basis may lead to improvements in speech perception for more adult listeners and improve how CIs are tailored. Some listeners may also need a longer period of time to acclimate to a new program.

Channel discrimination along all contacts of the cochlear implant electrode array and its relation to speech perception

OBJECTIVE

To test the channel discrimination of cochlear implant (CI) users along all contacts of the electrode array and assess whether this is related to speech perception.

DESIGN

CI recipients were tested with a custom-made channel discrimination test. They were asked to distinguish a target stimulus from two reference stimuli in a three-alternative forced choice (3AFC) task. The target stimulus was evoked using current steering, with current steering coefficients (α) of 1, 0.5 and 0.25. The test provided a discrimination score (Dα) for each electrode contact along the array.

STUDY SAMPLE

Thirty adults implanted with a CI from Advanced Bionics.

RESULTS

Large variations in Dα scores were observed, both across the electrode array and between subjects. Statistical analysis revealed a significant channel-to-channel variability in Dα score (p < 0.01). Further, there was a significant relationship between subjects' Dα scores and their speech perception in quiet (p < 0.001).

CONCLUSIONS

The large variations in Dα score emphasise the importance of testing pitch discrimination across the complete electrode array. The relationship between Dα score and speech perception indicates that pitch discrimination might be a contributing factor to the performance of individual implant users.

Simulating speech processing with cochlear implants: How does channel interaction affect learning in neural networks?

We introduce a novel machine learning approach for investigating speech processing with cochlear implants (CIs)—prostheses used to replace a damaged inner ear. Concretely, we use a simple perceptron and a deep convolutional network to classify speech spectrograms that are modified to approximate CI-delivered speech. Implant-delivered signals suffer from reduced spectral resolution, chiefly due to a small number of frequency channels and a phenomenon called channel interaction. The latter involves the spread of information from neighboring channels to similar populations of neurons and can be modeled by linearly combining adjacent channels. We find that early during training, this input modification degrades performance if the networks are first pre-trained on high-resolution speech—with a larger number of channels, and without added channel interaction. This suggests that the spectral degradation caused by channel interaction alters the signal to conflict with perceptual expectations acquired from high-resolution speech. We thus predict that a reduction of channel interaction will accelerate learning in CI users who are implanted after having adapted to high-resolution speech during normal hearing. (The code for replicating our experiments is available online: https://github.com/clips/SimulatingCochlearImplants).

Impedance Measures During in vitro Cochlear Implantation Predict Array Positioning

OBJECTIVE

Improper electrode placement during cochlear implant (CI) insertion can adversely affect speech perception outcomes. However, the intraoperative methods to determine positioning are limited. Because measures of electrode impedance can be made quickly, the goal of this study was to assess the relationship between CI impedance and proximity to adjacent structures.

METHODS

An Advanced Bionics CI array was inserted into a clear, plastic cochlea one electrode contact at a time in a saline bath (nine trials). At each insertion depth, response to biphasic current pulses was used to calculate access resistance (Ra), polarization resistance (Rp), and polarization capacitance (Cp). These measures were correlated to actual proximity as assessed by microscopy using linear regression models.

RESULTS

Impedance increased with insertion depth and proximity to the inner wall. Specifically, Ra increased, Cp decreased, and Rp slightly increased. Incorporating all impedance measures afforded a prediction model (r = 0.88) while optimizing for sub-mm positioning afforded a model with 78.3% specificity.

CONCLUSION

Impedance in vitro greatly changes with electrode insertion depth and proximity to adjacent structures in a predicable manner.

SIGNIFICANCE

Assessing proximity of the CI to adjacent structures is a significant first step in qualifying the electrode-neural interface. This information should aid in CI fitting, which should help maximize hearing and speech outcomes with a CI. Additionally, knowledge of the relationship between impedance and positioning could have utility in other tissue implants in the brain, retina, or spinal cord.

Psychophysical Tuning Curves as a Correlate of Electrode Position in Cochlear Implant Listeners

Speech understanding abilities vary widely among cochlear implant (CI) listeners. A potential source of this variability is the electrode-neuron interface (ENI), which includes peripheral factors such as electrode position and integrity of remaining spiral ganglion neurons. Suboptimal positioning of the electrode array has been associated with poorer speech outcomes; however, postoperative computerized tomography (CT) scans are often not available to clinicians. CT-estimated electrode-to-modiolus distance (distance from the inner wall of the cochlea) has been shown to account for some variability in behavioral thresholds. However, psychophysical tuning curves (PTCs) may provide additional insight into site-specific variation in channel interaction. Thirteen unilaterally implanted adults with the Advanced Bionics HiRes90K device participated. Behavioral thresholds and PTCs were collected for all available electrodes with steered quadrupolar (sQP) configuration, using a modified threshold sweep procedure, used in Bierer et al. (Trends Hear 19:1-12, 2015). PTC bandwidths were quantified to characterize channel interaction across the electrode array, and tip shifts were assessed to identify possible contributions of neural dead regions. Broader PTC bandwidths were correlated with electrodes farther from the modiolus, but not correlated with sQP threshold, though a trend was observed. Both measures were affected by scalar location, and PTC tip shifts were observed for electrodes farther from the modiolus. sQP threshold was the only variable correlated with word recognition. These results suggest PTCs may be used as a site-specific measure of channel interaction that correlates with electrode position in some CI listeners.

