Binaural Cues Research Articles

Low-frequency limit for binaural timing cues

Sound localization uses both interaural time differences (ITDs) and interaural level differences (ILDs). The ITD cue has a high-frequency boundary at 1500 Hz but is assumed to be limited only by the lowest audible frequency. We tested whether there is a low-frequency boundary for the ITD cue. Nineteen young, normal-hearing subjects participated in four experiments, in which 500-ms, 70-phon pure tones with various binaural cues were presented as a function of frequency from 62.5 to 2000 Hzin octave steps. The 70-phon level assured that audibility did not confound performance. Experiment 1 served as a control, in which a 10-dB ILD cue produced ∼90% correct lateralization at all frequencies. Experiment 2 measured lateralization accuracy with a 90-degree ITD cue, Experiment 3 measured the just-noticeable-difference from a 0-degree ITD standard, while Experiment 4 measured the binaural masking level difference in white noise. As expected, the listeners could not use the timingcue to perform these binaural tasks at 2000 Hz; Surprisingly, they showed significantly poorer performance at 62.5 Hz than mid frequencies. The band-pass characteristic result suggested a low-frequency boundary for binaural timing cues. This low boundary is not due to audibility but likely physical or physiological processes in the ear.

Subjective speech enhancement performance of a hearable with transparency and spatial frequency-weighted dereverberation

Preserving natural binaural cues combined with directionality and robust dereverberation is crucial for successful sound enhancement via hearables. In this study, we subjectively evaluate the performance of a real-time hearable prototype that aims to enhance frontal speech and spatial cues in complex environments using a new dereverberation algorithm and acoustic transparency. Our real-time algorithm exploits the spatial frequency-weighted coherence-to-diffuse ratio estimated between the conchal microphones and produces directional binaural gains using a Wiener filter for sound enhancement. The new hearable system can be customized or adapted to a given environment for optimum performance in different frequency bands and preserves natural binaural cues for all individuals by symmetrical spatial filtering. We use the real-time prototype to evaluate the perceived speech intelligibility in noise by a Matrix framework for normal-hearing subjects in a reverberant classroom. We also use a multi-stimulus with hidden reference and anchor-like framework to evaluate the perceived speech quality and listening comfort of several stimuli simulated for HATS in the same environment. Results show that speech intelligibility and listening comfort in noise can be significantly improved for the real-time hearable using the new algorithm with optimized parameters compared with several state-of-the-art coherence-based algorithms implemented in a similar device.

Cortical temporal integration can account for limits of temporal perception: investigations in the binaural system

The auditory system has exquisite temporal coding in the periphery which is transformed into a rate-based code in central auditory structures, like auditory cortex. However, the cortex is still able to synchronize, albeit at lower modulation rates, to acoustic fluctuations. The perceptual significance of this cortical synchronization is unknown. We estimated physiological synchronization limits of cortex (in humans with electroencephalography) and brainstem neurons (in chinchillas) to dynamic binaural cues using a novel system-identification technique, along with parallel perceptual measurements. We find that cortex can synchronize to dynamic binaural cues up to approximately 10 Hz, which aligns well with our measured limits of perceiving dynamic spatial information and utilizing dynamic binaural cues for spatial unmasking, i.e. measures of binaural sluggishness. We also find that the tracking limit for frequency modulation (FM) is similar to the limit for spatial tracking, demonstrating that this sluggish tracking is a more general perceptual limit that can be accounted for by cortical temporal integration limits.

Mapping the Auditory Space of Culex pipiens Female Mosquitoes in 3D.

