Interaural Temporal 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.

What, if anything, is coincidence detection?

Neural delays and coincidence detection are neural operations posited as key elements in temporal processing, in the auditory system and beyond. Although longstanding physiological evidence for coincidence detectors exists at a phenomenological level, mechanistic examinations have only recently been performed. We review intracellular recordings from two cell types which are traditionally regarded as prototypes of coincidence detectors. Neurons in the medial superior olive (MSO) are sensitive to “coincidences” in action potentials from the ipsi- and contralateral ear, making the neurons sensitive to interaural temporal differences. Octopus cells in the cochlear nucleus are sensitive to “coincidences” of action potentials across inputs tuned to a wide range of frequencies. We found that in both cases, the spike output of neurons does not simply reflect coincidences of input spikes. When inputs vary in strength, their sequence of activation is an important determinant of response strength. Maximal output is not just determined by the maximal number of coincidences, but reflects temporal input structure in interaction with intrinsic (membrane) properties. These observations suggest a simpler mechanism of temporal sensitivity than traditional lag and coincidence proposals.

Interhemispheric auditory cortical relationships in asymmetric hearing loss

Directional acoustic trauma, sudden hearing loss with incomplete recovery, Meniere’s disease, and cerebellopontine angle tumors are common clinical conditions that present with asymmetric hearing loss (ASL). Presentation of input signals from both ears for interhemispheric processing may be necessary to achieve the best hearing outcomes. To review temporal and spatial interhemispheric functional relationships revealed by microelectrode mapping studies in an animal model of ASL and by functional imaging studies in human single-sided deafness (SSD). Animal and human cohort comparison studies by investigators at the University of California. In animals with mild-to-moderate ASL, interaural temporal difference is a strong driver of cortical plastic change. Six months after hearing loss induction, the two cortices reorganize into a state of relative local hemispheric autonomy. In humans with SSD, temporal and spatial cortical plastic change is evidenced by reduced interhemispheric mean difference of the M100 p...

Lateralization of simulated sources and echoes differing in frequency based on interaural temporal differences.

This study examined listeners' ability to process interaural temporal differences (ITDs) in one of two sequential sounds when the two differed in spectral content. A correlational analysis assessed weights given to ITDs of simulated source and echo pulses for echo delays of 8-128 ms for conditions in which responses were based on the source or echo, a 3000-Hz Gaussian (target) pulse. The other (distractor) pulse was spectrally centered at 1500, 2000, 3000, 4000, or 5000 Hz. Also measured were proportion correct and proportion of responses predicted from the weights. Regardless of whether the echo or source pulse served as the target, target weight, and proportion correct increased with increasing distractor frequency, consistent with low-frequency dominance [Divenyi, J. Acoust. Soc. Am. 91, 1078-1084 (1992)]. Effects of distractor frequency were observed at echo delays out to 128 ms when the source served as the target, but only out to 64 ms when the echo served as the target. At echo delays beyond 8 ms, recency effects were exhibited with higher proportions correct obtained for judgments based on the echo pulse than the source pulse.

The influence of pause, attack, and decay duration of the ongoing envelope on sound lateralization.

Klein-Hennig et al. [J. Acoust. Soc. Am. 129, 3856-3872 (2011)] introduced a class of high-frequency stimuli for which the envelope shape can be altered by independently varying the attack, hold, decay, and pause durations. These stimuli, originally employed for testing the shape dependence of human listeners' sensitivity to interaural temporal differences (ITDs) in the ongoing envelope, were used to measure the lateralization produced by fixed interaural disparities. Consistent with the threshold ITD data, a steep attack and a non-zero pause facilitate strong ITD-based lateralization. In contrast, those conditions resulted in the smallest interaural level-based lateralization.