Voice Discrimination by Adults with Cochlear Implants: the Benefits of Early Implantation for Vocal-Tract Length Perception

Cochlear implant (CI) users find it extremely difficult to discriminate between talkers, which may partially explain why they struggle to understand speech in a multi-talker environment. Recent studies, based on findings with postlingually deafened CI users, suggest that these difficulties may stem from their limited use of vocal-tract length (VTL) cues due to the degraded spectral resolution transmitted by the CI device. The aim of the present study was to assess the ability of adult CI users who had no prior acoustic experience, i.e., prelingually deafened adults, to discriminate between resynthesized "talkers" based on either fundamental frequency (F0) cues, VTL cues, or both. Performance was compared to individuals with normal hearing (NH), listening either to degraded stimuli, using a noise-excited channel vocoder, or non-degraded stimuli. Results show that (a) age of implantation was associated with VTL but not F0 cues in discriminating between talkers, with improved discrimination for those subjects who were implanted at earlier age; (b) there was a positive relationship for the CI users between VTL discrimination and speech recognition score in quiet and in noise, but not with frequency discrimination or cognitive abilities; (c) early-implanted CI users showed similar voice discrimination ability as the NH adults who listened to vocoded stimuli. These data support the notion that voice discrimination is limited by the speech processing of the CI device. However, they also suggest that early implantation may facilitate sensory-driven tonotopicity and/or improve higher-order auditory functions, enabling better perception of VTL spectral cues for voice discrimination.

Assessment of Spectral and Temporal Resolution in Cochlear Implant Users Using Psychoacoustic Discrimination and Speech Cue Categorization

OBJECTIVES

This study was conducted to measure auditory perception by cochlear implant users in the spectral and temporal domains, using tests of either categorization (using speech-based cues) or discrimination (using conventional psychoacoustic tests). The authors hypothesized that traditional nonlinguistic tests assessing spectral and temporal auditory resolution would correspond to speech-based measures assessing specific aspects of phonetic categorization assumed to depend on spectral and temporal auditory resolution. The authors further hypothesized that speech-based categorization performance would ultimately be a superior predictor of speech recognition performance, because of the fundamental nature of speech recognition as categorization.

DESIGN

Nineteen cochlear implant listeners and 10 listeners with normal hearing participated in a suite of tasks that included spectral ripple discrimination, temporal modulation detection, and syllable categorization, which was split into a spectral cue-based task (targeting the /ba/-/da/ contrast) and a timing cue-based task (targeting the /b/-/p/ and /d/-/t/ contrasts). Speech sounds were manipulated to contain specific spectral or temporal modulations (formant transitions or voice onset time, respectively) that could be categorized. Categorization responses were quantified using logistic regression to assess perceptual sensitivity to acoustic phonetic cues. Word recognition testing was also conducted for cochlear implant listeners.

RESULTS

Cochlear implant users were generally less successful at utilizing both spectral and temporal cues for categorization compared with listeners with normal hearing. For the cochlear implant listener group, spectral ripple discrimination was significantly correlated with the categorization of formant transitions; both were correlated with better word recognition. Temporal modulation detection using 100- and 10-Hz-modulated noise was not correlated either with the cochlear implant subjects' categorization of voice onset time or with word recognition. Word recognition was correlated more closely with categorization of the controlled speech cues than with performance on the psychophysical discrimination tasks.

CONCLUSIONS

When evaluating people with cochlear implants, controlled speech-based stimuli are feasible to use in tests of auditory cue categorization, to complement traditional measures of auditory discrimination. Stimuli based on specific speech cues correspond to counterpart nonlinguistic measures of discrimination, but potentially show better correspondence with speech perception more generally. The ubiquity of the spectral (formant transition) and temporal (voice onset time) stimulus dimensions across languages highlights the potential to use this testing approach even in cases where English is not the native language.

