Values Of Interaural Time Difference Research Articles

Aging effects on the binaural interaction component of the auditory brainstem response in the Mongolian gerbil: Effects of interaural time and level differences

The effect of interaural time difference (ITD) and interaural level difference (ILD) on wave 4 of the binaural and summed monaural auditory brainstem responses (ABRs) as well as on the DN1 component of the binaural interaction component (BIC) of the ABR in young and old Mongolian gerbils (Meriones unguiculatus) was investigated. Measurements were made at a fixed sound pressure level (SPL) and a fixed level above visually detected ABR threshold to compensate for individual hearing threshold differences. In both stimulation modes (fixed SPL and fixed level above visually detected ABR threshold) an effect of ITD on the latency and the amplitude of wave 4 as well as of the BIC was observed. With increasing absolute ITD values BIC latencies were increased and amplitudes were decreased. ILD had a much smaller effect on these measures. Old animals showed a reduced amplitude of the DN1 component. This difference was due to a smaller wave 4 in the summed monaural ABRs of old animals compared to young animals whereas wave 4 in the binaural-evoked ABR showed no age-related difference. In old animals the small amplitude of the DN1 component was correlated with small binaural-evoked wave 1 and wave 3 amplitudes. This suggests that the reduced peripheral input affects central binaural processing which is reflected in the BIC.

Interaural Time Difference Discrimination as a Function of Frequency and Time Differences

Interaural Time Difference (ITD) is the main sound source localization cues by the human auditory system in the horizontal plane, ITD discrimination was measured for Gaussian white noise of various frequency bands and ITD in order to explore the perceptual mechanism of ITD. Seven values of ITD are selected discretely and inhomogeneous in accordance with qualitative analysis of perceptual characteristics for ITD and the signal was divided into 12 sub-bands based on the division method of critical bands on low frequency. The experiment results demonstrated that: the thresholds of ITD are more significant with frequency, they are smaller at 500 Hz, a maximum value especially when it reaches about 1500 Hz; the thresholds increase as the reference values of ITD increase. ITD the JND value increases with the ITD has tended to increase, the JND value is small when the ITD value change is more severe, but the trend is more consistent, when ITD large changes gently. This work will provide basic dates for comprehensive exploring of perceptual characteristics of the human ear and theory support for audio efficient compression.

Lateralization of stimuli with alternating interaural time differences: The role of monaural envelope cues.

A temporally acute binaural system can help to resolve inherent fluctuations in binaural information that are often present in complex auditory scenes. Using a broadband noise stimulus that rapidly alternates between two different values of interaural time difference (ITD), the ability of the binaural system to hear the lateral position resulting from one of the ITD values was investigated. Results show that listeners are able to accurately lateralize brief noise tokens of only 3-7 ms in duration. In two subsequent experiments, the role of an amplitude modulation (AM) imposed on the ITD-switching stimulus used in the first experiment was tested. For wideband stimuli, the temporal position of the ITD target relative to the phase of the AM did not influence absolute lateralization or detection performance. When the stimuli were narrowband, however, detection of the ITD target was best when temporally positioned in the rising portion of the AM. These experiments illustrate that the auditory system is capable of making accurate lateral estimates of very brief moments of ITD information. Furthermore, for these instantaneous changes in ITD information, the stimulus bandwidth can influence the role of envelope cues for the readout of binaural information.

Characterizing perceptual properties of a binaurally modulated stimulus.

The majority of everyday listening situations involve a complex mixture of multiple sound sources. Assuming a spectro-temporally sparse target signal, e.g., speech, the binaural cues in these complex listening environments do not simply result from a single sound source, but are often rapidly switching between the most dominant sources at any given moment. To investigate the perception of rapidly switching interaural time difference (ITD) cues, a noise stimulus that periodically alternates between two different values of ITD was created. This stimulus appears to evoke a purely binaural percept of modulation, which is the focus of the studies presented here. Results indicate that listeners can reliably discriminate this ITD-switching stimulus from a stimulus composed of stationary ITD cues. Frequency discrimination of this ITD-switching stimulus was tested in a separate experiment, which showed that listeners are generally able to discriminate a 50% change in modulation frequency for reference rates below 16 Hz. The final study investigated if modulation masking exists between the monaural and binaural auditory pathways. Although a statistically significant increase in modulation detection thresholds was observed when both types of modulations were presented together, the increase was relatively small, indicating only a weak interaction.