Time and intensity with Tino

Over a period of decades Constantine (Tino) Trahiotis has been a contemplative and creative scholar, an excellent teacher, a helpful collaborator, as well as a loyal friend. Tino, along with Bob Bilger and Erv Hafter, provided this author's introduction to the broader science of hearing beyond the particular perspective of my graduate training. At the same time, my attempts to interpret models of binaural interaction with Tino led to the review chapters on binaural modeling that the two of us jointly authored in the 1990s. Tino's demonstration of the unexpected dependence on bandwidth of the laterality of bands of noise presented with interaural temporal differences of large magnitude motivated me to think about the tradeoff between “straightness” and “centrality,” which appears to be helpful in considering the lateralization of many binaural stimuli. Finally, Tino, along with Les Bernstein, insisted on a local implementation of my models in their laboratory which became one source (along with significant others) for the binaural modeling toolbox developed by Michael Akeroyd that is presently in wide circulation. This talk will review these facets of my life with Tino, and comment on my current perspectives on related issues. [Work supported by DARPA and Cisco Research.]

Binaural interference: effects of temporal interferer fringe and interstimulus interval.

Binaural interference refers to the phenomenon in which the potency of binaural cues conveyed by a "target" stimulus occupying one spectral region is degraded by the presence of an "interferer" stimulus occupying a spectral region remote from the target. It is typified by conditions in which thresholds for detection of interaural temporal difference conveyed by a high-frequency target are elevated when the target is accompanied by a spectrally remote low-frequency interferer. This study explored effects of temporal relations between targets and interferers on binaural interference. In the first experiment, duration by which the interferer preceded and/or trailed the target (onset and offset "fringes") was varied. Results indicated binaural interference decreased with total duration of the temporal fringe, but did not depend on whether that duration was composed of onset, offset, or onset + offset fringes. In the second experiment, binaural interference was measured as a function of the interstimulus interval (ISI) between the two presentations of the target. Results indicated that shorter ISIs increased thresholds in both the interferer and no-interferer conditions, but did not affect binaural interference. These results suggest that the mechanisms underlying the effects of manipulations of the interferer temporal fringe and manipulation of the ISI are essentially independent.

Adaptation in Sound Localization Processing Induced by Interaural Time Difference in Amplitude Envelope at High Frequencies

BackgroundWhen a second sound follows a long first sound, its location appears to be perceived away from the first one (the localization/lateralization aftereffect). This aftereffect has often been considered to reflect an efficient neural coding of sound locations in the auditory system. To understand determinants of the localization aftereffect, the current study examined whether it is induced by an interaural temporal difference (ITD) in the amplitude envelope of high frequency transposed tones (over 2 kHz), which is known to function as a sound localization cue.Methodology/Principal FindingsIn Experiment 1, participants were required to adjust the position of a pointer to the perceived location of test stimuli before and after adaptation. Test and adapter stimuli were amplitude modulated (AM) sounds presented at high frequencies and their positional differences were manipulated solely by the envelope ITD. Results showed that the adapter's ITD systematically affected the perceived position of test sounds to the directions expected from the localization/lateralization aftereffect when the adapter was presented at ±600 µs ITD; a corresponding significant effect was not observed for a 0 µs ITD adapter. In Experiment 2, the observed adapter effect was confirmed using a forced-choice task. It was also found that adaptation to the AM sounds at high frequencies did not significantly change the perceived position of pure-tone test stimuli in the low frequency region (128 and 256 Hz).Conclusions/SignificanceThe findings in the current study indicate that ITD in the envelope at high frequencies induces the localization aftereffect. This suggests that ITD in the high frequency region is involved in adaptive plasticity of auditory localization processing.

Binaural displays based on interaural phase compensation.

The asymmetry of many neural tuning curves to the interaural time delay (ITD) of wideband stimuli around their maximum implies that such external temporal differences are compensated by some internal phase‐delay mechanism rather than a system of axonal time delays. An internal phase delay explains why the ITD that generates maximum neural activity (best delay) is generally less than a half period of the tuning curve’s modulation (“π‐limit”). The interaural phase circle (IPD circle) and the smoothed interaural phase spectrum (SIPS) are new binaural displays, which are based on these physiological findings. The IPD‐circle displays interaural phase difference (IPD) and interaural phase coherence (IPC) of single frequency channels in a polar diagram as vector angle and magnitude, respectively. Its two‐dimensionality facilitates the study of IPD and IPC fluctuations. The SIPS shows an activation matrix of binaural coincidence detectors across their best frequencies and internal phase delays. It serves similarly to the traditional cross‐correlogram as a display of interaural temporal differences across frequency. Its computation is based on the interaural phase spectrum, which is smoothed in frequency‐dimension by the auditory filter shapes. These displays describe neural ITD tuning curves with their asymmetry, and could provide a basis for future physiologically orientated models.

