Detection of interaural correlation by neurons in the superior olivary complex, inferior colliculus and auditory cortex of the unanesthetized rabbit

A critical binaural cue important for sound localization and detection of signals in noise is the interaural time difference (ITD), or difference in the time of arrival of sounds at each ear. The ITD can be determined by cross-correlating the sounds at the two ears and finding the ITD where the correlation is maximal. The amount of interaural correlation is affected by properties of spaces and can therefore be used to assess spatial attributes. To examine the neural basis for sensitivity to the overall level of the interaural correlation, we identified subcollicular neurons and neurons in the inferior colliculus (IC) and auditory cortex of unanesthetized rabbits that were sensitive to ITDs and examined their responses as the interaural correlation was varied. Neurons at each brain level could show linear or non-linear responses to changes in interaural correlation. The direction of the non-linearities in most neurons was to increase the slope of the response change for correlations near 1.0. The proportion of neurons with non-linear responses was similar in subcollicular and IC neurons but increased in the auditory cortex. Non-linear response functions to interaural correlation were not related to the type of response as determined by the tuning to ITDs across frequencies. The responses to interaural correlation were also not related to the frequency tuning of the neuron, unlike the responses to ITD, which broadens for neurons tuned to lower frequencies. The neural discriminibility of the ITD using frozen noise in the best neurons was similar to the behavioral acuity in humans at a reference correlation of 1.0. However, for other reference ITDs the neural discriminibility was more linear and generally better than the human discriminibility of the interaural correlation, suggesting that stimulus rather than neural variability is the basis for the decline in human performance at lower levels of interaural correlation.     

Coding of interaural time differences of transients in auditory cortex of Rattus norvegicus: implications for the evolution of mammalian sound localization.

We obtained quantitative evidence on the coding of interaural time differences (ITDs) of click stimuli by 40 single neurons in the auditory cortex of anesthetized albino rats. Most of the neurons (31/40) received an excitatory input from the contralateral ear, and an inhibitory input from the ipsilateral ear (EI cells). These neurons expressed their sensitivity to ITDs in a sigmoidal relation between spike count and ITD, with maximal responses associated with contralateral-leading ITDs. The mean ITD dynamic range was 590 microseconds. The dynamic ranges typically encompassed at least part of the behaviorally-relevant range (about +/- 130 microseconds). Variations in ITD from 130 microseconds favoring one ear to 130 microseconds favoring the other ear caused spike response rate changes, on average, of 29.5%. These data are similar to those previously presented for the central auditory systems of larger mammals, whose auditory localization acuity is significantly better than that of the rat. We argue, therefore, that the sound localization mechanisms based on transient ITDs have not evolved in a fashion that covaries with interaural distance, and that there exists a mismatch between the ITDs the rat will encounter in the free field, and the ITDs which are encoded by its nervous system. This may be one reason why sound localization acuity has a roughly inverse relation to interaural distance.     

Influence of thresholds on amplitudes in vestibular evoked myogenic potentials

Objectives

To investigate the relationship between the threshold and the interaural amplitude difference ratio (IADR) in cervical vestibular evoked myogenic potential (cVEMP) testing and pursuit the clinical significance of the parameters.

Materials and methods

cVEMP responses were recorded while the SCM contraction was controlled using a pressure cuff. The intensities of the sound stimulation decreased from 95 dB nHL by 5 dB, until no responses were evoked. Thresholds, interaural threshold difference (ITD), amplitudes, and interaural amplitude difference ratio at the stimulation of 95 dB nHL were calculated and the relationship between them was examined.

Results

All subjects showed cVEMP responses bilaterally. Thresholds measured were overall 76 dB nHL and most (92%) ears showed the ITD of 0 or 5 dB. The amplitudes of cVEMP responses showed a positive correlation with the sound intensities, and more specifically with the sound intensity above each threshold value. There was no significant difference in IADR values by the ITD.

Conclusions

Based on our study, the ITD is less than 10 dB in most normal subjects and estimation of threshold should be added to cVEMP testing for probing vestibular asymmetry. Getting a threshold might be helpful in determining whether the abnormal interaural amplitude difference ratio is related to the abnormal ITD.     

