Binaural Cues Research Articles

Enhancement of interaural level differences for bilateral cochlear implant users

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

Improved spatialization performance for joint speech dereverberation and noise reduction in binaural hearing aids

ObjectiveIn this work we propose a unified solution for joint speech dereverberation, noise reduction and spatial preservation of either diffuse noise fields or single-point noise sources in binaural hearing aid applications, which is based on the multichannel Wiener filter (MWF) with preservation of the interaural coherence (MWF-IC) method. MethodsThe original interaural coherence (IC) auxiliary penalty cost function in the MWF-IC, which comprises information about both interfering noise and late reverberation component, is modified to consider the interference noise information only. It provides a new regularization form for preserving both interaural time difference (ITD) and IC binaural cues of the residual noise. This approach is theoretically supported by recent findings about the relationship between IC and ITD binaural cues for point acoustic sources. ResultsExtensive simulation results with objective measures, as well as psychoacoustic experiments with normal hearing volunteers indicate that the proposed method outperforms the preservation of spatial localization cues of its original version, keeping approximately the same performance in terms of noise-reduction and speech-dereverberation. ConclusionThe proposed scheme expands the MWF-IC original spatial cue preservation capability from only diffuse noise fields to either diffuse noise fields or a single-point noise source. SignificanceThis is the first MWF-based speech dereverberation and noise reduction method for binaural hearing aid that provides perceptually relevant preservation of acoustic scenarios comprised of one speech source and either a single-point noise source or a diffuse noise field.

Reweighting of Binaural Localization Cues Induced by Lateralization Training

Normal-hearing listeners adapt to alterations in sound localization cues. This adaptation can result from the establishment of a new spatial map of the altered cues or from a stronger relative weighting of unaltered compared to altered cues. Such reweighting has been shown for monaural vs. binaural cues. However, studies attempting to reweight the two binaural cues, interaural differences in time (ITD) and level (ILD), yielded inconclusive results. This study investigated whether binaural-cue reweighting can be induced by lateralization training in a virtual audio-visual environment. Twenty normal-hearing participants, divided into two groups, completed the experiment consisting of 7 days of lateralization training, preceded and followed by a test measuring the binaural-cue weights. Participants’ task was to lateralize 500-ms bandpass-filtered (2–4 kHz) noise bursts containing various combinations of spatially consistent and inconsistent binaural cues. During training, additional visual cues reinforced the azimuth corresponding to ITDs in one group and ILDs in the other group and the azimuthal ranges of the binaural cues were manipulated group-specifically. Both groups showed a significant increase of the reinforced-cue weight from pre- to posttest, suggesting that participants reweighted the binaural cues in the expected direction. This reweighting occurred within the first training session. The results are relevant as binaural-cue reweighting likely occurs when normal-hearing listeners adapt to new acoustic environments. Reweighting might also be a factor underlying the low contribution of ITDs to sound localization of cochlear-implant listeners as they typically do not experience reliable ITD cues with clinical devices.

Combined Electric and Acoustic Stimulation (EAS) in Children: Investigating Benefit Afforded by Bilateral Versus Unilateral Acoustic Hearing.

Electric and acoustic stimulation (EAS) with preserved hearing in the implanted ear provides benefit for speech understanding, spatial hearing, and quality of life in adults. However, there is limited research on EAS outcomes in children. The aims of this study were to estimate the magnitude of EAS-related benefit on speech understanding in children with preserved acoustic hearing and to determine what role acoustic interaural time difference (ITD) sensitivity may have on said EAS benefit. Six children with acoustic hearing preservation and 20 children with normal hearing (NH) were recruited to participate. Speech recognition was assessed via an eight-loudspeaker array with speech presented from one loudspeaker at 0 degree and restaurant noise from all other loudspeakers (45-315 degrees). ITD thresholds were measured for a 250-Hz signal presented acoustically via insert earphones. Only one EAS listener demonstrated significant benefit from bilateral acoustic hearing as compared with acoustic hearing from a single ear. ITD thresholds were poor in the range of 302 to 1000+ ms and were considerably poorer than ITD thresholds for the NH group. These data suggest that children with acoustic hearing preservation may not exhibit initial EAS benefit for speech recognition in semi-diffuse noise; however, because none exhibited a decrement in performance with bilateral acoustic stimulation, EAS fittings are recommended to provide binaural acoustic access allowing for EAS adaptation to binaural cues over time. Future research should address the emergence of EAS benefit, binaural cue sensitivity, and the role of EAS experience in children and adults.