Horizontal-Plane Localization of Noise and Speech Signals by Postlingually Deafened Adults Fitted With Bilateral Cochlear Implants*

The main purpose of the study was to assess the ability of adults with bilateral cochlear implants to localize noise and speech signals in the horizontal plane. A second objective was to measure the change in localization performance in these adults between approximately 5 and 15 mo after activation. A third objective was to evaluate the relative roles of interaural level difference (ILD) and interaural temporal difference (ITD) cues in localization by these subjects. Twenty-two adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ cochlear implants, were tested in a modified source identification task. Subjects were tested individually in an anechoic chamber, which contained an array of 43 numbered loudspeakers extending from -90 degrees to +90 degrees azimuth. On each trial, a 200-msec signal (either a noise burst or a speech sample) was presented from one of 17 active loudspeakers (span: +/-80 degrees ), and the subject had to identify which source from the 43 loudspeakers in the array produced the signal. Subjects were tested in three conditions: left device only active, right device only active, and both devices active. Twelve of the 22 subjects were retested approximately 10 mo after their first test. In Experiment 2, the spectral content and rise-decay time of the noise stimulus were manipulated. The relationship between source azimuth and response azimuth was characterized in terms of the adjusted constant error (ĉ). (1) With both devices active, ĉ for the noise stimulus varied from 8.1 degrees to 43.4 degrees (mean: 24.1 degrees ). By comparison, ĉ for a group of listeners with normal hearing ranged from 3.5 degrees to 7.8 degrees (mean: 5.6 degrees ). When subjects listened in unilateral mode (with one device turned off), ĉ was at or near chance (50.5 degrees ) in all cases. However, when considering unilateral performance on each subject's better side, average ĉ for the speech stimulus was 47.9 degrees , which was significantly (but only slightly) better than chance. (2) When listening bilaterally, error score was significantly lower for the speech stimulus (mean ĉ = 21.5 degrees ) than for the noise stimulus (mean ĉ = 24.1 degrees ). (3) As a group, the 12 subjects who were retested 10 mo after their first visit showed no significant improvement in localization performance during the intervening time. However, two subjects who performed very poorly during their first visit showed dramatic improvement (error scores were halved) over the intervening time. In Experiment 2, removing the high-frequency content of noise signals resulted in significantly poorer performance, but removing the low-frequency content or increasing the rise-decay time did not have an effect. In agreement with previously reported data, subjects with bilateral cochlear implants localized sounds in the horizontal plane remarkably well when using both of their devices, but they generally could not localize sounds when either device was deactivated. They could localize the speech signal with slightly, but significantly better accuracy than the noise, possibly due to spectral differences in the signals, to the availability of envelope ITD cues with the speech but not the noise signal, or to more central factors related to the social salience of speech signals. For most subjects the remarkable ability to localize sounds has stabilized by 5 mo after activation. However, for some subjects who perform poorly initially, there can be substantial improvement past 5 mo. Results from Experiment 2 suggest that ILD cues underlie localization ability for noise signals, and that ITD cues do not contribute.

