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Sound location coding has been extensively studied at the central nucleus of the mammalian inferior colliculus (CNIC), supporting a population code. However, this population code has not been extensively characterized on the single-trial level with simultaneous recordings or at other anatomical regions like the dorsal cortex of inferior colliculus (DCIC), which is relevant for learning-induced experience dependent plasticity. To address these knowledge gaps, here we made in two complementary ways large-scale recordings of DCIC populations from awake mice in response to sounds delivered from 13 different frontal horizontal locations (azimuths): volumetric two-photon calcium imaging with ~700 cells simultaneously recorded at a relatively low temporal resolution, and high-density single-unit extracellular recordings with ~20 cells simultaneously recorded at a high temporal resolution. Independent of the method, the recorded DCIC population responses revealed substantial trial-to-trial variation (neuronal noise) which was significantly correlated across pairs of neurons (noise correlations) in the passively listening condition. Nevertheless, decoding analysis supported that these noisy response patterns encode sound location on the single-trial basis, reaching errors that match the discrimination ability of mice. The detected noise correlations contributed to minimize the error of the DCIC population code of sound azimuth. Altogether these findings point out that DCIC can encode sound location in a similar format to what has been proposed for CNIC, opening exciting questions about how noise correlations could shape this code in the context of cortico-collicular input and experience-dependent plasticity., Competing Interests: JB, HA, RP No competing interests declared, BB Reviewing editor, eLife, (© 2024, Boffi et al.)

This study investigated the impact of bilateral bone conduction (BC) stimulation and sensorineural hearing loss on spatial release from masking, binaural intelligibility level difference, and lateralization. The study involved two groups of adults with mild to moderate sensorineural hearing loss: one group of 21 participants with symmetric hearing loss and another group of nine participants with asymmetric hearing loss. All tests were conducted through BC and air conduction (AC) headsets with non-individualized virtual positions of the sound sources and linear amplification based on individual hearing thresholds. The findings revealed a bilateral benefit for both groups of hearing-impaired individuals, with symmetric hearing loss yielding better results than asymmetric hearing loss. AC stimulation provided approximately twice the benefit in terms of dB compared to BC stimulation. A large part of this benefit originated from a favorable signal-to-noise ratio due to noise reduction from the head shadow. However, binaural processing was present in both hearing-impaired groups with bilateral BC stimulation. The ability to lateralize sounds based on interaural time delays was significantly impaired in participants with both types of hearing loss when stimulation was by BC. Despite these challenges, the study underscores the benefits of bilateral fitting of BC hearing aids, even in individuals with mild to moderate sensorineural hearing loss, whether symmetric or asymmetric., Competing Interests: Declarations. Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

Human hearing is robust to noise, but the basis of this robustness is poorly understood. Several lines of evidence are consistent with the idea that the auditory system adapts to sound components that are stable over time, potentially achieving noise robustness by suppressing noise-like signals. Yet background noise often provides behaviorally relevant information about the environment and thus seems unlikely to be completely discarded by the auditory system. Motivated by this observation, we explored whether noise robustness might instead be mediated by internal models of noise structure that could facilitate the separation of background noise from other sounds. We found that detection, recognition, and localization in real-world background noise were better for foreground sounds positioned later in a noise excerpt, with performance improving over the initial second of exposure to a noise. These results are consistent with both adaptation-based and model-based accounts (adaptation increases over time and online noise estimation should benefit from acquiring more samples). However, performance was also robust to interruptions in the background noise and was enhanced for intermittently recurring backgrounds, neither of which would be expected from known forms of adaptation. Additionally, the performance benefit observed for foreground sounds occurring later within a noise excerpt was reduced for recurring noises, suggesting that a noise representation is built up during exposure to a new background noise and then maintained in memory. These findings suggest that noise robustness is supported by internal models-"noise schemas"-that are rapidly estimated, stored over time, and used to estimate other concurrent sounds., Competing Interests: Competing interests statement:The authors declare no competing interest.

Blind football players use head movements to accurately identify sound location when trapping a ball. Accurate sound localization is likely important for motor learning of ball trapping in blind football. However, whether head movements affect the acquisition of ball-trapping skills remains unclear. Therefore, this study examined the effect of head movements on skill acquisition during ball trapping. Overall, 20 sighted male college students were recruited and assigned to one of the following two groups: the conventional training group, where they were instructed to move leftward and rightward to align their body with the ball's trajectory, and the head-movement-focused group, where they were instructed to follow the ball with their faces until the ball touched their feet, in addition to the conventional training instructions. Both groups underwent a 2-day training for ball trapping according to the specific instructions. The head-movement-focused group showed a decrease in errors in ball trapping at near distances and with larger downward head rotations in the sagittal plane compared to the conventional training group, indicating that during the skill acquisition training for ball trapping, the sound source can be localized more accurately using larger head rotations toward the ball. These results may help beginner-level players acquire better precision in their movements while playing blind football., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Mieda, Kokubu. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Auditory spatial attention detection (ASAD) seeks to determine which speaker in a surround sound field a listener is focusing on based on the one's brain biosignals. Although existing studies have achieved ASAD from a single-trial electroencephalogram (EEG), the huge inter-subject variability makes them generally perform poorly in cross-subject scenarios. Besides, most ASAD methods do not take full advantage of topological relationships between EEG channels, which are crucial for high-quality ASAD. Recently, some advanced studies have introduced graph-based brain topology modeling into ASAD, but how to calculate edge weights in a graph to better capture actual brain connectivity is worthy of further investigation. To address these issues, we propose a new ASAD method in this paper. First, we model a multi-channel EEG segment as a graph, where differential entropy serves as the node feature, and a static adjacency matrix is generated based on inter-channel mutual information to quantify brain functional connectivity. Then, different subjects' EEG graphs are encoded into a shared embedding space through a total variation graph neural network. Meanwhile, feature distribution alignment based on multi-kernel maximum mean discrepancy is adopted to learn subject-invariant patterns. Note that we align EEG embeddings of different subjects to reference distributions rather than align them to each other for the purpose of privacy preservation. A series of experiments on open datasets demonstrate that the proposed model outperforms state-of-the-art ASAD models in cross-subject scenarios with relatively low computational complexity, and feature distribution alignment improves the generalizability of the proposed model to a new subject., Competing Interests: Declaration of competing interest None., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The neurological basis for perceptual awareness remains unclear, and theories disagree as to whether sensory cortices per se generate awareness. Critically, neural activity in the sensory cortices is only a neural correlate of consciousness (NCC) if it closely matches the contents of perceptual awareness. Research in vision and touch suggest that contralateral activity in sensory cortices is an NCC. Similarly, research in hearing with two sound sources (left and right) presented over headphones also suggests that a candidate NCC called the auditory awareness negativity (AAN) matches perceived location of sound. The current study used 13 different sound sources presented over loudspeakers for natural localization cues and measured event-related potentials to a threshold stimulus in a sound localization task. Preregistered Bayesian mixed models provided moderate evidence against an overall AAN and very strong evidence against its lateralization. Because of issues regarding data quantity and quality, exploratory analyses with aggregated data from multiple loudspeakers were conducted. Results provided moderate evidence for an overall AAN and strong evidence against its lateralization. Nonetheless, the interpretations of these results remain inconclusive. Therefore, future research should reduce the number of conditions and/or test over several sessions to procure a sufficient amount of data. Taken at face value, the results may suggest issues with AAN as an NCC of auditory awareness, as it does not laterally map onto experiences in a free-field auditory environment, in contrast to the NCCs of vision and touch., (© 2024 The Author(s). Psychophysiology published by Wiley Periodicals LLC on behalf of Society for Psychophysiological Research.)

Neural decoding is a tool for understanding how activities from a population of neurons inside the brain relate to the outside world and for engineering applications such as brain-machine interfaces. However, neural decoding studies mainly focused on different decoding algorithms rather than different neuron types which could use different coding strategies. In this study, we used two-photon calcium imaging to assess three auditory spatial decoders (space map, opponent channel, and population pattern) in excitatory and inhibitory neurons in the dorsal inferior colliculus of male and female mice. Our findings revealed a clustering of excitatory neurons that prefer similar interaural level difference (ILD), the primary spatial cues in mice, while inhibitory neurons showed random local ILD organization. We found that inhibitory neurons displayed lower decoding variability under the opponent channel decoder, while excitatory neurons achieved higher decoding accuracy under the space map and population pattern decoders. Further analysis revealed that the inhibitory neurons' preference for ILD off the midline and the excitatory neurons' heterogeneous ILD tuning account for their decoding differences. Additionally, we discovered a sharper ILD tuning in the inhibitory neurons. Our computational model, linking this to increased presynaptic inhibitory inputs, was corroborated using monaural and binaural stimuli. Overall, this study provides experimental and computational insight into how excitatory and inhibitory neurons uniquely contribute to the coding of sound locations., (Copyright © 2024 the authors.)