Nonlinguistic Outcome Measures in Adult Cochlear Implant Users Over the First Year of Implantation

OBJECTIVES

Postlingually deaf cochlear implant users' speech perception improves over several months after implantation due to a learning process which involves integration of the new acoustic information presented by the device. Basic tests of hearing acuity might evaluate sensitivity to the new acoustic information and be less sensitive to learning effects. It was hypothesized that, unlike speech perception, basic spectral and temporal discrimination abilities will not change over the first year of implant use. If there were limited change over time and the test scores were correlated with clinical outcome, the tests might be useful for acute diagnostic assessments of hearing ability and also useful for testing speakers of any language, many of which do not have validated speech tests.

DESIGN

Ten newly implanted cochlear implant users were tested for speech understanding in quiet and in noise at 1 and 12 months postactivation. Spectral-ripple discrimination, temporal-modulation detection, and Schroeder-phase discrimination abilities were evaluated at 1, 3, 6, 9, and 12 months postactivation.

RESULTS

Speech understanding in quiet improved between 1 and 12 months postactivation (mean 8% improvement). Speech in noise performance showed no statistically significant improvement. Mean spectral-ripple discrimination thresholds and temporal-modulation detection thresholds for modulation frequencies of 100 Hz and above also showed no significant improvement. Spectral-ripple discrimination thresholds were significantly correlated with speech understanding. Low FM detection and Schroeder-phase discrimination abilities improved over the period. Individual learning trends varied, but the majority of listeners followed the same stable pattern as group data.

CONCLUSIONS

Spectral-ripple discrimination ability and temporal-modulation detection at 100-Hz modulation and above might serve as a useful diagnostic tool for early acute assessment of cochlear implant outcome for listeners speaking any native language.

Current Focusing to Reduce Channel Interaction for Distant Electrodes in Cochlear Implant Programs

Speech understanding abilities are highly variable among cochlear implant (CI) listeners. Poor electrode-neuron interfaces (ENIs) caused by sparse neural survival or distant electrode placement may lead to increased channel interaction and reduced speech perception. Currently, it is not possible to directly measure neural survival in CI listeners; therefore, obtaining information about electrode position is an alternative approach to assessing ENIs. This information can be estimated with computerized tomography (CT) imaging; however, postoperative CT imaging is not often available. A reliable method to assess channel interaction, such as the psychophysical tuning curve (PTC), offers an alternative way to identify poor ENIs. This study aimed to determine (a) the within-subject relationship between CT-estimated electrode distance and PTC bandwidths, and (b) whether using focused stimulation on channels with suspected poor ENI improves vowel identification and sentence recognition. In 13 CI listeners, CT estimates of electrode-to-modiolus distance and PTCs bandwidths were measured for all available electrodes. Two test programs were created, wherein a subset of electrodes used focused stimulation based on (a) broad PTC bandwidth (Tuning) and (b) far electrode-to-modiolus distance (Distance). Two control programs were also created: (a) Those channels not focused in the Distance program (Inverse-Control), and (b) an all-channel monopolar program (Monopolar-Control). Across subjects, scores on the Distance and Tuning programs were significantly higher than the Inverse-Control program, and similar to the Monopolar-Control program. Subjective ratings were similar for all programs. These findings suggest that focusing channels suspected to have a high degree of channel interaction result in quite different outcomes, acutely.

Evaluating Multipulse Integration as a Neural-Health Correlate in Human Cochlear Implant Users: Effects of Stimulation Mode

Previous psychophysical studies have shown that a steep detection-threshold-versus-stimulation-rate function (multipulse integration; MPI) is associated with laterally positioned electrodes producing a broad neural excitation pattern. These findings are consistent with steep MPI depending on either a certain width of neural excitation allowing a large population of neurons operating at a low point on their dynamic range to respond to an increase in stimulation rate or a certain slope of excitation pattern that allows recruitment of neurons at the excitation periphery. Results of the current study provide additional support for these mechanisms by demonstrating significantly flattened MPI functions in narrow bipolar than monopolar stimulation. The study further examined the relationship between the steepness of the psychometric functions for detection (d' versus log current level) and MPI. In contrast to findings in monopolar stimulation, current data measured in bipolar stimulation suggest that steepness of the psychometric functions explained a moderate amount of the across-site variance in MPI. Steepness of the psychometric functions, however, cannot explain why MPI flattened in bipolar stimulation, since slopes of the psychometric functions were comparable in the two stimulation modes. Lastly, our results show that across-site mean MPI measured in monopolar and bipolar stimulation correlated with speech recognition in opposite signs, with steeper monopolar MPI being associated with poorer performance but steeper bipolar MPI being associated with better performance. If steeper MPI requires broad stimulation of the cochlea, the correlation between monopolar MPI and speech recognition can be interpreted as the detrimental effect of poor spectral resolution on speech recognition. Assuming bipolar stimulation produces narrow excitation, and MPI measured in bipolar stimulation reflects primarily responses of the on-site neurons, the correlation between bipolar MPI and speech recognition can be understood in light of the importance of neural survival for speech recognition.