Auditory Midbrain and Nerve Responses to Sinusoidal Variations in Interaural Correlation

The human sensitivity to interaural temporal differences in the acoustic waveforms to the two ears shows remarkable acuity but is also very sluggish. Fast changes in binaural parameters are not detectable, and this inability contrasts sharply with the excellent temporal resolution of the monaural auditory system. We studied the response of binaural neurons in the inferior colliculus of the cat to sinusoidal changes in the interaural correlation of broadband noise. Responses to the same waveforms were also obtained from auditory nerve fibers and further analyzed with a coincidence analysis. Overall, the auditory nerve and inferior colliculus showed a similar ability to code changes in interaural correlation. This ability extended to modulation frequencies an order of magnitude higher than the highest frequencies detected binaurally in humans. We conclude that binaural sluggishness is not caused by a lack of temporal encoding of fast binaural changes at the level of the midbrain. We hypothesize that there is no neural substrate at the level of the midbrain or higher to read out this temporal code and that this constitutes a low-pass "sluggishness" filter.

Underwater Hearing and Sound Localization with and without an Air Interface

Underwater hearing acuity and sound localization are improved by the presence of an air interface around the pinnae and inside the external ear canals. Hearing threshold and the ability to localize sound sources are reduced underwater. The resonance frequency of the external ear is lowered when the external ear canal is filled with water, and the impedance-matching ability of the middle ear is significantly reduced due to elevation of the ambient pressure, the water-mass load on the tympanic membrane, and the addition of a fluid-air interface during submersion. Sound lateralization on land is largely explained by the mechanisms of interaural intensity differences and interaural temporal or phase differences. During submersion, these differences are largely lost due to the increase in underwater sound velocity and cancellation of the head's acoustic shadow effect because of the similarity between the impedance of the skull and the surrounding water. Ten scuba divers wearing a regular opaque face mask or an opaque ProEar 2000 (Safe Dive, Ltd., Hofit, Israel) mask that enables the presence of air at ambient pressure in and around the ear made a dive to a depth of 3 m in the open sea. Four underwater speakers arranged on the horizontal plane at 90-degree intervals and at a distance of 5 m from the diver were used for testing pure-tone hearing thresholds (PTHT), the reception threshold for the recorded sound of a rubber-boat engine, and sound localization. For sound localization, the sound of the rubber boat's engine was randomly delivered by one speaker at a time at 40 dB HL above the recorded sound of a rubber-boat engine, and the diver was asked to point to the sound source. The azimuth was measured by the diver's companion using a navigation board. Underwater PTHT with both masks were significantly higher for frequencies of 250 to 6000 Hz when compared with the thresholds on land (p <0.0001). No differences were found in the PTHT or the reception threshold for the recorded sound of a rubber-boat engine for dry or wet ear conditions. There was no difference in the sound localization error between the regular mask and the ProEar 2000 mask. The presence of air around the pinna and inside the external ear canal did not improve underwater hearing sensitivity or sound localization. These results support the argument that bone conduction plays the main role in underwater hearing.

The benefit of practice for sound localization without sight

An experiment was designed to determine whether normally sighted human subjects would be able to adapt to the handicapping effects of sudden deprivation of visual cues on horizontal plane sound localization. Two groups of sighted normal-hearing young adults participated. One group was allowed the benefit of sight. The other group was blindfolded. Measurements of accuracy and the time to respond were made daily over the course of five consecutive days, in a semi-reverberant sound proof booth that modeled listening in a small office. Sound localization was assessed using an array of eight speakers that surrounded the subject in space. Each day, one block of 120 trials was presented for each of three stimuli, two one-third octave noise bands, centred at 0.5 and 4 kHz, and broadband noise, to assess the utilization of interaural temporal difference cues, interaural level difference cues and binaural and spectral cues in combination. Blindfolded subjects were relatively less accurate than sighted subjects. Both groups showed gains with practice, the blindfolded group to a greater degree, largely due to improvements in the use of spectral cues. The blindfolded group took longer to respond than the sighted group, but showed greater decrements in response time with practice.