Echolocation is a vital method of spatial orientation for many visually impaired individuals who are willing to and able to learn it. Blind echolocators use a variety of sounds, such as mouth clicks, cane taps, or specialized sound-emitting devices, to perceive their surroundings. In our study, we examined the effectiveness of several different sounds used in echolocation by conducting trials with 12 blind and 14 sighted volunteers. None of the participants had received formal training in echolocation, though a number identified as self-taught experts. The sounds tested included those played from a loudspeaker, generated by a mechanical clicker, or made by the participants themselves. The task given to the participants was to identify the direction and distance to an obstacle measuring 1x2 meters in an outdoor environment, with the obstacle placed in one of nine possible positions. Our findings indicated that the blind participants displayed significantly better echolocation skills when compared to the sighted participants. The results of the blind participants were also strongly divided into two distinct subgroups-totally blind participants performed much better than those which were legally blind, but had some residual vision. In terms of sound comparisons, we found that sounds with a center frequency near 3-4kHz and a wide spectrum provided higher accuracy rates than those with lower frequency peaks. Sighted participants performed best with 3kHz and 4kHz percussion sounds, while the blind group performed best with blue and pink noise. The loudspeaker generated tones generally yielded better results than those generated by the participant (using a mechanical clicker, mouth clicks or hand claps). These results may be useful in developing training programs that teach echolocation as well as artificial sounds to improve echolocation effectiveness., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Bujacz et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Despite the growing interest in studying binaural fusion, there is little consensus over its definition or how it is best measured. This review seeks to describe the complexities of binaural fusion, highlight measurement challenges, provide guidelines for rigorous perceptual measurements, and provide a working definition that encompasses this information. First, it is argued that binaural fusion may be multidimensional and might occur in one domain but not others, such as fusion in the spatial but not the spectral domain or vice versa. Second, binaural fusion may occur on a continuous scale rather than on a binary one. Third, binaural fusion responses are highly idiosyncratic, which could be a result of methodology, such as the specific experimental instructions, suggesting a need to explicitly report the instructions given. Fourth, it is possible that direct ("Did you hear one sound or two?") and indirect ("Where did the sound come from?" or "What was the pitch of the sound?") measurements of fusion will produce different results. In conclusion, explicit consideration of these attributes and reporting of methodology are needed for rigorous interpretation and comparison across studies and listener populations., (© 2024 Acoustical Society of America.)

Sound localization in the front-back dimension is reported to be challenging, with individual differences. We investigated whether auditory discrimination processing in the brain differs based on front-back sound localization ability. This study conducted an auditory oddball task using speakers in front of and behind the participants. We used event-related brain potentials to examine the deviance detection process between groups that could and could not discriminate front-back sound localization. The results indicated that mismatch negativity (MMN) occurred during the deviance detection process, and P2 amplitude differed between standard and deviant locations in both groups. However, the latency of MMN was shorter in the group that could discriminate front-back sounds than in the group that could not. Additionally, N1 amplitude increased for deviant locations compared to standard ones only in the discriminating group. In conclusion, the sensory memories matching process based on traces of previously presented stimuli (MMN, P2) occurred regardless of discrimination ability. However, the response to changes in the physical properties of sounds (MMN latency, N1 amplitude) differed depending on the ability to discriminate front-back sounds. Our findings suggest that the brain may have different processing strategies for the two directions even without subjective recognition of the front-back direction of incoming sounds., Competing Interests: Declaration of competing interest None of the authors have potential conflicts of interest to be disclosed., (Copyright © 2024. Published by Elsevier B.V.)

Objective: To design and develop a Portable Auditory Localization Acclimation Training (PALAT) system capable of producing psychoacoustically accurate localization cues; evaluate the training effect against a proven full-scale, laboratory-grade system under three listening conditions; and determine if the PALAT system is sensitive to differences among electronic level-dependent hearing protection devices (HPDs)., Background: In-laboratory auditory localization training has demonstrated the ability to improve localization performance with the open (natural) ear, that is, unoccluded, and while wearing HPDs. The military requires a portable system capable of imparting similar training benefits as those demonstrated in laboratory experiments., Method: In a full-factorial repeated measures design experiment, 12 audiometrically normal participants completed localization training and testing using an identical, optimized training protocol on two training systems under three listening conditions (open ear, TEP-100, and ComTac™ III). Statistical tests were performed on mean absolute accuracy score and front-back reversal errors., Results: No statistical difference existed between the PALAT and laboratory-grade DRILCOM systems on two dependent localization accuracy measurements at all stages of training. In addition, the PALAT system detected the same localization performance differences among the three listening conditions., Conclusion: The PALAT system imparted similar training benefits as the DRILCOM system and was sensitive to HPD localization performance differences., Application: The user-operable PALAT system and optimized training protocol can be employed by the military, law enforcement, and various industries, to improve auditory localization performance in conditions where auditory situation awareness is critical to safety.

Purpose: The purpose of this study was to determine whether providing realistic auditory or somatosensory cues to spatial location would affect measures of vestibulo-ocular reflex gain in a rotary chair testing (RCT) context., Method: This was a fully within-subject design. Thirty young adults age 18-30 years (16 men, 14 women by self-identification) completed sinusoidal harmonic acceleration testing in a rotary chair under five different conditions, each at three rotational frequencies (0.01, 0.08, and 0.32 Hz). We recorded gain as the ratio of the amplitude of eye movement to chair movement using standard clinical procedures. The five conditions consisted of two without spatial information (silence, tasking via headphones) and three with either auditory (refrigerator sound, tasking via speaker) or somatosensory (fan) information. Two of the conditions also included mental tasking (tasking via headphones, tasking via speaker) and differed only in terms of the spatial localizability of the verbal instructions. We used linear mixed-effects modeling to compare pairs of conditions, specifically examining the effects of the availability of spatial cues in the environment. This study was preregistered on Open Science Framework (https://osf.io/2gqcf/)., Results: Results showed significant effects of frequency in all conditions ( p < .05), but the only pairs of conditions that were significantly different were those including tasking in one condition but not the other (e.g., tasking via headphones vs. silence). Post hoc equivalence testing showed that the lack of significance in the other comparisons could be confirmed as not meaningfully different., Conclusions: These findings suggest that the presence of externally localizable sensory information, whether auditory or somatosensory, does not affect measures of gain in RCT to any relevant degree. However, these findings also contribute to the increasing body of evidence suggesting that mental engagement ("tasking") does increase gain whether or not it is provided via localizable instructions.

Hypothesis: Acoustic localization accuracy metrics currently employed in clinical literature both overestimate and underestimate performance benefit of cochlear implantation (CI) for single-sided deafness (SSD)., Background: Although localization in SSD with CI has been investigated, performance characterization has relied heavily on average error. Although attractively concise, this measure may misrepresent performance. Here, we characterize frequency-specific localization on a granular level in subjects with CI for SSD as a critical analysis of localization outcome metrics., Methods: Eight CI recipients with SSD were recruited. Stimuli of broadband (BBN) and narrowband noise (NBN) at low (500 Hz), mid (1000 Hz), and high (4000 Hz) frequencies were presented in a semianechoic chamber. Localization accuracy was quantified in mean angular error (MAE) and linear regression slope., Results: Use of a CI for SSD subjects improved localization performance by slope for all stimuli ( p ≤ 0.0033) to a level that was equal to normal-hearing controls at 1 and 4 kHz ( p ≥ 0.2281). MAE was also significantly improved for SSD subjects using CI for BBN stimuli ( p ≪ 0.0001); however, no statistically significant improvement in MAE was seen for NBN ( p ≥ 0.5773) with CI use. Descriptive analysis of individual subject performance highlights the reasons for contradictory results., Conclusion: There is inherent challenge in characterizing localization benefit for individuals with CI for SSD. Our data demonstrate the limitations of utilization of average error as the sole metric for outcome benefit. We emphasize the importance of continued research investigating alternative outcome measures as we work toward a more refined understanding of the potential benefits and limitations of cochlear implantation for SSD., Competing Interests: The authors disclose no conflicts of interest., (Copyright © 2024, Otology & Neurotology, Inc.)