Factors Affecting Outcomes in Cochlear Implant Recipients Implanted With a Perimodiolar Electrode Array Located in Scala Tympani

OBJECTIVE

To identify primary biographic and audiologic factors contributing to cochlear implant (CI) performance variability in quiet and noise by controlling electrode array type and electrode position within the cochlea.

BACKGROUND

Although CI outcomes have improved over time, considerable outcome variability still exists. Biographic, audiologic, and device-related factors have been shown to influence performance. Examining CI recipients with consistent array type and electrode position may allow focused investigation into outcome variability resulting from biographic and audiologic factors.

METHODS

Thirty-nine adults (40 ears) implanted for at least 6 months with a perimodiolar electrode array known (via computed tomography [CT] imaging) to be in scala tympani participated. Test materials, administered CI only, included monosyllabic words, sentences in quiet and noise, and spectral ripple discrimination.

RESULTS

In quiet, scores were high with mean word and sentence scores of 76 and 87%, respectively; however, sentence scores decreased by an average of 35 percentage points when noise was added. A principal components (PC) analysis of biographic and audiologic factors found three distinct factors, PC1 Age, PC2 Duration, and PC3 Pre-op Hearing. PC1 Age was the only factor that correlated, albeit modestly, with speech recognition in quiet and noise. Spectral ripple discrimination strongly correlated with speech measures.

CONCLUSION

For these recipients with consistent electrode position, PC1 Age was related to speech recognition performance. Consistent electrode position may have contributed to high speech understanding in quiet. Inter-subject variability in noise may have been influenced by auditory/cognitive processing, known to decline with age, and mechanisms that underlie spectral resolution ability.

The Electrically Evoked Compound Action Potential: From Laboratory to Clinic

The electrically evoked compound action potential (eCAP) represents the synchronous firing of a population of electrically stimulated auditory nerve fibers. It can be directly recorded on a surgically exposed nerve trunk in animals or from an intra-cochlear electrode of a cochlear implant. In the past two decades, the eCAP has been widely recorded in both animals and clinical patient populations using different testing paradigms. This paper provides an overview of recording methodologies and response characteristics of the eCAP, as well as its potential applications in research and clinical situations. Relevant studies are reviewed and implications for clinicians are discussed.

Spectro-temporal cues enhance modulation sensitivity in cochlear implant users

Although speech understanding is highly variable amongst cochlear implants (CIs) subjects, the remarkably high speech recognition performance of many CI users is unexpected and not well understood. Numerous factors, including neural health and degradation of the spectral information in the speech signal of CIs, likely contribute to speech understanding. We studied the ability to use spectro-temporal modulations, which may be critical for speech understanding and discrimination, and hypothesize that CI users adopt a different perceptual strategy than normal-hearing (NH) individuals, whereby they rely more heavily on joint spectro-temporal cues to enhance detection of auditory cues. Modulation detection sensitivity was studied in CI users and NH subjects using broadband "ripple" stimuli that were modulated spectrally, temporally, or jointly, i.e., spectro-temporally. The spectro-temporal modulation transfer functions of CI users and NH subjects was decomposed into spectral and temporal dimensions and compared to those subjects' spectral-only and temporal-only modulation transfer functions. In CI users, the joint spectro-temporal sensitivity was better than that predicted by spectral-only and temporal-only sensitivity, indicating a heightened spectro-temporal sensitivity. Such an enhancement through the combined integration of spectral and temporal cues was not observed in NH subjects. The unique use of spectro-temporal cues by CI patients can yield benefits for use of cues that are important for speech understanding. This finding has implications for developing sound processing strategies that may rely on joint spectro-temporal modulations to improve speech comprehension of CI users, and the findings of this study may be valuable for developing clinical assessment tools to optimize CI processor performance.

Effects of age and hearing mechanism on spectral resolution in normal hearing and cochlear-implanted listeners

Spectral resolution limits speech perception with a cochlear implant (CI) in post-lingually deaf adults. However, the development of spectral resolution in pre-lingually deaf implanted children is not well understood. Acoustic spectral resolution was measured as a function of age (school-age versus adult) in CI and normal-hearing (NH) participants using spectral ripple discrimination (SRD). A 3-alternative forced-choice task was used to obtain SRD thresholds at five ripple depths. Effects of age and hearing method on SRD and spectral modulation transfer function (SMTF) slope (reflecting frequency resolution) and x-intercept (reflecting across-channel intensity resolution) were examined. Correlations between SRD, SMTF parameters, age, and speech perception in noise were studied. Better SRD in NH than CI participants was observed at all depths. SRD thresholds and SMTF slope correlated with speech perception in CI users. When adjusted for floor performance, x-intercept did not correlate with SMTF slope or speech perception. Age and x-intercept correlations were positive and significant in NH but not CI children suggesting that across-channel intensity resolution matures during school-age in NH children. No evidence for maturation of spectral resolution beyond early school-age in pre-lingually deaf implanted CI users was found in the present study.