The task of directional hearing faces most animals that possess ears. They approach this task in different ways, but a common trait is the use of binaural cues to find the direction to the source of sound. In insects, the task is further complicated by their small size and, hence, minute temporal and level differences between two ears. A single symmetric flagellar particle velocity receiver, such as the antenna of a mosquito, should not be able to discriminate between the two opposite directions along the vector of the sound wave. Paired antennae of mosquitoes presume the usage of binaural hearing, but its mechanisms are expected to be significantly different from the ones typical for the pressure receivers. However, the directionality of flagellar auditory organs has received little attention. Here, we measured the in-flight orientation of antennae in female Culex pipiens pipiens mosquitoes and obtained a detailed physiological mapping of the Johnston's organ directionality at the level of individual sensory units. By combining these data, we created a three-dimensional model of the mosquito's auditory space. The orientation of the antennae was found to be coordinated with the neuronal asymmetry of the Johnston's organs to maintain a uniformly shaped auditory space, symmetric relative to a flying mosquito. The overlap of the directional characteristics of the left and right sensory units was found to be optimal for binaural hearing focused primarily in front of, above and below a flying mosquito.

Fast binaural processing but sluggish masker representation reconfiguration.

Perceptual organization of complex acoustic scenes requires fast binaural processing for accurate localization or lateralization based on short single-source-dominated glimpses. This sensitivity also manifests in the ability to detect rapid oscillating interaural time and phase differences as well as interaural correlation. However, binaural processing has also been termed "sluggish" based on experiments that require binaural detection in a masker with an additional binaural cue change in temporal proximity. The present study shows that the temporal integration windows obtained from data on binaural sluggishness cannot account for the detection of rapid binaural oscillations. A model with fast IPD encoding but a slower process of updating the internal representation of the masker IPD statistics accounted for both experiments of the "fast" and the "sluggish" categories.

Spectral weighting functions for localization of complex sound. II. The effect of competing noise.

Spectral weighting of sound localization cues was measured in the presence of three levels of competing noise presented in the free field. Target stimuli were complex tones containing seven tonal components, presented from an ∼120° range of frontal azimuths. Competitors were two independent Gaussian noises presented from 90° left and right azimuth at one of three levels yielding +9, 0, and -6 dB signal-to-noise ratio. Results revealed the greatest perceptual weight for components within the interaural time difference (ITD) "dominance region," which was found previously to peak around the 800-Hz component in quiet [Folkerts and Stecker (2022) J. Acoust. Soc. Am. 151, 3409-3425]. Here, peak weights were shifted toward lower-frequency components (i.e., 400 Hz) in all competing noise conditions. These results contradict the hypothesis of a shift in the peak weights toward higher frequencies based on previous behavioral localization performance in competing noise but are consistent with binaural cue sensitivity, availability, and reliability; measured low-frequency ITD cues within the dominance region were least disrupted by the presence of competing noise.

Experimental methods to study the sound source localization by distance in humans

The review presents current methods used for researches of the auditory distance perception. The monaural and binaural cues of stationary and moving sources localization are considered. The role of binaural hearing in estimating the distance to a sound source is discussed in detail. The involvement of localization cues in absolute and relative distance estimation is described. The advantages and limitations of different experimental practices for forming virtual sound images are discussed. The special section discusses approaches to the creation of moving sound images. The results of auditory distance estimations obtained by different methods for stationary and moving sound sources are summarized. The review includes the results of the authors' own studies and a description of promising experimental and applied approaches of this research field.

Lateralization of binaural envelope cues measured with a mobile cochlear-implant research processora).

Bilateral cochlear implant (BICI) listeners do not have full access to the binaural cues that normal hearing (NH) listeners use for spatial hearing tasks such as localization. When using their unsynchronized everyday processors, BICI listeners demonstrate sensitivity to interaural level differences (ILDs) in the envelopes of sounds, but interaural time differences (ITDs) are less reliably available. It is unclear how BICI listeners use combinations of ILDs and envelope ITDs, and how much each cue contributes to perceived sound location. The CCi-MOBILE is a bilaterally synchronized research processor with the untested potential to provide spatial cues to BICI listeners. In the present study, the CCi-MOBILE was used to measure the ability of BICI listeners to perceive lateralized sound sources when single pairs of electrodes were presented amplitude-modulated stimuli with combinations of ILDs and envelope ITDs. Young NH listeners were also tested using amplitude-modulated high-frequency tones. A cue weighting analysis with six BICI and ten NH listeners revealed that ILDs contributed more than envelope ITDs to lateralization for both groups. Moreover, envelope ITDs contributed to lateralization for NH listeners but had negligible contribution for BICI listeners. These results suggest that the CCi-MOBILE is suitable for binaural testing and developing bilateral processing strategies.

