Binaural fusion and listening effort in children who use bilateral cochlear implants: a psychoacoustic and pupillometric study

The temporal mismatch across listening sides affects cortical auditory evoked responses in normal hearing listeners and cochlear implant users with contralateral acoustic hearing

Combining a cochlear implant with contralateral acoustic hearing typically enhances speech understanding, although this improvement varies among CI users and can lead to an interference effect. This variability may be associated with the effectiveness of the integration between electric and acoustic stimulation, which might be affected by the temporal mismatch between the two listening sides. Finding methods to compensate for the temporal mismatch might contribute to the optimal adjustment of bimodal devices and to improve hearing in CI users with contralateral acoustic hearing. The current study investigates cortical auditory evoked potentials (CAEPs) in normal hearing listeners (NH) and CI users with contralateral acoustic hearing. In NH, the amplitude of the N1 peak and the maximum phase locking value (PLV) were analyzed under monaural, binaural, and binaural temporally mismatched conditions. In CI users, CAEPs were measured when listening with CI only (CIS_only), acoustically only (AS_only) and with both sides together (CIS+AS). When listening with CIS+AS, various interaural delays were introduced between the electric and acoustic stimuli. In NH listeners, interaural temporal mismatch resulted in decreased N1 amplitude and PLV. Moreover, PLV is suggested as a more sensitive measure to investigate the integration of information between the two listening sides. CI users showed varied N1 latencies between the AS_only and CIS_only listening conditions, with increased N1 amplitude when the temporal mismatch was compensated. A tendency towards increased PLV was also observed, however, to a lesser extent than in NH listeners, suggesting a limited integration between electric and acoustic stimulation. This work highlights the potential of CAEPs measurement to investigate cortical processing of the information between two listening sides in NH and bimodal CI users.

Listening Effort Measured With Pupillometry in Cochlear Implant Users Depends on Sound Level, But Not on the Signal to Noise Ratio When Using the Matrix Test

OBJECTIVES

We investigated whether listening effort is dependent on task difficulty for cochlear implant (CI) users when using the Matrix speech-in-noise test. To this end, we measured peak pupil dilation (PPD) at a wide range of signal to noise ratios (SNR) by systematically changing the noise level at a constant speech level, and vice versa.

DESIGN

A group of mostly elderly CI users performed the Dutch/Flemish Matrix test in quiet and in multitalker babble at different SNRs. SNRs were set relative to the speech-recognition threshold (SRT), namely at SRT, and 5 and 10 dB above SRT (0 dB, +5 dB, and +10 dB re SRT). The latter 2 conditions were obtained by either varying speech level (at a fixed noise level of 60 dBA) or by varying noise level (with a fixed speech level). We compared these PPDs with those of a group of typical hearing (TH) listeners. In addition, listening effort was assessed with subjective ratings on a Likert scale.

RESULTS

PPD for the CI group did not significantly depend on SNR, whereas SNR significantly affected PPDs for TH listeners. Subjective effort ratings depended significantly on SNR for both groups. For CI users, PPDs were significantly larger, and effort was rated higher when speech was varied, and noise was fixed for CI users. By contrast, for TH listeners effort ratings were significantly higher and performance scores lower when noise was varied, and speech was fixed.

CONCLUSIONS

The lack of a significant effect of varying SNR on PPD suggests that the Matrix test may not be a feasible speech test for measuring listening effort with pupillometric measures for CI users. A rating test appeared more promising in this population, corroborating earlier reports that subjective measures may reflect different dimensions of listening effort than pupil dilation. Establishing the SNR by varying speech or noise level can have subtle, but significant effects on measures of listening effort, and these effects can differ between TH listeners and CI users.

Review of Binaural Processing With Asymmetrical Hearing Outcomes in Patients With Bilateral Cochlear Implants

Bilateral cochlear implants (BiCIs) result in several benefits, including improvements in speech understanding in noise and sound source localization. However, the benefit bilateral implants provide among recipients varies considerably across individuals. Here we consider one of the reasons for this variability: difference in hearing function between the two ears, that is, interaural asymmetry. Thus far, investigations of interaural asymmetry have been highly specialized within various research areas. The goal of this review is to integrate these studies in one place, motivating future research in the area of interaural asymmetry. We first consider bottom-up processing, where binaural cues are represented using excitation-inhibition of signals from the left ear and right ear, varying with the location of the sound in space, and represented by the lateral superior olive in the auditory brainstem. We then consider top-down processing via predictive coding, which assumes that perception stems from expectations based on context and prior sensory experience, represented by cascading series of cortical circuits. An internal, perceptual model is maintained and updated in light of incoming sensory input. Together, we hope that this amalgamation of physiological, behavioral, and modeling studies will help bridge gaps in the field of binaural hearing and promote a clearer understanding of the implications of interaural asymmetry for future research on optimal patient interventions.

Quality of Life Impact of Cochlear Implantation for Single-Sided Deafness: Assessing the Interrelationship of Objective and Subjective Measures

OBJECTIVE

To determine the effect on quality of life (QOL) of cochlear implantation (CI) for single-sided deafness (SSD) and asymmetric hearing loss (AHL) using the first psychometrically developed CI-specific QOL tool for English-speaking patients and to assess its relationship to objective perceptual measures.

STUDY DESIGN

Retrospective cohort study.

SETTING

Tertiary-care medical center.

PATIENTS

English-speaking adults with SSD or AHL.

INTERVENTIONS

Unilateral CI.

MAIN OUTCOME MEASURES

Cochlear Implant Quality of Life (CIQOL) score, CI-alone speech-in-quiet (SIQ) score (CNC and AzBio), binaural speech-in-noise (SIN) threshold, binaural azimuthal sound localization (SL) error.

RESULTS

At the most recent postoperative evaluation (median, 9.3 months postimplantation), 25 of 28 subjects (89%) had a CIQOL improvement, with the improvement considered clinically beneficial (>3 points) for 18 of 28 subjects (64%). Group-mean CIQOL improvement was observed at the first postoperative visit and did not change significantly thereafter. Objective perceptual measures (SL, SIQ, SIN) continued to improve over 12 months after implantation. Linear mixed-model regression analyses showed a moderate positive correlation between SIN and SIQ improvements (r = 0.50 to 0.59, p < 0.0001) and a strong positive correlation between the improvement in the two SIQ measures (r = 0.89, p < 0.0001). No significant relationships were observed ( p > 0.05) among QOL or the objective perceptual measures.

CONCLUSIONS

QOL improved for the majority of subjects implanted for SSD and AHL. Different time courses for improvement in QOL and audiologic tests, combined with the lack of significant relationships among them, suggest that QOL outcomes reflect different aspects of the CI experience than those captured by speech-understanding and localization measures. SIQ may substitute for SIN when clinical constraints exist.

Interaural speech asymmetry predicts bilateral speech intelligibility but not listening effort in adults with bilateral cochlear implants

Introduction

Bilateral cochlear implants (BiCIs) can facilitate improved speech intelligibility in noise and sound localization abilities compared to a unilateral implant in individuals with bilateral severe to profound hearing loss. Still, many individuals with BiCIs do not benefit from binaural hearing to the same extent that normal hearing (NH) listeners do. For example, binaural redundancy, a speech intelligibility benefit derived from having access to duplicate copies of a signal, is highly variable among BiCI users. Additionally, patients with hearing loss commonly report elevated listening effort compared to NH listeners. There is some evidence to suggest that BiCIs may reduce listening effort compared to a unilateral CI, but the limited existing literature has not shown this consistently. Critically, no studies to date have investigated this question using pupillometry to quantify listening effort, where large pupil sizes indicate high effort and small pupil sizes indicate low effort. Thus, the present study aimed to build on existing literature by investigating the potential benefits of BiCIs for both speech intelligibility and listening effort.

