Binaural Localization Cues Research Articles

Further investigation of horizontal sound localization in noise

The accuracy of sound localization suffers significantly in a noisy environment. Noise can have an impact on the binaural localization cues such as interaural time difference (ITD) and interaural level difference (ILD). This impact is not only related to the signal-to-noise ratio (SNR), but also to the noise level. Therefore, it's crucial to understand how to describe binaural localization cues in noise. This study investigates the effect of noise level (ranging from 80 dB to 95 dB) and SNR (ranging from -15 dB to -5 dB) on the localization of horizontal sound sources at different azimuths. By analyzing the interaction between different binaural cues and localization accuracy in noise, the results demonstrate that there is a strong association between localization accuracy and interaural partial loudness difference. A follow-up experiment with modified ILD confirmed that the interaural partial loudness difference might more accurately reflect the auditory localization properties of the horizontal sound source in noise.

Spatially selective active noise control systems.

Active noise control (ANC) systems are commonly designed to achieve maximal sound reduction regardless of the incident direction of the sound. When desired sound is present, the state-of-the-art methods add a separate system to reconstruct it. This can result in distortion and latency. In this work, we propose a multi-channel ANC system that only reduces sound from undesired directions, and the system truly preserves the desired sound instead of reproducing it. The proposed algorithm imposes a spatial constraint on the hybrid ANC cost function to achieve spatial selectivity. Based on a six-channel microphone array on a pair of augmented eyeglasses, results show that the system minimized only noise coming from undesired directions. The control performance could be maintained even when the array was heavily perturbed. The proposed algorithm was also compared with the existing methods in the literature. Not only did the proposed system provide better noise reduction, but it also required much less effort. The binaural localization cues did not need to be reconstructed since the system preserved the physical sound wave from the desired source.

Reweighting of Binaural Localization Cues in Bilateral Cochlear-Implant Listeners

Normal-hearing (NH) listeners rely on two binaural cues, the interaural time (ITD) and level difference (ILD), for azimuthal sound localization. Cochlear-implant (CI) listeners, however, rely almost entirely on ILDs. One reason is that present-day clinical CI stimulation strategies do not convey salient ITD cues. But even when presenting ITDs under optimal conditions using a research interface, ITD sensitivity is lower in CI compared to NH listeners. Since it has recently been shown that NH listeners change their ITD/ILD weighting when only one of the cues is consistent with visual information, such reweighting might add to CI listeners’ low perceptual contribution of ITDs, given their daily exposure to reliable ILDs but unreliable ITDs. Six bilateral CI listeners completed a multi-day lateralization training visually reinforcing ITDs, flanked by a pre- and post-measurement of ITD/ILD weights without visual reinforcement. Using direct electric stimulation, we presented 100- and 300-pps pulse trains at a single interaurally place-matched electrode pair, conveying ITDs and ILDs in various spatially consistent and inconsistent combinations. The listeners’ task was to lateralize the stimuli in a virtual environment. Additionally, ITD and ILD thresholds were measured before and after training. For 100-pps stimuli, the lateralization training increased the contribution of ITDs slightly, but significantly. Thresholds were neither affected by the training nor correlated with weights. For 300-pps stimuli, ITD weights were lower and ITD thresholds larger, but there was no effect of training. On average across test sessions, adding azimuth-dependent ITDs to stimuli containing ILDs increased the extent of lateralization for both 100- and 300-pps stimuli. The results suggest that low-rate ITD cues, robustly encoded with future CI systems, may be better exploitable for sound localization after increasing their perceptual weight via training.

Reweighting of Binaural Localization Cues Induced by Lateralization Training

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

The lateralization and reliability of spatial mismatch negativity elicited by auditory deviants with virtual spatial location

Mismatch negativity (MMN) is an intensively studied event-related potential component that reflects pre-attentive auditory processing. Existing spatial MMN (sMMN) studies usually use loud-speakers in different locations or deliver sound with binaural localization cues through earphones to elicit MMN, which either was practically complicated or sounded unnatural to the subjects. In the present study, we generated head related transfer function (HRTF)-based spatial sounds and verified that the HRTF-based sounds retained the left and the right spatial localization cues. We further used them as deviants to elicit sMMN with conventional oddball paradigm. Results showed that sMMN was successfully elicited by the HRTF-based deviants in 18 of 21 healthy subjects in two separate sessions. Furthermore, the left deviants elicited higher sMMN amplitudes in the right hemisphere compared to the left hemisphere, while the right deviants elicited sMMN with similar amplitudes in both hemispheres, which supports a combination of contralateral and right-hemispheric dominance in spatial auditory information processing. In addition, the sMMN in response to the right deviants showed good test-retest reliability, while the sMMN in response to the left deviants had weak test-retest reliability. These findings implicate that HRTF-based sMMN could be a robust paradigm to investigate spatial localization and discrimination abilities.