Head‐related transfer function–reserved time‐frequency masking for robust binaural sound source localization

Various time-frequency (T-F) masks are being applied to sound source localization tasks. Moreover, deep learning has dramatically advanced T-F mask estimation. However, existing masks are usually designed for speech separation tasks and are suitable only for single-channel signals. A novel complex-valued T-F mask is proposed that reserves the head-related transfer function (HRTF), customized for binaural sound source localization. In addition, because the convolutional neural network that is exploited to estimate the proposed mask takes binaural spectral information as the input and output, accurate binaural cues can be preserved. Compared with conventional T-F masks that emphasize single speech source–dominated T-F units, HRTF-reserved masks eliminate the speech component while keeping the direct propagation path. Thus, the estimated HRTF is capable of extracting more reliable localization features for the final direction of arrival estimation. Hence, binaural sound source localization guided by the proposed T-F mask is robust under noisy and reverberant acoustic environments. The experimental results demonstrate that the new T-F mask is superior to conventional T-F masks and lead to the better performance of sound source localization in adverse environments.

Comparing Methods for Pairing Electrodes Across Ears With Cochlear Implants.

Currently, bilateral cochlear implants (CIs) are independently programmed in clinics using frequency allocations based on the relative location of a given electrode from the end of each electrode array. By pairing electrodes based on this method, bilateral CI recipients may have decreased sensitivity to interaural time differences (ITD) and/or interaural level differences (ILD), two cues critical for binaural tasks. There are multiple different binaural measures that can potentially be used to determine the optimal way to pair electrodes across the ears. Previous studies suggest that the optimal electrode pairing between the left and right ears may vary depending on the binaural task used. These studies, however, have only used one reference location or a single bilateral CI user. In both instances, it is difficult to determine if the results that were obtained reflect a measurement error or a systematic difference across binaural tasks. It is also difficult to determine from these studies if the differences between the three cues vary across electrode regions, which could result from differences in the availability of binaural cues across frequency regions. The purpose of this study was to determine if, after experience-dependent adaptation, there are systematic differences in the optimal pairing of electrodes at different points along the array for the optimal perception of ITD, ILD, and pitch. Data from seven bilateral Nucleus users was collected and analyzed. Participants were tested with ITD, ILD, and pitch-matching tasks using five different reference electrodes in one ear, spaced across the array. Comparisons were conducted to determine if the optimal bilateral electrode pairs systematically differed in different regions depending on whether they were measured based on ITD sensitivity, ILD sensitivity, or pitch matching, and how those pairs differed from the pairing in the participants' clinical programs. Results indicate that there was a significant difference in the optimal pairing depending on the cue measured, but only at the basal end of the array. The results suggest that optimal electrode pairings differ depending on the cue measured to determine optimal pairing, at least for the basal end of the array. This also suggests that the improvements seen when using optimally paired electrodes may be tied to the particular percept being measured both to determine electrode pairing and to assess performance, at least for the basal end of the array.

Transmission of Binaural Cues by Bilateral Cochlear Implants: Examining the Impacts of Bilaterally Independent Spectral Peak-Picking, Pulse Timing, and Compression.

Acoustic hearing listeners use binaural cues—interaural time differences (ITDs) and interaural level differences (ILDs)—for localization and segregation of sound sources in the horizontal plane. Cochlear implant users now often receive two implants (bilateral cochlear implants [BiCIs]) rather than one, with the goal to provide access to these cues. However, BiCI listeners often experience difficulty with binaural tasks. Most BiCIs use independent sound processors at each ear; it has often been suggested that such independence may degrade the transmission of binaural cues, particularly ITDs. Here, we report empirical measurements of binaural cue transmission via BiCIs implementing a common “n-of-m” spectral peak-picking stimulation strategy. Measurements were completed for speech and nonspeech stimuli presented to an acoustic manikin “fitted” with BiCI sound processors. Electric outputs from the BiCIs and acoustic outputs from the manikin’s in-ear microphones were recorded simultaneously, enabling comparison of electric and acoustic binaural cues. For source locations away from the midline, BiCI binaural cues, particularly envelope ITD cues, were found to be degraded by asymmetric spectral peak-picking. In addition, pulse amplitude saturation due to nonlinear level mapping yielded smaller ILDs at higher presentation levels. Finally, while individual pulses conveyed a spurious “drifting” ITD, consistent with independent left and right processor clocks, such variation was not evident in transmitted envelope ITDs. Results point to avenues for improvement of BiCI technology and may prove useful in the interpretation of BiCI spatial hearing outcomes reported in prior and future studies.