Two congruent cues are better than one: Impact of ITD-ILD combinations on reaction time for sound lateralization.

This letter presents a reaction time analysis of a sound lateralization test. Sounds from various directions were synthesized using interaural time-level difference (ITD-ILD) combinations, and human subjects performed left/right detection. Stimuli from the sides yielded quicker reactions and better class accuracy than from the front. Congruent ITD-ILD cues significantly improved both metrics. For opposing ITD-ILD cues, subjects' choices were mostly driven by the ITD, and the responses were significantly slower. The findings, obtained with an easily accessible methodology, corroborate the integrated processing of the binaural cues and promote the use of multiple congruent binaural cues in headphone reproduction.

Effect of Reverberation on Neural Responses to Natural Speech in Rabbit Auditory Midbrain: No Evidence for a Neural Dereverberation Mechanism.

Reverberation is ubiquitous in everyday acoustic environments. It degrades both binaural cues and the envelope modulations of sounds and thus can impair speech perception. Still, both humans and animals can accurately perceive reverberant stimuli in most everyday settings. Previous neurophysiological and perceptual studies have suggested the existence of neural mechanisms that partially compensate for the effects of reverberation. However, these studies were limited by their use of either highly simplified stimuli or rudimentary reverberation simulations. To further characterize how reverberant stimuli are processed by the auditory system, we recorded single-unit (SU) and multiunit (MU) activity from the inferior colliculus (IC) of unanesthetized rabbits in response to natural speech utterances presented with no reverberation ("dry") and in various degrees of simulated reverberation (direct-to-reverberant energy ratios (DRRs) ranging from 9.4 to -8.2 dB). Linear stimulus reconstruction techniques (Mesgarani et al., 2009) were used to quantify the amount of speech information available in the responses of neural ensembles. We found that high-quality spectrogram reconstructions could be obtained for dry speech and in moderate reverberation from ensembles of 25 units. However, spectrogram reconstruction quality deteriorated in severe reverberation for both MUs and SUs such that the neural degradation paralleled the degradation in the stimulus spectrogram. Furthermore, spectrograms reconstructed from responses to reverberant stimuli resembled spectrograms of reverberant speech better than spectrograms of dry speech. Overall, the results provide no evidence for a dereverberation mechanism in neural responses from the rabbit IC when studied with linear reconstruction techniques.

Audiovisual Training in Virtual Reality Improves Auditory Spatial Adaptation in Unilateral Hearing Loss Patients.

Unilateral hearing loss (UHL) leads to an alteration of binaural cues resulting in a significant increment of spatial errors in the horizontal plane. In this study, nineteen patients with UHL were recruited and randomized in a cross-over design into two groups; a first group (n = 9) that received spatial audiovisual training in the first session and a non-spatial audiovisual training in the second session (2 to 4 weeks after the first session). A second group (n = 10) received the same training in the opposite order (non-spatial and then spatial). A sound localization test using head-pointing (LOCATEST) was completed prior to and following each training session. The results showed a significant decrease in head-pointing localization errors after spatial training for group 1 (24.85° ± 15.8° vs. 16.17° ± 11.28°; p < 0.001). The number of head movements during the spatial training for the 19 participants did not change (p = 0.79); nonetheless, the hand-pointing errors and reaction times significantly decreased at the end of the spatial training (p < 0.001). This study suggests that audiovisual spatial training can improve and induce spatial adaptation to a monaural deficit through the optimization of effective head movements. Virtual reality systems are relevant tools that can be used in clinics to develop training programs for patients with hearing impairments.

Two-Year Outcomes of Cochlear Implant Use for Children With Unilateral Hearing Loss: Benefits and Comparison to Children With Normal Hearing.

