Interaural Level Difference Cues Research Articles

Magnified interaural level differences enhance spatial release from masking in bilateral cochlear implant users.

Bilateral cochlear implant (BiCI) usage makes binaural benefits a possibility for implant users. Yet, limited access to interaural time difference (ITD) cues and reduced saliency of interaural level difference (ILD) cues restricts perceptual benefits of spatially separating a target from masker sounds for BiCI users. Here, we explore whether magnifying ILD cues improves intelligibility of masked speech for BiCI listeners in a "symmetrical-masker" configuration, which controls for long-term positive target-to-masker ratio (TMR) at the ear nearer the target from naturally occurring ILD cues. We magnified ILDs by estimating moment-to-moment ITDs in 1-octave-wide frequency bands, and applying corresponding ILDs to the target-masker mixtures reaching the two ears at each time in each frequency band. We conducted two experiments, one with NH listeners using vocoded stimuli and one with BiCI users. ILD magnification significantly improved intelligibility in both experiments. BiCI listeners showed no benefit of spatial separation between target and maskers with natural ILDs, even for the largest target-masker separation. Because ILD magnification is applied to the mixed signals at each ear, the strategy does not alter the TMR in either ear at any time; improvements to masked speech intelligibility are thus likely from improved perceptual separation of the competing sources.

Neural mechanisms of spatial auditory attention with magnified interaural level difference cues

Bilateral cochlear implant users struggle in spatial release from masking (SRM) tasks, likely due to restricted access to interaural time difference (ITD) cues. Instead, they must rely on interaural level difference (ILD) cues; however, our previous behavioral experiments suggest that magnification of ILDs can facilitate SRM. Here, we probed the neural mechanisms underlying the benefit of magnified ILDs. We tested 18 normal-hearing subjects in an anechoic chamber. Listeners heard target and masker sequences of object and color words from opposite (left and right) quarterfields and were asked to detect color words in the target stream. Both streams were spatialized using either 50 μS ITD (ITD50), 500 μS ITD (ITD500), a broadband 10 dB ILD (ILD10), or the largest naturally occurring frequency-specific ILD (70 degrees; ILD70n). We recorded task-elicited hemodynamic responses in dorsolateral prefrontal cortex (DLPFC) using functional Near-Infrared Spectroscopy. Subjects performed best in the ITD500 and ILD10 conditions. Hemodynamic response magnitudes were smaller for ITD50 than for all other conditions, consistent with frontal activity increasing when perceptual segregation is possible and spatial attention can be deployed successfully. These data show that magnified ILD cues enhance SRM and that the benefit ILDs confer arises because listeners can engage cognitive attentional processes.

Acoustical verification of binaural features in hearing aids

Hearing aid verification systems are designed to verify sound pressure levels in a patient’s ear canal, as well as ensure hearing aid features are performing to company specifications. In addition to verifying sound output, verification systems can analyze frequency specific compression, prescription targets, directional microphones, noise reduction performance, etc. However, current verification systems are unable to verify several advanced digital signal processing (DSP) features that companies have included in their hearing aids over the recent years. Particularly, binaural features which require both hearing aids to be analyzed simultaneously to measure interaural cues. Here, interaural cues were analyzed from recordings made on an acoustic manikin in an anechoic chamber across various noise reduction and directional microphone features using open-fit and occluding coupling compared to an unaided condition. During data collection, settings in the fitting software did not always align to hearing aid coupling (i.e., open versus occluding dome conditions), an error likely to occur clinically. For some noise reduction hearing aid conditions, a large interaural level difference (ILD) was introduced to one hearing aid, distorting the sound source location. ILD cues measured here, across different DSP and coupling conditions, suggest verification protocols should include additional features, such as binaural measurements.

Bilateral Cochlear Implant Processing of Coding Strategies With CCi-MOBILE, an Open-Source Research Platform.

While speech understanding for cochlear implant (CI) users in quiet is relatively effective, listeners experience difficulty in identification of speaker and sound location. To assist for better residual hearing abilities and speech intelligibility support, bilateral and bimodal forms of assisted hearing is becoming popular among CI users. Effective bilateral processing calls for testing precise algorithm synchronization and fitting between both left and right ear channels in order to capture interaural time and level difference cues (ITD and ILDs). This work demonstrates bilateral implant algorithm processing using a custom-made CI research platform - CCi-MOBILE, which is capable of capturing precise source localization information and supports researchers in testing bilateral CI processing in real-time naturalistic environments. Simulation-based, objective, and subjective testing has been performed to validate the accuracy of the platform. The subjective test results produced an RMS error of ±8.66° for source localization, which is comparable to the performance of commercial CI processors.