Methods

Twelve BiCI adults were tested in three listening conditions: Better Ear, Poorer Ear, and Bilateral. Stimuli were IEEE sentences presented from a loudspeaker at 0° azimuth in quiet. Participants were asked to repeat back the sentences, and responses were scored by an experimenter while changes in pupil dilation were measured.

Results

On average, participants demonstrated similar speech intelligibility in the Better Ear and Bilateral conditions, and significantly worse speech intelligibility in the Poorer Ear condition. Despite similar speech intelligibility in the Better Ear and Bilateral conditions, pupil dilation was significantly larger in the Bilateral condition.

Discussion

These results suggest that the BiCI users tested in this study did not demonstrate binaural redundancy in quiet. The large interaural speech asymmetries demonstrated by participants may have precluded them from obtaining binaural redundancy, as shown by the inverse relationship between the two variables. Further, participants did not obtain a release from effort when listening with two ears versus their better ear only. Instead, results indicate that bilateral listening elicited increased effort compared to better ear listening, which may be due to poor integration of asymmetric inputs.

Gamma-Band Modulation in Parietal Area as the Electroencephalographic Signature for Performance in Auditory-Verbal Working Memory: An Exploratory Pilot Study in Hearing and Unilateral Cochlear Implant Children

This pilot study investigates the neurophysiological patterns of visual and auditory verbal working memory (VWM) in unilateral cochlear implant users (UCIs). We compared the task-related electroencephalogram (EEG) power spectral density of 7- to 13-year-old UCIs (n = 7) with a hearing control group (HC, n = 10) during the execution of a three-level n-back task with auditory and visual verbal (letters) stimuli. Performances improved as memory load decreased regardless of sensory modality (SM) and group factors. Theta EEG activation over the frontal area was proportionally influenced by task level; the left hemisphere (LH) showed greater activation in the gamma band, suggesting lateralization of VWM function regardless of SM. However, HCs showed stronger activation patterns in the LH than UCIs regardless of SM and in the parietal area (PA) during the most challenging audio condition. Linear regressions for gamma activation in the PA suggest the presence of a pattern-supporting auditory VWM only in HCs. Our findings seem to recognize gamma activation in the PA as the signature of effective auditory VWM. These results, although preliminary, highlight this EEG pattern as a possible cause of the variability found in VWM outcomes in deaf children, opening up new possibilities for interdisciplinary research and rehabilitation intervention.

Spectral and binaural loudness summation of equally loud narrowband signals in single-sided-deafness and bilateral cochlear implant users

Recent studies on loudness perception of binaural broadband signals in hearing impaired listeners found large individual differences, suggesting the use of such signals in hearing aid fitting. Likewise, clinical cochlear implant (CI) fitting with narrowband/single-electrode signals might cause suboptimal loudness perception in bilateral and bimodal CI listeners. Here spectral and binaural loudness summation in normal hearing (NH) listeners, bilateral CI (biCI) users, and unilateral CI (uCI) users with normal hearing in the unaided ear was investigated to assess the relevance of binaural/bilateral fitting in CI users. To compare the three groups, categorical loudness scaling was performed for an equal categorical loudness noise (ECLN) consisting of the sum of six spectrally separated third-octave noises at equal loudness. The acoustical ECLN procedure was adapted to an equivalent procedure in the electrical domain using direct stimulation. To ensure the same broadband loudness in binaural measurements with simultaneous electrical and acoustical stimulation, a modified binaural ECLN was introduced and cross validated with self-adjusted loudness in a loudness balancing experiment. Results showed a higher (spectral) loudness summation of the six equally loud narrowband signals in the ECLN in CI compared to NH. Binaural loudness summation was found for all three listener groups (NH, uCI, and biCI). No increased binaural loudness summation could be found for the current uCI and biCI listeners compared to the NH group. In uCI loudness balancing between narrowband signals and single electrodes did not automatically result in a balanced loudness perception across ears, emphasizing the importance of binaural/bilateral fitting.

Cortical imbalance following delayed restoration of bilateral hearing in deaf adolescents

Unilateral auditory deprivation in early childhood can lead to cortical strengthening of inputs from the stimulated side, yet the impact of this on bilateral processing when inputs are later restored beyond an early sensitive period is unknown. To address this, we conducted a longitudinal study with 13 bilaterally profoundly deaf adolescents who received unilateral access to sound via a cochlear implant (CI) in their right ear in early childhood before receiving bilateral access to sound a decade later via a second CI in their left ear. Auditory‐evoked cortical responses to unilateral and bilateral stimulation were measured repeatedly using electroencephalogram from 1 week to 14 months after activation of their second CI. Early cortical responses from the newly implanted ear and bilateral stimulation were atypically lateralized to the left ipsilateral auditory cortex. Duration of unilateral deafness predicted an unexpectedly stronger representation of inputs from the newly implanted, compared to the first implanted ear, in left auditory cortex. Significant initial reductions in responses were observed, yet a left‐hemisphere bias and unequal weighting of inputs favoring the long‐term deaf ear did not converge to a balanced state observed in the binaurally developed system. Bilateral response enhancement was significantly reduced in left auditory cortex suggesting deficits in ipsilateral response inhibition of new, dominant, inputs during bilateral processing. These findings paradoxically demonstrate the adaptive capacity of the adolescent auditory system beyond an early sensitive period for bilateral input, as well as restrictions on its potential to fully reverse cortical imbalances driven by long‐term unilateral deafness.

Attention to Speech and Music in Young Children with Bilateral Cochlear Implants: A Pupillometry Study

Early bilateral cochlear implants (CIs) may enhance attention to speech, and reduce cognitive load in noisy environments. However, it is sometimes difficult to measure speech perception and listening effort, especially in very young children. Behavioral measures cannot always be obtained in young/uncooperative children, whereas objective measures are either difficult to assess or do not reliably correlate with behavioral measures. Recent studies have thus explored pupillometry as a possible objective measure. Here, pupillometry is introduced to assess attention to speech and music in noise in very young children with bilateral CIs (N = 14, age: 17–47 months), and in the age-matched group of normally-hearing (NH) children (N = 14, age: 22–48 months). The results show that the response to speech was affected by the presence of background noise only in children with CIs, but not NH children. Conversely, the presence of background noise altered pupil response to music only in in NH children. We conclude that whereas speech and music may receive comparable attention in comparable listening conditions, in young children with CIs, controlling for background noise affects attention to speech and speech processing more than in NH children. Potential implementations of the results for rehabilitation procedures are discussed.

Interdisciplinary Approaches to the Study of Listening Effort in Young Children with Cochlear Implants

Very early bilateral implantation is thought to significantly reduce the attentional effort required to acquire spoken language, and consequently offer a profound improvement in quality of life. Despite the early intervention, however, auditory and communicative outcomes in children with cochlear implants remain poorer than in hearing children. The distorted auditory input via the cochlear implants requires more auditory attention resulting in increased listening effort and fatigue. Listening effort and fatigue may critically affect attention to speech, and in turn language processing, which may help to explain the variation in language and communication abilities. However, measuring attention to speech and listening effort is demanding in infants and very young children. Three objective techniques for measuring listening effort are presented in this paper that may address the challenges of testing very young and/or uncooperative children with cochlear implants: pupillometry, electroencephalography, and functional near-infrared spectroscopy. We review the studies of listening effort that used these techniques in paediatric populations with hearing loss, and discuss potential benefits of the systematic evaluation of listening effort in these populations.

