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.     

The effects of experimentally induced conductive hearing loss on spectral and temporal aspects of sound transmission through the ear

Conductive hearing loss (CHL) is known to produce hearing deficits, including deficits in sound localization ability. The differences in sound intensities and timing experienced between the two tympanic membranes are important cues to sound localization (ILD and ITD, respectively). Although much is known about the effect of CHL on hearing levels, little investigation has been conducted into the actual impact of CHL on sound location cues. This study investigated effects of CHL induced by earplugs on cochlear microphonic (CM) amplitude and timing and their corresponding effect on the ILD and ITD location cues. Acoustic and CM measurements were made in 5 chinchillas before and after earplug insertion, and again after earplug removal using pure tones (500?Hz to 24?kHz). ILDs in the unoccluded condition demonstrated position and frequency dependence where peak far-lateral ILDs approached 30?dB for high frequencies. Unoccluded ear ITD cues demonstrated positional and frequency dependence with increased ITD cue for both decreasing frequency (±420?μs at 500?Hz, ±310?μs for 1-4?kHz) and increasingly lateral sound source locations. Occlusion of the ear canal with foam plugs resulted in a mild, frequency-dependent conductive hearing loss of 10-38?dB (mean 31?±?3.9?dB) leading to a concomitant frequency dependent increase in ILDs at all source locations. The effective ITDs increased in a frequency dependent manner with ear occlusion as a direct result of the acoustic properties of the plugging material, the latter confirmed via acoustical measurements using a model ear canal with varying volumes of acoustic foam. Upon ear plugging with acoustic foam, a mild CHL is induced. Furthermore, the CHL induced by acoustic foam results in substantial changes in the magnitudes of both the ITD and ILD cues to sound location.     

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.     

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.     

Sensitivity to Interaural Time Differences Conveyed in the Stimulus Envelope: Estimating Inputs of Binaural Neurons Through the Temporal Analysis of Spike Trains

Sound-source localization in the horizontal plane relies on detecting small differences in the timing and level of the sound at the two ears, including differences in the timing of the modulated envelopes of high-frequency sounds (envelope interaural time differences (ITDs)). We investigated responses of single neurons in the inferior colliculus (IC) to a wide range of envelope ITDs and stimulus envelope shapes. By a novel means of visualizing neural activity relative to different portions of the periodic stimulus envelope at each ear, we demonstrate the role of neuron-specific excitatory and inhibitory inputs in creating ITD sensitivity (or the lack of it) depending on the specific shape of the stimulus envelope. The underlying binaural brain circuitry and synaptic parameters were modeled individually for each neuron to account for neuron-specific activity patterns. The model explains the effects of envelope shapes on sensitivity to envelope ITDs observed in both normal-hearing listeners and in neural data, and has consequences for understanding how ITD information in stimulus envelopes might be maximized in users of bilateral cochlear implants—for whom ITDs conveyed in the stimulus envelope are the only ITD cues available.     

Interaural time and level difference thresholds for acoustically presented signals in post-lingually deafened adults fitted with bilateral cochlear implants using CIS+ processing