Changes in interaural time sensitivity with interaural level differences in the inferior colliculus

We measured interaural time difference (ITD) sensitivity of 72 cells in the inferior colliculus of the anaesthetised guinea pig as a function of frequency and interaural level difference (ILD). For many units there was a "null" frequency, where varying the ILD made no difference to the position of the peak of the ITD sensitivity. This null frequency was not necessarily at the characteristic frequency (CF): it occurred at CF in less than a third of the neurons for which we had sufficient data (14/50). Equally often, the null occurred below (15/50) and less often, above CF (8/50). The remaining (13/50) neurons showed clear phase changes, but these were erratic or parallel and no null could be attributed. In 33 of the 37 neurons with an identifiable null frequency, the peak ITD moved towards the recording side with increasing ILD, for frequencies above the null, and away for frequencies below the null. The changes in ITD sensitivity expressed as phase were maximally about 0.2-0.3 cycles. Many of the changes in response phase with ILD are in the same direction and magnitude as changes in the phase locking with sound level in auditory nerve fibres. Thus, these changes in phase sensitivity at the basilar membrane and auditory nerve are maintained through to ITD tuning in the IC. This is consistent with a coincidence detection mechanism. However, some of the more complex phenomena which we observe are consistent with convergence at the IC.     

Sensitivity of the auditory middle latency response of the guinea pig to interaural level and time differences

This study examines the extent to which the auditory middle latency response (MLR) of the guinea pig is sensitive to sound localization cues such as interaural level and time differences (ILD and ITD, respectively). The MLR was recorded with an epidural electrode placed over the auditory cortex of an anesthetized guinea pig. Click stimuli were presented monaurally or binaurally with various ILDs and ITDs. The MLR was much larger for contralateral stimulation than for ipsilateral stimulation, and its amplitude was intermediate for diotic stimulation. The MLR amplitude was sensitive to both ILD and ITD: it decreased as the ipsilateral stimulus increased in level or advanced in time relative to the contralateral stimulus. The steep slope of the amplitude-versus-ITD function fell within an ITD range of +/-330 micros, namely the guinea pig's physiological ITD range. The response reduction that resulted from increasing the relative level of the ipsilateral level could be cancelled out by advancing the contralateral onset time relative to the ipsilateral onset time. This parallels the "time-intensity trading" in sound lateralization exhibited in human psychophysics. The results imply that the binaural interaction in the guinea pig MLR reflects aspects of neural processes that are involved in sound localization.     

Neuronal sensitivity to interaural time differences in the sound envelope in the auditory cortex of the pallid bat

Interaural time differences in the envelope of a sound (envelope ITDs) can potentially provide spatial information at high frequencies where interaural phase differences (IPDs) are not available. Interaural intensity differences (IIDs) also provide important spatial information at high frequencies. Both IIDs and envelope ITDs can influence spatial perception at high frequencies, but behavioral and physiological studies suggest that IIDs dominate perception. This study examines envelope ITD sensitivity in the auditory cortex of the pallid bat, a species that uses passive sound localization at the low end of its audible range to find prey. Its auditory system is entirely 'high-frequency' in that phase-locking does not occur at the low end of its audible range. If the bat uses ITDs, they must be derived from the envelope of the signal. A previous study of envelope ITD sensitivity in its inferior colliculus (IC) reported that neurons are sensitive to the small +/-70 micros range of available ITDs. This study extends these findings to the cortical level to assess the transformation of ITD sensitivity and the binaural response properties that underlie this sensitivity. Two measures of sensitivity were used. The dynamic ITD range measures the range of ITDs over which the maximum response of a neuron decreases by 80%. When presented with square-wave amplitude-modulated tones statically delayed in arrival time, the average dynamic ITD range in the IC is 304 micros, but dropped to 175 micros in auditory cortex. IC neurons average a 38% change in maximum response over the relevant ITD range, while cortical neurons average a 67% change. Also measured were time-intensity trading ratios, which index the extent to which a change in IID can cause a shift the dynamic ITD range. Average trading ratios are approximately the same in the IC and auditory cortex (17.9 micros/dB vs. 16.7 micros/dB, respectively). Binaural interactions changed from the IC to auditory cortex. In IC, ITD sensitivity is an inhibitory, subtractive process in which ITDs reduce the response evoked by contralateral monaural stimulation. In the auditory cortex, both binaural inhibition and facilitation occur. In the majority of cortical neurons, IID and ITD functions were remarkably similar in shape, having stepped, step-peaked or peaked functions. The binaural interactions (inhibition and/or facilitation) evoked by ITDs and IIDs were also typically similar. These results suggest that IIDs and envelope ITDs are having similar effects on output of the same binaural comparator system.     