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

Introduction: Cochlear implant (CI) recipients with normal hearing (NH) in the contralateral ear experience a significant improvement in sound source localization when listening with the CI in combination with their NH-ear (CI + NH) as compared to with the NH-ear alone. The improvement in localization is primarily due to sensitivity to interaural level differences (ILDs). Sensitivity to interaural timing differences (ITDs) may be limited by auditory aging, frequency-to-place mismatches, the signal coding strategy, and duration of CI use. The present report assessed the sensitivity of ILD and ITD cues in CI + NH listeners who were recipients of long electrode arrays that provide minimal frequency-to-place mismatches and were mapped with a coding strategy that presents fine structure cues on apical channels. Methods: Sensitivity to ILDs and ITDs for localization was assessed using broadband noise (BBN), as well as high-pass (HP) and low-pass (LP) filtered noise for adult CI + NH listeners. Stimuli were 200-ms noise bursts presented from 11 speakers spaced evenly over an 180° arc. Performance was quantified in root-mean-squared error and response patterns were analyzed to evaluate the consistency, accuracy, and side bias of the responses. Fifteen listeners completed the task at the 2-year post-activation visit; seven listeners repeated the task at a later annual visit. Results: Performance at the 2-year visit was best with the BBN and HP stimuli and poorer with the LP stimulus. Responses to the BBN and HP stimuli were significantly correlated, consistent with the idea that CI + NH listeners primarily use ILD cues for localization. For the LP stimulus, some listeners responded consistently and accurately and with limited side bias, which may indicate sensitivity to ITD cues. Two of the 7 listeners who repeated the task at a later annual visit experienced a significant improvement in performance with the LP stimulus, which may indicate that sensitivity to ITD cues may improve with long-term CI use. Conclusions: CI recipients with a NH-ear primarily use ILD cues for sound source localization, though some may use ITD cues as well. Sensitivity to ITD cues may improve with long-term CI listening experience.

Web-based remote testing as a viable option for measuring binaural hearing abilities

Web-based remote testing can increase access to clinically diverse populations in hearing research. However, whether remote testing affects data quality compared to in-lab testing is unknown. Data for binaural hearing tasks may be particularly vulnerable to commercial-grade hardware limitations and background noise from the testing environment. We replicated two published studies on binaural hearing abilities using a web-based behavioral experimental platform (Gorilla.sc). In Experiment 1, we replicated an experiment by Goupell et al. [JASA 133(4) (2013)] that tested intra-cranial lateralization to interaural time and level difference cues. In Experiment 2, we replicated an experiment by Goupell et al. [JASA 140(3) (2016)] that investigated binaural and contralateral unmasking of speech. Two groups of college-aged young adults were tested: one with verified normal hearing (NH) and one with self-reported NH. Testing took place in participants’ homes using their own computer and audio hardware. Performance was compared between the two groups of participants as well as with published data collected in-lab. For both experiments, performance was similar for in-lab and web-based remote testing. We found no effect of verified versus self-reported NH. These results suggest that remote testing may be a feasible option for binaural hearing experiments. [Work funded by NIH-NIDCD.]

Improved accuracy and computational efficiency in virtual acoustic rendering using principal components-based amplitude panning

Virtual loudspeaker arrays can be rendered over headphones using a head-related impulse response (HRIR) dataset, but these techniques often introduce undesirable spatial and timbral artifacts that are associated with distortions in the interaural level difference (ILD) cue. Here, a new and efficient virtual acoustic rendering technique that minimizes ILD distortions is described and evaluated. The technique uses principal components analysis (PCA) performed on the time-domain minimum-phase representation of the directionally dependent HRIRs. The resulting principal components (PCs) act as virtual acoustic filters, and the PCA scores are implemented as panning gains. These panning gains map the PC filters to source locations, similar to vector-base amplitude panning (VBAP). Thus, the technique is termed principal components-based amplitude panning (PCBAP). A spatial and spectral error analysis of ILD shows similar reconstruction accuracy is obtained using PCBAP with 15 filters compared to a 256-loudspeaker VBAP array. PCBAP is also scalable to simulate large loudspeaker arrays with a fixed set of PC filters and only small increases in computational requirements. Results from psychophysical testing suggest that perceptual equivalence between a PCBAP-rendered source and a reference source rendered with the original HRIR dataset is directionally dependent but can be achieved with as few as 10–20 PCs.

