Interaural Level Differences Research Articles

Effect of inter-aural temporal envelope differences on inter-aural time difference thresholds for amplitude modulated noise.

The inter-aural time difference (ITD) and inter-aural level difference (ILD) are important acoustic cues for horizontal localization and spatial release from masking. These cues are encoded based on inter-aural comparisons of tonotopically matched binaural inputs. Therefore, binaural coherence or the interaural spectro-temporal similarity is a pre-requisite for encoding ITD and ILD. The modulation depth of envelope is an important envelope characteristic that helps in encoding the envelope-ITD. However, inter-aural difference in modulation depth can result in reduced binaural coherence and poor representation of binaural cues as in the case with reverberation, noise and compression in cochlear implants and hearing aids. This study investigated the effect of inter-aural modulation depth difference on the ITD thresholds for an amplitude-modulated noise in normal hearing young adults. An amplitude modulated high pass filtered noise with varying modulation depth differences was presented sequentially through headphones. In one ear, the modulation depth was retained at 90% and in the other ear it varied from 90% to 50%. The ITD thresholds for modulation frequencies of 8 Hz and 16 Hz were estimated as a function of the inter-aural modulation depth difference. The Friedman test findings revealed a statistically significant increase in the ITD threshold with an increase in the inter-aural modulation depth difference for 8 Hz and 16 Hz. The results indicate that the inter-aural differences in the modulation depth negatively impact ITD perception for an amplitude-modulated high pass filtered noise.

Head-related transfer functions of rabbits within the front horizontal plane

The head-related transfer function (HRTF) describes the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6kHz that increased gain by 12 to 23dB depending on source azimuth; and a notch at 7.6kHz and peak at 9.8kHz visible for most azimuths. Overall, frequencies above 4kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16kHz varied monotonically with azimuth over ±31dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5kHz and 1.5kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs (5–16kHz), whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs (0.2–1.5kHz). Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.

An inhibitory glycinergic projection from the cochlear nucleus to the lateral superior olive.

Auditory brainstem neurons in the lateral superior olive (LSO) receive excitatory input from the ipsilateral cochlear nucleus (CN) and inhibitory transmission from the contralateral CN via the medial nucleus of the trapezoid body (MNTB). This circuit enables sound localization using interaural level differences. Early studies have observed an additional inhibitory input originating from the ipsilateral side. However, many of its details, such as its origin, remained elusive. Employing electrical and optical stimulation of afferents in acute mouse brainstem slices and anatomical tracing, we here describe a glycinergic projection to LSO principal neurons that originates from the ipsilateral CN. This inhibitory synaptic input likely mediates inhibitory sidebands of LSO neurons in response to acoustic stimulation.

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.

Development of an adaptive test of musical scene analysis abilities for normal-hearing and hearing-impaired listeners

Auditory scene analysis (ASA) is the process through which the auditory system makes sense of complex acoustic environments by organising sound mixtures into meaningful events and streams. Although music psychology has acknowledged the fundamental role of ASA in shaping music perception, no efficient test to quantify listeners’ ASA abilities in realistic musical scenarios has yet been published. This study presents a new tool for testing ASA abilities in the context of music, suitable for both normal-hearing (NH) and hearing-impaired (HI) individuals: the adaptive Musical Scene Analysis (MSA) test. The test uses a simple ‘yes–no’ task paradigm to determine whether the sound from a single target instrument is heard in a mixture of popular music. During the online calibration phase, 525 NH and 131 HI listeners were recruited. The level ratio between the target instrument and the mixture, choice of target instrument, and number of instruments in the mixture were found to be important factors affecting item difficulty, whereas the influence of the stereo width (induced by inter-aural level differences) only had a minor effect. Based on a Bayesian logistic mixed-effects model, an adaptive version of the MSA test was developed. In a subsequent validation experiment with 74 listeners (20 HI), MSA scores showed acceptable test–retest reliability and moderate correlations with other music-related tests, pure-tone-average audiograms, age, musical sophistication, and working memory capacities. The MSA test is a user-friendly and efficient open-source tool for evaluating musical ASA abilities and is suitable for profiling the effects of hearing impairment on music perception.