OBJECTIVES: The main purpose of the study was to measure thresholds for interaural time differences (ITDs) and interaural level differences (ILDs) for acoustically presented noise signals in adults with bilateral cochlear implants (CIs). A secondary purpose was to assess the correlation between the ILD and ITD thresholds and error scores in a horizontal-plane localization task, to test the hypothesis that localization by individuals with bilateral implants is mediated by the processing of ILD cues. DESIGN: Eleven adults, all postlingually deafened and all bilaterally fitted with MED-EL COMBI 40+ CIs, were tested in ITD and ILD discrimination tasks in which signals were presented acoustically through headphones that fit over their two devices. The stimulus was a 200-msec burst of Gaussian noise bandpass filtered from 100 to 4000 Hz. A two-interval forced-choice adaptive procedure was used in which the subject had to respond on each trial whether the lateral positions of the two sound images (with the interaural difference favoring the left and right sides in the two intervals) moved from left-to-right or right-to-left. RESULTS: In agreement with previously reported data, ITD thresholds for the subjects with bilateral implants were poor. The best threshold was approximately 400 microsec, and only five of 11 subjects tested achieved thresholds <1000 microsec. In contrast, ILD thresholds were relatively good; mean threshold was 3.8 dB with the initial compression circuit on the implant devices activated and 1.9 dB with the compression deactivated. The ILD and ITD thresholds were higher than previously reported thresholds obtained with direct electrical stimulation (generally, <1.0 dB and 100 to 200 microsec, respectively). When the data from two outlying subjects were omitted, ILD thresholds were highly correlated with total error score in a horizontal-plane localization task, computed for sources near midline (r = 0.87, p < 0.01). CONCLUSIONS: The higher ILD and ITD thresholds obtained in this study with acoustically presented signals (when compared with prior data with direct electrical stimulation) can be attributed-at least partially-to the signal processing carried out by the CI in the former case. The processing strategy effectively leaves only envelope information as a basis for ITD discrimination, which, for the acoustically presented noise stimuli, is mainly coded in the onset information. The operation of the compression circuit reduces the ILDs in the signal, leading to elevated ILD thresholds for the acoustically presented signals in this condition. The large magnitude of the ITD thresholds indicates that ITDs could not have contributed to the performance in the horizontal-plane localization task. Overall, the results suggest that for subjects using bilateral implants, localization of noise signals is mediated entirely by ILD cues, with little or no contribution from ITD information.     

Modulation Enhancement in the Electrical Signal Improves Perception of Interaural Time Differences with Bimodal Stimulation

Interaural timing cues are important for sound source localization and for binaural unmasking of speech that is spatially separated from interfering sounds. Users of a cochlear implant (CI) with residual hearing in the non-implanted ear (bimodal listeners) can only make very limited use of interaural timing cues with their clinical devices. Previous studies showed that bimodal listeners can be sensitive to interaural time differences (ITDs) for simple single- and three-channel stimuli. The modulation enhancement strategy (MEnS) was developed to improve the ITD perception of bimodal listeners. It enhances temporal modulations on all stimulated electrodes, synchronously with modulations in the acoustic signal presented to the non-implanted ear, based on measurement of the amplitude peaks occurring at the rate of the fundamental frequency in voiced phonemes. In the first experiment, ITD detection thresholds were measured using the method of constant stimuli for five bimodal listeners for an artificial vowel, processed with either the advanced combination encoder (ACE) strategy or with MEnS. With MEnS, detection thresholds were significantly lower, and for four subjects well within the physically relevant range. In the second experiment, the extent of lateralization was measured in three subjects with both strategies, and ITD sensitivity was determined using an adaptive procedure. All subjects could lateralize sounds based on ITD and sensitivity was significantly better with MEnS than with ACE. The current results indicate that ITD cues can be provided to bimodal listeners with modified sound processing.     

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.     

Sound-Localization Experiments with Barn Owls in Virtual Space: Influence of Interaural Time Difference on Head-Turning Behavior

Specific cues in a sound signal are naturally linked to certain parameters in acoustic space. In the barn owl, interaural time difference (ITD) varies mainly with azimuth, while interaural level difference (ILD) varies mainly with elevation. Previous data suggested that ITD is indeed the main cue for azimuthal sound localization in this species, while ILD is an important cue for elevational sound localization. The exact contributions of these parameters could be tested only indirectly because it was not possible to generate a stimulus that contained all relevant spatial information on the one hand, and allowed for a clean separation of these parameters on the other hand. Virtual auditory worlds offer this opportunity. Here we show that barn owls responded to azimuthal variations in virtual space in the same way as to variations in free-field stimuli. We interpret the increase of turning angle with sound-source azimuths (up to +/- 140 degrees) such that the owls did not experience front/back confusions with virtual stimuli. We then separated the influence of ITD from the influence of all other stimulus parameters by fixing the overall ITD in virtual stimuli to a constant value (+100 micros or +100 micros) while leaving all other sound characteristics unchanged. This manipulation influenced both azimuthal and elevational components of head arms. Since the owls' azimuthal head-turn amplitude always resembled the value signified by the ITD, these data demonstrated that azimuthal sound localization is influenced only by ITD both in the frontal hemisphere and in large parts of the rear hemisphere. ILDs did not have an influence on azimuthal components of head turns. While response latency to normal virtual stimuli was found to be largely independent of stimulus position, response delays of the head turns became longer if the ITD information pointed into a different hemisphere as the other cues of the sounds.     