The Importance of Access to Bilateral Hearing through Cochlear Implants in Children

Children with hearing loss require early access to sound in both ears to support their development. In this article, we describe barriers to providing bilateral hearing and developmental consequences of delays during early sensitive periods. Barriers include late identification of hearing loss in one or both ears and delayed access to intervention with hearing devices such as cochlear implants. Effects of delayed bilateral input on the auditory pathways and brain are discussed as well as behavioral effects on speech perception and other developmental outcomes including language and academics. Evidence for these effects has supported an evolution in cochlear implant candidacy in children that was started with unilateral implantation in children with profound deafness bilaterally to bilateral implantation to implantation of children with asymmetric hearing loss including children with single-side deafness. Opportunities to enhance the developmental benefits of bilateral hearing in children with hearing loss are also discussed including efforts to improve binaural/spatial hearing and consideration of concurrent vestibular deficits which are common in children with hearing loss.

Functional Consequences of Poor Binaural Hearing in Development: Evidence From Children With Unilateral Hearing Loss and Children Receiving Bilateral Cochlear Implants

Poor binaural hearing in children was hypothesized to contribute to related cognitive and academic deficits. Children with unilateral hearing have normal hearing in one ear but no access to binaural cues. Their cognitive and academic deficits could be unique from children receiving bilateral cochlear implants (CIs) at young ages who have poor access to spectral cues and impaired binaural sensitivity. Both groups are at risk for vestibular/balance deficits which could further contribute to memory and learning challenges. Eighty-eight children (43 male:45 female, aged 9.89  ±  3.40 years), grouped by unilateral hearing loss (n = 20), bilateral CI (n = 32), and typically developing (n = 36), completed a battery of sensory, cognitive, and academic tests. Analyses revealed that children in both hearing loss groups had significantly poorer skills (accounting for age) on most tests than their normal hearing peers. Children with unilateral hearing loss had more asymmetric speech perception than children with bilateral CIs (p < .0001) but balance and language deficits (p = .0004, p < .0001, respectively) were similar in the two hearing loss groups (p > .05). Visuospatial memory deficits occurred in both hearing loss groups (p = .02) but more consistently across tests in children with unilateral hearing loss. Verbal memory was not significantly different than normal (p > .05). Principal component analyses revealed deficits in a main cluster of visuospatial memory, oral language, mathematics, and reading measures (explaining 46.8% data variability). The remaining components revealed clusters of self-reported hearing, balance and vestibular function, and speech perception deficits. The findings indicate significant developmental impacts of poor binaural hearing in children.

A comparison of simultaneously-obtained measures of listening effort: pupil dilation, verbal response time and self-rating

OBJECTIVE

The aim of this study was to assess to what extent simultaneously-obtained measures of listening effort (task-evoked pupil dilation, verbal response time [RT], and self-rating) could be sensitive to auditory and cognitive manipulations in a speech perception task. The study also aimed to explore the possible relationship between RT and pupil dilation.

DESIGN

A within-group design was adopted. All participants were administered the Matrix Sentence Test in 12 conditions (signal-to-noise ratios [SNR] of -3, -6, -9 dB; attentional resources focussed vs divided; spatial priors present vs absent).

STUDY SAMPLE

Twenty-four normal-hearing adults, 20-41 years old (M = 23.5), were recruited in the study.

RESULTS

A significant effect of the SNR was found for all measures. However, pupil dilation discriminated only partially between the SNRs. Neither of the cognitive manipulations were effective in modulating the measures. No relationship emerged between pupil dilation, RT and self-ratings.

CONCLUSIONS

RT, pupil dilation, and self-ratings can be obtained simultaneously when administering speech perception tasks, even though some limitations remain related to the absence of a retention period after the listening phase. The sensitivity of the three measures to changes in the auditory environment differs. RTs and self-ratings proved most sensitive to changes in SNR.

Dichotic listening performance and effort as a function of spectral resolution and interaural symmetry

One potential benefit of bilateral cochlear implants is reduced listening effort in speech-on-speech masking situations. However, the symmetry of the input across ears, possibly related to spectral resolution, could impact binaural benefits. Fifteen young adults with normal hearing performed digit recall with target and interfering digits presented to separate ears and attention directed to the target ear. Recall accuracy and pupil size over time (used as an index of listening effort) were measured for unprocessed, 16-channel vocoded, and 4-channel vocoded digits. Recall accuracy was significantly lower for dichotic (with interfering digits) than for monotic listening. Dichotic recall accuracy was highest when the target was less degraded and the interferer was more degraded. With matched target and interferer spectral resolution, pupil dilation was lower with more degradation. Pupil dilation grew more shallowly over time when the interferer had more degradation. Overall, interferer spectral resolution more strongly affected listening effort than target spectral resolution. These results suggest that interfering speech both lowers performance and increases listening effort, and that the relative spectral resolution of target and interferer affect the listening experience. Ignoring a clearer interferer is more effortful.

Binaural Pitch Fusion in Children With Normal Hearing, Hearing Aids, and Cochlear Implants

OBJECTIVES

Binaural pitch fusion is the perceptual integration of stimuli that evoke different pitches between the ears into a single auditory image. Adults who use hearing aids (HAs) or cochlear implants (CIs) often experience abnormally broad binaural pitch fusion, such that sounds differing in pitch by as much as 3 to 4 octaves are fused across ears, leading to spectral averaging and speech perception interference. The main goal of this study was to measure binaural pitch fusion in children with different hearing device combinations and compare results across groups and with adults. A second goal was to examine the relationship of binaural pitch fusion to interaural pitch differences or pitch match range, a measure of sequential pitch discriminability.

DESIGN

Binaural pitch fusion was measured in children between the ages of 6.1 and 11.1 years with bilateral HAs (n = 9), bimodal CI (n = 10), bilateral CIs (n = 17), as well as normal-hearing (NH) children (n = 21). Depending on device combination, stimuli were pure tones or electric pulse trains delivered to individual electrodes. Fusion ranges were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the range that fused with the reference stimulus. Interaural pitch match functions were measured using sequential presentation of reference and comparison stimuli, and varying the comparison stimulus to find the pitch match center and range.

RESULTS

Children with bilateral HAs had significantly broader binaural pitch fusion than children with NH, bimodal CI, or bilateral CIs. Children with NH and bilateral HAs, but not children with bimodal or bilateral CIs, had significantly broader fusion than adults with the same hearing status and device configuration. In children with bilateral CIs, fusion range was correlated with several variables that were also correlated with each other: pure-tone average in the second implanted ear before CI, and duration of prior bilateral HA, bimodal CI, or bilateral CI experience. No relationship was observed between fusion range and pitch match differences or range.

CONCLUSIONS

The findings suggest that binaural pitch fusion is still developing in this age range and depends on hearing device combination but not on interaural pitch differences or discriminability.

Effects of long-term unilateral cochlear implant use on large-scale network synchronization in adolescents

Unilateral cochlear implantation (CI) limits deafness-related changes in the auditory pathways but promotes abnormal cortical preference for the stimulated ear and leaves the opposite ear with little protection from auditory deprivation. In the present study, time-frequency analyses of event-related potentials elicited from stimuli presented to each ear were used to determine effects of unilateral CI use on cortical synchrony. CI-elicited activity in 34 adolescents (15.4±1.9 years of age) who had listened with unilateral CIs for most of their lives prior to bilateral implantation were compared to responses elicited by a 500Hz tone-burst in normal hearing peers. Phase-locking values between 4 and 60Hz were calculated for 171 pairs of 19-cephalic recording electrodes. Ear specific results were found in the normal hearing group: higher synchronization in low frequency bands (theta and alpha) from left ear stimulation in the right hemisphere and more high frequency activity (gamma band) from right ear stimulation in the left hemisphere. In the CI group, increased phase synchronization in the theta and beta frequencies with bursts of gamma activity were elicited by the experienced-right CI between frontal, temporal and parietal cortical regions in both hemispheres, consistent with increased recruitment of cortical areas involved in attention and higher-order processes, potentially to support unilateral listening. By contrast, activity was globally desynchronized in response to initial stimulation of the naïve-left ear, suggesting decoupling of these pathways from the cortical hearing network. These data reveal asymmetric auditory development promoted by unilateral CI use, resulting in an abnormally mature neural network.