Loudness of low-frequency pure tones lateralized by interaural time differences.

Directional loudness is that phenomenon by which the loudness of a sound may vary according to the localization of its source. This phenomenon has been mainly observed for high-frequency sounds, for sources located in the horizontal plane. Because of the acoustic shadow of the head, the left and right ear pressures are modified depending on the source azimuth and the global loudness resulting from a summation process may vary accordingly. Directional loudness has also been reported to occur at 400 Hz, where shadowing effects are usually rather small. It might therefore be suspected that directional loudness effects could be influenced by other parameters involved in the localization process. This study assessed the loudness of low-frequency pure tones (200 and 400 Hz) lateralized with headphones by applying an interaural time difference (ITD) but no interaural level difference. It showed small but significant variations of loudness with respect to ITD at a low loudness level (40 phon): ITD values associated with virtual azimuths of ±60° and ±90° led to a small but significant increase in loudness (up to 1.25 dB). However, there was no such effect at a moderate loudness level (70 phon).

Discrimination of auditory motion patterns: The mismatch negativity study

The aim of the present study is to test whether mismatch negativity (MMN) response can be elicited by changes in auditory motion dynamics. The discrimination of auditory motion patterns was investigated using psychophysical and electrophysiological methods in the same group of subjects. Auditory event-related potentials (ERP) were recorded for stationary midline noises and moving noises shifting to the left/right from the head midline. Two patterns of auditory motion were used with gradual (Motion) and stepwise (Step) movements which started and ended at the same loci. Auditory motion was produced by linear and abrupt changes of interaural time differences (ITD) in binaurally presented stimuli.In Experiment 1, ERPs were recorded for stationary midline standards and for Motion and Step deviants. It was found that Step deviants result in larger MMN amplitudes than Motion deviants with the same distance travelled, which implies that information contained in the stimulus midportion could be involved in the processing of the auditory motion. The threshold ITD values for the detection of Step and Motion stimuli displacement obtained during psychoacoustic tests were greater than the minimal ITD changes which elicited significant MMN. Experiment 2 demonstrated that Step deviants elicited significant MMNs in the context of Motion standards, although these stimuli could not be discriminated behaviourally. MMNs elicited by Step deviants in different acoustic contexts are discussed from the viewpoint of different brain processes underlying the discrimination of the abrupt ITD change.These results suggest that the early cortical mechanism of auditory motion processing reflected by MMN could not be considered as a spatial discriminator of the onset/offset stimulus positions, that is, a simple onset-offset detector. Combining psychoacoustic data with MMN results we may conclude that motion discrimination in auditory system might be better at the preattentive level.

A Method of Speech Segregation in Noisy Environment

On the basis of zero-crossings detection, a new method of speech segregation is proposed using these clues of Interaural time difference(ITD), zero-crossings peak amplitude(ZCPA) and zero-crossings power. The estimation of ITD is utilizing the statistical properties of zero-crossings detected from binaural filter bank outputs in order to get more reliable ITD estimation in noisy environment. Signal phase ambiguity in the high frequencies is solved by correcting the signals in the high frequency channels by means of the ITD values calculated by ZCPA algorithm in the low channels. Spline interpolation is used for sound segregation in order to make sound signal segregated correspond to the original sound signals as much as possible. The test for sound segregation in various mixture sources is made. The results show that the method can make can segregate various mixed sound sources effectively. The advantages of the proposed method are the robustness to noise, the less computational complexity and no need to train the masks for sound segregation.

Localization of sound in rooms. V. Binaural coherence and human sensitivity to interaural time differences in noise.