OFFSET RESPONSES IN CONDITIONS OF AUDITORY SPATIAL MASKING IN HUMANS

The influence of relative spatial positions of sound stimuli and background masker on the event related potentials (ERPs) evoked by sound offset was investigated. Sound stimuli were presented dichotically, the position of sound images was manipulated using interaural level differences. Test signals were presented in silence or against the background of a masker. Signal and masker were either co-located or separated by 90 or 180 deg of azimuth. Co-location of signal and masker resulted in amplitude decrease and latency increase in the N1, P2 and N2 components. When angular distance between signal and masker increased, the amplitude recovered almost to the initial level and the latency of all components became shorter. The present findings are in line with the view that offset response is essential for target stimulus detection in the background noise.

A comparison of binaural cues transmitted by clinically available cochlear-implant stimulation strategies

Cochlear implants (Cis) effectively restore speech understanding in quiet for many recipients. However, bilateral CI listeners continue to perform poorer than acoustic-hearing listeners on spatial-hearing tasks (e.g., speech understanding in noise and sound localization). One explanation is that current clinically available stimulation strategies and unsynchronized sound processors degrade the fidelity of binaural cues. However, no conclusive comparison of the binaural cues transmitted by different stimulation strategies has been made. Therefore, this study compared binaural cues transmitted by two clinically available cochlear implant stimulation strategies. Two unsynchronized Cochlear Ltd. Nucleus 6 sound processors were placed on a binaural mannequin centered in a horizontal ring of loudspeakers in an anechoic chamber. Acoustic stimuli were presented from the loudspeakers and the resulting electrical pulse trains were recorded for both a peak-picking stimulation strategy and constantly stimulating strategy. Envelope interaural time differences and interaural level differences were extracted from the recordings and compared between stimulation strategies. Compared to the constantly stimulating strategy, results show that the peak-picking strategy led to more variable and less monotonic binaural cues, which could constrain sound-localization abilities. These findings argue for the bilateral synchronization of CIs in multiple domains, including electrode selection in peak-picking stimulation strategies and pulse timing.

The localization of virtual source reproduced by the crosstalk cancellation system under different direct-to-reverberation energy ratios conditions

This study explores the localization of virtual sources reproduced by the crosstalk cancellation (CTC) system under reverberation environments with different direct-to-reverberation energy ratios (DRRs). Binaural room impulse responses in rectangle rooms are simulated by high-order image source method. Two independent key variables, the reverberation time (RT) and the distance of the sound source to the listener, are employed to manipulate DRR through simulation. Subsequently, a subjective localization experiment based on the CTC system is virtually conducted using earphone reproduction. The results show that the azimuth localization in horizontal plane is almost impervious by RT but is subjected to the change of the source distance. Specifically, as the distance of sound source increases, the perceived azimuth of lateral virtual sources exhibits a bias towards the front, accompanied by larger variances. The results indicate that reflections with small delays have a more detrimental effect on sound localization compared to the late reverberation with large intensity, which may be associated with the inhibition of late reflections based on the precedence effect. Additionally, the localization data are analyzed and explained using binaural cues, including the interaural cross-correlation coefficient, the interaural time difference (ITD), and interaural level difference (ILD).

Low-frequency limit for binaural timing cues

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

The impact of phase on auditory situational awareness

A person’s situational awareness is the ability to understand and correctly interpret the world as it exists around them. One component of auditory situational awareness is sound localization, or the ability to identify where a sound is in space relative to the listener. Different aural cues contribute to a listener’s sound localization: the difference in the time it takes for a sound to reach one ear compared to the other (interaural time difference, or ITD) and the difference in the level of the sound at each ear (interaural intensity difference, or IID). The relative contribution of these interaural differences changes with frequency regime, with ITD/IPD being more dominant at low frequencies and IID being more dominant at high frequencies. These cues, as well as a spectral coloration dependent upon the shape of the listener’s pinna and concha bowl, contribute to what is called the head-related transfer function (HRTF). This study examines the impact of a phase inversion in one ear on a person’s auditory situational awareness. Testing was conducted in accordance with ASA/ANSI S3.71-2019, Method 3 to measure the impact on response time, using full spectrum stimuli as well as band-limited stimuli to focus testing on each aural cue.

Acoustical verification of binaural features in hearing aids

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

Age differences in binaural and working memory abilities in school-going children.