Pupillometry Assessment of Speech Recognition and Listening Experience in Adult Cochlear Implant Patients

Objective

The aim of the present study was to investigate the pupillary response to word identification in cochlear implant (CI) patients. Authors hypothesized that when task difficulty (i.e., addition of background noise) increased, pupil dilation markers such as the peak dilation or the latency of the peak dilation would increase in CI users, as already observed in normal-hearing and hearing-impaired subjects.

Methods

Pupillometric measures in 10 CI patients were combined to standard speech recognition scores used to evaluate CI outcomes, namely, speech audiometry in quiet and in noise at +10 dB signal-to-noise ratio (SNR). The main outcome measures of pupillometry were mean pupil dilation, maximal pupil dilation, dilation latency, and mean dilation during return to baseline or retention interval. Subjective hearing quality was evaluated by means of one self-reported fatigue questionnaire, and the Speech, Spatial, and Qualities (SSQ) of Hearing scale.

Results

All pupil dilation data were transformed to percent change in event-related pupil dilation (ERPD, %). Analyses show that the peak amplitudes for both mean pupil dilation and maximal pupil dilation were higher during the speech-in-noise test. Mean peak dilation was measured at 3.47 ± 2.29% noise vs. 2.19 ± 2.46 in quiet and maximal peak value was detected at 9.17 ± 3.25% in noise vs. 8.72 ± 2.93% in quiet. Concerning the questionnaires, the mean pupil dilation during the retention interval was significantly correlated with the spatial subscale score of the SSQ Hearing scale [r(8) = −0.84, p = 0.0023], and with the global score [r(8) = −0.78, p = 0.0018].

Conclusion

The analysis of pupillometric traces, obtained during speech audiometry in quiet and in noise in CI users, provided interesting information about the different processes engaged in this task. Pupillometric measures could be indicative of listening difficulty, phoneme intelligibility, and were correlated with general hearing experience as evaluated by the SSQ of Hearing scale. These preliminary results show that pupillometry constitutes a promising tool to improve objective quantification of CI performance in clinical settings.

Age-Related Changes in Listening Effort for Children and Teenagers With Normal Hearing and Cochlear Implants

OBJECTIVES

A clinically viable measure of listening effort is crucial in safeguarding the educational success of hard-of-hearing students enrolled in mainstream schools. To this end, a novel behavioral paradigm of listening effort targeting school-age children has been designed and reported in Hsu et al. (2017). The current article consists of two follow-up experiments investigating the effects of noise, processing depth, and age in a similar paradigm, first in a group of participants with normal hearing (NH) followed by a sample of school-age cochlear implant (CI) users. Research objectives include the construction of normative values of listening effort and comparing outcomes between age-matched NH and CI participants.

DESIGN

In Experiment 1, the listening effort dual-task paradigm was evaluated in a group of 90 NH participants with roughly even age distribution between 6 and 26 years. The primary task asked a participant to verbally repeat each of the target words presented in either quiet or noise, while the secondary task consisted of categorization true-or-false questions "animal" and "dangerous," representing two levels of semantic processing depth. Two outcome measures were obtained for each condition: a classic word recognition score (WRS) and an average response time (RT) measured during the secondary task. The RT was defined as the main listening effort metric throughout the study. Each NH participant's long-term memory retrieval speed and working memory capacity were also assessed through standardized tests. It was hypothesized that adding noise would negatively affect both WRS and RT, whereas an increase in age would see significant improvement in both measures. A subsequent Experiment 2 administered a shortened version of the paradigm to 14 school-age CI users between 5 and 14 years old at a university clinic. The patterns of results from the CI group were expected to approximate those of the NH group, except with larger between-subject variability.

RESULTS

For NH participants, while WRS was significantly affected by age and noise levels, RT was significantly affected by age, noise levels, and depth of processing. RT was significantly correlated with long-term memory retrieval speed but not with working memory capacity. There was also a significant interaction effect between age and noise levels for both WRS and RT. The RT data set from the NH group served as a basis to establish age-dependent 95% prediction intervals for expected future observations. For CI participants, the effect of age on the two outcome measures was more visible when target words were presented in quiet. Depending on the condition, between 35.7% and 72.7% of the children with CI exhibited higher-than-norms listening effort as measured by categorization processing times.

CONCLUSION

Listening effort appears to decrease with age from early school-age years to late teenage years. The effects of background noise and processing depth are comparable with those reported in Hsu et al. (2017). Future studies interested in expanding the paradigm's clinical viability should focus on the reduction of testing time while maintaining or increasing the sensitivity and external validity of its outcome measures.

Spatial Hearing by Bilateral Cochlear Implant Users With Temporal Fine-Structure Processing

Several studies have demonstrated the advantages of the bilateral vs. unilateral cochlear implantation in listeners with bilateral severe to profound hearing loss. However, it remains unclear to what extent bilaterally implanted listeners have access to binaural cues, e.g., accurate processing of interaural timing differences (ITDs) for low-frequency sounds (<1.5 kHz) and interaural level differences (ILDs) for high frequencies (>3 kHz). We tested 25 adult listeners, bilaterally implanted with MED-EL cochlear implant (CI) devices, with and without fine-structure (FS) temporal processing as encoding strategy in the low-frequency channels. In order to assess whether the ability to process binaural cues was affected by fine-structure processing, we performed psychophysical ILD and ITD sensitivity measurements and free-field sound localization experiments. We compared the results of the bilaterally implanted listeners with different numbers of FS channels. All CI listeners demonstrated good sensitivity to ILDs, but relatively poor to ITD cues. Although there was a large variability in performance, some bilateral CI users showed remarkably good localization skills. The FS coding strategy for bilateral CI hearing did not improve fine-structure ITD processing for spatial hearing on a group level. However, some CI listeners were able to exploit weakly informative temporal cues to improve their low-frequency spatial perception.

Effects of Bimodal and Bilateral Cochlear Implant Use on a Nonauditory Working Memory Task: Reading Span Tests Over 2 Years Following Cochlear Implantation

Purpose A growing body of evidence indicates that treatment of hearing loss by provision of hearing aids leads to improvements in auditory and visual working memory. The purpose of this study was to assess whether similar working memory benefits are observed following provision of cochlear implants (CIs). Method Fifteen adults with postlingually acquired severe bilateral sensorineural hearing loss completed the prospective longitudinal study. Participants were candidates for bilateral cochlear implantation with some aidable hearing in each ear. Implantation surgeries were carried out sequentially, approximately 1 year apart. Working memory was measured with the visual Reading Span Test (Daneman & Carpenter, 1980) at 5 time points: pre-operatively following a 6-month bilateral hearing aid trial, after 6 and 12 months of bimodal (CI plus contralateral hearing aid) listening experience following the 1st CI surgery and activation, and again after 6 and 12 months of bilateral CI listening experience following the 2nd CI surgery and activation. Results Compared to the preoperative baseline, CI listening experience yielded significant improvements in participants' ability to recall test words in the correct serial order after 12 months in the bimodal condition. Individual performance outcomes were variable, but almost all participants showed increases in task performance over the course of the study. Conclusions These results suggest that, similar to appropriate interventions with hearing aids, treatment of hearing loss with CIs can yield working memory benefits. A likely mechanism is the freeing of cognitive resources previously devoted to effortful listening.