On the influence of non-individual binaural cues and the impact of level normalization on auditory distance estimation of nearby sound sources

Nearby sound sources provide distinct binaural cues, mainly in the form of interaural level differences, which vary with respect to distance and azimuth. However, there is a long-standing controversy regarding whether humans can actually utilize binaural cues for distance estimation of nearby sources. Therefore, we conducted three experiments using non-individual binaural synthesis. In Experiment 1, subjects had to estimate the relative distance of loudness-normalized and non-normalized nearby sources in static and dynamic binaural rendering in a multi-stimulus comparison task under anechoic conditions. Loudness normalization was used as a plausible method to compensate for noticeable intensity differences between stimuli. With the employed loudness normalization, nominal distance did not significantly affect distance ratings for most conditions despite the presence of non-individual binaural distance cues. In Experiment 2, subjects had to judge the relative distance between loudness-normalized sources in dynamic binaural rendering in a forced-choice task. Below chance performance in this more sensitive task revealed that the employed loudness normalization strongly affected distance estimation. As this finding indicated a general issue with loudness normalization for studies on relative distance estimation, Experiment 3 directly tested the validity of loudness normalization and a frequently used amplitude normalization. Results showed that both normalization methods lead to remaining (incorrect) intensity cues, which subjects most likely used for relative distance estimation. The experiments revealed that both examined normalization methods have consequential drawbacks. These drawbacks might in parts explain conflicting findings regarding the effectiveness of binaural cues for relative distance estimation in the literature.

Instrumental Quality Predictions and Analysis of Auditory Cues for Algorithms in Modern Headphone Technology.

Smart headphones or hearables use different types of algorithms such as noise cancelation, feedback suppression, and sound pressure equalization to eliminate undesired sound sources or to achieve acoustical transparency. Such signal processing strategies might alter the spectral composition or interaural differences of the original sound, which might be perceived by listeners as monaural or binaural distortions and thus degrade audio quality. To evaluate the perceptual impact of these distortions, subjective quality ratings can be used, but these are time consuming and costly. Auditory-inspired instrumental quality measures can be applied with less effort and may also be helpful in identifying whether the distortions impair the auditory representation of monaural or binaural cues. Therefore, the goals of this study were (a) to assess the applicability of various monaural and binaural audio quality models to distortions typically occurring in hearables and (b) to examine the effect of those distortions on the auditory representation of spectral, temporal, and binaural cues. Results showed that the signal processing algorithms considered in this study mainly impaired (monaural) spectral cues. Consequently, monaural audio quality models that capture spectral distortions achieved the best prediction performance. A recent audio quality model that predicts monaural and binaural aspects of quality was revised based on parts of the current data involving binaural audio quality aspects, leading to improved overall performance indicated by a mean Pearson linear correlation of 0.89 between obtained and predicted ratings.

Multi-Tone Phase Coding of Interaural Time Difference for Sound Source Localization With Spiking Neural Networks

Mammals exhibit remarkable capability of detecting and localizing sound sources in complex acoustic environments by using binaural cues in the spiking manner. Emulating the auditory process for sound source localization (SSL) by mammals, we propose a computational model for accurate and robust SSL under the neuromorphic spiking neural network (SNN) framework. The center of this model is a Multi-Tone Phase Coding (MTPC) scheme, which encodes the interaural time difference (ITD) between binaural pure tones into discriminative spike patterns that can be directly classified by SNNs. As such, SSL can be implemented as an event-driven task on highly efficient, neuromorphic parallel processors. We evaluate the proposed computational model on a directional audio dataset recorded from a microphone array in a realistic acoustic environment with background noise, obstruction, reflection, and other interferences. We report superior localization capability with a mean absolute error (MAE) of 1.02 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> or 100% classification accuracy with an angle resolution of 5 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> , which surpasses other SNN-based biologically plausible neuromorphic approaches by a relatively large margin and on par with human performance in similar tasks. This study opens up many application opportunities in human-robot interaction where energy efficiency is crucial. As a case study, we successfully deploy the proposed SSL system in a robotic platform to track the speaker and orient the robot's attention.