Peripheral auditory processing, the precedence effect and responses of single units in the inferior colliculus

The purpose of this paper is to illustrate how interactions occurring within the auditory periphery are relevant to the interpretation of neurophysiological data obtained in recent studies seeking to find physiological correlates of ‘the precedence effect’. Similar information was presented orally at the recent International Symposium on the Central Auditory System held in Salamanca, Spain. The physiological data of interest are responses from single neural units in the inferior colliculus recorded following stimulation by successive pairs of binaural clicks. We show how peripheral, monaural, within-filter interactions of such successive clicks can produce internal values of interaural temporal differences (ITDs) and interaural intensitive differences (IIDs) that can differ greatly from those present in the external stimulus. These interactions can produce unintended internal ITDs and IIDs that are well ‘outside the tuning range’ of the single unit being studied. When this occurs, the responses of the single units to the pairs of clicks would be expected to diminish. We also discuss how hair cell-related adaptation and compression can also lead to diminished responses. It is suggested that effects resulting from peripheral interactions should be taken into account or evaluated quantitatively before other factors, including central inhibition, are invoked.

Original Article: The effect of blindness on horizontal plane sound source identification: El efecto de la ceguera en la identificatión de la fuente sonora en el piano horizontal

The effect of blindness on sound source identification was studied. Four groups of normally-hearing adults, two sighted and two blind, participated. Subjects were tested using arrays of four and eight loudspeakers, surrounding them in the horizontal plane. One sighted group was tested in quiet. The other groups were tested in continuous 60-dB SPL white noise. Three 75-dB SPL 300-ms stimuli were localized: one-third octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. Broadband noise was easiest to localize (both binaural and spectral cues available), and the 0.5-kHz noise band was the most difficult (primarily interaural temporal difference cue available). Subjects with late-onset blindness achieved significantly higher scores than the early blind and blindfolded sighted subjects. The percentage correct decreased with an increase in the number of speakers, but background noise had no effect. The results attest to the benefit of early visual experience for spatial hearing in adulthood, and demonstrate the negative impact of sudden loss of sight.

Temporal resolution in an interaural temporal difference discrimination task with distracter stimuli

Interaural temporal difference (ITD) thresholds were determined for brief (14-ms) 500-Hz tone bursts in a two-interval forced-choice paradigm in which the interstimulus interval was 36 ms. Preliminary data indicate that the ITD threshold can be affected by the presence of a third (distracter) burst temporally centered between the two primary bursts on each trial: When the distracter has the same ITD as the final burst of the pair, the ITD threshold is unchanged or decreases relative to in the no-distracter condition; when it has the same ITD as the first burst, the threshold increases. The results are consistent with the hypothesis that, when presented with a rapid three-burst triad, the binaural system processes the ITD of the first burst normally (i.e., as if presented in isolation), but confuses or averages the ITDs of the latter two bursts. Follow-up experiments will measure the effects of the discrete nature of the distracter (i.e., by removing the silent gap between the distracter and the burst that shares its ITD), and the frequency of the distracter on the magnitude of its effect. Results will be discussed in terms of binaural temporal resolution of onset versus post-onset information. [Supported by NIDCD.]

The effect of aging on horizontal plane sound localization

An experiment was conducted to determine the effect of aging on sound localization. Seven groups of 16 subjects, aged 10-81 years, were tested. Sound localization was assessed using six different arrays of four or eight loudspeakers that surrounded the subject in the horizontal plane, at a distance of 1 m. For two 4-speaker arrays, one loudspeaker was positioned in each spatial quadrant, on either side of the midline or the interaural axis, respectively. For four 8-speaker arrays, two loudspeakers were positioned in each quadrant, one close to the midline and the second separated from the first by 15 degrees, 30 degrees, 45 degrees, or 60 degrees. Three different 300-ms stimuli were localized: two one-third-octave noise bands, centered at 0.5 and 4 kHz, and broadband noise. The stimulus level (75 dB SPL) was well above hearing threshold for all subjects tested. Over the age range studied, percent-correct sound-source identification judgments decreased by 12%-15%. Performance decrements were apparent as early as the third decade of life. Broadband noise was easiest to localize (both binaural and spectral cues were available), and the 0.5-kHz noise band, the most difficult to localize (primarily interaural temporal difference cue available). Accuracy was relatively higher in front of than behind the head, and errors were largely front/back mirror image reversals. A left-sided superiority was evident until the fifth decade of life. The results support the conclusions that the processing of spectral information becomes progressively less efficient with aging, and is generally worse for sources on the right side of space.

