Interaural Level Differences Research Articles

Effect of extreme adaptive frequency compression in bimodal listeners on sound localization and speech perception

Objectives: This study aimed to improve access to high-frequency interaural level differences (ILD), by applying extreme frequency compression (FC) in the hearing aid (HA) of 13 bimodal listeners, using a cochlear implant (CI) and conventional HA in opposite ears.Design: An experimental signal-adaptive frequency-lowering algorithm was tested, compressing frequencies above 160 Hz into the individual audible range of residual hearing, but only for consonants (adaptive FC), thus protecting vowel formants, with the aim to preserve speech perception. In a cross-over design with at least 5 weeks of acclimatization between sessions, bimodal performance with and without adaptive FC was compared for horizontal sound localization, speech understanding in quiet and in noise, and vowel, consonant and voice-pitch perception.Results: On average, adaptive FC did not significantly affect any of the test results. Yet, two subjects who were fitted with a relatively weak frequency compression ratio, showed improved horizontal sound localization. After the study, four subjects preferred adaptive FC, four preferred standard frequency mapping, and four had no preference. Noteworthy, the subjects preferring adaptive FC were those with best performance on all tasks, both with and without adaptive FC.Conclusion: On a group level, extreme adaptive FC did not change sound localization and speech understanding in bimodal listeners. Possible reasons are too strong compression ratios, insufficient residual hearing or that the adaptive switching, although preserving vowel perception, may have been ineffective to produce consistent ILD cues. Individual results suggested that two subjects were able to integrate the frequency-compressed HA input with that of the CI, and benefitted from enhanced binaural cues for horizontal sound localization.

Auditory Mismatch Negativity in Response to Changes of Counter-Balanced Interaural Time and Level Differences.

Interaural time differences (ITD) and interaural level differences (ILD) both signal horizontal sound source location. To achieve a unified percept of our acoustic environment, these two cues require integration. In the present study, we tested this integration of ITD and ILD with electroencephalography (EEG) by measuring the mismatch negativity (MMN). The MMN can arise in response to spatial changes and is at least partly generated in auditory cortex. In our study, we aimed at testing for an MMN in response to stimuli with counter-balanced ITD/ILD cues. To this end, we employed a roving oddball paradigm with alternating sound sequences in two types of blocks: (a) lateralized stimuli with congruently combined ITD/ILD cues and (b) midline stimuli created by counter-balanced, incongruently combined ITD/ILD cues. We observed a significant MMN peaking at about 112–128 ms after change onset for the congruent ITD/ILD cues, for both lower (0.5 kHz) and higher carrier frequency (4 kHz). More importantly, we also observed significant MMN peaking at about 129 ms for incongruently combined ITD/ILD cues, but this effect was only detectable in the lower frequency range (0.5 kHz). There were no significant differences of the MMN responses for the two types of cue combinations (congruent/incongruent). These results suggest that—at least in the lower frequency ranges (0.5 kHz)—ITD and ILD are processed independently at the level of the MMN in auditory cortex.

Enhanced Excitatory Connectivity and Disturbed Sound Processing in the Auditory Brainstem of Fragile X Mice.

Hypersensitivity to sounds is one of the prevalent symptoms in individuals with Fragile X syndrome (FXS). It manifests behaviorally early during development and is often used as a landmark for treatment efficacy. However, the physiological mechanisms and circuit-level alterations underlying this aberrant behavior remain poorly understood. Using the mouse model of FXS (Fmr1 KO), we demonstrate that functional maturation of auditory brainstem synapses is impaired in FXS. Fmr1 KO mice showed a greatly enhanced excitatory synaptic input strength in neurons of the lateral superior olive (LSO), a prominent auditory brainstem nucleus, which integrates ipsilateral excitation and contralateral inhibition to compute interaural level differences. Conversely, the glycinergic, inhibitory input properties remained unaffected. The enhanced excitation was the result of an increased number of cochlear nucleus fibers converging onto one LSO neuron, without changing individual synapse properties. Concomitantly, immunolabeling of excitatory ending markers revealed an increase in the immunolabeled area, supporting abnormally elevated excitatory input numbers. Intrinsic firing properties were only slightly enhanced. In line with the disturbed development of LSO circuitry, auditory processing was also affected in adult Fmr1 KO mice as shown with single-unit recordings of LSO neurons. These processing deficits manifested as an increase in firing rate, a broadening of the frequency response area, and a shift in the interaural level difference function of LSO neurons. Our results suggest that this aberrant synaptic development of auditory brainstem circuits might be a major underlying cause of the auditory processing deficits in FXS.SIGNIFICANCE STATEMENT Fragile X Syndrome (FXS) is the most common inheritable form of intellectual impairment, including autism. A core symptom of FXS is extreme sensitivity to loud sounds. This is one reason why individuals with FXS tend to avoid social interactions, contributing to their isolation. Here, a mouse model of FXS was used to investigate the auditory brainstem where basic sound information is first processed. Loss of the Fragile X mental retardation protein leads to excessive excitatory compared with inhibitory inputs in neurons extracting information about sound levels. Functionally, this elevated excitation results in increased firing rates, and abnormal coding of frequency and binaural sound localization cues. Imbalanced early-stage sound level processing could partially explain the auditory processing deficits in FXS.