A notched-noise precursor affects both diotic and dichotic notched-noise masking

The present study investigates how diotic and dichotic masked thresholds, in a notched-noise masking paradigm, are affected by activation of the Medial OlivoCochlear (MOC) reflex. Thresholds were obtained for a 500-Hz pure tone diotic or a dichotic signal, S (S0 or Sπ respectively), in the presence of a simultaneous or forward diotic masker (bandpass noise with no notch or a 400-Hz notch). A diotic precursor sound (bandpass noise with a 400- or 800-Hz notch) was presented prior to the signal and masker to activate the MOC reflex. For simultaneous- and forward-masking conditions, the decrease in masked thresholds as a notch was introduced in the masker was larger for the diotic than for the dichotic condition. This resulted in a reduced binaural masking level difference (BMLD) for the masker with a notch. The precursor augmented these two effects. The results indicate that the effect of the precursor, eliciting the MOC reflex, is less pronounced when binaural cues are processed.

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.

Quantization-Aware Binaural MWF Based Noise Reduction Incorporating External Wireless Devices

For hearing-impaired listeners, both ambient noise suppression and binaural cues preservation of directional sources are required, such that a complete spatial awareness of the acoustic scene can be obtained. It was shown that the binaural multichannel Wiener filter (MWF) with partial noise preservation can achieve joint noise reduction and binaural cues preservation and incorporating an external microphone signal improves the performance of binaural MWFs. Motivated by this, we propose a binaural MWF incorporating external wireless devices in this paper. First, we theoretically analyze the performance of the MWF in terms of output signal-to-noise ratio (SNR) and binaural cues preservation errors. As in practice the external devices are power driven with a limited amount of battery resource and the power consumption heavily depends on the transmission rate, given an expected noise reduction performance we then optimize the bit-rate for a single external microphone. Further, we consider to minimize the total power consumption over multiple external devices under a constraint on the output SNR, which turns out to be a rate distribution problem. The proposed rate-distributed binaural MWF is evaluated using a hearing-aid setup with various dynamics. It is shown that the proposed method can obtain a desired SNR at a much lower bit-rate, and an expected trade-off between SNR gain and binaural cues preservation accuracy can be obtained by optimizing the bit-rates. Increasing the bit-rates improves both instrumental speech quality and speech intelligibility.

Comparison of two cortical measures of binaural hearing acuity

Objective Multiple studies have demonstrated binaural hearing deficits in the aging and those with hearing loss. Consequently, there is great interest in developing efficient clinical tests of binaural hearing acuity to improve diagnostic assessments and to assist clinicians when fitting binaural hearing aids and/or cochlear implants. Design Two cortical measures of interaural phase difference sensitivity, the acoustic change complex (ACC) and interaural phase modulation following response (IPM-FR), were compared on three metrics using five different stimulus interaural phase differences (IPDs; 0°, ±22.5°, ±45°, ±67.5° and ±90°). These metrics were scalp topography, time-to-detect, and input–output characteristics. Study sample Ten young, normal-hearing listeners. Results Scalp topography qualitatively differed between ACC and IPM-FR. The IPM-FR demonstrated better time-to-detect performance on smaller (±22.5° and ±45°) but not larger (67.5°, and ±90°) IPDs. Input–output characteristics of each response were similar. Conclusions The IPM-FR may be a faster and more efficient tool for assessing neural sensitivity to subtle IPD changes. However, the ACC may be useful for research or clinical questions concerned with the topographic representation of binaural cues.

Performance Analysis of the Extended Binaural MVDR Beamformer With Partial Noise Estimation

Besides reducing undesired noise sources and limiting speech distortion, another important objective of a binaural noise reduction algorithm is the preservation of the binaural cues of all sound sources in the acoustic scene. In this paper, we consider the binaural minimum variance distortionless response beamformer with partial noise estimation (BMVDR-N), which allows to trade off between noise reduction performance and binaural cue preservation of the noise component by mixing the output signals of the BMVDR beamformer with the noisy reference microphone signals. For a directional noise source, it has been shown that incorporating an external microphone in addition to the head-mounted microphones enables both the noise reduction performance as well as the interaural time and level difference cues of the noise component to be improved in the output signals. In this paper, we consider an arbitrary noise field and analytically show that incorporating an external microphone in the BMVDR-N beamformer enables 1) a larger output signal-to-noise ratio (SNR) for the same mixing parameter, 2) the same output SNR for a larger mixing parameter, and 3) the same desired output magnitude squared coherence (MSC) of the noise component for a smaller mixing parameter to be obtained. The derived analytical expressions are firstly validated using simulated anechoic acoustic transfer functions, where the listener’s head is modelled as a rigid sphere. Experimental results using recorded signals for a binaural hearing device setup in a reverberant environment also show that in a realistic scenario incorporating an external microphone in the BMVDR-N beamformer significantly improves the output SNR and reduces the mixing parameter that is required to obtain a desired output MSC of the noise component compared to using only the head-mounted microphones.