Low-frequency fine-structure cues allow for the online use of lexical stress during spoken-word recognition in spectrally degraded speech

English listeners use suprasegmental cues to lexical stress during spoken-word recognition. Prosodic cues are, however, less salient in spectrally degraded speech, as provided by cochlear implants. The present study examined how spectral degradation with and without low-frequency fine-structure information affects normal-hearing listeners' ability to benefit from suprasegmental cues to lexical stress in online spoken-word recognition. To simulate electric hearing, an eight-channel vocoder spectrally degraded the stimuli while preserving temporal envelope information. Additional lowpass-filtered speech was presented to the opposite ear to simulate bimodal hearing. Using a visual world paradigm, listeners' eye fixations to four printed words (target, competitor, two distractors) were tracked, while hearing a word. The target and competitor overlapped segmentally in their first two syllables but mismatched suprasegmentally in their first syllables, as the initial syllable received primary stress in one word and secondary stress in the other (e.g., "'admiral," "'admi'ration"). In the vocoder-only condition, listeners were unable to use lexical stress to recognize targets before segmental information disambiguated them from competitors. With additional lowpass-filtered speech, however, listeners efficiently processed prosodic information to speed up online word recognition. Low-frequency fine-structure cues in simulated bimodal hearing allowed listeners to benefit from suprasegmental cues to lexical stress during word recognition.

Fitting prelingually deafened adult cochlear implant users based on electrode discrimination performance

OBJECTIVE

This study investigated the hypotheses that (1) prelingually deafened CI users do not have perfect electrode discrimination ability and (2) the deactivation of non-discriminable electrodes can improve auditory performance.

DESIGN

Electrode discrimination difference limens were determined for all electrodes of the array. The subjects' basic map was subsequently compared to an experimental map, which contained only discriminable electrodes, with respect to speech understanding in quiet and in noise, listening effort, spectral ripple discrimination and subjective appreciation.

STUDY SAMPLE

Subjects were six prelingually deafened, late implanted adults using the Nucleus cochlear implant.

RESULTS

Electrode discrimination difference limens across all subjects and electrodes ranged from 0.5 to 7.125, with significantly larger limens for basal electrodes. No significant differences were found between the basic map and the experimental map on auditory tests. Subjective appreciation was found to be significantly poorer for the experimental map.

CONCLUSIONS

Prelingually deafened CI users were unable to discriminate between all adjacent electrodes. There was no difference in auditory performance between the basic and experimental map. Potential factors contributing to the absence of improvement with the experimental map include the reduced number of maxima, incomplete adaptation to the new frequency allocation, and the mainly basal location of deactivated electrodes.

Reducing Channel Interaction Through Cochlear Implant Programming May Improve Speech Perception: Current Focusing and Channel Deactivation

Speech perception among cochlear implant (CI) listeners is highly variable. High degrees of channel interaction are associated with poorer speech understanding. Two methods for reducing channel interaction, focusing electrical fields, and deactivating subsets of channels were assessed by the change in vowel and consonant identification scores with different program settings. The main hypotheses were that (a) focused stimulation will improve phoneme recognition and (b) speech perception will improve when channels with high thresholds are deactivated. To select high-threshold channels for deactivation, subjects’ threshold profiles were processed to enhance the peaks and troughs, and then an exclusion or inclusion criterion based on the mean and standard deviation was used. Low-threshold channels were selected manually and matched in number and apex-to-base distribution. Nine ears in eight adult CI listeners with Advanced Bionics HiRes90k devices were tested with six experimental programs. Two, all-channel programs, (a) 14-channel partial tripolar (pTP) and (b) 14-channel monopolar (MP), and four variable-channel programs, derived from these two base programs, (c) pTP with high- and (d) low-threshold channels deactivated, and (e) MP with high- and (f) low-threshold channels deactivated, were created. Across subjects, performance was similar with pTP and MP programs. However, poorer performing subjects (scoring < 62% correct on vowel identification) tended to perform better with the all-channel pTP than with the MP program (1 > 2). These same subjects showed slightly more benefit with the reduced channel MP programs (5 and 6). Subjective ratings were consistent with performance. These finding suggest that reducing channel interaction may benefit poorer performing CI listeners.

The Impact of Auditory Spectral Resolution on Listening Effort Revealed by Pupil Dilation

OBJECTIVES

This study measured the impact of auditory spectral resolution on listening effort. Systematic degradation in spectral resolution was hypothesized to elicit corresponding systematic increases in pupil dilation, consistent with the notion of pupil dilation as a marker of cognitive load.