Binaural masking release in symmetric listening conditions with spectro-temporally modulated maskers.

Speech reception thresholds (SRTs) decrease as target and maskers are spatially separated (spatial release from masking, SRM). The current study systematically assessed how SRTs and SRM for a frontal target in a spatially symmetric masker configuration depend on spectro-temporal masker properties, the availability of short-time interaural level difference (ILD) and interaural time difference (ITD), and informational masking. Maskers ranged from stationary noise to single, interfering talkers and were modified by head-related transfer functions to provide: (i) different binaural cues (ILD, ITD, or both) and (ii) independent maskers in each ear ("infinite ILD"). Additionally, a condition was tested in which only information from short-time spectro-temporal segments of the ear with a favorable signal-to-noise ratio (better-ear glimpses) was presented. For noise-based maskers, ILD, ITD, and spectral changes related to masker location contributed similarly to SRM, while ILD cues played a larger role if temporal modulation was introduced. For speech maskers, glimpsing and perceived location contributed roughly equally and ITD contributed less. The "infinite ILD" condition might suggest better-ear glimpsing limitations resulting in a maximal SRM of 12 dB for maskers with low or absent informational masking. Comparison to binaural model predictions highlighted the importance of short-time processing and helped to clarify the contribution of the different binaural cues and mechanisms.

Modeling speech localization, talker identification, and word recognition in a multi-talker setting.

This study introduces a model for solving three different auditory tasks in a multi-talker setting: target localization, target identification, and word recognition. The model was used to simulate psychoacoustic data from a call-sign-based listening test involving multiple spatially separated talkers [Brungart and Simpson (2007). Percept. Psychophys. 69(1), 79-91]. The main characteristics of the model are (i) the extraction of salient auditory features ("glimpses") from the multi-talker signal and (ii) the use of a classification method that finds the best target hypothesis by comparing feature templates from clean target signals to the glimpses derived from the multi-talker mixture. The four features used were periodicity, periodic energy, and periodicity-based interaural time and level differences. The model results widely exceeded probability of chance for all subtasks and conditions, and generally coincided strongly with the subject data. This indicates that, despite their sparsity, glimpses provide sufficient information about a complex auditory scene. This also suggests that complex source superposition models may not be needed for auditory scene analysis. Instead, simple models of clean speech may be sufficient to decode even complex multi-talker scenes.

Binaural sensitivity in children who use bilateral cochlear implants.

Children who are deaf and receive bilateral cochlear implants (BiCIs) perform better on spatial hearing tasks using bilateral rather than unilateral inputs; however, they underperform relative to normal-hearing (NH) peers. This gap in performance is multi-factorial, including the inability of speech processors to reliably deliver binaural cues. Although much is known regarding binaural sensitivity of adults with BiCIs, less is known about how the development of binaural sensitivity in children with BiCIs compared to NH children. Sixteen children (ages 9-17 years) were tested using synchronized research processors. Interaural time differences and interaural level differences (ITDs and ILDs, respectively) were presented to pairs of pitch-matched electrodes. Stimuli were 300-ms, 100-pulses-per-second, constant-amplitude pulse trains. In the first and second experiments, discrimination of interaural cues (either ITDs or ILDs) was measured using a two-interval left/right task. In the third experiment, subjects reported the perceived intracranial position of ITDs and ILDs in a lateralization task. All children demonstrated sensitivity to ILDs, possibly due to monaural level cues. Children who were born deaf had weak or absent sensitivity to ITDs; in contrast, ITD sensitivity was noted in children with previous exposure to acoustic hearing. Therefore, factors such as auditory deprivation, in particular, lack of early exposure to consistent timing differences between the ears, may delay the maturation of binaural circuits and cause insensitivity to binaural differences.