Influence of interaural time- and level differences on the binaural interaction components in normal adults

BACKGROUND: Binaural interaction components of auditory brainstem responses have long been studied and its beta-wave has been shown to be correlated with directional hearing ability. As patients with auditory processing disorders such as patients with some sorts of learning disabilities frequently have difficulties with binaural processing it seems sensible to use binaural interaction components in the diagnosis of these disorders. METHODS: In order to obtain normal values and to investigate the influence of interaural time (ITD) and level differences (ILD) binaural interaction components were measured in 21 adults. Interaural time differences varied between 0 and 1.2 ms and interaural level differences between 0 and 30 dB. RESULTS: beta-latencies increased significantly as interaural time differences increased while beta-amplitudes did not change significantly. In measurements with higher interaural time differences detection of the beta-wave was only rarely possible. The effect of interaural level differences on beta-latencies was less pronounced and not significant as well as the influence of interaural level differences on beta-amplitudes. CONCLUSIONS: In the present study it could be shown that the beta-wave of the binaural interaction waveform is present in almost every normal hearing subject. As amplitudes show a considerable variation beta-latencies seem to be of higher diagnostic value than amplitudes.     

Bone-conducted sound lateralization of interaural time difference and interaural intensity difference in children and a young adult with bilateral microtia and atresia of the ears

Bone-conducted sound lateralization tests to determine interaural time difference (ITD) and interaural intensity difference (IID) were conducted by means of a self-recording apparatus in 20 children and a young adult with bilateral microtia and atresia of the ear. This apparatus changes ITD automatically from 0 to 2,000 micros at 50 micros/s and IID from 1 to 40 dB at 1 dB's. When ITD exceeds approximately 200 micros/s and IID exceeds 5 dB in normal subjects the sounds are recognized separately. The test stimulus was a continuous narrow-band noise at 500 Hz and 30 dB SL applied to the right and left mastoids through bone vibrators. In the patients with bilateral atresia of the ears, ITD results revealed approximately normal thresholds of discrimination in half the patients and IID results revealed threshold elevation in only 10%. It is noted that bone-conducted sound lateralization abilities of ITD or IID are maintained in many of these patients.     

Identification of binaural integration deficits in children with the Competing Words Subtest: Standard score versus interaural asymmetry

The Competing Words Subtest is a commonly used dichotic listening test for assessing binaural integration in children suspected of having an auditory processing disorder. In 124 children, standard scores from the subtest suggested a binaural integration deficit in 23% of the children tested. Because standard scores are derived from the combined scores of both ears during the test, children with normal performance in one ear and weak performance in the other ear may be overlooked. For these children, a measure of interaural asymmetry may be a more sensitive indicator of a binaural integration deficit. When an age-appropriate criterion for interaural asymmetry from the Competing Words Subtest was used, the incidence of a binaural integration deficit increased to 51% of the children tested. Four typical patterns of dichotic listening performance were identified based on results from the two scoring techniques.     

Sensitivity for interaural time and intensity difference of auditory midbrain neurons in the grassfrog

The sensitivity for interaural time (ITD) and intensity (IID) difference was investigated for single units in the auditory midbrain of the grassfrog. A temporally structured stimulus was used which was presented by means of a closed sound system. At best frequency (BF) the majority of units was selective for ITD as indicated by an asymmetrically (73%) or symmetrically (7%) shaped ITD-rate histogram. About 20% appeared to be nonselective. Units with a symmetrical rate histogram had BFs well above 0.9 kHz, whereas for the other categories no relationship with BF was observed. Most units had a selectivity for ITD which was rather independent from frequency and absolute intensity level. In 62% of the units interaural time difference could be traded by interaural intensity difference. In most cases this so-called time-intensity trading could be explained by the intensity-latency characteristics of auditory nerve fibres. About 20% was sensitive to IID only and 5% to ITD only. A binaural model is proposed which is based on the intensity-rate and intensity-latency characteristics of auditory nerve fibres, the linear summation of excitatory and inhibitory post synaptic potentials in second order neurons, and spatiotemporal integration at the level of third order neurons. By variation of only a small number of parameters, namely strengths and time constants of the connectivities, the range of experimentally observed response patterns could be reproduced.     