Human Auditory Detection and Discrimination Measured with the Pupil Dilation Response

In the standard Hughson-Westlake hearing tests (Carhart and Jerger 1959), patient responses like a button press, raised hand, or verbal response are used to assess detection of brief test signals such as tones of varying pitch and level. Because of its reliance on voluntary responses, Hughson-Westlake audiometry is not suitable for patients who cannot follow instructions reliably, such as pre-lingual infants (Northern and Downs 2002). As an alternative approach, we explored the use of the pupillary dilation response (PDR), a short-latency component of the orienting response evoked by novel stimuli, as an indicator of sound detection. The pupils of 31 adult participants (median age 24 years) were monitored with an infrared video camera during a standard hearing test in which they indicated by button press whether or not they heard narrowband noises centered at 1, 2, 4, and 8 kHz. Tests were conducted in a quiet, carpeted office. Pupil size was summed over the first 1750 ms after stimulus delivery, excluding later dilations linked to expenditure of cognitive effort (Kahneman and Beatty 1966; Kahneman et al. 1969). The PDR yielded thresholds comparable to the standard test at all center frequencies tested, suggesting that the PDR is as sensitive as traditional methods of assessing detection. We also tested the effects of repeating a stimulus on the habituation of the PDR. Results showed that habituation can be minimized by operating at near-threshold stimulus levels. At sound levels well above threshold, the PDR habituated but could be recovered by changing the frequency or sound level, suggesting that the PDR can also be used to test stimulus discrimination. Given these features, the PDR may be useful as an audiometric tool or as a means of assessing auditory discrimination in those who cannot produce a reliable voluntary response.

Binaural hearing is impaired in children with hearing loss who use bilateral hearing aids

This paper asked whether children fitted with bilateral hearing aids (BHA) develop normal perception of binaural cues which are the basis of spatial hearing. Data from children with BHA (n = 26, age = 12.6 ± 2.84 years) were compared to data from a control group (n = 12, age = 12.36 ± 2.83 years). Stimuli were 250 Hz click-trains of 36 ms and a 40 ms consonant-vowel /da/ at 1 Hz presented through ER3A insert-earphones unilaterally or bilaterally. Bilateral stimuli were presented at different interaural level difference (ILD) and interaural timing difference (ITD) conditions. Participants indicated whether the sound came from the left or right side (lateralization) or whether one sound or two could be heard (binaural fusion). BHA children lateralized ILDs similarly to the control group but had impaired lateralization of ITDs. Longer response times relative to controls suggest that lateralization of ITDs was challenging for children with BHA. Most, but not all, of the BHA group were able to fuse click and speech sounds similarly to controls. Those unable to fuse showed particularly poor ITD lateralization. Results suggest that ITD perception is abnormal in children using BHAs, suggesting persistent effects of hearing loss that are not remediated by present clinical rehabilitation protocols.

Behavioral Measures of Listening Effort in School-Age Children: Examining the Effects of Signal-to-Noise Ratio, Hearing Loss, and Amplification

OBJECTIVES

Increased listening effort in school-age children with hearing loss (CHL) could compromise learning and academic achievement. Identifying a sensitive behavioral measure of listening effort for this group could have both clinical and research value. This study examined the effects of signal-to-noise ratio (SNR), hearing loss, and personal amplification on 2 commonly used behavioral measures of listening effort: dual-task visual response times (visual RTs) and verbal response times (verbal RTs).

DESIGN

A total of 82 children (aged 6-13 years) took part in this study; 37 children with normal hearing (CNH) and 45 CHL. All children performed a dual-task paradigm from which both measures of listening effort (dual-task visual RT and verbal RT) were derived. The primary task was word recognition in multi-talker babble in three individually selected SNR conditions: Easy, Moderate, and Hard. The secondary task was a visual monitoring task. Listening effort during the dual-task was quantified as the change in secondary task RT from baseline (single-task visual RT) to the dual-task condition. Listening effort based on verbal RT was quantified as the time elapsed from the onset of the auditory stimulus to the onset of the verbal response when performing the primary (word recognition) task in isolation. CHL completed the task aided and/or unaided to examine the effect of amplification on listening effort.

RESULTS

Verbal RTs were generally slower in the more challenging SNR conditions. However, there was no effect of SNR on dual-task visual RT. Overall, verbal RTs were significantly slower in CHL versus CNH. No group difference in dual-task visual RTs was found between CNH and CHL. No effect of amplification was found on either dual-task visual RTs or verbal RTs.

CONCLUSIONS

This study compared dual-task visual RT and verbal RT measures of listening effort in the child population. Overall, verbal RTs appear more sensitive than dual-task visual RTs to the negative effects of SNR and hearing loss. The current findings extend the literature on listening effort in the pediatric population by demonstrating that, even for speech that is accurately recognized, school-age CHL show a greater processing speed decrement than their normal-hearing counterparts, a decrement that could have a negative impact on learning and academic achievement in the classroom.

Sound Localization in Toddlers with Normal Hearing and with Bilateral Cochlear Implants Revealed Through a Novel "Reaching for Sound" Task

BACKGROUND

Spatial hearing abilities in children with bilateral cochlear implants (BiCIs) are typically improved when two implants are used compared with a single implant. However, even with BiCIs, spatial hearing is still worse compared to normal-hearing (NH) age-matched children. Here, we focused on children who were younger than three years, hence in their toddler years. Prior research with this age focused on measuring discrimination of sounds from the right versus left.

PURPOSE

This study measured both discrimination and sound location identification in a nine-alternative forced-choice paradigm using the "reaching for sound" method, whereby children reached for sounding objects as a means of capturing their spatial hearing abilities.

RESEARCH DESIGN

Discrimination was measured with sounds randomly presented to the left versus right, and loudspeakers at fixed angles ranging from ±60° to ±15°. On a separate task, sound location identification was measured for locations ranging from ±60° in 15° increments.

STUDY SAMPLE

Thirteen children with BiCIs (27-42 months old) and fifteen age-matched (NH).

DATA COLLECTION AND ANALYSIS

Discrimination and sound localization were completed for all subjects. For the left-right discrimination task, participants were required to reach a criterion of 4/5 correct trials (80%) at each angular separation prior to beginning the localization task. For sound localization, data was analyzed in two ways. First, percent correct scores were tallied for each participant. Second, for each participant, the root-mean-square-error was calculated to determine the average distance between the response and stimulus, indicative of localization accuracy.

RESULTS

All BiCI users were able to discriminate left versus right at angles as small as ±15° when listening with two implants; however, performance was significantly worse when listening with a single implant. All NH toddlers also had >80% correct at ±15°. Sound localization results revealed root-mean-square errors averaging 11.15° in NH toddlers. Children in the BiCI group were generally unable to identify source location on this complex task (average error 37.03°).

CONCLUSIONS

Although some toddlers with BiCIs are able to localize sound in a manner consistent with NH toddlers, for the majority of toddlers with BiCIs, sound localization abilities are still emerging.

Effects of Additional Low-Pass-Filtered Speech on Listening Effort for Noise-Band-Vocoded Speech in Quiet and in Noise

Objectives:

Residual acoustic hearing in electric–acoustic stimulation (EAS) can benefit cochlear implant (CI) users in increased sound quality, speech intelligibility, and improved tolerance to noise. The goal of this study was to investigate whether the low-pass–filtered acoustic speech in simulated EAS can provide the additional benefit of reducing listening effort for the spectrotemporally degraded signal of noise-band–vocoded speech.

Design:

Listening effort was investigated using a dual-task paradigm as a behavioral measure, and the NASA Task Load indeX as a subjective self-report measure. The primary task of the dual-task paradigm was identification of sentences presented in three experiments at three fixed intelligibility levels: at near-ceiling, 50%, and 79% intelligibility, achieved by manipulating the presence and level of speech-shaped noise in the background. Listening effort for the primary intelligibility task was reflected in the performance on the secondary, visual response time task. Experimental speech processing conditions included monaural or binaural vocoder, with added low-pass–filtered speech (to simulate EAS) or without (to simulate CI).

Results:

In Experiment 1, in quiet with intelligibility near-ceiling, additional low-pass–filtered speech reduced listening effort compared with binaural vocoder, in line with our expectations, although not compared with monaural vocoder. In Experiments 2 and 3, for speech in noise, added low-pass–filtered speech allowed the desired intelligibility levels to be reached at less favorable speech-to-noise ratios, as expected. It is interesting that this came without the cost of increased listening effort usually associated with poor speech-to-noise ratios; at 50% intelligibility, even a reduction in listening effort on top of the increased tolerance to noise was observed. The NASA Task Load indeX did not capture these differences.