Binaural hearing is the interplay of acoustic cues (interaural time differences: ITD, interaural level differences: ILD, and spectral cues) and cognitive abilities (e.g., working memory, attention). The current study investigated the effect of developmental age on auditory binaural resolution and working memory and the association between them (if any) in school-going children. Fifty-seven normal-hearing school-going children aged 6-15y were recruited for the study. The participants were divided into three groups: Group 1 (n=17, Mage=7.1y±0.72y), Group 2 (n=23; Mage=10.2y±0.8y), Group 3 (n=17; Mage: 14.1y ±1.3y). Group 4, with normal hearing young adults (n=20; Mage=21.1y± 3.2y), was included for comparing the maturational changes in former groups with adult values. Tests of binaural resolution (ITD and ILD thresholds) and auditory working memory (forward and backward digit span and 2n-back digit) were administered to all the participants. Results indicated a main effect of age on spatial resolution and working memory, with the median of lower age groups (Group 1 & Group 2) being significantly poorer (p<0.01) than the higher age groups (Group 3 & Group 4). Groups 2, 3, and 4 performed significantly better than Group 1 (p<0.001) on the forward span and ILD task. Groups 3 and 4 had significantly better ITD (p=0.04), backward span (p=0.02), and 2n-back scores than Group 2. A significant correlation between scores on working memory tasks and spatial resolution thresholds was also found. On discriminant function analysis, backward span and ITD emerged as sensitive measures for segregating older groups (Group 3 & Group 4) from younger groups (Group 1 & Group 2). The present study showed that the ILD thresholds and forward digit span mature by nine years. However, the backward digit span score continued to mature beyond 15y. This finding can be attributed to the influence of auditory attention (a working memory process) on the binaural resolution, which is reported to mature till late adolescence.

Web-based psychoacoustics of binaural hearing: Two validation experiments.

Web-based testing is an appealing option for expanding psychoacoustics research outside laboratory environments due to its simple logistics. For example, research participants partake in listening tasks using their own computer and audio hardware and can participate in a comfortable environment of their choice at their own pace. However, it is unknown how deviations from conventional in-lab testing affect data quality, particularly in binaural hearing tasks that traditionally require highly precise audio presentation. Here, we used an online platform to replicate two published in-lab experiments: lateralization to interaural time and level differences (ITD and ILD, experiment I) and dichotic and contralateral unmasking of speech (experiment II) in normal-hearing (NH) young adults. Lateralization data collected online were strikingly similar to in-lab results. Likewise, the amount of unmasking measured online and in-lab differed by less than 1 dB, although online participants demonstrated higher speech reception thresholds overall than those tested in-lab by up to ∼7 dB. Results from online participants who completed a hearing screening versus those who self-reported NH did not differ significantly. We conclude that web-based psychoacoustics testing is a viable option for assessing binaural hearing abilities among young NH adults and discuss important considerations for online study design.

Randomizing spectral cues used to resolve front-back reversals in sound-source localization.

Front-back reversals (FBRs) in sound-source localization tasks due to cone-of-confusion errors on the azimuth plane occur with some regularity, and their occurrence is listener-dependent. There are fewer FBRs for wideband, high-frequency sounds than for low-frequency sounds presumably because the sources of low-frequency sounds are localized on the basis of interaural differences (interaural time and level differences), which can lead to ambiguous responses. Spectral cues can aid in determining sound-source locations for wideband, high-frequency sounds, and such spectral cues do not lead to ambiguous responses. However, to what extent spectral features might aid sound-source localization is still not known. This paper explores conditions in which the spectral profile of two-octave wide noise bands, whose sources were localized on the azimuth plane, were randomly varied. The experiment demonstrated that such spectral profile randomization increased FBRs for high-frequency noise bands, presumably because whatever spectral features are used for sound-source localization were no longer as useful for resolving FBRs, and listeners relied on interaural differences for sound-source localization, which led to response ambiguities. Additionally, head rotation decreased FBRs in all cases, even when FBRs increased due to spectral profile randomization. In all cases, the occurrence of FBRs was listener-dependent.