Integration of deep learning with expectation maximization for spatial cue-based speech separation in reverberant conditions

In this paper, we formulate a blind source separation (BSS) framework, which allows integrating U-Net based deep learning source separation network with probabilistic spatial machine learning expectation maximization (EM) algorithm for separating speech in reverberant conditions. Our proposed model uses a pre-trained deep learning convolutional neural network, U-Net, for clustering the interaural level difference (ILD) cues and machine learning expectation maximization (EM) algorithm for clustering the interaural phase difference (IPD) cues. The integrated model exploits the complementary strengths of the two approaches to BSS: the strong modeling power of supervised neural networks and the ease of unsupervised machine learning algorithms, whose few parameters can be estimated on as little as a single segment of an audio mixture. The results show an average improvement of 4.3 dB in signal to distortion ratio (SDR) and 4.3% in short time speech intelligibility (STOI) over the EM based source separation algorithm MESSL-GS (model-based expectation–maximization source separation and localization with garbage source) and 4.5 dB in SDR and 8% in STOI over deep learning convolutional neural network (U-Net) based speech separation algorithm SONET under the reverberant conditions ranging from anechoic to those mostly encountered in the real world.

Synchronized Automatic Gain Control in Bilateral Cochlear Implant Recipients Yields Significant Benefit in Static and Dynamic Listening Conditions.

Individuals with bilateral cochlear implants (BiCIs) rely mostly on interaural level difference (ILD) cues to localize stationary sounds in the horizontal plane. Independent automatic gain control (AGC) in each device can distort this cue, resulting in poorer localization of stationary sound sources. However, little is known about how BiCI listeners perceive sound in motion. In this study, 12 BiCI listeners’ spatial hearing abilities were assessed for both static and dynamic listening conditions when the sound processors were synchronized by applying the same compression gain to both devices as a means to better preserve the original ILD cues. Stimuli consisted of band-pass filtered (100–8000 Hz) Gaussian noise presented at various locations or panned over an array of loudspeakers. In the static listening condition, the distance between two sequentially presented stimuli was adaptively varied to arrive at the minimum audible angle, the smallest spatial separation at which the listener can correctly determine whether the second sound was to the left or right of the first. In the dynamic listening condition, participants identified if a single stimulus moved to the left or to the right. Velocity was held constant and the distance the stimulus traveled was adjusted using an adaptive procedure to determine the minimum audible movement angle. Median minimum audible angle decreased from 17.1° to 15.3° with the AGC synchronized. Median minimum audible movement angle decreased from 100° to 25.5°. These findings were statistically significant and support the hypothesis that synchronizing the AGC better preserves ILD cues and results in improved spatial hearing abilities. However, restoration of the ILD cue alone was not enough to bridge the large performance gap between BiCI listeners and normal-hearing listeners on these static and dynamic spatial hearing measures.

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.

Solving the cocktail party problem for bilateral cochlear implant users

It is well-known that listeners with hearing impairment have trouble solving the cocktail party problem. Bilateral cochlear implant users have shown little or no perceptual abilities in this regard. This is due in large part to poor encoding of interaural time difference cues by these devices. Interaural level difference cues, which are well-encoded by bilateral cochlear implants, are not thought to facilitate benefit beyond improved signal-to-noise ratio benefits from head-shadow. An approach to mitigating this problem will be presented, that magnifies interaural level difference cues in real time to increase the perceptual distance between sound sources that are separated in the horizontal plane. The effects of ILD magnification on source localization and speech intelligibility in cocktail-party settings will be discussed.

Localization Uncertainty in Time-Amplitude Stereophonic Reproduction

This article studies the effects of inter-channel time and level differences in stereophonic reproduction on perceived localization uncertainty, which is defined as how difficult it is for a listener to tell where a sound source is located. Towards this end, a computational model of localization uncertainty is proposed first. The model calculates inter-aural time and level difference cues, and compares them to those associated to free-field point-like sources. The comparison is carried out using a particular distance functional that replicates the increased uncertainty observed experimentally with inconsistent inter-aural time and level difference cues. The model is validated by formal listening tests, achieving a Pearson correlation of 0.99. The model is then used to predict localization uncertainty for stereophonic setups and a listener in central and off-central positions. Results show that amplitude methods achieve a slightly lower localization uncertainty for a listener positioned exactly in the center of the sweet spot. As soon as the listener moves away from that position, the situation reverses, with time-amplitude methods achieving a lower localization uncertainty.