Behavioral sensitivity to interaural time differences in the rabbit

An important cue for sound localization and separation of signals from noise is the interaural time difference (ITD). Humans are able to localize sounds within 1-2 degrees and can detect very small changes in the ITD (10-20micros). In contrast, many animals localize sounds with less precision than humans. Rabbits, for example, have sound localization thresholds of approximately 22 degrees . There is only limited information about behavioral ITD discrimination in animals with poor sound localization acuity that are typically used for the neural recordings. For this study, we measured behavioral discrimination of ITDs in the rabbit for a range of reference ITDs from 0 to +/-300micros. The behavioral task was conditioned avoidance and the stimulus was band-limited noise (500-1500Hz). Across animals, the average discrimination threshold was 50-60micros for reference ITDs of 0 to +/-200micros. There was no trend in the thresholds across this range of reference ITDs. For a reference ITD of +/-300micros, which is near the limit of the physiological window defined by the head width in this species, the discrimination threshold increased to approximately 100micros. The ITD discrimination in rabbits less acute than in cats, which have a similar head size. This result supports the suggestion that ITD discrimination, like sound localization [see Heffner, 1997. Acta Otolaryngol. 532 (Suppl.), 46-53] is determined by factors other than head size.     

Are Interaural Time and Level Differences Represented by Independent or Integrated Codes in the Human Auditory Cortex?

Sound localization is important for orienting and focusing attention and for segregating sounds from different sources in the environment. In humans, horizontal sound localization mainly relies on interaural differences in sound arrival time and sound level. Despite their perceptual importance, the neural processing of interaural time and level differences (ITDs and ILDs) remains poorly understood. Animal studies suggest that, in the brainstem, ITDs and ILDs are processed independently by different specialized circuits. The aim of the current study was to investigate whether, at higher processing levels, they remain independent or are integrated into a common code of sound laterality. For that, we measured late auditory cortical potentials in response to changes in sound lateralization elicited by perceptually matched changes in ITD and/or ILD. The responses to the ITD and ILD changes exhibited significant morphological differences. At the same time, however, they originated from overlapping areas of the cortex and showed clear evidence for functional coupling. These results suggest that the auditory cortex contains an integrated code of sound laterality, but also retains independent information about ITD and ILD cues. This cue-related information might be used to assess how consistent the cues are, and thus, how likely they would have arisen from the same source.     

Reweighting of Binaural Localization Cues in Bilateral Cochlear-Implant Listeners

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

The influence of externalization and spatial cues on the generation of auditory brainstem responses and middle latency responses

The effect of externalization and spatial cues on the generation of auditory brainstem responses (ABRs) and middle latency responses (MLRs) was investigated in this study. Most previous evoked potential studies used click stimuli with variations of interaural time (ITDs) and interaural level differences (ILDs) which merely led to a lateralization of sound inside the subject's head. In contrast, in the present study potentials were elicited by a virtual acoustics stimulus paradigm with 'natural' spatial cues and compared to responses to a diotic, non-externalized reference stimulus. Spatial sound directions were situated on the horizontal plane (corresponding to variations in ITD, ILD, and spectral cues) or the midsagittal plane (variation of spectral cues only). An optimized chirp was used which had proven to be advantageous over the click since it compensates for basilar membrane dispersion. ABRs and MLRs were recorded from 32 scalp electrodes and both binaural potentials (B) and binaural difference potentials (BD, i.e., the difference between binaural and summed monaural responses) were investigated. The amplitudes of B and BD to spatial stimuli were not higher than those to the diotic reference. ABR amplitudes decreased and latencies increased with increasing laterality of the sound source. A rotating dipole source exhibited characteristic patterns in dependence on the stimulus laterality. For the MLR data, stimulus laterality was reflected in the latency of component N(a). In addition, dipole source analysis revealed a systematic magnitude increase for the dipole contralateral to the azimuthal position of the sound source. For the variation of elevation, the right dipole source showed a stronger activation for stimuli away from the horizontal plane. The results indicate that at the level of the brainstem and primary auditory cortex binaural interaction is mostly affected by interaural cues (ITD, ILD). Potentials evoked by stimuli with natural combinations of ITD, ILD, and spectral cues were not larger than those elicited by diotic chirps.     