High frequency sensitivity to interaural onset time differences in the bat inferior colliculus

Many neurons in the auditory midbrain are tuned to binaural cues. Two prominent binaural cues are the interaural level difference (ILD) and the interaural time difference (ITD). The ITD cue can further be subdivided into the ongoing envelope ITD cues and transient onset ITD cues. More is known about the sensitivity of single neurons to ongoing envelope ITDs compared to transient onset ITDs in the mammalian auditory system, particularly in bats. The current study examines the response properties of single neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, to onset ITDs in response to high frequency pure tones. Measures of neurons’ dynamic ITD response revealed an average change of 36% of its maximum response within the behaviorally relevant range of ITDs (±50 µs). Across all IC neurons, we measured an average time-intensity trading ratio of 30 µs/dB in the sensitivity of the ITD response function to changing ILDs. Minimum and maximum ITD responses were clustered within a narrow range of ITDs. The average peak in the ITD response function was at 268 µs, a finding that is consistent with other non-echolocating mammals. Some ITD-sensitive neurons also showed weak facilitation of maximum response during binaural stimulation, compared to monaural stimulation. These results suggest that echolocating bats possess the potential to use onset ITD cues to assist in the azimuthal sound localization of ultrasonic frequencies.

The Clear-Speech Benefit for School-Age Children: Speech-in-Noise and Speech-in-Speech Recognition.

Purpose Talkers often modify their speech when communicating with individuals who struggle to understand speech, such as listeners with hearing loss. This study evaluated the benefit of clear speech in school-age children and adults with normal hearing for speech-in-noise and speech-in-speech recognition. Method Masked sentence recognition thresholds were estimated for school-age children and adults using an adaptive procedure. In Experiment 1, the target and masker were summed and presented over a loudspeaker located directly in front of the listener. The masker was either speech-shaped noise or two-talker speech, and target sentences were produced using a clear or conversational speaking style. In Experiment 2, stimuli were presented over headphones. The two-talker speech masker was diotic (M0). Clear and conversational target sentences were presented either in-phase (T0) or out-of-phase (Tπ) between the two ears. The M0Tπ condition introduces a segregation cue that was expected to improve performance. Results For speech presented over a single loudspeaker (Experiment 1), the clear-speech benefit was independent of age for the noise masker, but it increased with age for the two-talker masker. Similar age effects for the two-talker speech masker were seen under headphones with diotic presentation (M0T0), but comparable clear-speech benefit as a function of age was observed with a binaural cue to facilitate segregation (M0Tπ). Conclusions Consistent with prior research, children showed a robust clear-speech benefit for speech-in-noise recognition. Immaturity in the ability to segregate target from masker speech may limit young children's ability to benefit from clear-speech modifications for speech-in-speech recognition under some conditions. When provided with a cue that facilitates segregation, children as young as 4-7 years of age derived a clear-speech benefit in a two-talker masker that was similar to the benefit experienced by adults.

Fiber-Specific Changes in White Matter Microstructure in Individuals With X-Linked Auditory Neuropathy.