Sound-source localization is based on spatial cues arising due to interactions of sound waves with the torso, head and ears. Here, we evaluated neural responses to free-field sound sources in the central nucleus of the inferior colliculus (CIC), the medial geniculate body (MGB) and the primary auditory cortex (A1) of Mongolian gerbils. Using silicon probes we recorded from anaesthetized gerbils positioned in the centre of a sound-attenuating, anechoic chamber. We measured rate-azimuth functions (RAFs) with broad-band noise of varying levels presented from loudspeakers spanning 210° in azimuth and characterized RAFs by calculating spatial centroids, Equivalent Rectangular Receptive Fields (ERRFs), steepest slope locations and spatial-separation thresholds. To compare neuronal responses with behavioural discrimination thresholds from the literature we performed a neurometric analysis based on signal-detection theory. All structures demonstrated heterogeneous spatial tuning with a clear dominance of contralateral tuning. However, the relative amount of contralateral tuning decreased from the CIC to A1. In all three structures spatial tuning broadened with increasing sound-level. This effect was strongest in CIC and weakest in A1. Neurometric spatial-separation thresholds compared well with behavioural discrimination thresholds for locations directly in front of the animal. Our findings contrast with those reported for another rodent, the rat, which exhibits homogenous and sharply delimited contralateral spatial tuning. Spatial tuning in gerbils resembles more closely the tuning reported in A1 of cats, ferrets and non-human primates. Interestingly, gerbils, in contrast to rats, share good low-frequency hearing with carnivores and non-human primates, which may account for the observed spatial tuning properties., (© 2024 The Author(s). European Journal of Neuroscience published by Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Objective: Evaluate sound localization accuracy of subjects with single-sided deafness (SSD) with active transcutaneous bone conduction implants (atBCIs)., Study Design: Prospective case-control study., Setting: Tertiary referral center., Patients: Ten SSD patients (with ATBCIS) and 10 controls., Interventions: Localization was assessed in a semianechoic chamber using a 24-speaker array. Stimuli included broadband noise (BBN) and narrowband noise (NBN). Perceived stimulus angle was recorded and compared with presented location. Statistical analyses were performed using ANOVA and Wilcoxon rank sum tests., Main Outcome Measures: The primary outcome measures were as follows: 1) mean angular error (MAE) error (°) and regression slope and 2) subjective benefit assessment (Speech Spatial Qualities questionnaire)., Results: Subjects with SSD demonstrated worse localization by MAE and regression slope compared with controls for both broadband noise (p < 0.0001) and narrowband noise at 500 Hz and 1000 kHz (p < 0.0001). There was no statistically significant difference (p = 0.1090) in slope between all groups at 4000 Hz. There was no significant difference in slope or MAE aided compared with unaided. Localization ability varied widely within the SSD cohort, with some individuals showing some ability in the unaided condition, best at 4000 Hz. Although SSQ confirmed particular difficulty in the spatial hearing domain, all domains improved with device use., Conclusions: Localization ability for individuals with SSD falls into a somewhat bimodal distribution. Some have fair localization, particularly at high frequencies, that is preserved but not improved with the atBCI. Others have minimal to no localization ability at any frequency, with no apparent device benefit., (Copyright © 2024, Otology & Neurotology, Inc.)

Affective stimuli in our environment indicate reward or threat and thereby relate to approach and avoidance behavior. Previous findings suggest that affective stimuli may bias visual perception, but it remains unclear whether similar biases exist in the auditory domain. Therefore, we asked whether affective auditory voices (angry vs. neutral) influence sound distance perception. Two VR experiments (data collection 2021-2022) were conducted in which auditory stimuli were presented via loudspeakers located at positions unknown to the participants. In the first experiment ( N = 44), participants actively placed a visually presented virtual agent or virtual loudspeaker in an empty room at the perceived sound source location. In the second experiment ( N = 32), participants were standing in front of several virtual agents or virtual loudspeakers and had to indicate the sound source by directing their gaze toward the perceived sound location. Results in both preregistered experiments consistently showed that participants estimated the location of angry voice stimuli at greater distances than the location of neutral voice stimuli. We discuss that neither emotional nor motivational biases can account for these results. Instead, distance estimates seem to rely on listeners' representations regarding the relationship between vocal affect and acoustic characteristics. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

How we move our bodies affects how we perceive sound. For instance, head movements help us to better localize the source of a sound and to compensate for asymmetric hearing loss. However, many auditory experiments are designed to restrict head and body movements. To study the role of movement in hearing, we developed a behavioral task called sound-seeking that rewarded freely moving mice for tracking down an ongoing sound source. Over the course of learning, mice more efficiently navigated to the sound. Next, we asked how sound-seeking was affected by hearing loss induced by surgical removal of the malleus from the middle ear. After bilateral hearing loss sound-seeking performance drastically declined and did not recover. In striking contrast, after unilateral hearing loss mice were only transiently impaired and then recovered their sound-seek ability over about a week. Throughout recovery, unilateral mice increasingly relied on a movement strategy of sequentially checking potential locations for the sound source. In contrast, the startle reflex (an innate auditory behavior) was preserved after unilateral hearing loss and abolished by bilateral hearing loss without recovery over time. In sum, mice compensate with body movement for permanent unilateral damage to the peripheral auditory system. Looking forward, this paradigm provides an opportunity to examine how movement enhances perception and enables resilient adaptation to sensory disorders., (© 2024. The Author(s).)

In real-world listening situations, individuals typically utilize head and eye movements to receive and enhance sensory information while exploring acoustic scenes. However, the specific patterns of such movements have not yet been fully characterized. Here, we studied how movement behavior is influenced by scene complexity, varied in terms of reverberation and the number of concurrent talkers. Thirteen normal-hearing participants engaged in a speech comprehension and localization task, requiring them to indicate the spatial location of a spoken story in the presence of other stories in virtual audio-visual scenes. We observed delayed initial head movements when more simultaneous talkers were present in the scene. Both reverberation and a higher number of talkers extended the search period, increased the number of fixated source locations, and resulted in more gaze jumps. The period preceding the participants' responses was prolonged when more concurrent talkers were present, and listeners continued to move their eyes in the proximity of the target talker. In scenes with more reverberation, the final head position when making the decision was farther away from the target. These findings demonstrate that the complexity of the acoustic scene influences listener behavior during speech comprehension and localization in audio-visual scenes., (© 2024. The Author(s).)

Purpose: First-generation bone bridges (BBs) have demonstrated favorable safety and audiological benefits in patients with conductive hearing loss. However, studies on the effects of second-generation BBs are limited, especially among children. In this study, we aimed to explore the surgical and audiological effects of second-generation BBs in patients with bilateral congenital microtia., Methods: This single-center prospective study included nine Mandarin-speaking patients with bilateral microtia. All the patients underwent BCI Generation 602 (BCI602; MED-EL, Innsbruck, Austria) implant surgery between September 2021 and June 2023. Audiological and sound localization tests were performed under unaided and BB-aided conditions., Results: The transmastoid and retrosigmoid sinus approaches were implemented in three and six patients, respectively. No patient underwent preoperative planning, lifts were unnecessary, and no sigmoid sinus or dural compression occurred. The mean function gain at 0.5-4.0 kHz was 28.06 ± 4.55-dB HL. The word recognition scores improved significantly in quiet under the BB aided condition. Signal-to-noise ratio reduction by 10.56 ± 2.30 dB improved the speech reception threshold in noise. Patients fitted with a unilateral BB demonstrated inferior sound source localization after the initial activation., Conclusions: Second-generation BBs are safe and effective for patients with bilateral congenital microtia and may be suitable for children with mastoid hypoplasia without preoperative three-dimensional reconstruction., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Purpose: The interaural time difference (ITD) is a primary horizontal-plane sound localization cue computed in the auditory brainstem. ITDs are accessible in the temporal fine structure of pure tones with a frequency of no higher than about 1400 Hz. How listeners' ITD sensitivity transitions from very best sensitivity near 700 Hz to impossible to detect within 1 octave currently lacks a fully compelling physiological explanation. Here, it was hypothesized that the rapid decline in ITD sensitivity is dictated not by a central neural limitation but by initial peripheral sound encoding, specifically, the low-frequency (apical) portion of the cochlear excitation pattern produced by a pure tone., Methods: ITD sensitivity was measured in 16 normal-hearing listeners as a joint function of frequency (900-1500 Hz) and level (10-50 dB sensation level)., Results: Performance decreased with increasing frequency and decreasing sound level. The slope of performance decline was 90 dB/octave, consistent with the low-frequency slope of the cochlear excitation pattern., Conclusion: Fine-structure ITD sensitivity near 1400 Hz may be conveyed primarily by "off-frequency" activation of neurons tuned to lower frequencies near 700 Hz. Physiologically, this could be realized by having neurons sensitive to fine-structure ITD up to only about 700 Hz. A more extreme model would have only a single narrow channel near 700 Hz that conveys fine-structure ITDs. Such a model is a major simplification and departure from the classic formulation of the binaural display, which consists of a matrix of neurons tuned to a wide range of relevant frequencies and ITDs., (© 2024. The Author(s) under exclusive licence to Association for Research in Otolaryngology.)