Conclusions:

The dual-task results provide partial evidence for a potential decrease in listening effort as a result of adding low-frequency acoustic speech to noise-band–vocoded speech. Whether these findings translate to CI users with residual acoustic hearing will need to be addressed in future research because the quality and frequency range of low-frequency acoustic sound available to listeners with hearing loss may differ from our idealized simulations, and additional factors, such as advanced age and varying etiology, may also play a role.

Binaural Pitch Fusion in Bilateral Cochlear Implant Users

OBJECTIVES

Binaural pitch fusion is the fusion of stimuli that evoke different pitches between the ears into a single auditory image. Individuals who use hearing aids or bimodal cochlear implants (CIs) experience abnormally broad binaural pitch fusion, such that sounds differing in pitch by as much as 3-4 octaves are fused across ears, leading to spectral averaging and speech perception interference. The goal of this study was to determine if adult bilateral CI users also experience broad binaural pitch fusion.

DESIGN

Stimuli were pulse trains delivered to individual electrodes. Fusion ranges were measured using simultaneous, dichotic presentation of reference and comparison stimuli in opposite ears, and varying the comparison stimulus to find the range that fused with the reference stimulus.

RESULTS

Bilateral CI listeners had binaural pitch fusion ranges varying from 0 to 12 mm (average 6.1 ± 3.9 mm), where 12 mm indicates fusion over all electrodes in the array. No significant correlations of fusion range were observed with any subject factors related to age, hearing loss history, or hearing device history, or with any electrode factors including interaural electrode pitch mismatch, pitch match bandwidth, or within-ear electrode discrimination abilities.

CONCLUSIONS

Bilateral CI listeners have abnormally broad fusion, similar to hearing aid and bimodal CI listeners. This broad fusion may explain the variability of binaural benefits for speech perception in quiet and in noise in bilateral CI users.

The Pupil Dilation Response to Auditory Stimuli: Current State of Knowledge

The measurement of cognitive resource allocation during listening, or listening effort, provides valuable insight in the factors influencing auditory processing. In recent years, many studies inside and outside the field of hearing science have measured the pupil response evoked by auditory stimuli. The aim of the current review was to provide an exhaustive overview of these studies. The 146 studies included in this review originated from multiple domains, including hearing science and linguistics, but the review also covers research into motivation, memory, and emotion. The present review provides a unique overview of these studies and is organized according to the components of the Framework for Understanding Effortful Listening. A summary table presents the sample characteristics, an outline of the study design, stimuli, the pupil parameters analyzed, and the main findings of each study. The results indicate that the pupil response is sensitive to various task manipulations as well as interindividual differences. Many of the findings have been replicated. Frequent interactions between the independent factors affecting the pupil response have been reported, which indicates complex processes underlying cognitive resource allocation. This complexity should be taken into account in future studies that should focus more on interindividual differences, also including older participants. This review facilitates the careful design of new studies by indicating the factors that should be controlled for. In conclusion, measuring the pupil dilation response to auditory stimuli has been demonstrated to be sensitive method applicable to numerous research questions. The sensitivity of the measure calls for carefully designed stimuli.

Interaural Time Difference Perception with a Cochlear Implant and a Normal Ear

Currently there is a growing population of cochlear-implant (CI) users with (near) normal hearing in the non-implanted ear. This configuration is often called SSD (single-sided deafness) CI. The goal of the CI is often to improve spatial perception, so the question raises to what extent SSD CI listeners are sensitive to interaural time differences (ITDs). In a controlled lab setup, sensitivity to ITDs was investigated in 11 SSD CI listeners. The stimuli were 100-pps pulse trains on the CI side and band-limited click trains on the acoustic side. After determining level balance and the delay needed to achieve synchronous stimulation of the two ears, the just noticeable difference in ITD was measured using an adaptive procedure. Seven out of 11 listeners were sensitive to ITDs, with a median just noticeable difference of 438 μs. Out of the four listeners who were not sensitive to ITD, one listener reported binaural fusion, and three listeners reported no binaural fusion. To enable ITD sensitivity, a frequency-dependent delay of the electrical stimulus was required to synchronize the electric and acoustic signals at the level of the auditory nerve. Using subjective fusion measures and refined by ITD sensitivity, it was possible to match a CI electrode to an acoustic frequency range. This shows the feasibility of these measures for the allocation of acoustic frequency ranges to electrodes when fitting a CI to a subject with (near) normal hearing in the contralateral ear.

Verbal Response Times as a Potential Indicator of Cognitive Load During Conventional Speech Audiometry With Matrix Sentences

This study examined verbal response times-that is, the duration from stimulus offset to voice onset-as a potential measure of cognitive load during conventional testing of speech-in-noise understanding. Response times were compared with a measure of perceived effort as assessed by listening effort scaling. Three listener groups differing in age and hearing status participated in the study. Testing was done at two target intelligibility levels (80%, 95%) and with two noise types (stationary and fluctuating). Verbal response times reflected effects of intelligibility level, noise type, and listener group. Response times were shorter for 95% compared with 80% target intelligibility, shorter for fluctuating compared with stationary noise, and shorter for young listeners compared with older listeners. Responses were also faster for the older listeners with near normal hearing compared with the older hearing-aid users. In contrast, subjective listening effort scaling predominantly revealed effects of target intelligibility level but did not show consistent noise-type or listener-group effects. These findings show that verbal response times and effort scalings tap into different domains of listening effort. Verbal response times can be easily assessed during conventional speech audiometry and have the potential to show effects beyond performance measures and subjective effort estimates.

Effects of Hearing Impairment and Hearing Aid Amplification on Listening Effort: A Systematic Review

OBJECTIVES

To undertake a systematic review of available evidence on the effect of hearing impairment and hearing aid amplification on listening effort. Two research questions were addressed: Q1) does hearing impairment affect listening effort? and Q2) can hearing aid amplification affect listening effort during speech comprehension?

DESIGN

English language articles were identified through systematic searches in PubMed, EMBASE, Cinahl, the Cochrane Library, and PsycINFO from inception to August 2014. References of eligible studies were checked. The Population, Intervention, Control, Outcomes, and Study design strategy was used to create inclusion criteria for relevance. It was not feasible to apply a meta-analysis of the results from comparable studies. For the articles identified as relevant, a quality rating, based on the 2011 Grading of Recommendations Assessment, Development, and Evaluation Working Group guidelines, was carried out to judge the reliability and confidence of the estimated effects.

RESULTS

The primary search produced 7017 unique hits using the keywords: hearing aids OR hearing impairment AND listening effort OR perceptual effort OR ease of listening. Of these, 41 articles fulfilled the Population, Intervention, Control, Outcomes, and Study design selection criteria of: experimental work on hearing impairment OR hearing aid technologies AND listening effort OR fatigue during speech perception. The methods applied in those articles were categorized into subjective, behavioral, and physiological assessment of listening effort. For each study, the statistical analysis addressing research question Q1 and/or Q2 was extracted. In seven articles more than one measure of listening effort was provided. Evidence relating to Q1 was provided by 21 articles that reported 41 relevant findings. Evidence relating to Q2 was provided by 27 articles that reported 56 relevant findings. The quality of evidence on both research questions (Q1 and Q2) was very low, according to the Grading of Recommendations Assessment, Development, and Evaluation Working Group guidelines. We tested the statistical evidence across studies with nonparametric tests. The testing revealed only one consistent effect across studies, namely that listening effort was higher for hearing-impaired listeners compared with normal-hearing listeners (Q1) as measured by electroencephalographic measures. For all other studies, the evidence across studies failed to reveal consistent effects on listening effort.