Auditory neuropathy (AN) is the term used to describe a group of hearing disorders, in which the hearing impairment occurs as a result of abnormal auditory nerve function. While our understanding of this condition has advanced significantly over recent years, the ability to determine the site of lesion and the extent of dysfunction in affected individuals remains a challenge. To this end, we investigated potential axonal degeneration in the white matter tracts of the brainstem in individuals with X-linked AN. We hypothesized that individuals with X-linked AN would show focal degeneration within the VIII nerve and/or auditory brainstem tracts, and the degree of degeneration would correlate with the extent of auditory perceptual impairment. This was achieved using a higher-order diffusion magnetic resonance imaging (dMRI)-based quantitative measure called apparent fiber density as obtained from a technique called single-shell 3-tissue constrained spherical deconvolution and analyzed with the fixel-based analysis framework. Eleven subjects with genetically confirmed X-linked AN and 11 controls with normal hearing were assessed using behavioral and objective auditory measures. dMRI data were also collected for each participant. Fixel-based analysis of the brainstem region showed that subjects with X-linked AN had significantly lower apparent fiber density in the VIII nerve compared with controls, consistent with axonal degeneration in this region. Subsequent analysis of the auditory brainstem tracts specifically showed that degeneration was also significant in these structures overall. The apparent fiber density findings were supported by objective measures of auditory function, such as auditory brainstem responses, electrocochleography, and otoacoustic emissions, which showed VIII nerve activity was severely disrupted in X-linked AN subjects while cochlear sensory hair cell function was relatively unaffected. Moreover, apparent fiber density results were significantly correlated with temporal processing ability (gap detection task) in affected subjects, suggesting that the degree of VIII nerve degeneration may impact the ability to resolve temporal aspects of an acoustic signal. Auditory assessments of sound detection, speech perception, and the processing of binaural cues were also significantly poorer in the X-linked AN group compared with the controls with normal hearing. The results of this study suggest that the dMRI-based measure of apparent fiber density may provide a useful adjunct to existing auditory assessments in the characterization of the site of lesion and extent of dysfunction in individuals with AN. Additionally, the ability to determine the degree of degeneration has the potential to guide rehabilitation strategies in the future.

Reaching to sounds in virtual reality: A multisensory-motor approach to promote adaptation to altered auditory cues

When localising sounds in space the brain relies on internal models that specify the correspondence between the auditory input reaching the ears, initial head-position and coordinates in external space. These models can be updated throughout life, setting the basis for re-learning spatial hearing abilities in adulthood. In addition, strategic behavioural adjustments allow people to quickly adapt to atypical listening situations. Until recently, the potential role of dynamic listening, involving head-movements or reaching to sounds, have remained largely overlooked. Here, we exploited visual virtual reality (VR) and real-time kinematic tracking, to study the role of active multisensory-motor interactions when hearing individuals adapt to altered binaural cues (one ear plugged and muffed). Participants were immersed in a VR scenario showing 17 virtual speakers at ear-level. In each trial, they heard a sound delivered from a real speaker aligned with one of the virtual ones and were instructed to either reach-to-touch the perceived sound source (Reaching group), or read the label associated with the speaker (Naming group). Participants were free to move their heads during the task and received audio-visual feedback on their performance. Most importantly, they performed the task under binaural or monaural listening. Results show that both groups adapted rapidly to monaural listening, improving sound localisation performance across trials and changing their head-movement behaviour. Reaching the sounds induced faster and larger sound localisation improvements, compared to just naming its position. This benefit was linked to progressively wider head-movements to explore auditory space, selectively in the Reaching group. In conclusion, reaching to sounds in an immersive visual VR context proved most effective for adapting to altered binaural listening. Head-movements played an important role in adaptation, pointing to the importance of dynamic listening when implementing training protocols for improving spatial hearing.

Age-Related Deficits in Electrophysiological and Behavioral Measures of Binaural Temporal Processing.

Binaural processing, particularly the processing of interaural phase differences, is important for sound localization and speech understanding in background noise. Age has been shown to impact the neural encoding and perception of these binaural temporal cues even in individuals with clinically normal hearing sensitivity. This work used a new electrophysiological response, called the interaural phase modulation-following response (IPM-FR), to examine the effects of age on the neural encoding of interaural phase difference cues. Relationships between neural recordings and performance on several behavioral measures of binaural processing were used to determine whether the IPM-FR is predictive of interaural phase difference sensitivity and functional speech understanding deficits. Behavioral binaural frequency modulation detection thresholds were measured to assess sensitivity to interaural phase differences while spatial release-from-masking thresholds were used to assess speech understanding abilities in spatialized noise. Thirty adults between the ages of 35 to 74 years with normal low-frequency hearing thresholds were used in this study. Data showed that older participants had weaker neural responses to the interaural phase difference cue and were less able to take advantage of binaural cues for speech understanding compared to younger participants. Results also showed that the IPM-FR was predictive of performance on the binaural frequency modulation detection task, but not on the spatial release-from-masking task after accounting the effects of age. These results confirm previous work that showed that the IPM-FR reflects age-related declines in binaural temporal processing and provide further evidence that this response may represent a useful objective tool for assessing binaural function. However, further research is needed to understand how the IPM-FR is related to speech understanding abilities.