Modeling perceived externalization of a static, lateral sound image

Perceived externalization is a relevant feature to create an immersive acoustic environment with headphone reproduction. In the present study, listener-specific acoustic transfer characteristics for an azimuth angle of 90° were modified to investigate the role of monaural spectral cues, interaural level differences (ILDs), and temporal fluctuations of ILDs on perceived externalization in anechoic and reverberant environments. Listeners’ ratings suggested that each acoustic cue was important for perceived externalization. If only one correct acoustic cue remained in the ear signals, the sound image could not be perceived as fully externalized. Reverberation did reduce but not eliminate the influences of monaural spectral and ILD cues on perceived externalization. Additionally, the spectral details of the ipsilateral ear signal were more important for perceived externalization than those in the contralateral ear signal. A computational model was proposed to quantify those relationships and predict externalization ratings by comparing the acoustic cues extracted from the target (modified) and template (non-processed) binaural signals after several auditory processing steps. The accuracy of predicted externalization ratings was higher than 90% under all experimental conditions.

Electrophysiological responses to lateral shifts are not consistent with opponent-channel processing of interaural level differences.

Cortical encoding of auditory space relies on two major peripheral cues, interaural time difference (ITD) and interaural level difference (ILD) of the sounds arriving at a listener's ears. In much of the precortical auditory pathway, ITD and ILD cues are processed independently, and it is assumed that cue integration is a higher order process. However, there remains debate on how ITDs and ILDs are encoded in the cortex and whether they share a common mechanism. The present study used electroencephalography (EEG) to measure evoked cortical potentials from narrowband noise stimuli with imposed binaural cue changes. Previous studies have similarly tested ITD shifts to demonstrate that neural populations broadly favor one spatial hemifield over the other, which is consistent with an opponent-channel model that computes the relative activity between broadly tuned neural populations. However, it is still a matter of debate whether the same coding scheme applies to ILDs and, if so, whether processing the two binaural cues is distributed across similar regions of the cortex. The results indicate that ITD and ILD cues have similar neural signatures with respect to the monotonic responses to shift magnitude; however, the direction of the shift did not elicit responses equally across cues. Specifically, ITD shifts evoked greater responses for outward than inward shifts, independently of the spatial hemifield of the shift, whereas ILD-shift responses were dependent on the hemifield in which the shift occurred. Active cortical structures showed only minor overlap between responses to cues, suggesting the two are not represented by the same pathway.NEW & NOTEWORTHY Interaural time differences (ITDs) and interaural level differences (ILDs) are critical to locating auditory sources in the horizontal plane. The higher order perceptual feature of auditory space is thought to be encoded together by these binaural differences, yet evidence of their integration in cortex remains elusive. Although present results show some common effects between the two cues, key differences were observed that are not consistent with an ITD-like opponent-channel process for ILD encoding.

Improving Localization and Speech Reception in Noise for Bilateral Cochlear Implant Recipients.

This study looked at different methods to preserve interaural level difference (ILD) cues for bilateral cochlear implant (BiCI) recipients. One possible distortion to ILD is from automatic gain control (AGC). Localization accuracy of BiCI recipients using default versus increased AGC threshold and linked AGCs versus independent AGCs was examined. In addition, speech reception in noise was assessed using linked versus independent AGCs and enabling and disabling Autosensitivity™ Control. Subjective information via a diary and questionnaire was also collected about maps with linked and independent AGCs during a take-home experience. Localization accuracy improved in the increased AGC threshold and the linked AGCs conditions. Increasing the AGC threshold resulted in a 4° improvement in root mean square error averaged across all speaker locations. Using linked AGCs, BiCI participants experienced an 8° improvement for all speaker locations and a 19° improvement at the speaker location most affected by the AGC. Speech reception threshold in noise improved by an average of 2.5 dB when using linked AGCs versus independent AGCs. In addition, the effect of linked AGCs on speech in noise was compared with that of Autosensitivity™ Control. The Speech, Spatial, and Qualities of Hearing Scale-12 question comparative survey showed an improvement when using maps with linked AGCs. These findings support the hypothesis that ILD cues may be preserved by increasing the AGC threshold or linking AGCs.

Modeling underwater hearing and sound localization in the frog Xenopus laevis.

Animals that are small compared to sound wavelengths face the challenge of localizing a sound source since the main cues to sound direction-interaural time differences (ITD) and interaural level differences (ILD)-both depend on size. Remarkably, the majority of terrestrial vertebrates possess internally coupled ears (ICE) with an air-filled cavity connecting the two eardrums and producing an inherently directional middle-ear system. Underwater, longer wavelengths and faster sound-speed reduce both ITD and ILD cues. Nonetheless, many animals communicate through and localize underwater sound. Here, a typical representative equipped with ICE is studied: the fully aquatic clawed frog Xenopus laevis. It is shown that two factors improve underwater sound-localization quality. First, inflated lungs function as Helmholtz resonator and generate directional amplitude differences between eardrum vibrations in the high-frequency (1.7-2.2 kHz) and low-frequency (0.8-1.2 kHz) range of the male advertisement calls. Though the externally arriving ILDs practically vanish, the perceived internal level differences are appreciable, more than 10 dB. As opposed to, e.g., lizards with thin and flexible eardrums, plate-like eardrums are shown to be Xenopus' second key to successfully handling aquatic surroundings. Based on ICE, both plate-like eardrums and inflated lungs functioning as Helmholtz resonators explain the phonotaxis performance of Xenopus.