CONCLUSION

In summary, we could only identify scientific evidence from physiological measurement methods, suggesting that hearing impairment increases listening effort during speech perception (Q1). There was no scientific, finding across studies indicating that hearing aid amplification decreases listening effort (Q2). In general, there were large differences in the study population, the control groups and conditions, and the outcome measures applied between the studies included in this review. The results of this review indicate that published listening effort studies lack consistency, lack standardization across studies, and have insufficient statistical power. The findings underline the need for a common conceptual framework for listening effort to address the current shortcomings.

Cortisol, Chromogranin A, and Pupillary Responses Evoked by Speech Recognition Tasks in Normally Hearing and Hard-of-Hearing Listeners: A Pilot Study

Pupillometry is one method that has been used to measure processing load expended during speech understanding. Notably, speech perception (in noise) tasks can evoke a pupil response. It is not known if there is concurrent activation of the sympathetic nervous system as indexed by salivary cortisol and chromogranin A (CgA) and whether such activation differs between normally hearing (NH) and hard-of-hearing (HH) adults. Ten NH and 10 adults with mild-to-moderate hearing loss (mean age 52 years) participated. Two speech perception tests were administered in random order: one in quiet targeting 100% correct performance and one in noise targeting 50% correct performance. Pupil responses and salivary samples for cortisol and CgA analyses were collected four times: before testing, after the two speech perception tests, and at the end of the session. Participants rated their perceived accuracy, effort, and motivation. Effects were examined using repeated-measures analyses of variance. Correlations between outcomes were calculated. HH listeners had smaller peak pupil dilations (PPDs) than NH listeners in the speech in noise condition only. No group or condition effects were observed for the cortisol data, but HH listeners tended to have higher cortisol levels across conditions. CgA levels were larger at the pretesting time than at the three other test times. Hearing impairment did not affect CgA. Self-rated motivation correlated most often with cortisol or PPD values. The three physiological indicators of cognitive load and stress (PPD, cortisol, and CgA) are not equally affected by speech testing or hearing impairment. Each of them seem to capture a different dimension of sympathetic nervous system activity.

Best Practices and Advice for Using Pupillometry to Measure Listening Effort: An Introduction for Those Who Want to Get Started

Within the field of hearing science, pupillometry is a widely used method for quantifying listening effort. Its use in research is growing exponentially, and many labs are (considering) applying pupillometry for the first time. Hence, there is a growing need for a methods paper on pupillometry covering topics spanning from experiment logistics and timing to data cleaning and what parameters to analyze. This article contains the basic information and considerations needed to plan, set up, and interpret a pupillometry experiment, as well as commentary about how to interpret the response. Included are practicalities like minimal system requirements for recording a pupil response and specifications for peripheral, equipment, experiment logistics and constraints, and different kinds of data processing. Additional details include participant inclusion and exclusion criteria and some methodological considerations that might not be necessary in other auditory experiments. We discuss what data should be recorded and how to monitor the data quality during recording in order to minimize artifacts. Data processing and analysis are considered as well. Finally, we share insights from the collective experience of the authors and discuss some of the challenges that still lie ahead.

Music perception improves in children with bilateral cochlear implants or bimodal devices

The objectives of this study were to determine if music perception by pediatric cochlear implant users can be improved by (1) providing access to bilateral hearing through two cochlear implants or a cochlear implant and a contralateral hearing aid (bimodal users) and (2) any history of music training. The Montreal Battery of Evaluation of Musical Ability test was presented via soundfield to 26 bilateral cochlear implant users, 8 bimodal users and 16 children with normal hearing. Response accuracy and reaction time were recorded via an iPad application. Bilateral cochlear implant and bimodal users perceived musical characteristics less accurately and more slowly than children with normal hearing. Children who had music training were faster and more accurate, regardless of their hearing status. Reaction time on specific subtests decreased with age, years of musical training and, for implant users, better residual hearing. Despite effects of these factors on reaction time, bimodal and bilateral cochlear implant users' responses were less accurate than those of their normal hearing peers. This means children using bilateral cochlear implants and bimodal devices continue to experience challenges perceiving music that are related to hearing impairment and/or device limitations during development.

Measuring listening-related effort and fatigue in school-aged children using pupillometry

Stress and fatigue from effortful listening may compromise well-being, learning, and academic achievement in school-aged children. The aim of this study was to investigate the effect of a signal-to-noise ratio (SNR) typical of those in school classrooms on listening effort (behavioral and pupillometric) and listening-related fatigue (self-report and pupillometric) in a group of school-aged children. A sample of 41 normal-hearing children aged 8-11years performed a narrative speech-picture verification task in a condition with recommended levels of background noise ("ideal": +15dB SNR) and a condition with typical classroom background noise levels ("typical": -2dB SNR). Participants showed increased task-evoked pupil dilation in the typical listening condition compared with the ideal listening condition, consistent with an increase in listening effort. No differences were found between listening conditions in terms of performance accuracy and response time on the behavioral task. Similarly, no differences were found between listening conditions in self-report and pupillometric markers of listening-related fatigue. This is the first study to (a) examine listening-related fatigue in children using pupillometry and (b) demonstrate physiological evidence consistent with increased listening effort while listening to spoken narratives despite ceiling-level task performance accuracy. Understanding the physiological mechanisms that underpin listening-related effort and fatigue could inform intervention strategies and ultimately mitigate listening difficulties in children.

Simultaneous bilateral cochlear implants: Developmental advances do not yet achieve normal cortical processing

BACKGROUND

Simultaneous bilateral cochlear implantation promotes symmetric development of bilateral auditory pathways but binaural hearing remains abnormal. To evaluate whether bilateral cortical processing remains impaired in such children, cortical activity to unilateral and bilateral stimuli was assessed in a unique cohort of 16 children who received bilateral cochlear implants (CIs) simultaneously at 1.97 ± 0.86 years of age and had ~4 years of CI experience, providing the first opportunity to assess electrically driven cortical development in the absence of reorganized asymmetries from sequential implantation.

METHODS

Cortical activity to unilateral and bilateral stimuli was measured using multichannel electro-encephalography. Cortical processing in children with bilateral CIs was compared with click-elicited activity in 13 normal hearing children matched for time-in-sound. Source activity was localized using the Time Restricted, Artefact and Coherence source Suppression (TRACS) beamformer method.

RESULTS

Consistent with dominant crossed auditory pathways, normal P1 activity (~100 ms) was weaker to ipsilateral stimuli relative to contralateral and bilateral stimuli and both auditory cortices preferentially responded to the contralateral ear. Right hemisphere dominance was evident overall. Children with bilateral CIs maintained the expected right dominance but differences from normal included: (i) minimal changes between ipsilateral, contralateral and bilateral stimuli, (ii) weaker than normal contralateral stimulus preference, (iii) symmetric activity to bilateral stimuli, and (iv) increased occipital lobe recruitment during bilateral relative to unilateral stimulation. Between-group contrasts demonstrated lower than normal activity in the inferior parieto-occipital lobe (suggesting deficits in sensory integration) and greater than normal left frontal lobe activity (suggesting increased attention), even during passive listening.

CONCLUSIONS

Together, findings suggest that early simultaneous bilateral cochlear implantation promotes normal-like auditory symmetry but that abnormalities in cortical processing consequent to deafness and/or electrical stimulation through two independent speech processors persist.