DESIGN

Spectral resolution of sentences was varied with two different vocoders: (1) a noise-channel vocoder with a variable number of spectral channels; and (2) a vocoder designed to simulate front-end processing of a cochlear implant, including peak-picking channel selection with variable synthesis filter slopes to simulate spread of neural excitation. Pupil dilation was measured after subject-specific luminance adjustment and trial-specific baseline measures. Mixed-effects growth curve analysis was used to model pupillary responses over time.

RESULTS

For both types of vocoder, pupil dilation grew with each successive degradation in spectral resolution. Within each condition, pupillary responses were not related to intelligibility scores, and the effect of spectral resolution on pupil dilation persisted even when only analyzing trials in which responses were 100% correct.

CONCLUSIONS

Intelligibility scores alone were not sufficient to quantify the effort required to understand speech with poor resolution. Degraded spectral resolution results in increased effort required to understand speech, even when intelligibility is at 100%. Pupillary responses were a sensitive and highly granular measurement to reveal changes in listening effort. Pupillary responses might potentially reveal the benefits of aural prostheses that are not captured by speech intelligibility performance alone as well as the disadvantages that are overcome by increased listening effort.

Assessing the Electrode-Neuron Interface with the Electrically Evoked Compound Action Potential, Electrode Position, and Behavioral Thresholds

Variability in speech perception scores among cochlear implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual cochlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.

Spectrotemporal Modulation Detection and Speech Perception by Cochlear Implant Users

Spectrotemporal modulation (STM) detection performance was examined for cochlear implant (CI) users. The test involved discriminating between an unmodulated steady noise and a modulated stimulus. The modulated stimulus presents frequency modulation patterns that change in frequency over time. In order to examine STM detection performance for different modulation conditions, two different temporal modulation rates (5 and 10 Hz) and three different spectral modulation densities (0.5, 1.0, and 2.0 cycles/octave) were employed, producing a total 6 different STM stimulus conditions. In order to explore how electric hearing constrains STM sensitivity for CI users differently from acoustic hearing, normal-hearing (NH) and hearing-impaired (HI) listeners were also tested on the same tasks. STM detection performance was best in NH subjects, followed by HI subjects. On average, CI subjects showed poorest performance, but some CI subjects showed high levels of STM detection performance that was comparable to acoustic hearing. Significant correlations were found between STM detection performance and speech identification performance in quiet and in noise. In order to understand the relative contribution of spectral and temporal modulation cues to speech perception abilities for CI users, spectral and temporal modulation detection was performed separately and related to STM detection and speech perception performance. The results suggest that that slow spectral modulation rather than slow temporal modulation may be important for determining speech perception capabilities for CI users. Lastly, test–retest reliability for STM detection was good with no learning. The present study demonstrates that STM detection may be a useful tool to evaluate the ability of CI sound processing strategies to deliver clinically pertinent acoustic modulation information.

Cochlear implant users' spectral ripple resolution

This study revisits the issue of the spectral ripple resolution abilities of cochlear implant (CI) users. The spectral ripple resolution of recently implanted CI recipients (implanted during the last 10 years) were compared to those of CI recipients implanted 15 to 20 years ago, as well as those of normal-hearing and hearing-impaired listeners from previously published data from Henry, Turner, and Behrens [J. Acoust. Soc. Am. 118, 1111-1121 (2005)]. More recently, implanted CI recipients showed significantly better spectral ripple resolution. There is no significant difference in spectral ripple resolution for these recently implanted subjects compared to hearing-impaired (acoustic) listeners. The more recently implanted CI users had significantly better pre-operative speech perception than previously reported CI users. These better pre-operative speech perception scores in CI users from the current study may be related to better performance on the spectral ripple discrimination task; however, other possible factors such as improvements in internal and external devices cannot be excluded.

Relationships Among Peripheral and Central Electrophysiological Measures of Spatial and Spectral Selectivity and Speech Perception in Cochlear Implant Users

OBJECTIVES

The ability to perceive speech is related to the listener's ability to differentiate among frequencies (i.e., spectral resolution). Cochlear implant (CI) users exhibit variable speech-perception and spectral-resolution abilities, which can be attributed in part to the extent of electrode interactions at the periphery (i.e., spatial selectivity). However, electrophysiological measures of peripheral spatial selectivity have not been found to correlate with speech perception. The purpose of this study was to evaluate auditory processing at the periphery and cortex using both simple and spectrally complex stimuli to better understand the stages of neural processing underlying speech perception. The hypotheses were that (1) by more completely characterizing peripheral excitation patterns than in previous studies, significant correlations with measures of spectral selectivity and speech perception would be observed, (2) adding information about processing at a level central to the auditory nerve would account for additional variability in speech perception, and (3) responses elicited with spectrally complex stimuli would be more strongly correlated with speech perception than responses elicited with spectrally simple stimuli.