We examine the effect of auditory cues occurring in reality on redirection. Specifically, we set two hypotheses: the auditory cues emanating from fixed positions in reality (Fixed sound, FS) increase the noticeability of redirection, while the auditory cues whose positions are manipulated consistently with the visual manipulation (Redirected sound, RDS) decrease the noticeability of redirection. To verify these hypotheses, we implemented an experimental environment that virtually reproduced FS and RDS conditions using binaural recording, and then we conducted a user study ( N=18) to investigate the detection thresholds (DTs) for rotational manipulation and the sound localization accuracy of the auditory cues under FS and RDS, as well as the baseline condition without auditory cues (No sound, NS). The results show, against the hypotheses, FS gave a wider range of DTs than NS, while RDS gave a similar range of DTs to NS. Combining these results with those of sound localization accuracy reveals that, rather than the auditory cues affecting the participants' spatial perception in VR, the visual manipulation made their sound localization less accurate, which would be a reason for the increased range of DTs under FS. Furthermore, we conducted a follow-up user study ( N=11) to measure the sound localization accuracy of FS where the auditory cues were actually placed in a real setting, and we found that the accuracy tended to be similar to that of virtually reproduced FS, suggesting the validity of the auditory cues used in this study. Given these findings, we also discuss potential applications.

Objectives: To assess the influence of three factors using retrospective chart review: age at which 2nd cochlear implant (CI) is implanted, prior hearing aid (HA) experience in the 2nd CI ear, and long-term experience with bilateral cochlear implants (BICIs) on sound localization in children with sequential BICIs., Methods: Mean absolute error (MAE) in localizing speech noise of 60 children with sequential BICIs was compared across four age groups of the 2nd CI (1-5.0; 5.1-10.0; 10.1-14.0; & 14.1-19.0 years) and two extents of prior HA experience (more than and less than one year). MAE was also longitudinally analyzed after 4-6 years of experience with BICI involving 18 participants out of 60., Results: Children who received 2nd CI before five years of age demonstrated significantly better localization than those who received it after ten years of age. More than one year of prior HA experience in the 2nd CI ear and extensive experience with sequential BICIs significantly enhanced localization performance. Inter-implant intervals and age at the 2nd CI showed a significant positive correlation with the MAE (poorer localization)., Conclusion: The results indicate that age at 2nd CI is important in developing sound localization skills. Based on the results, obtaining 2nd CI within the first five years of life and no later than ten years old is recommended. The results also suggest that longer use of amplification before 2nd CI and prolonged BICI experience significantly fosters localization development., Competing Interests: Declaration of competing interest The authors report there are no competing interests to declare., (Copyright © 2024 Elsevier B.V. All rights reserved.)

When individuals are exposed to two pure tones with close frequencies presented separately in each ear, they perceive a third sound known as binaural beats (BB), characterized by a frequency equal to the difference between the two tones. Previous research has suggested that BB may influence brain activity, potentially benefiting attention and relaxation. In this study, we hypothesized that the impact of BB on cognition and EEG is linked to the spatial characteristics of the sound. Participants listened to various types of spatially moving sounds (BB, panning and alternate beeps) at 6Hz and 40Hz frequencies. EEG measurements were conducted throughout the auditory stimulation, and participants completed questionnaires on relaxation, affect, and a sustained attention task. The results indicated that binaural, panning sounds and alternate beeps had a more pronounced effect on electrical brain activity than the control condition. Additionally, an improvement in relaxation was observed with these sounds at both 6Hz and 40Hz. Overall, these findings support our hypothesis that the impact of auditory stimulation lies in the spatial attributes rather than the sensation of beating itself., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Sudre et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

People with single-sided deafness (SSD) or asymmetric hearing loss (AHL) have particular difficulty understanding speech in noisy listening situations and in sound localization. The objective of this multicenter study is to evaluate the effect of a cochlear implant (CI) in adults with single-sided deafness (SSD) or asymmetric hearing loss (AHL), particularly regarding sound localization and speech intelligibility with additional interest in electric-acoustic pitch matching. A prospective longitudinal study at 7 European tertiary referral centers was conducted including 19 SSD and 16 AHL subjects undergoing cochlear implantation. Sound localization accuracy was investigated in terms of root mean square error and signed bias before and after implantation. Speech recognition in quiet and speech reception thresholds in noise for several spatial configurations were assessed preoperatively and at several post-activation time points. Pitch perception with CI was tracked using pitch matching. Data up to 12 months post activation were collected. In both SSD and AHL subjects, CI significantly improved sound localization for sound sources on the implant side, and thus overall sound localization. Speech recognition in quiet with the implant ear improved significantly. In noise, a significant head shadow effect was found for SSD subjects only. However, the evaluation of AHL subjects was limited by the small sample size. No uniform development of pitch perception with the implant ear was observed. The benefits shown in this study confirm and expand the existing body of evidence for the effectiveness of CI in SSD and AHL. Particularly, improved localization was shown to result from increased localization accuracy on the implant side.

Congenital single-sided deafness (SSD) leads to an aural preference syndrome that is characterized by overrepresentation of the hearing ear in the auditory system. Cochlear implantation (CI) of the deaf ear is an effective treatment for SSD. However, the newly introduced auditory input in congenital SSD often does not reach expectations in late-implanted CI recipients with respect to binaural hearing and speech perception. In a previous study, a reduction of the interaural time difference (ITD) sensitivity has been shown in unilaterally congenitally deaf cats (uCDCs). In the present study, we focused on the interaural level difference (ILD) processing in the primary auditory cortex. The uCDC group was compared with hearing cats (HCs) and bilaterally congenitally deaf cats (CDCs). The ILD representation was reorganized, replacing the preference for the contralateral ear with a preference for the hearing ear, regardless of the cortical hemisphere. In accordance with the previous study, uCDCs were less sensitive to interaural time differences than HCs, resulting in unmodulated ITD responses, thus lacking directional information. Such incongruent ITDs and ILDs cannot be integrated for binaural sound source localization. In normal hearing, the predominant effect of each ear is excitation of the auditory cortex in the contralateral cortical hemisphere and inhibition in the ipsilateral hemisphere. In SSD, however, auditory pathways reorganized such that the hearing ear produced greater excitation in both cortical hemispheres and the deaf ear produced weaker excitation and preserved inhibition in both cortical hemispheres., Competing Interests: The authors declare no competing financial interests., (Copyright © 2024 the authors.)

Piano tone localization at the performer's listening point is a multisensory process involving audition, vision, and upper limb proprioception. The consequent representation of the auditory scene, especially in experienced pianists, is likely also influenced by their memory about the instrument keyboard. Disambiguating such components is not obvious, and first requires an analysis of the acoustic tone localization process to assess the role of auditory feedback in forming this scene. This analysis is complicated by the acoustic behavior of the piano, which does not guarantee the activation of the auditory precedence effect during a tone attack, nor can it provide robust interaural differences during the subsequent free evolution of the sound. In a tone localization task using a Disklavier upright piano (which can be operated remotely and configured to have its hammers hit a damper instead of producing a tone), twenty-three expert musicians, including pianists, successfully recognized the angular position of seven evenly distributed notes across the keyboard. The experiment involved listening to either full piano tones or just the key mechanical noise, with no additional feedback from other senses. This result suggests that the key mechanical noise alone activated the localization process without support from vision and/or limb proprioception. Since the same noise is present in the onset of the full tones, the key mechanics of our piano created a touch precursor in such tones that may be responsible of their correct angular localization by means of the auditory precedence effect. However, the significance of pitch cues arriving at a listener after the touch precursor was not measured when full tones were presented. As these cues characterize a note and, hence, the corresponding key position comprehensively, an open question remains regarding the contribution of pianists' spatial memory of the instrument keyboard to tone localization., (© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).)