Children with severe-to-profound unilateral hearing loss, including cases of single-sided deafness (SSD), lack access to binaural cues that support spatial hearing, such as recognizing speech in complex multisource environments and sound source localization. Listening in a monaural condition negatively impacts communication, learning, and quality of life for children with SSD. Cochlear implant (CI) use may restore binaural hearing abilities and improve outcomes as compared to alternative treatments or no treatment. This study investigated performance over 24 months of CI use in young children with SSD as compared to the better hearing ear alone and to children with bilateral normal hearing (NH). Eighteen children with SSD who received a CI between the ages of 3.5 and 6.5 years as part of a prospective clinical trial completed assessments of word recognition in quiet, masked sentence recognition, and sound source localization at regular intervals out to 24-month postactivation. Eighteen peers with bilateral NH, matched by age at the group level, completed the same test battery. Performance at 24-month postactivation for the SSD group was compared to the performance of the NH group. Children with SSD have significantly poorer speech recognition in quiet, masked sentence recognition, and localization both with and without the use of the CI than their peers with NH. The SSD group experienced significant benefits with the CI+NH versus the NH ear alone on measures of isolated word recognition, masked sentence recognition, and localization. These benefits were realized within the first 3 months of use and were maintained through the 24-month postactivation interval. Young children with SSD who use a CI experience significant isolated word recognition and bilateral spatial hearing benefits, although their performance remains poorer than their peers with NH.

Effects of frequency-to-place mismatch in children listening to simualted unilateral and bilateral cochlear implant configurations

Binaural cues are important for spatial speech recognition. While binaural cues can be partly restored by bilateral cochlear implants or a unilateral cochlear implant in patients with single-sided deafness, the benefits may be limited by frequency-to-place mismatch in both cases. Previous studies reported worse recognition of spatially separated speech as frequency-to-place mismatch increased in typical-hearing adults listening to acoustic simulations of bilateral cochlear implants (Thomas et al., 2022). This work also revealed that children were more sensitive to the spectral degradation and less able to use spatial cues for spatial speech recognition. The present study investigates the effects of frequency-to-place mismatch on binaural fusion in typical hearing children (7–9 years old) listening to acoustic simulations of bi- and unilateral cochlear implant configurations. The stimuli in the left ear were either unprocessed or processed by a 16-channel sinewave vocoder with a fixed insertion depth while the stimuli in the right ear were processed by 16-channel sinewave vocoders with varying insertion depth. Speech recognition threshold results from N = 10 children show an effect of interaural insertion depth mismatch upon speech recognition thresholds, with no differences by age. Comparisons with adult data will likewise be reported.

Quantifying the impact of hearing-aid microphone placement and head-mounted devices on binaural cues from head-related transfer functions

Emerging techniques for assessment and training of spatial hearing use virtual reality (VR) devices to present auditory-visual cues. Sounds may be processed using head-related transfer functions (HRTFs), which capture directional acoustic cues, and visuals may be presented using a head-mounted display (HMD). For users of hearing devices such as hearing aids or cochlear implants, microphone placement may alter the binaural cues carried by the HRTF. Cues may also be distorted in presentations that combine HMD visuals with loudspeaker audio. This work seeks to understand these impacts by measuring and creating a database of HRTFs in the presence of hearing aids and HMDs. HRTFs were recorded within an anechoic chamber, using a standard manikin in a loudspeaker array located on the horizontal plane. Recordings were made with real sound sources of 5.625° resolution and virtual sources of 1° resolution. We will compare interaural time and level differences (ITDs and ILDs) from HRTFs measured through hearing-aid microphones with varied microphone placement (i.e., behind-the-ear and in-the-ear) and the presence versus absence of an HMD during the HRTF recording. Results will provide insights regarding HRTFs for individuals with hearing devices, as well as characterization of the HMD impact on VR-guided individual HRTF measurement.

Brain plasticity and auditory spatial adaptation in patients with unilateral hearing loss.