Binaural responses in the auditory midbrain of chicken (Gallus gallus).

The auditory midbrain is the location in which neurons represent binaural acoustic information necessary for sound localization. The external nucleus of the midbrain inferior colliculus (IC) of the barn owl is a classic example of an auditory space map, but it is unknown to what extent the principles underlying its formation generalize to other, less specialized animals. We characterized the spiking responses of 139 auditory neurons in the IC of the chicken (Gallus gallus) invivo, focusing on their sensitivities to the binaural localization cues of interaural time (ITD) and level (ILD) differences. Most units were frequency-selective, with best frequencies distributed unevenly into low-frequency and high-frequency (>2kHz) clusters. Many units showed sensitivity to either ITD (65%) or ILD (66%) and nearly half to both (47%). ITD selectivity was disproportionately more common among low-frequency units, while ILD-only selective units were predominantly tuned to high frequencies. ILD sensitivities were diverse, and we thus developed a decision tree defining five types. One rare type with a bell-like ILD tuning was also selective for ITD but typically not frequency-selective, and thus matched the characteristics of neurons in the auditory space map of the barn owl. Our results suggest that generalist birds such as the chicken show a prominent representation of ITD and ILD cues in the IC, providing complementary information for sound localization, according to the duplex theory. A broadband response type narrowly selective for both ITD and ILD may form the basis for a representation of auditory space.

Vision-induced reweighting of binaural localization cues

Normal-hearing listeners apply frequency-dependent weights when combining the two binaural cues (interaural time and level difference, ITD and ILD) to determine the perceived sound source azimuth. Cochlear-implant (CI) listeners, however, rely almost entirely on ILDs. Since current CI systems do not reliably convey ITD information, CI listeners might learn to ignore ITDs and focus on ILDs instead. We investigated whether this reweighting of binaural cues is generally possible. 20 normal-hearing participants, assigned to two groups, completed an experiment in a virtual audio-visual environment. The experiment consisted of a pre-test to establish the initial ITD/ILD weights, a seven-day training, in which visual feedback reinforced one of the cues, and a post-test to measure the effect of training on the weights. Participants’ task was to lateralize octave-wide bands of noise (centered at 2.8 kHz) containing various combinations of spatially inconsistent ITD and ILD. In both groups, the lateralization bias ...

The source and effects of binaural cue ambiguity in free-field stereo sound localization—Modeling simulation

Our recent study (Montagne and Zhou, JASA 2016) showed that binaural localization cues—interaural time (ITD) and level (ILD) differences—were more variably distributed when stimuli were presented stereophonically instead of from single speakers. We hypothesized that variability in listeners’ responses is directly related to variability in the binaural cues imposed by the stimulus. Here, we investigate the validity of this hypothesis by examining the distribution of ITDs and ILDs using a simulated binaural neural network. The peripheral component of this model includes an updated auditory-nerve model (Zilany et al., 2014) and the central component of this model incorporated binaural correlation and level difference analysis. The decision variable was made based on combined ITD and ILD distributions across frequencies. The modeled data were analyzed and interpreted with regard to results from a parallel behavioral test. The model results suggest that low-frequency ITDs are a major cue for sound source localization even they are ambiguously distributed with multiple peaks. On the other hand, the ILD cue can strongly modulate which ITD peak dominates the perceived source location. The network analysis further investigates potential neural mechanisms by examining decision-making based on an ILD-modulated ITD network vs. separate ITD and ILD networks.

Exploring binaural hearing in gerbils (Meriones unguiculatus) using virtual headphones