The sound localization behavior of the nocturnally hunting barn owl and its underlying neural computations is a textbook example of neuroethology. Differences in sound timing and level at the two ears are integrated in a series of well-characterized steps, from brainstem to inferior colliculus (IC), resulting in a topographical neural representation of auditory space. It remains an important question of brain evolution: How is this specialized case derived from a more plesiomorphic pattern? The present study is the first to match physiology and anatomical subregions in the non-owl avian IC. Single-unit responses in the chicken IC were tested for selectivity to different frequencies and to the binaural difference cues. Their anatomical origin was reconstructed with the help of electrolytic lesions and immunohistochemical identification of different subregions of the IC, based on previous characterizations in owl and chicken. In contrast to barn owl, there was no distinct differentiation of responses in the different subregions. We found neural topographies for both binaural cues but no evidence for a coherent representation of auditory space. The results are consistent with previous work in pigeon IC and chicken higher-order midbrain and suggest a plesiomorphic condition of multisensory integration in the midbrain that is dominated by lateral panoramic vision., (© 2024 The Author(s). The Journal of Comparative Neurology published by Wiley Periodicals LLC.)

Background: Binaural hearing enables better speech comprehension in noisy environments and is necessary for acoustic spatial orientation. This study investigates speech discrimination in noise with separated signal sources and measures sound localization. The aim was to study characteristics and reproducibility of two selected measurement techniques which seem to be suitable for description of the aforementioned aspects of binaural hearing., Materials and Methods: Speech reception thresholds (SRT) in noise and test-retest reliability were collected from 55 normal-hearing adults for a spatial setup of loudspeakers with angles of ± 45° and ± 90° using the Oldenburg sentence test. The investigations of sound localization were conducted in a semicircle and fullcircle setup (7 and 12 equidistant loudspeakers)., Results: SRT (S -45 N 45 : -14.1 dB SNR; S 45 N -45 : -16.4 dB SNR; S 0 N 90 : -13.1 dB SNR; S 0 N -90 : -13.4 dB SNR) and test-retest reliability (4 to 6 dB SNR) were collected for speech intelligibility in noise with separated signals. The procedural learning effect for this setup could only be mitigated with 120 training sentences. Significantly smaller SRT values, resulting in better speech discrimination, were found for the test situation of the right compared to the left ear. RMS values could be gathered for sound localization in the semicircle (1,9°) as well as in the fullcircle setup (11,1°). Better results were obtained in the retest of the fullcircle setup., Conclusion: When using the Oldenburg sentence test in noise with spatially separated signals, it is mandatory to perform a training session of 120 sentences in order to minimize the procedural learning effect. Ear-specific SRT values for speech discrimination in noise with separated signal sources are required, which is probably due to the right-ear advantage. A training is recommended for sound localization in the fullcircle setup., (© 2024. The Author(s).)

Objective: To investigate if cartilage conduction (CC) rerouting devices are noninferior to air-conduction (AC) rerouting devices for single-sided deafness (SSD) patients by measuring objective and subjective performance using speech-in-noise tests that resemble a realistic hearing environment, sound localization tests, and standardized questionnaires., Study Design: Prospective, single-subject randomized, crossover study., Setting: Anechoic room inside a university., Patients: Nine adults between 21 and 58 years of age with severe or profound unilateral sensorineural hearing loss., Interventions: Patients' baseline hearing was assessed; they then used both the cartilage conduction contralateral routing of signals device (CC-CROS) and an air-conduction CROS hearing aid (AC-CROS). Patients wore each device for 2 weeks in a randomly assigned order., Main Outcome Measures: Three main outcome measures were 1) speech-in-noise tests, measuring speech reception thresholds; 2) proportion of correct sound localization responses; and 3) scores on the questionnaires, "Abbreviated Profile of Hearing Aid Benefit" (APHAB) and "Speech, Spatial, and Qualities of Hearing Scale" with 12 questions (SSQ-12)., Results: Speech reception threshold improved significantly when noise was ambient, and speech was presented from the front or the poor-ear side with both CC-CROS and AC-CROS. When speech was delivered from the better-ear side, AC-CROS significantly improved performance, whereas CC-CROS had no significant effect. Both devices mainly worsened sound localization, whereas the APHAB and SSQ-12 scores showed benefits., Conclusion: CC-CROS has noninferior hearing-in-noise performance except when the speech was presented to the better ear under ambient noise. Subjective measures showed that the patients realized the effectiveness of both devices., Competing Interests: The authors disclose no conflicts of interest., (Copyright © 2024, Otology & Neurotology, Inc.)

Objectives: This study aimed to investigate the effects of an adhesive bone conduction device (aBCD) in children with congenital single-sided deafness (SSD). Specifically, we examined whether the aBCD elicits improvement in the speech perception ability of children with congenital SSD and whether using this device would adversely affect the horizontal localisation abilities of these children., Methods: Thirteen school-aged children with SSD and seven children with Normal Hearing (NH) were included in this study. Speech perception in noise was measured using the Mandarin Speech Test Materials and sound localisation performance was evaluated using broadband noise stimuli (0.5-20 kHz), randomly played from seven loudspeakers at different stimulus levels (65-, 70-, and 75-dB SPL)., Results: All children with SSD showed inferior speech perception and sound localisation performance compared with children with NH. The aBCD use remarkably improved the speech perception abilities of these children under quiet and noise conditions; however, their sound localisation abilities neither improved nor deteriorated., Conclusion: This study reveals the effectiveness and safety of a non-surgical aBCD in paediatric patients with SSD. Our results provide a theoretical basis for early hearing intervention with an aBCD in children with congenital SSD who are temporarily unable to undergo ear surgery., Level of Evidence: Level 3., (Copyright © 2024 Associação Brasileira de Otorrinolaringologia e Cirurgia Cérvico-Facial. Published by Elsevier España S.L.U. All rights reserved.)

Locating sound sources such as prey or predators is critical for survival in many vertebrates. Terrestrial vertebrates locate sources by measuring the time delay and intensity difference of sound pressure at each ear 1-5 . Underwater, however, the physics of sound makes interaural cues very small, suggesting that directional hearing in fish should be nearly impossible 6 . Yet, directional hearing has been confirmed behaviourally, although the mechanisms have remained unknown for decades. Several hypotheses have been proposed to explain this remarkable ability, including the possibility that fish evolved an extreme sensitivity to minute interaural differences or that fish might compare sound pressure with particle motion signals 7,8 . However, experimental challenges have long hindered a definitive explanation. Here we empirically test these models in the transparent teleost Danionella cerebrum, one of the smallest vertebrates 9,10 . By selectively controlling pressure and particle motion, we dissect the sensory algorithm underlying directional acoustic startles. We find that both cues are indispensable for this behaviour and that their relative phase controls its direction. Using micro-computed tomography and optical vibrometry, we further show that D. cerebrum has the sensory structures to implement this mechanism. D. cerebrum shares these structures with more than 15% of living vertebrate species, suggesting a widespread mechanism for inferring sound direction., (© 2024. The Author(s).)