Binaural recordings of noise in rooms were used to determine the relationship between binaural coherence and the effectiveness of the interaural time difference (ITD) as a cue for human sound localization. Experiments showed a strong, monotonic relationship between the coherence and a listener's ability to discriminate values of ITD. The relationship was found to be independent of other, widely varying acoustical properties of the rooms. However, the relationship varied dramatically with noise band center frequency. The ability to discriminate small ITD changes was greatest for a mid-frequency band. To achieve sensitivity comparable to mid-band, the binaural coherence had to be much larger at high frequency, where waveform ITD cues are imperceptible, and also at low frequency, where the binaural coherence in a room is necessarily large. Rivalry experiments with opposing interaural level differences (ILDs) found that the trading ratio between ITD and ILD increasingly favored the ILD as coherence decreased, suggesting that the perceptual weight of the ITD is decreased by increased reflections in rooms.

Responses to Interaural Time Delay in Human Cortex

Humans use differences in the timing of sounds at the two ears to determine the location of a sound source. Various models have been posited for the neural representation of these interaural time differences (ITDs). These models make opposing predictions about the lateralization of ITD processing in the human brain. The weighted-image model predicts that sounds leading in time at one ear activate maximally the opposite brain hemisphere for all values of ITD. In contrast, the π-limit model assumes that ITDs beyond half the period of the stimulus center frequency are not explicitly encoded in the brain and that such “long” ITDs activate maximally the side of the brain to which the sound is heard. A previous neuroimaging study revealed activity in the human inferior colliculus consistent with the π-limit. Here we show that cortical responses to sounds with ITDs within the π-limit are in line with the predictions of both models. However, contrary to the immediate predictions of both models, neural activation is bilateral for “long” ITDs, despite these being perceived as clearly lateralized. Furthermore, processing of long ITDs leads to higher activation in cortex than processing of short ITDs. These data show that coding of ITD in cortex is fundamentally different from coding of ITD in the brain stem. We discuss these results in the context of the two models.

Training-induced improvements on interaural level difference (ILD) and interaural time difference (ITD) discrimination in human adults

The two primary cues to the location of sound sources on the horizontal plane are interaural level differences (ILDs) and interaural time differences (ITDs). The malleability of the processing of these two cues in humans was investigated by examining how multiday practice affects the discrimination of different values of ILDs and ongoing ITDs presented over headphones in adults with normal hearing. On average, the listeners improved on both ILD and ITD discrimination, but the learning patterns differed between the two cue types. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, suggesting procedural learning. However, a subsequent slower‐improvement stage occurred solely for the ILD cue and showed some specificity to the stimulus used during training. Interestingly, for ILD discrimination, both the best and worst daily threshold estimates decreased with multiday training, indicating an improvement in fundamental processing capacity. In contrast, for ITD discrimination, the best threshold estimates remained unchanged but the worst decreased, suggesting an increased ability to access already existing capacities. One interpretation of these data is that training mediates the processing of ILDs and ITDs at a stage in the auditory pathway, where these two cues are processed separately. [Work supported by NIH/NIDCD.]

Time–intensity trading in bilateral congenital aural atresia patients

In an effort to examine the rules by which information of bilaterally applied bone-conducted signals arising from interaural time differences (ITD) and interaural intensity differences (IID) is combined, data were measured for continuous 500 Hz narrow band noise at 65–70 dB HL in 11 patients with bilateral congenital aural atresia. Time–intensity trading functions were obtained by shifting the sound image towards one side using ITD, and shifting back to a centered sound image by varying the IID in the same ear (auditory midline task). ITD values were varied from −600 to +600 μs at 200 μs steps, where negative values indicate delays to the right ear. The results indicate that time–intensity trading is present in patients with bilateral aural atresia. The gross response properties of time–intensity trading in response to bone-conducted signals were comparable in patients with bilateral aural atresia and normal-hearing subjects, though there was a larger inter-subject variability and higher discrimination thresholds across IIDs in the atresia group. These results suggest that the mature auditory brainstem has a potential to employ binaural cues later in life, although to a restricted degree. A binaural fitting of a bone-conducted hearing aid might optimize binaural hearing and improve sound lateralization, and we recommend now systematically bilateral fitting in aural atresia patients.