Spatial variation in signal and sensory precision both constrain auditory acuity at high frequencies

Sensory performance is constrained by the information in the stimulus and the precision of the involved sensory system(s). Auditory spatial acuity is robust across a broad range of sound frequencies and source locations, but declines at eccentric lateral angles. The basis of such variation is not fully understood. Low-frequency auditory spatial acuity is mediated by sensitivity to interaural time difference (ITD) cues. While low-frequency spatial acuity varies across azimuth and some physiological models predict strong medial bias in the precision of ITD sensitivity, human psychophysical ITD sensitivity appears to vary only slightly with reference ITD magnitude. Correspondingly, recent analyses suggest that spatial variation in human low-frequency acuity is well-accounted for by acoustic factors alone. Here we examine the matter of high-frequency auditory acuity, which is mediated by sensitivity to interaural level difference (ILD) cues. Using two different psychophysical tasks in human subjects, we demonstrate decreasing ILD acuity with increasing ILD magnitude. We then demonstrate that the multiplicative combination of spatially variant sensory precision and physical cue information (local slope of the ILD cue) provides improved prediction of classic high-frequency spatial acuity data. Finally, we consider correlates of magnitude dependent acuity in neurons that are sensitive to ILDs.

A Complex Panning Method for Near-Field Imaging

Conventional amplitude panning can be used to produce images of distant objects. An extended panning method is presented that can also produce image cues for the near-field region, by the control of Inter-aural Level Difference cues in the low frequency range below ∼1000 Hz. The approach has grown from an adaptive panning method that corrects for the dependence of image direction on head orientation. Stereo panning functions are derived from a general formulation. A single first order filter is required for each image. The method is tested by simulating a range of configurations using measured Head Related Transfer Functions, and also with listening tests. The results confirm the ability of the method to control near-field cues, while also compensating image direction for head rotation.

Early temporary asymmetrical hearing impairs behavioral and neural sensitivity to sound location

Children experiencing asymmetrical hearing early in life often display binaural hearing impairments that persist long after symmetric hearing is restored, suggesting abnormal central auditory development. Here, we test the hypothesis that abnormal neural coding of the interaural level difference (ILD) cue to sound location, resulting from asymmetrical hearing during development, underlies the hearing impairments. Guinea pigs were reared for 8 weeks from birth with a unilateral conductive hearing loss (CHL via earplug). After earplug removal we measured behavioral spatial acuity, the binaural interaction component (BIC) of the auditory brainstem response (ABR), and sensitivity to ILDs in auditory midbrain neurons (inferior colliculus, IC). Animals raised with asymmetric hearing displayed worse spatial acuity (~2x worse) for high-pass noise compared to age-matched controls, suggesting impaired sensitivity to ILDs. Physiologically, animals displayed abnormal BICs of the ABR indicating altered binaural proces...

Temporal constraints on neural and psychophysical sensitivity to interaural level difference cues to sound source location

Interaural differences in the timing (ITD) and level (ILD) of impinging sounds carry critical information about source location. However, in everyday listening environments, sounds are often decorrelated between the ears by reverberation and background noise, degrading the fidelity of both ITD and ILD cues. Similar distortions to ITD and ILD are also experienced by hearing-impaired humans who use hearing aids or cochlear implants, as these devices also degrade temporal and intensive features of the signal at each ear. Here, we demonstrate that behavioral ILD sensitivity (in humans) and neural ILD sensitivity (in single neurons of the chinchilla auditory midbrain) remain robust under stimulus conditions that render ITD cues undetectable. Neural and behavioral data were compared to the outputs of a model of ILD processing with a single free parameter, the duration of excitatory-inhibitory interaction. Behavioral, neural, and modeling data collectively suggest that ILD sensitivity depends on binaural integration of excitation and inhibition within a ≳3-ms temporal window, significantly longer than observed in lower brainstem neurons. This relatively slow integration potentiates a unique role for the ILD system in spatial hearing that may be of particular importance when informative ITD cues are unavailable. [Work supported by F32-DC013927 [ADB] and R01-DC011555 [DJT].]