DESIGN

Eleven adult CI users participated. Three experimental processor programs (MAPs) were created to vary the likelihood of electrode interactions within each participant. For each MAP, a subset of 7 of 22 intracochlear electrodes was activated: adjacent (MAP 1), every other (MAP 2), or every third (MAP 3). Peripheral spatial selectivity was assessed using the electrically evoked compound action potential (ECAP) to obtain channel-interaction functions for all activated electrodes (13 functions total). Central processing was assessed by eliciting the auditory change complex with both spatial (electrode pairs) and spectral (rippled noise) stimulus changes. Speech-perception measures included vowel discrimination and the Bamford-Kowal-Bench Speech-in-Noise test. Spatial and spectral selectivity and speech perception were expected to be poorest with MAP 1 (closest electrode spacing) and best with MAP 3 (widest electrode spacing). Relationships among the electrophysiological and speech-perception measures were evaluated using mixed-model and simple linear regression analyses.

RESULTS

All electrophysiological measures were significantly correlated with each other and with speech scores for the mixed-model analysis, which takes into account multiple measures per person (i.e., experimental MAPs). The ECAP measures were the best predictor. In the simple linear regression analysis on MAP 3 data, only the cortical measures were significantly correlated with speech scores; spectral auditory change complex amplitude was the strongest predictor.

CONCLUSIONS

The results suggest that both peripheral and central electrophysiological measures of spatial and spectral selectivity provide valuable information about speech perception. Clinically, it is often desirable to optimize performance for individual CI users. These results suggest that ECAP measures may be most useful for within-subject applications when multiple measures are performed to make decisions about processor options. They also suggest that if the goal is to compare performance across individuals based on a single measure, then processing central to the auditory nerve (specifically, cortical measures of discriminability) should be considered.

Peripheral and Central Contributions to Cortical Responses in Cochlear Implant Users

OBJECTIVES

The primary goal of this study was to describe relationships between peripheral and central electrophysiologic measures of auditory processing within individual cochlear implant (CI) users. The distinctiveness of neural excitation patterns resulting from the stimulation of different electrodes, referred to as 'spatial selectivity,' was evaluated. The hypothesis was that if central representations of spatial interactions differed across participants semi-independently of peripheral input, then the within-subject relationships between peripheral and central electrophysiologic measures of spatial selectivity would reflect those differences. Cross-subject differences attributable to processing central to the auditory nerve may help explain why peripheral electrophysiologic measures of spatial selectivity have not been found to correlate with speech perception.

DESIGN

Eleven adults participated in this and a companion study. All were peri- or post-lingually deafened with more than 1 year of CI experience. Peripheral spatial selectivity was evaluated at 13 cochlear locations using 13 electrodes as probes to elicit electrically evoked compound action potentials (ECAPs). Masker electrodes were varied across the array for each probe electrode to derive channel-interaction functions. The same 13 electrodes were used to evaluate spatial selectivity represented at a cortical level. Electrode pairs were stimulated sequentially to elicit the auditory change complex (ACC), an obligatory cortical potential suggestive of discrimination. For each participant, the relationship between ECAP channel-interaction functions (quantified as channel-separation indices) and ACC N1-P2 amplitudes was modeled using the saturating exponential function y = a * (1-e). Both a and b coefficients were varied using a least-squares approach to optimize the fits.

RESULTS

Electrophysiologic measures of spatial selectivity assessed at peripheral (ECAP) and central (ACC) levels varied across participants. The results indicate that differences in ACC amplitudes observed across participants for the same stimulus conditions were not solely the result of differences in peripheral excitation patterns. This finding supports the view that processing at multiple points along the auditory neural pathway from the periphery to the cortex may vary across individuals with different etiologies and auditory experiences.

CONCLUSIONS

The distinctiveness of neural excitation resulting from electrical stimulation varies across CI recipients, and this variability was observed in both peripheral and cortical electrophysiologic measures. The ACC amplitude differences observed across participants were partially independent from differences in peripheral neural spatial selectivity. These findings are clinically relevant because they imply that there may be limits (1) to the predictive ability of peripheral measures and (2) in the extent to which improving the selectivity of electrical stimulation via programming options (e.g., current focusing/steering) will result in more specific central neural excitation patterns or will improve speech perception.