Neurons of the lateral superior olive (LSO) in the auditory brainstem play a fundamental role in binaural sound localization. Previous theoretical studies developed various types of neuronal models to study the physiological functions of the LSO. These models were usually tuned to a small set of physiological data with specific aims in mind. Therefore, it is unclear whether and how they can be related to each other, how widely applicable they are, and which model is suitable for what purposes. In this study, we address these questions for six different single-compartment integrate-and-fire (IF) type LSO models. The models are divided into two groups depending on their subthreshold responses: passive (linear) models with only the leak conductance and active (nonlinear) models with an additional low-voltage-activated potassium conductance that is prevalent among the auditory system. Each of these two groups is further subdivided into three subtypes according to the spike generation mechanism: one with simple threshold-crossing detection and voltage reset, one with threshold-crossing detection plus a current to mimic spike shapes, and one with a depolarizing exponential current for spiking. In our simulations, all six models were driven by identical synaptic inputs and calibrated with common criteria for binaural tuning. The resulting spike rates of the passive models were higher for intensive inputs and lower for temporally structured inputs than those of the active models, confirming the active function of the potassium current. Within each passive or active group, the simulated responses resembled each other, regardless of the spike generation types. These results, in combination with the analysis of computational costs, indicate that an active IF model is more suitable than a passive model for accurately reproducing temporal coding of LSO. The simulation of realistic spike shapes with an extended spiking mechanism added relatively small computational costs., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Ashida et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Significance: It is important to know whether early-onset vision loss and late-onset vision loss are associated with differences in the estimation of distances of sound sources within the environment. People with vision loss rely heavily on auditory cues for path planning, safe navigation, avoiding collisions, and activities of daily living., Purpose: Loss of vision can lead to substantial changes in auditory abilities. It is unclear whether differences in sound distance estimation exist in people with early-onset partial vision loss, late-onset partial vision loss, and normal vision. We investigated distance estimates for a range of sound sources and auditory environments in groups of participants with early- or late-onset partial visual loss and sighted controls., Methods: Fifty-two participants heard static sounds with virtual distances ranging from 1.2 to 13.8 m within a simulated room. The room simulated either anechoic (no echoes) or reverberant environments. Stimuli were speech, music, or noise. Single sounds were presented, and participants reported the estimated distance of the sound source. Each participant took part in 480 trials., Results: Analysis of variance showed significant main effects of visual status (p<0.05) environment (reverberant vs. anechoic, p<0.05) and also of the stimulus (p<0.05). Significant differences (p<0.05) were shown in the estimation of distances of sound sources between early-onset visually impaired participants and sighted controls for closer distances for all conditions except the anechoic speech condition and at middle distances for all conditions except the reverberant speech and music conditions. Late-onset visually impaired participants and sighted controls showed similar performance (p>0.05)., Conclusions: The findings suggest that early-onset partial vision loss results in significant changes in judged auditory distance in different environments, especially for close and middle distances. Late-onset partial visual loss has less of an impact on the ability to estimate the distance of sound sources. The findings are consistent with a theoretical framework, the perceptual restructuring hypothesis, which was recently proposed to account for the effects of vision loss on audition., Competing Interests: Conflict of Interest Disclosure: None of the authors have reported a financial conflict of interest., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Optometry.)

Objective: Severely asymmetrical hearing loss (SAHL) is characterized by a moderately severe or severe hearing loss in one side and normal or mildly impaired controlateral hearing in the other. The Active tri-CROS combines the Contralateral Routing-of-Signal System (CROS, or BiCROS if the best ear is stimulated) and the stimulation of the worst ear by an in-the-canal hearing aid. This study aims to evaluate the benefit of the Active tri-CROS for SAHL patients., Study Design: This retrospective study was conducted from September 2019 to December 2020., Setting: Ambulatory, tertiary care., Patients: Patients were retrospectively included if they had received the Active tri-CROS system after having used a CROS or BiCROS system for SAHL for at least 3 years., Main Outcome Measures: Audiometric gain, signal-to-noise ratio, spatial localization, and the Abbreviated Profile of Hearing Aid Benefit and Tinnitus Handicap Inventory questionnaires were performed before equipment and after a month with the system., Results: Twenty patients (mean, 62 yr old) with a mean of 74.3 ± 8.7 dB HL on the worst ear were included. The mean tonal hearing gain on the worst ear was 20 ± 6 dB. Signal-to-noise ratio significantly rose from 1.43 ± 3.9 to 0.16 ± 3.4 dB ( p = 0.0001). Spatial localization was not significantly improved. The mean Tinnitus Handicap Inventory test score of the eight patients suffering from tinnitus rose from 45.5 ± 18.5 to 31 ± 25.2 ( p = 0.016)., Conclusions: The Active tri-CROS system is a promising new therapeutically solution for SAHL., Competing Interests: The authors disclose no conflicts of interest., (Copyright © 2024, Otology & Neurotology, Inc.)

Sound localization is a crucial aspect of human auditory perception. VR (virtual reality) technologies provide immersive audio platforms that allow human listeners to experience natural sounds based on their ability to localize sound. However, the simulations of sound generated by these platforms, which are based on the general head-related transfer function (HRTF), often lack accuracy in terms of individual sound perception and localization due to significant individual differences in this function. In this study, we aimed to investigate the disparities between the perceived locations of sound sources by users and the locations generated by the platform. Our goal was to determine if it is possible to train users to adapt to the platform-generated sound sources. We utilized the Microsoft HoloLens 2 virtual platform and collected data from 12 subjects based on six separate training sessions arranged in 2 weeks. We employed three modes of training to assess their effects on sound localization, in particular for studying the impacts of multimodal error, visual, and sound guidance in combination with kinesthetic/postural guidance, on the effectiveness of the training. We analyzed the collected data in terms of the training effect between pre- and post-sessions as well as the retention effect between two separate sessions based on subject-wise paired statistics. Our findings indicate that, as far as the training effect between pre- and post-sessions is concerned, the effect is proven to be statistically significant, in particular in the case wherein kinesthetic/postural guidance is mixed with visual and sound guidance. Conversely, visual error guidance alone was found to be largely ineffective. On the other hand, as far as the retention effect between two separate sessions is concerned, we could not find any meaningful statistical implication on the effect for all three error guidance modes out of the 2-week session of training. These findings can contribute to the improvement of VR technologies by ensuring they are designed to optimize human sound localization abilities.

The natural environment challenges the brain to prioritize the processing of salient stimuli. The barn owl, a sound localization specialist, exhibits a circuit called the midbrain stimulus selection network, dedicated to representing locations of the most salient stimulus in circumstances of concurrent stimuli. Previous competition studies using unimodal (visual) and bimodal (visual and auditory) stimuli have shown that relative strength is encoded in spike response rates. However, open questions remain concerning auditory-auditory competition on coding. To this end, we present diverse auditory competitors (concurrent flat noise and amplitude-modulated noise) and record neural responses of awake barn owls of both sexes in subsequent midbrain space maps, the external nucleus of the inferior colliculus (ICx) and optic tectum (OT). While both ICx and OT exhibit a topographic map of auditory space, OT also integrates visual input and is part of the global-inhibitory midbrain stimulus selection network. Through comparative investigation of these regions, we show that while increasing strength of a competitor sound decreases spike response rates of spatially distant neurons in both regions, relative strength determines spike train synchrony of nearby units only in the OT. Furthermore, changes in synchrony by sound competition in the OT are correlated to gamma range oscillations of local field potentials associated with input from the midbrain stimulus selection network. The results of this investigation suggest that modulations in spiking synchrony between units by gamma oscillations are an emergent coding scheme representing relative strength of concurrent stimuli, which may have relevant implications for downstream readout., Competing Interests: The authors declare no competing financial interests., (Copyright © 2024 the authors.)

Bayesian models have proven effective in characterizing perception, behavior, and neural encoding across diverse species and systems. The neural implementation of Bayesian inference in the barn owl's sound localization system and behavior has been previously explained by a non-uniform population code model. This model specifies the neural population activity pattern required for a population vector readout to match the optimal Bayesian estimate. While prior analyses focused on trial-averaged comparisons of model predictions with behavior and single-neuron responses, it remains unknown whether this model can accurately approximate Bayesian inference on single trials under varying sensory reliability, a fundamental condition for natural perception and behavior. In this study, we utilized mathematical analysis and simulations to demonstrate that decoding a non-uniform population code via a population vector readout approximates the Bayesian estimate on single trials for varying sensory reliabilities. Our findings provide additional support for the non-uniform population code model as a viable explanation for the barn owl's sound localization pathway and behavior., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Fischer et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Humans make use of small differences in the timing of sounds at the two ears-interaural time differences (ITDs)-to locate their sources. Despite extensive investigation, however, the neural representation of ITDs in the human brain is contentious, particularly the range of ITDs explicitly represented by dedicated neural detectors. Here, using magneto- and electro-encephalography (MEG and EEG), we demonstrate evidence of a sparse neural representation of ITDs in the human cortex. The magnitude of cortical activity to sounds presented via insert earphones oscillated as a function of increasing ITD-within and beyond auditory cortical regions-and listeners rated the perceptual quality of these sounds according to the same oscillating pattern. This pattern was accurately described by a population of model neurons with preferred ITDs constrained to the narrow, sound-frequency-dependent range evident in other mammalian species. When scaled for head size, the distribution of ITD detectors in the human cortex is remarkably like that recorded in vivo from the cortex of rhesus monkeys, another large primate that uses ITDs for source localization. The data solve a long-standing issue concerning the neural representation of ITDs in humans and suggest a representation that scales for head size and sound frequency in an optimal manner., Competing Interests: Declaration of interests The authors declare no competing interests., (Crown Copyright © 2024. Published by Elsevier Inc. All rights reserved.)