Preserving spatial perception in rooms using direct-sound driven dynamic range compression.

Fast-acting hearing-aid compression systems typically distort the auditory cues involved in the spatial perception of sounds in rooms by enhancing low-level reverberant energy portions of the sound relative to the direct sound. The present study investigated the benefit of a direct-sound driven compression system that adaptively selects appropriate time constants to preserve the listener's spatial impression. Specifically, fast-acting compression was maintained for time-frequency units dominated by the direct sound while the processing of the compressor was linearized for time-frequency units dominated by reverberation. This compression scheme was evaluated with normal-hearing listeners who indicated their perceived location and distribution of sound images in the horizontal plane for virtualized speech. The experimental results confirmed that both independent compression at each ear and linked compression across ears resulted in broader, sometimes internalized, sound images as well as image splits. In contrast, the linked direct-sound driven compression system provided the listeners with a spatial perception similar to that obtained with linear processing that served as the reference condition. The independent direct-sound driven compressor created a sense of movement of the sound between the two ears, suggesting that preserving the interaural level differences via linked compression is advantageous with the proposed direct-sound driven compression scheme.

Spatial Processing Is Frequency Specific in Auditory Cortex But Not in the Midbrain.

The cochlea behaves like a bank of band-pass filters, segregating information into different frequency channels. Some aspects of perception reflect processing within individual channels, but others involve the integration of information across them. One instance of this is sound localization, which improves with increasing bandwidth. The processing of binaural cues for sound location has been studied extensively. However, although the advantage conferred by bandwidth is clear, we currently know little about how this additional information is combined to form our percept of space. We investigated the ability of cells in the auditory system of guinea pigs to compare interaural level differences (ILDs), a key localization cue, between tones of disparate frequencies in each ear. Cells in auditory cortex believed to be integral to ILD processing (excitatory from one ear, inhibitory from the other: EI cells) compare ILDs separately over restricted frequency ranges which are not consistent with their monaural tuning. In contrast, cells that are excitatory from both ears (EE cells) show no evidence of frequency-specific processing. Both cell types are explained by a model in which ILDs are computed within separate frequency channels and subsequently combined in a single cortical cell. Interestingly, ILD processing in all inferior colliculus cell types (EE and EI) is largely consistent with processing within single, matched-frequency channels from each ear. Our data suggest a clear constraint on the way that localization cues are integrated: cortical ILD tuning to broadband sounds is a composite of separate, frequency-specific, binaurally sensitive channels. This frequency-specific processing appears after the level of the midbrain.SIGNIFICANCE STATEMENT For some sensory modalities (e.g., somatosensation, vision), the spatial arrangement of the outside world is inherited by the brain from the periphery. The auditory periphery is arranged spatially by frequency, not spatial location. Therefore, our auditory perception of location must be synthesized from physical cues in separate frequency channels. There are multiple cues (e.g., timing, level, spectral cues), but even single cues (e.g., level differences) are frequency dependent. The synthesis of location must account for this frequency dependence, but it is not known how this might occur. Here, we investigated how interaural-level differences are combined across frequency along the ascending auditory system. We found that the integration in auditory cortex preserves the independence of the different-level cues in different frequency regions.

Sensitivity to an Illusion of Sound Location in Human Auditory Cortex.