Transformations in processing interaural time differences between the superior olivary complex and inferior colliculus: beyond the Jeffress model

Interaural time differences (ITDs) are used to localize sounds and improve signal detection in noise. Encoding ITDs in neurons depends on specialized mechanisms for comparing inputs from the two ears. Most studies have emphasized how the responses of ITD-sensitive neurons are consistent with the tenets of the Jeffress model. The Jeffress model uses neuronal coincidence detectors that compare inputs from both sides and delay lines so that different neurons achieve coincidence at different ITDs. Although Jeffress-type models are successful at predicting sensitivity to ITDs in humans, in many respects they are a limited representation of the responses seen in neurons. In the superior olivary complex (SOC), ITD-sensitive neurons are distributed across both the medial (MSO) and lateral (LSO) superior olives. Similar response types are found in neurons sensitive to ITDs in two signal types: low-frequency sounds and envelopes of high-frequency sounds. Excitatory–excitatory interactions in the MSO are associated with peak-type responses, and excitatory–inhibitory interactions in the LSO are associated with trough-type responses. There are also neurons with responses intermediate between peak- and trough-type. In the inferior colliculus (IC), the same basic types remain, presumably due to inputs arising from the MSO and LSO. Using recordings from the SOC and IC, we describe how the response types can be described within a continuum that extends to very large values of ITD, and compare the functional organization at the two levels.

Origin of the binaural interaction component in wave P4 of the short-latency auditory evoked potentials in the cat: evaluation of serial depth recordings from the brainstem

There is no general agreement on the origin of the binaural interaction (BI) component in auditory brainstem responses (ABRs). To study this issue the ABRs to monaural and binaural clicks with various interaural time differences (ITDs) were simultaneously recorded from the vertex and from a recording electrode aiming at the superior olive (SO) in cats. Electrode path was along the fibers of the lateral lemniscus (LL). Binaural difference potentials (BDPs), which were computed by subtracting the sum of the two monaural responses from the binaural response, were obtained at systematic depths and across a range of ITD values. It was observed that only a specific BDP deflection recorded at the level at which lemniscal fibers terminate in the nuclei of LL coincided in time with the most prominent BDP in the cat’s vertex-recorded ABRs, the BDP in their wave P4. As ITD was increased, the latency shifts and amplitude decrements of the scalp-recorded far-field BDP wave exactly followed those recorded at this lemniscal near-field BDP locus. The data support our hypothesis that the BI component in wave P4 results from a binaural reduction in dischargings of axons ascending in the LL, with this reduction due to contralateral inhibition of the discharge activity of the inhibitory–excitatory units in the lateral nucleus of the SO. Furthermore, at the level of the SO, the BDP in the responses to contra-leading binaural clicks always had larger magnitudes than those evoked by ipsi-leading ones. This bilateral asymmetry is consistent with the view that the BDP in scalp-recorded ABRs is related to the function of sound lateralization.

Different patterns of human discrimination learning for two interaural cues to sound-source location.

Two of the primary cues used to localize the sources of sounds are interaural level differences (ILDs) and interaural time differences (ITDs). We conducted two experiments to explore how practice affects the human discrimination of values of ILDs and ongoing ITDs presented over headphones. We measured discrimination thresholds of 13 to 32 naive listeners in a variety of conditions during a pretest and again, 2 weeks later, during a posttest. Between those two tests, we trained a subset of listeners 1 h per day for 9 days on a single ILD or ITD condition. Listeners improved on both ILD and ITD discrimination. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, indicating conceptual or procedural learning. A subsequent slower-improvement stage, which occurred solely for the ILD cue, only affected conditions with the trained stimulus frequency, suggesting that stimulus processing had fundamentally changed. These different learning patterns indicate that practice affects the attention to, or low-level encoding of, ILDs and ITDs at sites at which the two cue types are processed separately. Thus, these data reveal differences in the effect of practice on ILD and ITD discrimination, and provide insight into the encoding of these two cues to sound-source location in humans.

Adaptive adjustment of connectivity in the inferior colliculus revealed by focal pharmacological inactivation.