Humans have the remarkable ability to integrate information from different senses, which greatly facilitates the detection, localization and identification of events in the environment. About 466 million people worldwide suffer from hearing loss. Yet, the impact of hearing loss on how the senses work together is rarely investigated. Here, we investigate how a common sensory impairment, asymmetric conductive hearing loss (AHL), alters the way our senses interact by examining human orienting behaviour with normal hearing (NH) and acute AHL. This type of hearing loss disrupts auditory localization. We hypothesized that this creates a conflict between auditory and visual spatial estimates and alters how auditory and visual inputs are integrated to facilitate multisensory spatial perception. We analysed the spatial and temporal properties of saccades to auditory, visual and audiovisual stimuli before and after plugging the right ear of participants. Both spatial and temporal aspects of multisensory integration were affected by AHL. Compared with NH, AHL caused participants to make slow, inaccurate and unprecise saccades towards auditory targets. Surprisingly, increased weight on visual input resulted in accurate audiovisual localization with AHL. This came at a cost: saccade latencies for audiovisual targets increased significantly. The larger the auditory localization errors, the less participants were able to benefit from audiovisual integration in terms of saccade latency. Our results indicate that observers immediately change sensory weights to effectively deal with acute AHL and preserve audiovisual accuracy in a way that cannot be fully explained by statistical models of optimal cue integration., (© 2024 The Authors. European Journal of Neuroscience published by Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Having two ears enables us to localize sound sources by exploiting interaural time differences (ITDs) in sound arrival. Principal neurons of the medial superior olive (MSO) are sensitive to ITD, and each MSO neuron responds optimally to a best ITD (bITD). In many cells, especially those tuned to low sound frequencies, these bITDs correspond to ITDs for which the contralateral ear leads, and are often larger than the ecologically relevant range, defined by the ratio of the interaural distance and the speed of sound. Using in vivo recordings in gerbils, we found that shortly after hearing onset the bITDs were even more contralaterally leading than found in adult gerbils, and travel latencies for contralateral sound-evoked activity clearly exceeded those for ipsilateral sounds. During the following weeks, both these latencies and their interaural difference decreased. A computational model indicated that spike timing-dependent plasticity can underlie this fine-tuning. Our results suggest that MSO neurons start out with a strong predisposition toward contralateral sounds due to their longer neural travel latencies, but that, especially in high-frequency neurons, this predisposition is subsequently mitigated by differential developmental fine-tuning of the travel latencies., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Sierksma, Borst. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Advancements in cochlear implants (CIs) have led to a significant increase in bilateral CI users, especially among children. Yet, most bilateral CI users do not fully achieve the intended binaural benefit due to potential limitations in signal processing and/or surgical implant positioning. One crucial auditory cue that normal hearing (NH) listeners can benefit from is the interaural time difference (ITD), i.e ., the time difference between the arrival of a sound at two ears. The ITD sensitivity is thought to be heavily relying on the effective utilization of temporal fine structure (very rapid oscillations in sound). Unfortunately, most current CIs do not transmit such true fine structure. Nevertheless, bilateral CI users have demonstrated sensitivity to ITD cues delivered through envelope or interaural pulse time differences, i.e ., the time gap between the pulses delivered to the two implants. However, their ITD sensitivity is significantly poorer compared to NH individuals, and it further degrades at higher CI stimulation rates, especially when the rate exceeds 300 pulse per second. The overall purpose of this research thread is to improve spatial hearing abilities in bilateral CI users. This study aims to develop electroencephalography (EEG) paradigms that can be used with clinical settings to assess and optimize the delivery of ITD cues, which are crucial for spatial hearing in everyday life. The research objective of this article was to determine the effect of CI stimulation pulse rate on the ITD sensitivity, and to characterize the rate-dependent degradation in ITD perception using EEG measures. To develop protocols for bilateral CI studies, EEG responses were obtained from NH listeners using sinusoidal-amplitude-modulated (SAM) tones and filtered clicks with changes in either fine structure ITD (ITD FS ) or envelope ITD (ITD ENV ). Multiple EEG responses were analyzed, which included the subcortical auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs) elicited by stimuli onset, offset, and changes. Results indicated that acoustic change complex (ACC) responses elicited by ITD ENV changes were significantly smaller or absent compared to those elicited by ITD FS changes. The ACC morphologies evoked by ITD FS changes were similar to onset and offset CAEPs, although the peak latencies were longest for ACC responses and shortest for offset CAEPs. The high-frequency stimuli clearly elicited subcortical ASSRs, but smaller than those evoked by lower carrier frequency SAM tones. The 40-Hz ASSRs decreased with increasing carrier frequencies. Filtered clicks elicited larger ASSRs compared to high-frequency SAM tones, with the order being 40 > 160 > 80> 320 Hz ASSR for both stimulus types. Wavelet analysis revealed a clear interaction between detectable transient CAEPs and 40-Hz ASSRs in the time-frequency domain for SAM tones with a low carrier frequency., Competing Interests: The authors declare that they have no competing interests., (© 2024 Hu et al.)

Ambient sound can mask acoustic signals. The current study addressed how echolocation in people is affected by masking sound, and the role played by type of sound and spatial (i.e. binaural) similarity. We also investigated the role played by blindness and long-term experience with echolocation, by testing echolocation experts, as well as blind and sighted people new to echolocation. Results were obtained in two echolocation tasks where participants listened to binaural recordings of echolocation and masking sounds, and either localized echoes in azimuth or discriminated echo audibility. Echolocation and masking sounds could be either clicks or broad band noise. An adaptive staircase method was used to adjust signal-to-noise ratios (SNRs) based on participants' responses. When target and masker had the same binaural cues (i.e. both were monoaural sounds), people performed better (i.e. had lower SNRs) when target and masker used different types of sound (e.g. clicks in noise-masker or noise in clicks-masker), as compared to when target and masker used the same type of sound (e.g. clicks in click-, or noise in noise-masker). A very different pattern of results was observed when masker and target differed in their binaural cues, in which case people always performed better when clicks were the masker, regardless of type of emission used. Further, direct comparison between conditions with and without binaural difference revealed binaural release from masking only when clicks were used as emissions and masker, but not otherwise (i.e. when noise was used as masker or emission). This suggests that echolocation with clicks or noise may differ in their sensitivity to binaural cues. We observed the same pattern of results for echolocation experts, and blind and sighted people new to echolocation, suggesting a limited role played by long-term experience or blindness. In addition to generating novel predictions for future work, the findings also inform instruction in echolocation for people who are blind or sighted., Competing Interests: Declaration of competing interest None., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Objective: To assess cochlear implant (CI) sound processor usage over time in children with single-sided deafness (SSD) and identify factors influencing device use., Study Design: Retrospective, chart review study., Setting: Pediatric tertiary referral center., Patients: Children with SSD who received CI between 2014 and 2020., Outcome Measure: Primary outcome was average daily CI sound processor usage over follow-up., Results: Fifteen children with SSD who underwent CI surgery were categorized based on age of diagnosis and surgery timing. Over an average of 4.3-year follow-up, patients averaged 4.6 hours/day of CI usage. Declining usage trends were noted over time, with the first 2 years postactivation showing higher rates. No significant usage differences emerged based on age, surgery timing, or hearing loss etiology., Conclusions: Long-term usage decline necessitates further research into barriers and enablers for continued CI use in pediatric SSD cases., Competing Interests: Conflicts of interest and disclosures: Dr. Kuthubutheen is an investigator for a medical device trial for Cochlear Corporation., (Copyright © 2024, Otology & Neurotology, Inc.)

The Journal of Comparative Physiology lived up to its name in the last 100 years by including more than 1500 different taxa in almost 10,000 publications. Seventeen phyla of the animal kingdom were represented. The honeybee (Apis mellifera) is the taxon with most publications, followed by locust (Locusta migratoria), crayfishes (Cambarus spp.), and fruitfly (Drosophila melanogaster). The representation of species in this journal in the past, thus, differs much from the 13 model systems as named by the National Institutes of Health (USA). We mention major accomplishments of research on species with specific adaptations, specialist animals, for example, the quantitative description of the processes underlying the axon potential in squid (Loligo forbesii) and the isolation of the first receptor channel in the electric eel (Electrophorus electricus) and electric ray (Torpedo spp.). Future neuroethological work should make the recent genetic and technological developments available for specialist animals. There are many research questions left that may be answered with high yield in specialists and some questions that can only be answered in specialists. Moreover, the adaptations of animals that occupy specific ecological niches often lend themselves to biomimetic applications. We go into some depth in explaining our thoughts in the research of motion vision in insects, sound localization in barn owls, and electroreception in weakly electric fish., (© 2024. The Author(s).)