Spectral and temporal analysis of simulated dead regions in cochlear implants

A cochlear implant (CI) electrode in a "cochlear dead region" will excite neighboring neural populations. In previous research that simulated such dead regions, stimulus information in the simulated dead region was either added to the immediately adjacent frequency regions or dropped entirely. There was little difference in speech perception ability between the two conditions. This may imply that there may be little benefit of ensuring that stimulus information on an electrode in a suspected cochlear dead region is transmitted. Alternatively, performance may be enhanced by a broader frequency redistribution, rather than adding stimuli from the dead region to the edges. In the current experiments, cochlear dead regions were introduced by excluding selected CI electrodes or vocoder noise-bands. Participants were assessed for speech understanding as well as spectral and temporal sensitivities as a function of the size of simulated dead regions. In one set of tests, the normal input frequency range of the sound processor was distributed among the active electrodes in bands with approximately logarithmic spacing ("redistributed" maps); in the remaining tests, information in simulated dead regions was dropped ("dropped" maps). Word recognition and Schroeder-phase discrimination performance, which require both spectral and temporal sensitivities, decreased as the size of simulated dead regions increased, but the redistributed and dropped remappings showed similar performance in these two tasks. Psychoacoustic experiments showed that the near match in word scores may reflect a tradeoff between spectral and temporal sensitivity: spectral-ripple discrimination was substantially degraded in the redistributed condition relative to the dropped condition while performance in a temporal modulation detection task degraded in the dropped condition but remained constant in the redistributed condition.

Using speech sounds to test functional spectral resolution in listeners with cochlear implants

In this study, spectral properties of speech sounds were used to test functional spectral resolution in people who use cochlear implants (CIs). Specifically, perception of the /ba/-/da/ contrast was tested using two spectral cues: Formant transitions (a fine-resolution cue) and spectral tilt (a coarse-resolution cue). Higher weighting of the formant cues was used as an index of better spectral cue perception. Participants included 19 CI listeners and 10 listeners with normal hearing (NH), for whom spectral resolution was explicitly controlled using a noise vocoder with variable carrier filter widths to simulate electrical current spread. Perceptual weighting of the two cues was modeled with mixed-effects logistic regression, and was found to systematically vary with spectral resolution. The use of formant cues was greatest for NH listeners for unprocessed speech, and declined in the two vocoded conditions. Compared to NH listeners, CI listeners relied less on formant transitions, and more on spectral tilt. Cue-weighting results showed moderately good correspondence with word recognition scores. The current approach to testing functional spectral resolution uses auditory cues that are known to be important for speech categorization, and can thus potentially serve as the basis upon which CI processing strategies and innovations are tested.

Objective measures of electrode discrimination with electrically evoked auditory change complex and speech-perception abilities in children with auditory neuropathy spectrum disorder

OBJECTIVES

This study aimed to (1) determine the sensitivity of the electrically evoked auditory change complex (eACC) to changes in stimulating electrode position; and (2) investigate the association between results of eACC measures and behavioral electrode discrimination and their association with speech-perception performance in pediatric cochlear implant (CI) users who have auditory neuropathy spectrum disorder (ANSD).

DESIGN

Fifteen children with ANSD ranging in age between 5.4 and 18.6 years participated in this study. All subjects used Cochlear Nucleus devices. For each subject, open-set speech-perception ability was assessed using the Phonetically Balanced Kindergarten word lists presented at 60 dB SPL, using monitored live voice in a sound booth. Behavioral and objective measures of electrode discrimination were assessed in a nonclinical test environment. The stimuli used to elicit these measures were 800 msec biphasic pulse trains delivered by a direct interface to the CI. Data were collected from two basic stimulation conditions. In the standard condition, the entire pulse train was delivered to a mid-array electrode (electrode 11 or 12) at the maximum comfortable level (C level). In the change condition, the stimulus was split into two 400 msec pulse train segments presented sequentially on two different electrodes. The stimulation level of the second 400 msec pulse train was loudness balanced to the C level of the mid-array electrode used in the standard condition. The separation between the pair of stimulating electrodes was systematically varied. For behavioral electrode-discrimination measures, each subject was required to determine whether he or she heard one or two sounds for stimuli presented in different stimulation conditions. For the eACC measures, two replicates of 100 artifact-free sweeps were recorded for each stimulation condition.

RESULTS

The eACC in response to changes in stimulating electrode position was recorded from all subjects with ANSD using direct electrical stimulation. Electrode-discrimination thresholds determined with the eACC and behavioral measures were consistent. Children with ANSD using CIs who showed poorer speech performance also required larger separations between the stimulating electrode pair to reliably elicit the eACC than subjects with better speech-perception performance. There was a robust correlation between electrode-discrimination capacities and speech-perception performances in subjects tested in this study. The effect of electrode separation on eACC amplitudes was not monotonic.

CONCLUSIONS

These results demonstrate the feasibility of using eACC to evaluate electrode-discrimination capacities in children with ANSD. These results suggest that the eACC elicited by changes in stimulating electrode position holds great promise as an objective tool for evaluating spectral-pattern detection in such subjects, which may be predictive of their potential speech-perception performance.