In the midbrain sound localization pathway of the barn owl, a map of auditory space is synthesized in the external nucleus of the inferior colliculus (ICX) and transmitted to the optic tectum. Early auditory experience shapes these maps of auditory space in part by modifying the tuning of the constituent neurons for interaural time difference (ITD), a primary cue for sound-source azimuth. Here we show that these adaptive modifications in ITD tuning correspond to changes in the pattern of connectivity within the inferior colliculus. We raised owls with an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. As reported previously, device rearing shifted the representation of ITD in the ICX and tectum but not in the primary source of input to the ICX, the central nucleus of the inferior colliculus (ICC). We applied the local anesthetic lidocaine (QX-314) iontophoretically in the ICC to inactivate small populations of neurons that represented particular values of frequency and ITD. We measured the effect of this inactivation in the optic tecta of a normal owl and owls raised with the device. In the normal owl, inactivation at a critical site in the ICC eliminated responses in the tectum to the frequency-specific ITD value represented at the site of inactivation in the ICC. The location of this site was consistent with the known pattern of ICC-ICX-tectum connectivity. In the device-reared owls, adaptive changes in the representation of ITD in the tectum corresponded to dramatic and predictable changes in the locations of the critical sites of inactivation in the ICC. Given that the abnormal representation of ITD in the tectum depended on frequency and was likely conveyed directly from the ICX, these results suggest that experience causes large-scale, frequency-specific adjustments in the pattern of connectivity between the ICC and the ICX.

Abnormal Auditory Experience Induces Frequency-Specific Adjustments in Unit Tuning for Binaural Localization Cues in the Optic Tectum of Juvenile Owls

Early auditory experience shapes the auditory spatial tuning of neurons in the barn owl's optic tectum in a frequency-dependent manner. We examined the basis for this adaptive plasticity in terms of changes in tuning for frequency-specific interaural time differences (ITDs) and level differences (ILDs), the dominant sound localization cues. We characterized broadband and narrowband ITD and ILD tuning in normal owls and in owls raised with an acoustic filtering device in one ear that caused frequency-dependent changes in sound timing and level. In normal owls, units were tuned to frequency-specific ITD and ILD values that matched those produced by sound sources located in their visual receptive fields. In contrast, in device-reared owls, ITD tuning at most sites was shifted from normal by approximately 55 microsec toward open-ear leading for 4 kHz stimuli and 15 microsec toward the opposite-ear leading for 8 kHz stimuli, reflecting the acoustic effects of the device. ILD tuning was shifted in the adaptive direction by approximately 3 dB for 4 kHz stimuli and 8 dB for 8 kHz stimuli, but these shifts were substantially smaller than expected based on the acoustic effects of the device. Most sites also exhibited conspicuously abnormal frequency-response functions, including a strong dependence on stimulus ITD and a reduction of normally robust responses to 6 kHz stimuli. The results demonstrate that the response properties of high-order auditory neurons in the optic tectum are adjusted during development to reflect the influence of frequency-specific features of the binaural localization cues experienced by the individual.

Early Visual Experience Shapes the Representation of Auditory Space in the Forebrain Gaze Fields of the Barn Owl

Auditory spatial information is processed in parallel forebrain and midbrain pathways. Sensory experience early in life has been shown to exert a powerful influence on the representation of auditory space in the midbrain space-processing pathway. The goal of this study was to determine whether early experience also shapes the representation of auditory space in the forebrain. Owls were raised wearing prismatic spectacles that shifted the visual field in the horizontal plane. This manipulation altered the relationship between interaural time differences (ITDs), the principal cue used for azimuthal localization, and locations of auditory stimuli in the visual field. Extracellular recordings were used to characterize ITD tuning in the auditory archistriatum (AAr), a subdivision of the forebrain gaze fields, in normal and prism-reared owls. Prism rearing altered the representation of ITD in the AAr. In prism-reared owls, unit tuning for ITD was shifted in the adaptive direction, according to the direction of the optical displacement imposed by the spectacles. Changes in ITD tuning involved the acquisition of unit responses to adaptive ITD values and, to a lesser extent, the elimination of responses to nonadaptive (previously normal) ITD values. Shifts in ITD tuning in the AAr were similar to shifts in ITD tuning observed in the optic tectum of the same owls. This experience-based adjustment of binaural tuning in the AAr helps to maintain mutual registry between the forebrain and midbrain representations of auditory space and may help to ensure consistent behavioral responses to auditory stimuli.