Contributions of Intrinsic Neural and Stimulus Variance to Binaural Sensitivity

The discrimination of a change in a stimulus is determined both by the magnitude of that change and by the variability in the neural response to the stimulus. When the stimulus is itself noisy, then the relative contributions of the neural (intrinsic) and stimulus induced variability becomes a critical question. We measured the contribution of intrinsic neural noise and interstimulus variability to the discrimination of interaural time differences (ITDs) and interaural correlation (IAC). We measured discharge rate versus characteristic frequency (CF) tone ITD functions, and CF-centered narrowband noise ITD and IAC functions in interleaved blocks in the same units in the inferior colliculus of urethane-anesthetized guinea pigs. Ten “frozen” tokens of noise were synthesized and the responses to each token were separately analyzed to allow the relative contributions of intrinsic and stimulus variability to be assessed. ITD and IAC discrimination thresholds were determined for a simulated two-interval forced-choice experiment, based on the firing rate distributions, using receiver operating characteristic analysis. On average, between stimulus variability contributed 19% (range, 1.5–30%) of the variance in noise ITD discrimination and 27% (range, 3–50%) in IAC discrimination. Noise ITD thresholds were slightly higher than tone ITD thresholds. Taking the mean of the thresholds for individual noise tokens gave a similar result to pooling across all noise tokens. This implies that although the stimulus induced variability is measurable, it is insignificant in relation to the intrinsic noise in ITD and IAC discrimination.     

Acoustic cues underlying auditory distance in barn owls

CONCLUSIONS: We conclude that: (1) among several cues examined, the monaural cue of direct-to-reverberant (D/R) ratio in the ipsilateral ear provides the most information about sound-source distance; (2) interaural level difference (ILD) provides less information about sound-source distance; and (3) a comprehensive theory of three-dimensional auditory localization must incorporate the fact that all of the major acoustic cues change with distance. OBJECTIVE: Neural mechanisms underlying auditory localization of distance are poorly understood. The present study was an initial step toward filling this gap in knowledge. MATERIALS AND METHODS: The binaural room impulse responses of adult barn owls were measured. The sound source was placed at various distances (up to 80 cm) and azimuths (0-90 degrees) relative to the owl's head, with the elevation kept at 0 degrees . RESULTS: We determined the value of each cue for a 3-10 kHz band, and found that: (1) D/R ratio of signal amplitudes provided the most information about sound-source distance; (2) the ipsilateral D/R ratio represented distance more clearly than the contralateral or binaural-average D/R ratios; (3) ILD of direct signals increased with decreasing distance under certain conditions; (3) interaural time difference (ITD) of direct signals increased with decreasing distance at 90 degrees azimuth; and (4) the spectral patterns of ILD and the monaural direct signals changed with distance in complex ways.     

Effects of interaural intensity and time disparity on transient evoked otoacoustic emissions

Monaural and binaural 11/s, 65 dB pe SPL clicks with interaural time and intensity disparities known to affect central auditory processing were used to study contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) in 10 subjects (20 ears). Psychophysical assessment of sound lateralization induced by the same stimuli was also conducted. TEOAEs were recorded to monaural (ipsilateral to the OAE recording probe) and to binaural clicks when clicks to the contralateral ear were synchronous and symmetrical in intensity, or, in the binaural intensity disparity conditions, synchronous but 10 dB higher or 10 dB lower in the ear contralateral to the OAE recording probe. When interaural time disparities were studied, the clicks to the contralateral ear were of the same intensity throughout, but 400 μs earlier or 400 μs later than to the ear with the probe. The TEOAE components at 13–15.8 ms showed suppression, relative to monaural responses, under all binaural conditions. This contralateral suppression did not correlate with the psychophysical findings. Suppression effects were more pronounced with binaural disparity than with binaurally symmetrical clicks. Thus, although contralateral click intensity was the same with time disparities, suppression was paradoxically enhanced compared to the binaurally symmetrical stimulation. To explain these results we propose that two factors are involved in TEOAE suppression with binaural clicks: (1) contralateral intensity and (2) interaural disparity (time or intensity). The latency of the suppressions observed, the effect of interaural disparity on these suppressions, coupled with the anatomical origin of the crossed efferent fibers and the disparity sensitivity of the superior olivary complex (SOC), all suggest SOC involvement in these TEOAE suppressions.     