Human listeners place greater weight on the beginning of a sound compared to the middle or end when determining sound location, creating an auditory illusion known as the Franssen effect. Here, we exploited that effect to test whether human auditory cortex (AC) represents the physical vs. perceived spatial features of a sound. We used functional magnetic resonance imaging (fMRI) to measure AC responses to sounds that varied in perceived location due to interaural level differences (ILD) applied to sound onsets or to the full sound duration. Analysis of hemodynamic responses in AC revealed sensitivity to ILD in both full-cue (veridical) and onset-only (illusory) lateralized stimuli. Classification analysis revealed regional differences in the sensitivity to onset-only ILDs, where better classification was observed in posterior compared to primary AC. That is, restricting the ILD to sound onset—which alters the physical but not the perceptual nature of the spatial cue—did not eliminate cortical sensitivity to that cue. These results suggest that perceptual representations of auditory space emerge or are refined in higher-order AC regions, supporting the stable perception of auditory space in noisy or reverberant environments and forming the basis of illusions such as the Franssen effect.

Effect of frequency region on binaural interference for interaural level differences

Binaural interference occurs when a spectrally remote diotic interferer affects the ability to detect changes in interaural differences in the target. For interaural time differences, the magnitude of binaural interference depends on the target and interferer locations. For interaural level differences (ILDs), it is unclear if the magnitude of binaural interference depends on target and interferer location. ILDs are highly frequency dependent; ILDs become larger for increasing frequency for sources in the free field. Therefore, we hypothesized that binaural interference for ILDs would be frequency dependent, and that both target and interferer frequency would affect thresholds. In ten young normal-hearing listeners, we measured ILD discrimination thresholds using single tones, once with no interferer and then in the presence of a diotic interferer. We tested five frequencies: 500, 1000, 2000, 4000, and 8000 Hz. Each combination of target and interferer frequency was tested, resulting in 25 conditions. ILD...

Effects of clinical hearing aid settings on sound localization cues

Sound localization cues, particularly interaural level difference (ILD) cues, are known to be affected by hearing aid processing such as wide-dynamic range compression and strong directional microphones. These distorted cues may negatively impact spatial awareness and communication in complex environments, two areas of challenge for new and experienced hearing aid users. Previously, we investigated frequency specific alterations to ILD and interaural time difference (ITD) cues using linear amplification in simulated reverberant rooms and with gaged vent sizes. In reverberation, ITD became erratic and ILD reduced; minimal effects of hearing aid venting were observed (Diedesch and Stecker, Am. Aud. Soc. 2016). Here, we applied that approach to hearing aid settings more typically encountered by clinical patients. Phonak Audeo receiver-in-the-canal (RIC) hearing aids were programmed to typical open-fit hearing impairments using clinically normal settings for compression algorithms and directional microphones. Recordings, collected using an acoustic manikin, were compared across hearing aid coupling (standard open and closed domes) in anechoic and simulated rooms. Frequency-specific ITD and ILD were quantified across coupling, room, and hearing aid settings. [Work supported by the F. V. Hunt Postdoctoral Research Fellowship, NIH R01-DC011548 (GCS), R01-DC011828 (FJG), and the VA RR&D NCRAR.]

Modification of the head-related transfer functions for auditory proximity in rooms

The human hearing uses two signals entering the ears to interpret surrounding spaces and sound sources, and certain acoustic impressions are often regarded favorably by the listeners. At times, music consumers may consider auditory proximity and intimacy as a desired sensation. In some scenarios, the acoustic reflections, often arriving from lateral angles, succeed in enhancing the perceived proximity, even though all acoustic events occur at far distances. In contrast, natural sounds emanating from a very close proximity are instinctively perceived as intimate. The perception of spatial sound is based on the binaural cues, which can be modeled with head-related transfer functions (HRTF). Earlier research has demonstrated that the interaural level difference for lateral incidence is the foremost difference between near and far-distance HRTFs. In the context of room acoustics, only lateral reflections arrive from angles which at high frequencies create traces of interaural level difference otherwise characteristic to near-field HRTF. This paper explores altering the auditory proximity of auralizations by introducing near-field-type interaural level differences to widely available far-field HRTFs. Analysis of the application of these modified HRTF to reflected sound provides more insight into the sources of auditory proximity.