Binaural cross-correlation and auditory localization in the barn owl: a theoretical study

The barn owl is a nocturnal predator that is able to capture mice in complete darkness using only sound to localize prey. Two binaural cues are used by the barn owl to determine the spatial position of a sound source: differences in the time of arrival of sounds at the two ears for the azimuth (interaural time differences (ITDs)) and differences in their amplitude for the elevation (interaural level differences (ILDs)). Neurophysiological investigations have revealed that two different neural pathways starting from the cochlea seem to be specialized for processing ITDs and ILDs. Much evidence suggests that in the barn owl the localization of the azimuth is based on a cross-correlation-like treatment of the auditory inputs at the two ears. In particular, in the external nucleus of the inferior colliculus (ICx), where cells are activated by specific values of ITD, neural activation has been recently observed to be dependent on some measure of the level of cross-correlation between the input auditory signals. However, it has also been observed that these neurons are less sensitive to noise than predicted by direct binaural cross-correlation. The mechanisms underlying such signal-to-noise improvement are not known. In this paper, by focusing on a model of the barn owl's neural pathway to the optic tectum dedicated to the localization of the azimuth, we study the mechanisms by which the ITD tuning of ICx units is achieved. By means of analytical examinations and computer simulations, we show that strong analogies exist between the process by which the barn owl evaluates the azimuth of a sound source and the generalized cross-correlation algorithm, one of the most robust methods for the estimate of time delays.

Pharmacological Specialization of Learned Auditory Responses in the Inferior Colliculus of the Barn Owl

Neural tuning for interaural time difference (ITD) in the optic tectum of the owl is calibrated by experience-dependent plasticity occurring in the external nucleus of the inferior colliculus (ICX). When juvenile owls are subjected to a sustained lateral displacement of the visual field by wearing prismatic spectacles, the ITD tuning of ICX neurons becomes systematically altered; ICX neurons acquire novel auditory responses, termed "learned responses," to ITD values outside their normal, pre-existing tuning range. In this study, we compared the glutamatergic pharmacology of learned responses with that of normal responses expressed by the same ICX neurons. Measurements were made in the ICX using iontophoretic application of glutamate receptor antagonists. We found that in early stages of ITD tuning adjustment, soon after learned responses had been induced by experience-dependent processes, the NMDA receptor antagonist D, L-2-amino-5-phosphonopentanoic acid (AP-5) preferentially blocked the expression of learned responses of many ICX neurons compared with that of normal responses of the same neurons. In contrast, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked learned and normal responses equally. After long periods of prism experience, preferential blockade of learned responses by AP-5 was no longer observed. These results indicate that NMDA receptors play a preferential role in the expression of learned responses soon after these responses have been induced by experience-dependent processes, whereas later in development or with additional prism experience (we cannot distinguish which), the differential NMDA receptor-mediated component of these responses disappears. This pharmacological progression resembles the changes that occur during maturation of glutamatergic synaptic currents during early development.

Representation of Binaural Spatial Cues in Field L of the Barn Owl Forebrain

This study examined the representation of spatial information in the barn owl Field L, the first telencephalic processing stage of the classical auditory pathway. Field L units were recorded extracellularly, and their responses to dichotically presented interaural time differences (ITD) and interaural level differences (ILD) were tested. We observed a variety of tuning profiles in Field L. Some sites were not sensitive to ITD or ILD. Other sites, especially those in the high-frequency region, were highly selective for values of ITD and ILD. These sites had multipeaked (commonly called "phase ambiguous") ITD tuning profiles and were tuned for a single value of ILD. The tuning properties of these sites are similar to those seen in the lateral shell of the central nucleus of the inferior colliculus. Although the tuning properties of Field L sites were similar to those observed in the inferior colliculus, the functional organization of this spatial information was fundamentally different. Whereas in the inferior colliculus spatial information is organized into global topographics maps, in Field L spatial information is organized into local clusters, with sites having similar binaural tuning properties grouped together. The representation of binaural cues in Field L suggests that it is involved in auditory space processing but at a lower level of information processing than the auditory archistriatum, a forebrain area that is specialized for processing spatial information, and that the levels of information processing in the forebrain space processing pathway are remarkably similar to those in the well-known midbrain space processing pathway.