Selectivity for temporal characteristics of sound and interaural time difference of auditory midbrain neurons in the grassfrog: A system theoretical approach

The selectivity for temporal characteristics of sound and interaural time difference (ITD) was investigated in the torus semicircularis (TS) of the grassfrog. Stimuli were delivered by means of a closed sound system and consisted of binaurally presented Poisson distributed condensation clicks, and pseudo-random (RAN) or equidistant (EQU) click trains of which ITD was varied. With RAN and EQU trains, 86% of the TS units demonstrated a clear selectivity for ITD. Most commonly, these units had monotonically increasing ITD-rate functions. In general, units responding to Poisson clicks, responded also to RAN and EQU trains. One category of units which showed strong time-locking had comparable selectivities for ITD with both stimulus ensembles. A second category of units showed a combined selectivity for temporal structure and ITD. These units responded exclusively to EQU trains in a nonsynchronized way. From the responses obtained with the Poisson click ensemble so-called Poisson system kernels were determined, in analogy to the Wiener-Volterra functional expansion for nonlinear systems. The kernel analysis was performed up to second order. Contralateral (CL) first order kernels usually had positive or combinations of positive and negative regions, indicating that the contralateral ear exerted an excitatory or combined excitatory-inhibitory influence upon the neural response. Ipsilateral (IL), units were characterized by first order kernels which were not significantly different from zero, or kernels in which a single negative region was present. A large variety of CL second order kernels has been observed whereas rarely IL second order kernels were encountered. About 35% of the units possessed nonzero second order cross kernels, which indicates that CL and IL neural processes are interacting in a nonlinear way. Units demonstrating a pronounced selectivity for ITD, were generally characterized by positive CL combined with negative IL first order kernels. Findings suggested that, in the grassfrog, neural selectivity for ITD mainly is established by linear interaction of excitatory and inhibitory processes originating from the CL and IL ear, respectively. Units exhibiting strong time-locking to Poisson clicks and RAN and EQU trains had significantly shorter response latencies than moderately time-locking units. In the first category of units, a substantial higher number of nonzero first and second order kernels was observed. It was concluded that nonlinearr response properties, as observed in TS units, most likely have to be ascribed to nonlinear characteristics of neural components located in the auditory nervous system peripheral to the torus semicircularis.     

The effects of age and interaural delay on detecting a change in interaural correlation: The role of temporal jitter

Duration thresholds for detecting a change in interaural correlation (from 0 to 1, or from 1 to 0) in the initial portion of a 1-second, broadband noise (0-10 kHz) were determined for younger and older adults in a two-interval, two-alternative forced choice paradigm as a function of the interaural delay between the noise bursts presented to each ear. When the interaural delay was 0 ms, older adults found it harder to detect a change in correlation from 0 to 1 than from 1 to 0. For younger adults, however, this pattern was reversed. For interaural delays greater than 0 ms, both younger adults and older adults found it easier to detect a change in interaural correlation from 0 to 1 for short interaural delays (1 ms) with the reverse being true for longer interaural delays (5 ms). It is shown that this pattern of results is expected if temporal jitter (loss of neural synchrony in the auditory system) increases with age and with interaural delay. The implications of these results for age-related changes in stream segregation are discussed.     

Evoked Potential Correlates of Interaural Phase Reversals

Human auditory evoked potentials were recorded from the vertex in a variety of interaural stimulus conditions over a broad range of intensities. The amplitude of the N1-P2 component of the evoked potential was greater for antiphasic than for homophasic stimuli, reflecting the loudness differences reported by others at low signal-to-noise ratios. At higher signal-to-noise ratios or in the absence of an external noise masker where loudness differences disappear, however, a difference between N1-P2 amplitudes evoked by homophasic and antiphasic stimuli persists