Objective: To assess the perceived benefit of cochlear implant (CI) use for children with unilateral hearing loss (UHL) and evaluate whether perceived abilities are associated with performance on measures of speech recognition and spatial hearing., Method: Nineteen children with moderate-to-profound UHL underwent cochlear implantation. The Speech Spatial and Qualities of Hearing Questionnaire modified for children (SSQ-C) were completed by parental proxy pre-operatively and at 3, 6, 9, 12, 18, and 24 months post-activation. Linear mixed models evaluated perceived benefits over the study period. Pearson correlations assessed the association between subjective report and performance on measures of word recognition with the CI alone and spatial hearing (speech recognition in spatially-separated noise and sound source localization) in the combined condition (CI plus contralateral ear)., Results: For the SSQ-C, parents reported significant improvements with CI use as compared to pre-operative perceptions (p < 0.001); improved perceptions were either maintained or continued to improve over the 2-year post-activation period. Perceived benefit did not significantly correlate with word recognition with the CI alone or spatial hearing outcomes in the combined condition., Conclusion: Families of children with UHL observed benefits of CI use early after cochlear implantation that was maintained with long-term device use. Responses to subjective measures may broaden our understanding of the experiences of pediatric CI users with UHL in addition to outcomes on typical measures of CI performance., Level of Evidence: 3 Laryngoscope, 134:919-925, 2024., (© 2023 The American Laryngological, Rhinological and Otological Society, Inc.)

Sound localization is essential to perceive the surrounding world and to interact with objects. This ability can be learned across time, and multisensory and motor cues play a crucial role in the learning process. A recent study demonstrated that when training localization skills, reaching to the sound source to determine its position reduced localization errors faster and to a greater extent as compared to just naming sources' positions, despite the fact that in both tasks, participants received the same feedback about the correct position of sound sources in case of wrong response. However, it remains to establish which features have made reaching to sound more effective as compared to naming. In the present study, we introduced a further condition in which the hand is the effector providing the response, but without it reaching toward the space occupied by the target source: the pointing condition. We tested three groups of participants (naming, pointing, and reaching groups) each while performing a sound localization task in normal and altered listening situations (i.e. mild-moderate unilateral hearing loss) simulated through auditory virtual reality technology. The experiment comprised four blocks: during the first and the last block, participants were tested in normal listening condition, while during the second and the third in altered listening condition. We measured their performance, their subjective judgments (e.g. effort), and their head-related behavior (through kinematic tracking). First, people's performance decreased when exposed to asymmetrical mild-moderate hearing impairment, more specifically on the ipsilateral side and for the pointing group. Second, we documented that all groups decreased their localization errors across altered listening blocks, but the extent of this reduction was higher for reaching and pointing as compared to the naming group. Crucially, the reaching group leads to a greater error reduction for the side where the listening alteration was applied. Furthermore, we documented that, across blocks, reaching and pointing groups increased the implementation of head motor behavior during the task (i.e., they increased approaching head movements toward the space of the sound) more than naming. Third, while performance in the unaltered blocks (first and last) was comparable, only the reaching group continued to exhibit a head behavior similar to those developed during the altered blocks (second and third), corroborating the previous observed relationship between the reaching to sounds task and head movements. In conclusion, this study further demonstrated the effectiveness of reaching to sounds as compared to pointing and naming in the learning processes. This effect could be related both to the process of implementing goal-directed motor actions and to the role of reaching actions in fostering the implementation of head-related motor strategies., (© 2024. The Author(s).)

Interaural time differences (ITDs) are a major cue for sound localization and change with increasing head size. Since the barn owl's head width more than doubles in the month after hatching, we hypothesized that the development of their ITD detection circuit might be modified by experience. To test this, we raised owls with unilateral ear inserts that delayed and attenuated the acoustic signal, and then measured the ITD representation in the brainstem nucleus laminaris (NL) when they were adults. The ITD circuit is composed of delay line inputs to coincidence detectors, and we predicted that plastic changes would lead to shorter delays in the axons from the manipulated ear, and complementary shifts in ITD representation on the two sides. In owls that received ear inserts starting around P14, the maps of ITD shifted in the predicted direction, but only on the ipsilateral side, and only in those tonotopic regions that had not experienced auditory stimulation prior to insertion. The contralateral map did not change. Thus, experience-dependent plasticity of the ITD circuit occurs in NL, and our data suggest that ipsilateral and contralateral delays are independently regulated. As a result, altered auditory input during development leads to long-lasting changes in the representation of ITD. Significance Statement The early life of barn owls is marked by increasing sensitivity to sound, and by increasing ITDs. Their prolonged post-hatch development allowed us to examine the role of altered auditory experience in the development of ITD detection circuits. We raised owls with a unilateral ear insert and found that their maps of ITD were altered by experience, but only in those tonotopic regions ipsilateral to the occluded ear that had not experienced auditory stimulation prior to insertion. This experience-induced plasticity allows the sound localization circuits to be customized to individual characteristics, such as the size of the head, and potentially to compensate for imbalanced hearing sensitivities between the left and right ears., (Copyright © 2023 the authors.)

There is currently a lack of prospective studies comparing multiple treatment options for single-sided deafness (SSD) in terms of long-term sound localization outcomes. This randomized controlled trial (RCT) aims to compare the objective and subjective sound localization abilities of SSD patients treated with a cochlear implant (CI), a bone conduction device (BCD), a contralateral routing of signals (CROS) hearing aid, or no treatment after two years of follow-up. About 120 eligible patients were randomized to cochlear implantation or to a trial period with first a BCD on a headband, then a CROS (or vice versa). After the trial periods, participants opted for a surgically implanted BCD, a CROS, or no treatment. Sound localization accuracy (in three configurations, calculated as percentage correct and root-mean squared error in degrees) and subjective spatial hearing (subscale of the Speech, Spatial and Qualities of hearing (SSQ) questionnaire) were assessed at baseline and after 24 months of follow-up. At the start of follow-up, 28 participants were implanted with a CI, 25 with a BCD, 34 chose a CROS, and 26 opted for no treatment. Participants in the CI group showed better sound localization accuracy and subjective spatial hearing compared to participants in the BCD, CROS, and no-treatment groups at 24 months. Participants in the CI and CROS groups showed improved subjective spatial hearing at 24 months compared to baseline. To conclude, CI outperformed the BCD, CROS, and no-treatment groups in terms of sound localization accuracy and subjective spatial hearing in SSD patients. TRIAL REGISTRATION Netherlands Trial Register (https://onderzoekmetmensen.nl): NL4457, CINGLE trial., Competing Interests: Declaration of Conflicting InterestsThe authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Timing cues such as interaural time differences (ITDs) and temporal pitch are pivotal for sound localization and source segregation, but their perception is degraded in cochlear-implant (CI) listeners as compared to normal-hearing listeners. In multi-electrode stimulation, intra-aural channel interactions between electrodes are assumed to be an important factor limiting access to those cues. The monaural asynchrony of stimulation timing across electrodes is assumed to mediate the amount of these interactions. This study investigated the effect of the monaural temporal electrode asynchrony (mTEA) between two electrodes, applied similarly in both ears, on ITD-based left/right discrimination sensitivity in five CI listeners, using pulse trains with 100 pulses per second and per electrode. Forward-masked spatial tuning curves were measured at both ears to find electrode separations evoking controlled degrees of across-electrode masking. For electrode separations smaller than 3 mm, results showed an effect of mTEA. Patterns were u/v-shaped, consistent with an explanation in terms of the effective pulse rate that appears to be subject to the well-known rate limitation in electric hearing. For separations larger than 7 mm, no mTEA effects were observed. A comparison to monaural rate-pitch discrimination in a separate set of listeners and in a matched setup showed no systematic differences between percepts. Overall, an important role of the mTEA in both binaural and monaural dual-electrode stimulation is consistent with a monaural pulse-rate limitation whose effect is mediated by channel interactions. Future CI stimulation strategies aiming at improved timing-cue encoding should minimize the stimulation delay between nearby electrodes that need to be stimulated successively., Competing Interests: Declaration of Conflicting InterestsThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.