Inter-aural separation during hearing by bilateral bone conduction stimulation

Cross-head transmission inherent in bone conduction (BC) hearing is one of the most important factors that limit the performance of BC binaural hearing compared to air conduction (AC) binaural hearing. In AC, cross-head transmission is imperceptible leading to a clear understanding of the nature and position of the sound source(s). In this study, the prominence of cross-head transmission in BC hearing is addressed using the fact that ipsilateral cochlear excitation can be canceled by controlled bilateral BC stimulation.A cancellation experiment was conducted on twenty participants with normal hearing at thirteen third-octave frequencies between 250 and 4000 Hz. Both stationary and transient BC stimulation at the mastoid was used. The technique employed multiple stages of masking enabling adjustments of the stimulation level and phase until the tones got canceled in the ipsilateral ear. In addition, the ear canal sound pressure was obtained for ipsilateral and contralateral BC stimulation in isolation, and with bilateral BC stimulation at perceptual cancellation.The inter-aural level differences of both the types of stimulations were found to be the same. Crosstalk was found to be the lowest around 2 kHz and the highest around 1 kHz. The unwrapped inter-aural phase difference from stationary signal cancellation showed an overall increase with frequency starting at around no difference (35°) at 250 Hz to reach 607° at 4 kHz. Cycle-adjusted inter-aural time difference was very low (61 µs) at 250 Hz and increased to 1.1 ms at 800 Hz before falling to 0.6 ms at 4 kHz. It was also found that the ear canal sound pressure was not cancelled at the same phase as the sound in the cochlea.

Closed-field Auditory Spatial Perception and Its Relationship to Musical Aptitude

Introduction: Musical aptitude is the innate ability of an individual to understand, appreciate, improvise, and have a good sense of pitch and rhythm, even without undergoing formal musical training. The present study aimed to understand the effect of musical aptitude on auditory spatial perception. Method: Forty nonmusicians were subjected to a musical aptitude test Mini Profile of Music Perception Skills (Mini-PROMS) based on which they were divided into two groups. Group I included 20 nonmusicians with good musical aptitude (NM-GA) and Group II comprised 20 nonmusicians with poor musical aptitude (NM-PA). Auditory spatial tests included interaural time difference (ITD) and interaural level difference (ILD) threshold tests and a closed-field spatial test called virtual acoustic space identification (VASI) test. Results: Kruskal–Wallis test revealed a significant difference between Group I (NM-GA) and Group II (NM-PA) in ITD (p &lt; 0.001), ILD (p = 0.002), and VASI (p = 0.012) tests, suggesting the role of musical aptitude in auditory spatial perception. Correlational analyses showed a moderate positive correlation between Mini-PROMS scores with VASI (r = 0.31, p = 0.04) and a moderate negative correlation with ILD (r = −0.3, p = 0.04) and ITD (r = −0.5, p = 0.001). Conclusion: This study defines a positive association between musical aptitude and auditory spatial perception. Further research should include a comparison of spatial skills among musicians and nonmusicians with good musical aptitude.

Spatial Auditory Masking Affects the Interhemispheric Asymmetry of Evoked Responses

Interhemispheric asymmetry of electrical brain activity was investigated in the conditions of spatial auditory masking. Moving test signals were presented either in silence or against the background of stationary maskers of various spatial positions. The spatial properties of the stimuli were defined by interaural level differences (ILD). Onset-energy responses (ON-responses), motion-onset responses (MOR) and OFF-responses were analyzed. To compute the topograms and to analyze asymmetry, the amplitudes of each component were averaged over the symmetric electrode clusters in the left and right hemispheres. The ON-responses showed a contralateral dominance of the N1 component in silence, and the degree of contralateral bias increased in masking conditions. Interhemispheric asymmetry of the P2 component was absent in silence. However, the P2 amplitude was higher in the right hemisphere in all combinations of masker and signal. The asymmetry of both deflections was maximal when the masker and the initial portion of the signal were separated by 180 degrees. On the contrary, the interhemispheric asymmetry of the motion-onset response was found only in silence: the cN1 deflection was biased to the side contralateral to the signal. The topography of the OFF-response was symmetrical under all experimental conditions.