The ability to localize sounds in patients with Unilateral Hearing Loss (UHL) is usually disrupted due to alteration in the integration of binaural cues. Nonetheless, some patients are able to compensate deficit using adaptive strategies. In this study, we explored the neural correlates underlying this adaptation. Twenty-one patients with UHL were separated into 3 groups using cluster analysis based on their binaural performance. The resulting clusters were referred to as better, moderate, and poorer performers cluster (BPC, MPC, and PPC). We measured the mismatch negativity (MMN) elicited by deviant sounds located at 10°, 20°, and 100° from a standard positioned at 50° ipsilateral to the deaf ear. The BPC exhibited significant MMN for all 3 deviants, similar to normal hearing (NH) subjects. In contrast, there was no significant MMN for 10° and 20° deviants for the PPC and for NH when one ear was plugged and muffed. Scalp distribution was maximal over central regions in BPC, while PPC showed more frontal MMN distribution. Thus, the BPC exhibited a contralateral activation pattern, similar to NH, while the PPC exhibited more symmetrical hemispheric activation. MMN can be used as a neural marker to reflect spatial adaptation in patients with UHL.

Binaural Hearing Using the ADHEAR Bone Conduction System in the Monaurally Occluded Ear

Introduction: The study aimed to investigate binaural cues in the rehabilitation of unilateral occluded ears with a bone conduction hearing aid. Methods: The study sample consisted of 40 adult volunteers with normal hearing. Unilateral pseudo-conductive hearing loss was induced by inserting an earplug into the external auditory canal (EAC) and silicone material in the concha for ear impression. The adaptive speech-in-noise test (Italian Matrix test) was performed in three spatial orientations to assess binaural cues (summation, squelch, and head shadow effects). All evaluations were performed in the normal condition, after EAC occlusion, and after application of an adhesive bone conduction hearing aid. Binaural contrast differences were calculated in the three conditions. Results: In the EAC occlusion condition, there was a significant increase in the signal-to-noise ratio (SNR) in both the S₀N₀ (2.4 dB) and the S₉₀N₋₉₀ (7.7 dB) settings, and a slight albeit significant increase in the S₀N₉₀ setting (1.35 dB). After fitting the BC hearing aid, there was a reduction of −1.8 dB SNR (p < 0.001) in the S₀N₀ setting and −2 dB (p = 0.003) in the S₉₀N₋₉₀ setting. There was no improvement in the SNR (p = 0.405) in evaluation of the squelch effect (S₀N₉₀). These data were corroborated by a better binaural contrast due to a reduction in the summation effect in the monaural occlusion condition and a subsequent reduction in binaural contrast after fitting the hearing aid due to an increase in the summation effect (−2.5 dB vs. 0.3 dB; p < 0.001). Conclusions: Application of a bone conduction hearing aid in unilateral pseudo-conductive hearing loss strengthens speech recognition of noise by improving the summation effect and impeding the shadow effect of the head; however, there appears to be no improvement in speech perception in noise due to spatial release from masking.

Spatial audio for interactive hearing research

The use of sound field synthesis for hearing research has gained popularity due to the ability to auralize a wide range of sound scenes in a controlled and reproducible way. We are interested in reproducing acoustic environments for interactive hearing research, allowing participants to move freely over an extended area in the reproduced sound field. While the physically accurate sound field reproduction using sound field synthesis is limited to the sweet spot, it is unclear how different perceptual measures vary across the reproduction area and how suitable sound field synthesis is to evaluate them. To investigate the viability of listening experiments and provide a database for modelling approaches, measurements of binaural cues were carried out in the Simulated Open Field Environment loudspeaker array. Results show that the binaural cues are reproduced well close to the center, but exhibit more variance than in the corresponding free field case. Off center, lower interaural coherence is observed, which can affect binaural unmasking and speech intelligibility. In this work, we study binaural cues and speech reception thresholds over a wide area in the loudspeaker array to investigate the feasibility of psychoacoustic experiments involving speech understanding.

Persistence and generalization of adaptive changes in auditory localization behavior following unilateral conductive hearing loss.