The Mongolian gerbil (Meriones unguiculatus) has become a key species in investigations of the neural processing of sound localization cues in mammals. While its sound localization has been tested extensively under free-field stimulation, many neurophysiological studies use headphones to present signals with binaural localization cues. The gerbil's behavioral sensitivity to binaural cues, however, is unknown for the lack of appropriate stimulation paradigms in awake behaving gerbils. We close this gap in knowledge by mimicking a headphone stimulation; we use free-field loudspeakers and apply cross-talk cancellation techniques to present pure tones with binaural cues via “virtual headphones” to gerbils trained in a sound localization task. All gerbils were able to lateralize sounds depending on the interaural time or level difference (ITD and ILD, respectively). For ITD stimuli, reliable responses were seen for frequencies ≤2.9 kHz, the highest frequency tested with ITD stimuli. ITD sensitivity was frequency-dependent with the highest sensitivity observed at 1 kHz. For stimuli with ITD outside the gerbil's physiological range, responses were cyclic indicating the use of phase information when lateralizing narrow-band sounds. For ILD stimuli, reliable responses were obtained for frequencies ≥2 kHz. The comparison of ITD and ILD thresholds with ITD and ILD thresholds derived from gerbils’ free-field performance suggests that ongoing ITD information is the main cue for sound localization at frequencies <2 kHz. At 2 kHz, ITD and ILD cues are likely used in a complementary way. Verification of the use of the virtual headphones suggests that they can serve as a suitable substitute for conventional headphones particularly at frequencies ≤2 kHz.

Six Degrees of Auditory Spatial Separation.

The location of a sound is derived computationally from acoustical cues rather than being inherent in the topography of the input signal, as in vision. Since Lord Rayleigh, the descriptions of that representation have swung between "labeled line" and "opponent process" models. Employing a simple variant of a two-point separation judgment using concurrent speech sounds, we found that spatial discrimination thresholds changed nonmonotonically as a function of the overall separation. Rather than increasing with separation, spatial discrimination thresholds first declined as two-point separation increased before reaching a turning point and increasing thereafter with further separation. This "dipper" function, with a minimum at 6 ° of separation, was seen for regions around the midline as well as for more lateral regions (30 and 45°). The discrimination thresholds for the binaural localization cues were linear over the same range, so these cannot explain the shape of these functions. These data and a simple computational model indicate that the perception of auditory space involves a local code or multichannel mapping emerging subsequent to the binaural cue coding.

Binaural Sound Source Localization based on Sub-band SNR Estimation

Sound Source Localization (SSL) has a wide application in speech separation, recognition and enhancement. Binaural sound source localization based on human spatial hearing mechanism is an important research field of SSL. The recent binaural SSL research is focused on the system robust against noise and reverberation. In order to improve the localization performance in degraded environment, this paper proposes an algorithm to adaptively select the ‘good’ sub-bands to compute the binaural localization cues. Firstly, sub-band Signal-Noise Ratio (SNR) is estimated based on the autocorrelation matrix of binaural sound signals. Then, Inter-aural Time Difference (ITD) is computed by adaptively selecting the sub-bands which have the high SNR. Since the ITD is calculated through the sub-bands which are less affected by the noise, the sound source azimuth is estimated more accurate. The simulation results show that compared to the conventional binaural SSL algorithm, the localization accuracy of the proposed algorithm has been improved significantly.

Reduced-bandwidth Multi-channel Wiener Filter based binaural noise reduction and localization cue preservation in binaural hearing aids

Binaural hearing aids allow for a wireless exchange of microphone signals between a left and a right device. A significant noise reduction performance improvement can be achieved compared to a monaural configuration (a single device) or a bilateral configuration (in which the devices work independently). In addition, the binaural localization cues, i.e. the Interaural Time Differences and Interaural Level Differences, can also be better preserved in a binaural procedure. It was previously proven that a binaural noise reduction procedure based on the Speech Distortion Weighted Multi-channel Wiener Filter (SDW-MWF) indeed preserves the speech localization cues, if all microphone signals can be exchanged. However, in practice, it may not be feasible to exchange all microphone signals between the devices, so that reduced-bandwidth SDW-MWF schemes (where only filtered combinations of microphone signals are exchanged) have to be utilized. This paper demonstrates that a straightforward reduced-bandwidth SDW-MWF scheme still preserves the speech ITD cues, but distorts the speech ILD cues, in a single speech source scenario. Novel reduced-bandwidth SDW-MWF schemes, which make use of a common spectral postfilter, are therefore introduced. Experiments in a reverberant environment demonstrate that the novel schemes reduce the ILD distortion, without severely degrading the noise reduction performance.