The effect of frequency modulation of the ability to judge dynamic changes in interaural level differences

The cues for apparent auditory motion include dynamic interaural temporal differences, level changes, and frequency modulation (FM) [Rosenblum, Perception 16, 175–186 (1987)]. The manner in which these cues interact was examined in a task in which listeners judged the point at which a stimulus having dynamic changes in interaural level difference (ILD) passed through the intracranial midline. In base-line conditions, stimuli were 2.0–2.5-s, 0.5- or 2-kHz sinusoids presented through headphones. In experimental conditions, a frequency-modulated tone was added. Frequency was modulated by a linear ramp or according to a Doppler-based algorithm. The midpoint of the frequency-sweep occurred either at the instant that the ILD passed through 0 dB, acting as a task cue, or at some other point. FM rate and magnitude, temporal position of frequency-sweep midpoint, and direction of apparent movement were randomized across trials. Results indicated that the presence of FM had a significant effect on the perception of intracranial midline. This effect was obtained across FM parameter values. Results were considered in terms of how dynamic cues for perceived motion perceptually interact. [Work supported by NIH and AFOSR.]

Cross-spectral and temporal factors in the precedence effect: discrimination suppression of the lag sound in free-field.

In an anechoic chamber, subjects were required to discriminate a 20 degrees azimuthal change in a lag sound's position in the presence of a lead sound coming from a different direction. Delay between lead and lag sounds was adaptively varied in several conditions to track discrimination suppression thresholds. In experiment 1, lead and lag stimuli were 5-ms, 1-octave, A-weighted noise bursts (65 dB), with lead and lag parametrically set to center frequencies of 0.5, 2.0, or 3.0 kHz. Discrimination suppression thresholds were higher when lead and lag center frequencies coincided (mean: 11.3 ms) than when they did not coincide (mean: 2.9 ms). These results support the "spectral overlap hypothesis" of Blauert and Divenyi [Acustica 66, 267-274 (1988)], but not the "localization strength hypothesis" later proposed by Divenyi [J. Acoust. Soc. Am. 91, 1078-1084 (1992)]. Spectral overlap and localization strength appear to be two relatively independent factors governing discrimination suppression. It is proposed here that localization strength is weighted more when stimuli are presented via headphones and the only cue to lateral position is the interaural temporal difference, while spectral overlap is weighted more for free-field presented stimuli. In experiment 2, lead and lag stimuli were 8-ms, 1.5-kHz A-weighted tone bursts (65 dB), with lead and lag rise times parametrically set to 0, 2, or 4 ms. In this case the amount of discrimination suppression increased as lead rise time became more abrupt or as lag rise time became more gradual. These results support the localization strength hypothesis: The greater the localization strength of the lead stimulus (independently assessed by measuring its minimum audible angle in isolation), the greater suppression it exerted on discriminability of the lag sound's position. It appears that for stimuli presented in the free-field, spectral overlap is the primary factor affecting discrimination suppression, but when overlap is held constant, abruptness of stimulus onsets governs the amount of suppression.

The effect of a free-field auditory target’s horizontal motion on its detectability.

Previous work has shown that the motion of an auditory signal, when simulated over earphones by presenting a binaural stimulus with a changing interaural temporal difference, does not enhance its detectability relative to that of a stationary signal [Grantham and Luethke, J. Acoust. Soc. Am. 83, 1117–1123 (1988)]. The present study will compare the detectability of stationary and moving auditory targets presented in free-field, where all cues for location, including interaural level differences and spectral information, will be available. Six normal-hearing subjects will be tested individually in a darkened anechoic chamber. Masked threshold will be determined in an adaptive, single-interval 2AFC procedure for targets that are either stationary or moving at various velocities. Three different signals will be employed: a 500-Hz tone, an 8000-Hz tone, and a broadband noise. Preliminary results from one subject indicate that motion may enhance the detectability of broadband noise targets, but not of the pure-tone targets. [Work supported by NIDCD.]