Blind modeling of binaural unmasking of speech in stationary maskers

The equalization cancellation (EC) model predicts the binaural masking level difference by equalizing interaural differences in level and time and increasing the signal-to-noise ratio (SNR) using destructive and constructive interferences. The EC model has been successfully combined with the speech intelligibility index (SII) to predict binaural speech intelligibility. Here a blind EC model is introduced that relies solely on the mixture of speech and noise, replacing the unrealistic requirement of the separated clean speech and noise signals in previous versions. The model uses two parallel EC paths, which either maximize or minimize the EC output level in each frequency band. If SNR is negative, minimization improves the SNR by removing the interferer component from the mixed signal. If SNR is positive, maximization improves the SNR by enhancing the target component. Either the minimizing or maximizing path in each frequency band is selected blindly based on envelope frequency-selective amplitude modula...

Interaural level difference-based model of speech localization in multi-talker environment

Horizontal localization is based on extraction of the interaural time and level differences (ITD, ILD). Even in complex scenes with multiple talkers and reverberation, the auditory system is remarkably good at estimating the individual talker locations. Several previous models proposed mechanisms that stressed the importance of ITD in the localization process. Here, we examine whether azimuth estimation in complex scenarios can be based solely on ILD. We implemented a model (based on Faller and Merimaa, 2004) in which azimuth estimation was based on ILDs of signal parts with high interaural correlation and with spectral profile matching that of the target. Comparison with experimental data (Kopco et al., 2010) showed that highly correlated parts of the signal, if available, provide reliable ILD estimates sufficient for precise target localization. However, for lateral target positions, at which the target dominates one ear but not the other, interaural correlation was too low to guide ILD extraction. In such cases, a new model based on finding maximum ILD provided good estimates even if maskers dominated in the worse ear. The combined model predictions matched the experimental data with target locations between -50° and 50° and for 4 maskers in reverberation. [Work supported by APVV-0452-12, H2020-MSCA-RISE-2015 #691229.]Horizontal localization is based on extraction of the interaural time and level differences (ITD, ILD). Even in complex scenes with multiple talkers and reverberation, the auditory system is remarkably good at estimating the individual talker locations. Several previous models proposed mechanisms that stressed the importance of ITD in the localization process. Here, we examine whether azimuth estimation in complex scenarios can be based solely on ILD. We implemented a model (based on Faller and Merimaa, 2004) in which azimuth estimation was based on ILDs of signal parts with high interaural correlation and with spectral profile matching that of the target. Comparison with experimental data (Kopco et al., 2010) showed that highly correlated parts of the signal, if available, provide reliable ILD estimates sufficient for precise target localization. However, for lateral target positions, at which the target dominates one ear but not the other, interaural correlation was too low to guide ILD extraction. In s...

A biologically inspired piezoelectric microphone

The parasitoid fly Ormia ochracea has the remarkable ability to locate crickets using audible sound. This ability is, in fact, remarkable as the fly’s hearing mechanism spans only 1.5 mm, which is 50 times smaller than the wavelength of sound emitted by the cricket. The hearing mechanism is, for all practical purposes, a point in space with no significant interaural time or level differences to draw from. It has been discovered that the fly's hearing organ utilizes multiple vibration modes to amplify interaural time and level differences (see Miles et al., J. Acoust. Soc. Am. 98 (6), December 1995). Here, we present a fully integrated mimic of the Ormia’s hearing mechanism capable of replicating the remarkable sound localization ability of the special fly. A silicon-micromachined prototype is presented which uses multiple piezoelectric sensing ports to simultaneously transduce two orthogonal vibration modes of the sensing structure, thereby enabling simultaneous measurement of sound pressure and pressure ...

A model of the ongoing precedence effect

The precedence effect is the observation that, when a sound is repeated after a delay of a few milliseconds, the interaural cues of the leading sound have greater influence on localization and lateralization judgments of the composite sound than do the cues of the lagging sound. Classic studies of the precedence effect with transient stimuli have suggested auditory mechanisms that emphasize earlier-arriving cues. More recent studies have shown, however, that non-transient sounds—steady, long-duration noises with slow onset—also give rise to the precedence effect. This ongoing precedence effect is difficult to explain with the abrupt-onset mechanisms invoked to explain the effect with transients. The present model of the ongoing effect assumes that listeners use interaural cues within auditory bands to form band-specific lateral positions, which are then integrated across bands. Within-band positions are estimated from the interaural level difference and the interaural phase during the rising side of bandpass envelope modulations. Cross-band integration includes a mechanism that favors cues that are consistent across wider frequency regions. Predictions from the model will be compared to published data. [Work supported by NIDCD R01 01625.]