Cerebral dominance in spatial hearing and working memory abilities in adults with normal hearing sensitivity

BackgroundCerebral dominance refers to the biological description of the brain, where one cerebral hemisphere is dominant over the other in certain cerebral functions. There is scanty literature on cerebral dominance and its impact on auditory spatial processing and working memory, which is explored in the study.MethodsA total of 45 participants with normal hearing were divided into three groups of 15 participants. The groups were categorized based on scores obtained on the alert scale of the cognitive style checklist as the bilateral dominant, left dominant, and the right dominant group. The spatial hearing was assessed using interaural time difference (ITD), the interaural level difference (ILD), and virtual acoustic space identification (VASI) tests, whereas the auditory working memory abilities were tested using forward span, backward span, ascending digit span, descending digit span, and 2n back tests.ResultsMONOVA results indicated that there is no significant main effect of cerebral dominance on all auditory working memory tests. In spatial hearing, although ILD and ILD thresholds were not influenced by cerebral dominance, the main effect of cerebral dominance was seen on VASI accuracy scores. Post-hoc analyses of VASI scores showed that the bilateral dominant group demonstrated significantly better spatial perception scores compared to the left and right dominant groups, with latter groups showing similar performance.ConclusionsWhile ITD and ILD tests fall short of revealing cerebral asymmetry, VASI’s power in capturing cerebral dominance effects makes it a valuable tool in spatial processing assessment. The study’s findings highlight the need for assessing cerebral dominance, before administering spatial hearing tests.

How to define thresholds for level and interaural-level-difference discrimination: Insights from scedasticities and distributions

Sensitivity to changes in the stimulus level at one or at both ears and to changes in the interaural level difference (ILD) between the two ears has been studied widely. Several different definitions of threshold and, for one of them, two different ways of averaging single-listener thresholds have been used (i.e., arithmetically and geometrically), but it is unclear which definition and which way of averaging is most suitable. Here, we addressed this issue by examining which of the differently defined thresholds yielded the highest degree of homoscedasticity (homogeneity of the variance). We also examined how closely the differently defined thresholds followed the normal distribution. We measured thresholds from a large number of human listeners as a function of stimulus duration in six experimental conditions, using an adaptive two-alternative forced-choice paradigm. Thresholds defined as the logarithm of the ratio of the intensities or amplitudes of the target and the reference stimulus (i.e., as the difference in their levels or ILDs; the most commonly used definition) were clearly heteroscedastic. Log-transformation of these latter thresholds, as sometimes performed, did not result in homoscedasticity. Thresholds defined as the logarithm of the Weber fraction for stimulus intensity and thresholds defined as the logarithm of the Weber fraction for stimulus amplitude (the most rarely used definition) were consistent with homoscedasticity, but the latter were closer to the ideal case. Thresholds defined as the logarithm of the Weber fraction for stimulus amplitude also followed the normal distribution most closely. The discrimination thresholds should therefore be expressed as the logarithm of the Weber fraction for stimulus amplitude and be averaged arithmetically across listeners. Other implications are discussed, and the obtained differences between the thresholds in different conditions are compared to the literature.

Web-based psychoacoustics: Hearing screening, infrastructure, and validation

Anonymous web-based experiments are increasingly used in many domains of behavioral research. However, online studies of auditory perception, especially of psychoacoustic phenomena pertaining to low-level sensory processing, are challenging because of limited available control of the acoustics, and the inability to perform audiometry to confirm normal-hearing status of participants. Here, we outline our approach to mitigate these challenges and validate our procedures by comparing web-based measurements to lab-based data on a range of classic psychoacoustic tasks. Individual tasks were created using jsPsych, an open-source JavaScript front-end library. Dynamic sequences of psychoacoustic tasks were implemented using Django, an open-source library for web applications, and combined with consent pages, questionnaires, and debriefing pages. Subjects were recruited via Prolific, a subject recruitment platform for web-based studies. Guided by a meta-analysis of lab-based data, we developed and validated a screening procedure to select participants for (putative) normal-hearing status based on their responses in a suprathreshold task and a survey. Headphone use was standardized by supplementing procedures from prior literature with a binaural hearing task. Individuals meeting all criteria were re-invited to complete a range of classic psychoacoustic tasks. For the re-invited participants, absolute thresholds were in excellent agreement with lab-based data for fundamental frequency discrimination, gap detection, and sensitivity to interaural time delay and level difference. Furthermore, word identification scores, consonant confusion patterns, and co-modulation masking release effect also matched lab-based studies. Our results suggest that web-based psychoacoustics is a viable complement to lab-based research. Source code for our infrastructure is provided.