Sound localization relies on the neural processing of binaural and monaural spatial cues generated by the physical properties of the head and body. Hearing loss in one ear compromises binaural computations, impairing the ability to localize sounds in the horizontal plane. With appropriate training, adult individuals can adapt to this binaural imbalance and largely recover their localization accuracy. However, it remains unclear how long this learning is retained or whether it generalizes to other stimuli. We trained ferrets to localize broadband noise bursts in quiet conditions and measured their initial head orienting responses and approach-to-target behavior. To evaluate the persistence of auditory spatial learning, we tested the sound localization performance of the animals over repeated periods of monaural earplugging that were interleaved with short or long periods of normal binaural hearing. To explore learning generalization to other stimulus types, we measured the localization accuracy before and after adaptation using different bandwidth stimuli presented against constant or amplitude-modulated background noise. Retention of learning resulted in a smaller initial deficit when the same ear was occluded on subsequent occasions. Each time, the animals' performance recovered with training to near pre-plug levels of localization accuracy. By contrast, switching the earplug to the contralateral ear resulted in less adaptation, indicating that the capacity to learn a new strategy for localizing sound is more limited if the animals have previously adapted to conductive hearing loss in the opposite ear. Moreover, the degree of adaptation to the training stimulus for individual animals was significantly correlated with the extent to which learning extended to untrained octave band target sounds presented in silence and to broadband targets presented in background noise, suggesting that adaptation and generalization go hand in hand. Together, these findings provide further evidence for plasticity in the weighting of monaural and binaural cues during adaptation to unilateral conductive hearing loss, and show that the training-dependent recovery in spatial hearing can generalize to more naturalistic listening conditions, so long as the target sounds provide sufficient spatial information.

Distance discrimination thresholds of proximal sound sources in a real anechoic environment

When a sound source off the midline is located close to the head, the interaural level difference changes dramatically with distance. Although distance discrimination thresholds for frontal sound sources have been comprehensively studied, the thresholds for real lateral sound sources in the proximal region have rarely been explored. The present study measured distance discrimination thresholds via a loudspeaker located at different distances and directions, both with fixed-emission and normalized intensities referring to the head center, to confirm the contribution of near-field binaural cues in a quantitative manner. The results demonstrated that the sound level is the main cue for distance discrimination, and binaural cues promote the ability to perceive differences in sound distances for lateral sources located at 0.5 m from the head center. The thresholds for lateral sources were significantly lower than those for sources in the median plane, both with and without level cues. However, when the sound sources were located at a distance of 1.0 m, the promotion of lateral thresholds was not significant. This study provides plausible just-noticeable differences for distance discrimination at various directions and distances from the head.

Aging alters across-hemisphere cortical dynamics during binaural temporal processing.

Differences in the timing and intensity of sounds arriving at the two ears provide fundamental binaural cues that help us localize and segregate sounds in the environment. Neural encoding of these cues is commonly represented asymmetrically in the cortex with stronger activation in the hemisphere contralateral to the perceived spatial location. Although advancing age is known to degrade the perception of binaural cues, less is known about how the neural representation of such cues is impacted by age. Here, we use electroencephalography (EEG) to investigate age-related changes in the hemispheric distribution of interaural time difference (ITD) encoding based on cortical auditory evoked potentials (CAEPs) and derived binaural interaction component (BIC) measures in ten younger and ten older normal-hearing adults. Sensor-level analyses of the CAEP and BIC showed age-related differences in global field power, where older listeners had significantly larger responses than younger for both binaural metrics. Source-level analyses showed hemispheric differences in auditory cortex activity for left and right lateralized stimuli in younger adults, consistent with a contralateral activation model for processing ITDs. Older adults, however, showed reduced hemispheric asymmetry across ITDs, despite having overall larger responses than younger adults. Further, when averaged across ITD condition to evaluate changes in cortical asymmetry over time, there was a significant shift in laterality corresponding to the peak components (P1, N1, P2) in the source waveform that also was affected by age. These novel results demonstrate across-hemisphere cortical dynamics during binaural temporal processing that are altered with advancing age.