The roles of inhibition and adaptation for spatial hearing in difficult listening conditions

The computation of binaural cues such as the Interaural Time Difference (ITD) and Interaural Level Difference (ILD) by the auditory system is known to play an important role in spatial hearing. It is not yet understood how such computations are performed in realistic acoustic environments where noise and reverberations are present. It has been hypothesized that robust sound localization is achieved through the extraction of the ITD information in the rising part of amplitude modulated (AM) sounds. Dietz et al. (2013) tested this hypothesis using psychoacoustics and MEG experiments. They presented AM sounds with ITDs varying during the course of one AM cycle. Their results showed that participants preferentially extracted the ITD information in the rising portion of the AM cycle. We designed a computational model of the auditory pathway to investigate the neural mechanisms involved in this process. Two mechanisms were tested. The first one corresponds to the adaptation in the auditory nerve fibers. The sec...

Psychoacoustics in the United Kingdom in the period 1900 to 1950

Much research in psychoacoustics during this period was conducted in the “Psychological Laboratory” in Cambridge (H. Bannister, A. F. Rawdon-Smith) and in Manchester (T. S. Littler, A. W. Ewing). Although Lord Rayleigh had published his influential duplex theory of sound localization in 1907, lingering doubts remained about whether interaural phase could truly be discriminated—rather, it had been proposed that somehow interaural phase differences were converted into interaural intensity differences. Bannister conducted a series of experiments to assess this idea and concluded that “binaural phase differences are appreciated in some manner which is distinct from the appreciation of binaural intensity differences.” Bannister also reported experiments on time-intensity trading in sound localization and reported that sometimes two images were perceived. Rawdon Smith and independently Littler and Ewing studied the phenomenon of auditory fatigue (temporary threshold shift, TTS). They showed that the maximum TTS produced by an intense fatiguing tone occurred for test tones with frequencies well above that of the fatiguing tone, a phenomenon that was repeatedly confirmed in later studies and that has only recently been explained. A common feature of the papers describing this work was a detailed description of the (often ingenious) apparatus that was developed to conduct the experiments.

Microsecond interaural time differences of acoustic transients are decoded by inhibitory-excitatory interactions in neurons of the lateral superior olive

The lateral superior olive (LSO) in the auditory brainstem generates sensitivity to interaural level differences (ILD), an important cue for sound localization, by comparing excitatory (E) input from the ipsilateral ear with inhibitory (I) input from the contralateral ear. Large axosomatic synapses (e.g., calyx of Held) point to the importance of precise temporal processing, but that is not easily reconciled with ILD detection. We propose that the IE interaction allows detection of interaural time differences (ITD) of acoustic transients, to which humans are exquisitely sensitive. We obtained in vivo whole cell recordings of LSO and MSO neurons in the gerbil, while presenting monaural and binaural clicks. We found that ITD functions to clicks in the LSO are surprisingly steep, in contrast to MSO neurons, which are considered the main ITD detectors. Intracellular LSO recordings show EPSPs generated by the ipsilateral click and IPSPs by the contralateral click, where IPSPs often arrive earlier. Binaural spiking is maximally suppressed when the EPSP coincides with the falling slope rather than the peak of the IPSP. We conclude that LSO neurons are more sensitive to ITDs of transients than MSO neurons. This clarifies the importance of timing specializations in the LSO circuit.

Audible sonar images generated with proprioception for target analysis

Some blind humans have demonstrated the ability to detect and classify objects with echolocation using palatal clicks. An audible-sonar robot mimics human click emissions, binaural hearing, and head movements to extract interaural time and level differences from target echoes. Targets of various complexity are examined by transverse displacements of the sonar and by target pose rotations that model movements performed by the blind. Controlled sonar movements executed by the robot provide data that model proprioception information available to blind humans for examining targets from various aspects. The audible sonar uses this sonar location and orientation information to form two-dimensional target images that are similar to medical diagnostic ultrasound tomograms. Simple targets, such as single round and square posts, produce distinguishable and recognizable images. More complex targets configured with several simple objects generate diffraction effects and multiple reflections that produce image artifacts. The presentation illustrates the capabilities and limitations of target classification from audible sonar images.