Cortical Representation of Interaural Time Difference Is Impaired by Deafness in Development: Evidence from Children with Early Long-term Access to Sound through Bilateral Cochlear Implants Provided Simultaneously

Accurate use of interaural time differences (ITDs) for spatial hearing may require access to bilateral auditory input during sensitive periods in human development. Providing bilateral cochlear implants (CIs) simultaneously promotes symmetrical development of bilateral auditory pathways but does not support normal ITD sensitivity. Thus, although binaural interactions are established by bilateral CIs in the auditory brainstem, potential deficits in cortical processing of ITDs remain. Cortical ITD processing in children with simultaneous bilateral CIs and normal hearing with similar time-in-sound was explored in the present study. Cortical activity evoked by bilateral stimuli with varying ITDs (0, ±0.4, ±1 ms) was recorded using multichannel electroencephalography. Source analyses indicated dominant activity in the right auditory cortex in both groups but limited ITD processing in children with bilateral CIs. In normal-hearing children, adult-like processing patterns were found underlying the immature P1 (∼100 ms) response peak with reduced activity in the auditory cortex ipsilateral to the leading ITD. Further, the left cortex showed a stronger preference than the right cortex for stimuli leading from the contralateral hemifield. By contrast, children with CIs demonstrated reduced ITD-related changes in both auditory cortices. Decreased parieto-occipital activity, possibly involved in spatial processing, was also revealed in children with CIs. Thus, simultaneous bilateral implantation in young children maintains right cortical dominance during binaural processing but does not fully overcome effects of deafness using present CI devices. Protection of bilateral pathways through simultaneous implantation might be capitalized for ITD processing with signal processing advances, which more consistently represent binaural timing cues.SIGNIFICANCE STATEMENT Multichannel electroencephalography demonstrated impairment of binaural processing in children who are deaf despite early access to bilateral auditory input by first finding that foundations for binaural hearing are normally established during early stages of cortical development. Although 4- to 7-year-old children with normal hearing had immature cortical responses, adult patterns in cortical coding of binaural timing cues were measured. Second, children receiving two cochlear implants in the same surgery maintained normal-like input from both ears, but this did not support significant effects of binaural timing cues in either auditory cortex. Deficits in parieto-occiptal areas further suggested impairment in spatial processing. Results indicate that cochlear implants working independently in each ear do not fully overcome deafness-related binaural processing deficits, even after long-term experience.

Bilateral cochlear implants in children: Effects of auditory experience and deprivation on auditory perception

Spatial hearing skills are essential for children as they grow, learn and play. These skills provide critical cues for determining the locations of sources in the environment, and enable segregation of important sounds, such as speech, from background maskers or interferers. Spatial hearing depends on availability of monaural cues and binaural cues. The latter result from integration of inputs arriving at the two ears from sounds that vary in location. The binaural system has exquisite mechanisms for capturing differences between the ears in both time of arrival and intensity. The major cues that are thus referred to as being vital for binaural hearing are: interaural differences in time (ITDs) and interaural differences in levels (ILDs). In children with normal hearing (NH), spatial hearing abilities are fairly well developed by age 4-5 years. In contrast, most children who are deaf and hear through cochlear implants (CIs) do not have an opportunity to experience normal, binaural acoustic hearing early in life. These children may function by having to utilize auditory cues that are degraded with regard to numerous stimulus features. In recent years there has been a notable increase in the number of children receiving bilateral CIs, and evidence suggests that while having two CIs helps them function better than when listening through a single CI, these children generally perform worse than their NH peers. This paper reviews some of the recent work on bilaterally implanted children. The focus is on measures of spatial hearing, including sound localization, release from masking for speech understanding in noise and binaural sensitivity using research processors. Data from behavioral and electrophysiological studies are included, with a focus on the recent work of the authors and their collaborators. The effects of auditory plasticity and deprivation on the emergence of binaural and spatial hearing are discussed along with evidence for reorganized processing from both behavioral and electrophysiological studies. The consequences of both unilateral and bilateral auditory deprivation during development suggest that the relevant set of issues is highly complex with regard to successes and the limitations experienced by children receiving bilateral cochlear implants. This article is part of a Special Issue entitled .

Balancing current levels in children with bilateral cochlear implants using electrophysiological and behavioral measures

Children have benefited from bilateral cochlear implants (CIs) over unilateral CIs despite often missing important periods in bilateral auditory development. This suggests a remarkable perceptual ability by children to "work around" abnormal changes in the auditory pathways. Nonetheless, these children rely primarily on interaural level differences as interaural timing cues are more difficult to access or detect. Mismatched levels provided to the two implants could distort interaural level cues thus compromising the benefits of bilateral CI use. We asked whether "balanced" or "centered" perception of bilateral input can be predicted by physiological or behavioral measures. Twenty-four children who had used unilateral CIs for 9.21 ± 2.66 years prior to bilateral implantation participated. "Balanced bilateral levels" were identified by responses occurring with a probability of 50% on the right side of the head and 50% on the left in a two choice lateralization task. Loudness judgments of current presented unilaterally by each implant were measured on a continuous visual scale. Maximum wave eV amplitudes were evoked unilaterally by each implant and matched amplitudes were identified. Balanced bilateral levels were predicted within 10 Clinical Units (CU) in 9 of 13 (69%) children using matched wave eV amplitudes. Bilaterally balanced levels were reasonably predicted by similar loudness judgments (<10% difference between CIs) in only 6 of 13 (46%) children. Results indicate that matching amplitudes of physiological responses can produce a balanced perception of bilateral input despite unilateral strengthening of the auditory pathways and can potentially be used clinically to provide a first approximation of balance/centered levels.

Early unilateral cochlear implantation promotes mature cortical asymmetries in adolescents who are deaf

Unilateral cochlear implant (CI) stimulation establishes hearing to children who are deaf but compromises bilateral auditory development if a second implant is not provided within ∼ 1.5 years. In this study we asked: 1) What are the cortical consequences of missing this early sensitive period once children reach adolescence? 2) What are the effects of unilateral deprivation on the pathways from the opposite ear? Cortical responses were recorded from 64-cephalic electrodes within the first week of bilateral CI activation in 34 adolescents who had over 10 years of unilateral right CI experience and in 16 normal hearing peers. Cortical activation underlying the evoked peaks was localized to areas of the brain using beamformer imaging. The first CI evoked activity which was more strongly lateralized to the contralateral left hemisphere than normal, with abnormal recruitment of the left prefrontal cortex (involved in cognition/attention), left temporo-parietal-occipital junction (multi-modal integration), and right precuneus (visual processing) region. CI stimulation in the opposite deprived ear evoked atypical cortical responses with abnormally large and widespread dipole activity across the cortex. Thus, using a unilateral CI to hear beyond the period of cortical maturation causes lasting asymmetries in the auditory system, requires recruitment of additional cortical areas to support hearing, and does little to protect the unstimulated pathways from effects of auditory deprivation. The persistence of this reorganization into maturity could signal a closing of a sensitive period for promoting auditory development on the deprived side.

Comparison of Interaural Electrode Pairing Methods for Bilateral Cochlear Implants

In patients with bilateral cochlear implants (CIs), pairing matched interaural electrodes and stimulating them with the same frequency band is expected to facilitate binaural functions such as binaural fusion, localization, and spatial release from masking. Because clinical procedures typically do not include patient-specific interaural electrode pairing, it remains the case that each electrode is allocated to a generic frequency range, based simply on the electrode number. Two psychoacoustic techniques for determining interaurally paired electrodes have been demonstrated in several studies: interaural pitch comparison and interaural time difference (ITD) sensitivity. However, these two methods are rarely, if ever, compared directly. A third, more objective method is to assess the amplitude of the binaural interaction component (BIC) derived from electrically evoked auditory brainstem responses for different electrode pairings; a method has been demonstrated to be a potential candidate for bilateral CI users. Here, we tested all three measures in the same eight CI users. We found good correspondence between the electrode pair producing the largest BIC and the electrode pair producing the maximum ITD sensitivity. The correspondence between the pairs producing the largest BIC and the pitch-matched electrode pairs was considerably weaker, supporting the previously proposed hypothesis that whilst place pitch might adapt over time to accommodate mismatched inputs, sensitivity to ITDs does not adapt to the same degree.