Age-related Hearing Loss and Ear Morphology Affect Vertical but not Horizontal Sound-Localization Performance

Several studies have attributed deterioration of sound localization in the horizontal (azimuth) and vertical (elevation) planes to an age-related decline in binaural processing and high-frequency hearing loss (HFHL). The latter might underlie decreased elevation performance of older adults. However, as the pinnae keep growing throughout life, we hypothesized that larger ears might enable older adults to localize sounds in elevation on the basis of lower frequencies, thus (partially) compensating their HFHL. In addition, it is not clear whether sound localization has already matured at a very young age, when the body is still growing, and the binaural and monaural sound-localization cues change accordingly. The present study investigated sound-localization performance of children (7-11 years), young adults (20-34 years), and older adults (63-80 years) under open-loop conditions in the two-dimensional frontal hemifield. We studied the effect of age-related hearing loss and ear size on localization responses to brief broadband sound bursts with different bandwidths. We found similar localization abilities in azimuth for all listeners, including the older adults with HFHL. Sound localization in elevation for the children and young adult listeners with smaller ears improved when stimuli contained frequencies above 7 kHz. Subjects with larger ears could also judge the elevation of sound sources restricted to lower frequency content. Despite increasing ear size, sound localization in elevation deteriorated in older adults with HFHL. We conclude that the binaural localization cues are successfully used well into later stages of life, but that pinna growth cannot compensate the more profound HFHL with age.

A hearing-inspired approach for distant-microphone speech recognition in the presence of multiple sources

This paper addresses the problem of speech recognition in reverberant multisource noise conditions using distant binaural microphones. Our scheme employs a two-stage fragment decoding approach inspired by Bregman's account of auditory scene analysis, in which innate primitive grouping ‘rules’ are balanced by the role of learnt schema-driven processes. First, the acoustic mixture is split into local time-frequency fragments of individual sound sources using signal-level primitive grouping cues. Second, statistical models are employed to select fragments belonging to the sound source of interest, and the hypothesis-driven stage simultaneously searches for the most probable speech/background segmentation and the corresponding acoustic model state sequence. The paper reports recent advances in combining adaptive noise floor modelling and binaural localisation cues within this framework. By integrating signal-level grouping cues with acoustic models of the target sound source in a probabilistic framework, the system is able to simultaneously separate and recognise the sound of interest from the mixture, and derive significant recognition performance benefits from different grouping cue estimates despite their inherent unreliability in noisy conditions. Finally, the paper will show that missing data imputation can be applied via fragment decoding to allow reconstruction of a clean spectrogram that can be further processed and used as input to conventional ASR systems. The best performing system achieves an average keyword recognition accuracy of 85.83% on the PASCAL CHiME Challenge task.

Binaural sensitivity changes between cortical on and off responses

Neurons exhibiting on and off responses with different frequency tuning have previously been described in the primary auditory cortex (A1) of anesthetized and awake animals, but it is unknown whether other tuning properties, including sensitivity to binaural localization cues, also differ between on and off responses. We measured the sensitivity of A1 neurons in anesthetized ferrets to 1) interaural level differences (ILDs), using unmodulated broadband noise with varying ILDs and average binaural levels, and 2) interaural time delays (ITDs), using sinusoidally amplitude-modulated broadband noise with varying envelope ITDs. We also assessed fine-structure ITD sensitivity and frequency tuning, using pure-tone stimuli. Neurons most commonly responded to stimulus onset only, but purely off responses and on-off responses were also recorded. Of the units exhibiting significant binaural sensitivity nearly one-quarter showed binaural sensitivity in both on and off responses, but in almost all (∼97%) of these units the binaural tuning of the on responses differed significantly from that seen in the off responses. Moreover, averaged, normalized ILD and ITD tuning curves calculated from all units showing significant sensitivity to binaural cues indicated that on and off responses displayed different sensitivity patterns across the population. A principal component analysis of ITD response functions suggested a continuous cortical distribution of binaural sensitivity, rather than discrete response classes. Rather than reflecting a release from inhibition without any functional significance, we propose that binaural off responses may be important to cortical encoding of sound-source location.

Identification of auditory cues utilized in human echolocation—Objective measurement results

We present objective measurements of source-to-ear responses obtained in a previously established experimental paradigm of human echolocation. We identify and analyze the binaural localization cues encoded in those responses and we discuss their significance with respect to the previously reported performance in this specific experimental paradigm. The cues encoded in both the echo (lagging) and the direct transmission and echo (leading and lagging) parts of the responses are examined and their significance in view of the “precedence effect” is discussed. The variation and salience of the binaural cues pertaining to extensions of the previous experimental paradigm are examined and interpreted. This work allows us to formulate more detailed hypotheses and to design more informative subjective listening tests in order to further investigate the viability of using the acquired sensory modality of human echolocation in real-world applications.

Postnatal development of sound pressure transformations by the head and pinnae of the cat: Binaural characteristics

There are three acoustical cues to sound location: Interaural time differences (ITDs), interaural level differences (ILDs), and monaural spectral shape cues. During development, the increasing interaural distance and pinnae size associated with a growing head and pinnae result in localization cues that change continuously until maturation is complete. Here the authors report measurements of both the physical dimensions of the head and pinnae, as well as acoustical measurements of the binaural localization cues of cats aged 1.3 weeks to adulthood. For a given source location, ILD magnitude tended to increase with both frequency and age. Moreover, the range of significant ILD production (approximately 10 dB) shifted with age from higher to lower frequencies. ITD magnitude increased with age. Partial correlation analyses revealed that increasing pinnae size accounted for approximately 31% of the variance in the development of ILDs while increasing head size accounted for virtually none. On the other hand, increases in both the head and pinnae sizes contributed to the development of the ITD cues accounting for approximately 71% and approximately 25% of the variance, respectively. ILD and ITD cues in cats reach maturity by approximately 16 and approximately 22 weeks, respectively, which match the time period over which the pinnae and head dimensions reach maturity.

Subjective and objective evaluation of localisation accuracy in wave field sysnthesis

Wave Field Synthesis (WFS) is a holophonic technique that relies on the reproduction of physical properties of sound fields in an extended listening area. Limitations of this technique are due to approximations regarding the mathematical basis and have been studied in [Ber88] [dB04]. The synthesis remains correct up to a corner frequency referred to as the spatial aliasing frequency and inside an area limited by the length of the WFS array and the position of the virtual sources. A Multichannel Equalization filter (MEQ) is applied to the driving limentation signals of the loudspeakers, as described in [Cor06]. The aim of the present study is to evaluate localisation in WFS depending on both listening and source positions. An objective analysis of the binaural signals is compared to the results of a localisation task for which spatialised high‐pass filtered white noises served as stimuli. The results of the psycho‐acoustic test are compared to the binaural localisation cues, namely ITD (interaural time differences) and ILD (interaural level differences). Above the aliasing frequency, ITD and ILD are conflicting and ILD errors account in most cases for the perceptual errors.

Stimulus-Specific Adaptations in the Gaze Control System of the Barn Owl

Abrupt orientation to novel stimuli is a critical, memory-dependent task performed by the brain. In the present study, we examined two gaze control centers of the barn owl: the optic tectum (OT) and the arcopallium gaze fields (AGFs). Responses of neurons to long sequences of dichotic sound bursts comprised of two sounds differing in the probability of appearance were analyzed. We report that auditory neurons in the OT and in the AGFs tend to respond stronger to rarely presented sounds (novel sounds) than to the same sounds when presented frequently. This history-dependent phenomenon, known as stimulus-specific adaptation (SSA), was demonstrated for rare sound frequencies, binaural localization cues [interaural time difference (ITD) and level difference (ILD)] and sound amplitudes. The manifestation of SSA in such a variety of independent acoustic features, in the midbrain and in the forebrain, supports the notion that SSA is involved in sensory memory and novelty detection. To track the origin of SSA, we analyzed responses of neurons in the external nucleus of the inferior colliculus (ICX; the source of auditory input to the OT) to similar sequences of sound bursts. Neurons in the ICX responded stronger to rare sound frequencies, but did not respond differently to rare ITDs, ILDs, or sound amplitudes. We hypothesize that part of the SSA reported here is computed in high-level networks, giving rise to novelty signals that modulate tectal responses in a context-dependent manner.

The effect of overlap-masking on binaural reverberant word intelligibility

Reverberation interferes with the ability to understand speech in rooms. Overlap-masking explains this degradation by assuming reverberant phonemes endure in time and mask subsequent reverberant phonemes. Most listeners benefit from binaural listening when reverberation exists, indicating that the listener's binaural system processes the two channels to reduce the reverberation. This paper investigates the hypothesis that the binaural word intelligibility advantage found in reverberation is a result of binaural overlap-masking release with the reverberation acting as masking noise. The tests utilize phonetically balanced word lists (ANSI-S3.2 1989), that are presented diotically and binaurally with recorded reverberation and reverberation-like noise. A small room, 62 m3, reverberates the words. These are recorded using two microphones without additional noise sources. The reverberation-like noise is a modified form of these recordings and has a similar spectral content. It does not contain binaural localization cues due to a phase randomization procedure. Listening to the reverberant words binaurally improves the intelligibility by 6.0% over diotic listening. The binaural intelligibility advantage for reverberation-like noise is only 2.6%. This indicates that binaural overlap-masking release is insufficient to explain the entire binaural word intelligibility advantage in reverberation.