Sound pressure transformations by the head and pinnae of the adult Chinchilla (Chinchilla lanigera)

There are three main cues to sound location: the interaural differences in time (ITD) and level (ILD) as well as the monaural spectral shape cues. These cues are generated by the spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although the chinchilla has been used for decades to study the anatomy, physiology, and psychophysics of audition, including binaural and spatial hearing, little is actually known about the sound pressure transformations by the head and pinnae and the resulting sound localization cues available to them. Here, we measured the directional transfer functions (DTFs), the directional components of the head-related transfer functions, for 9 adult chinchillas. The resulting localization cues were computed from the DTFs. In the frontal hemisphere, spectral notch cues were present for frequencies from ～6-18?kHz. In general, the frequency corresponding to the notch increased with increases in source elevation as well as in azimuth towards the ipsilateral ear. The ILDs demonstrated a strong correlation with source azimuth and frequency. The maximum ILDs were <10?dB for frequencies <5?kHz, and ranged from 10-30?dB for the frequencies >5?kHz. The maximum ITDs were dependent on frequency, yielding 236?μs at 4?kHz and 336?μs at 250?Hz. Removal of the pinnae eliminated the spectral notch cues, reduced the acoustic gain and the ILDs, altered the acoustic axis, and reduced the ITDs.     

Context Effects in the Discriminability of Spatial Cues

In order to investigate whether performance in an auditory spatial discrimination task depends on the prevailing listening conditions, we tested the ability of human listeners to discriminate target sounds with and without presentation of a preceding sound. Target sounds were either lateralized by means of interaural time differences (ITDs) of +400, 0, or −400 μs or interaural level differences (ILDs) with the same subjective intracranial locations. The preceding sound was always lateralized by means of ITD. This allowed for testing whether the effects of a preceding sound were location- or cue-specific. Preceding sounds and target sounds were randomly paired across trials. Listeners had to discriminate whether they perceived the target sounds as coming from the same or different intracranial locations. Finally, stimuli were selected so that, without any preceding sound, ITD and ILD cues were equally discriminable at all target lateralizations. Stimuli were 800 Hz-wide, 400-ms duration bands of noise centered at 500 Hz, presented over headphones. The duration of the preceding sound was randomly selected from a uniform distribution spanning from 1s to 2s. Results show that discriminability of both binaural cues was improved for midline target positions when preceding sound and targets were co-located, whereas it was impaired when preceding sound and targets came from different positions. No effect of the preceding sound was found for left or right target positions. These results are compatible with a purely bottom–up mechanism based on adaptive coding of ITD around the midline that may be combined with top–down mechanisms to increase localization accuracy in realistic listening conditions.     

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.     

Sound localization and sensitivity to interaural cues in bilateral users of the Med-El Combi 40/40+cochlear implant system.

OBJECTIVE: The purpose of this study was to investigate sound localization in subjects bilaterally implanted with MED-EL COMBI 40/40+ cochlear implants. In addition, the sensitivity to interaural cues was assessed. METHODS: In the localization test (11 subjects), CCITT noise (500 ms, original and HRTF-filtered, 70/75/80 dB sound pressure level) was presented from one of seven loudspeakers between -90 degrees and 90 degrees azimuth. The subject had to indicate which loudspeaker the noise was presented from. Sensitivity to interaural level differences (ILD) was assessed by performing localization tests (4 subjects) with the loudness of the two speech processors unbalanced to various degrees. To investigate the subjects' sensitivity to interaural time differences (ITD), lateralization was measured (7 subjects) as a function of the time difference between two Gaussian-like pulses, each directed to one of the subject's speech processor microphones by way of headphones. RESULTS: The judgments of all subjects significantly correlated with the positions of the loudspeakers. The scatter in the judged azimuth measured by the standard deviation of the responses was on average 27.5 degrees . Unbalanced loudness of the speech processors produced a bias in azimuth toward the speech processor with the louder volume setting. The mean rate of shift was 1.4 degrees per unit on the Wuerzburg loudness scale. Six of seven subjects showed a significant sensitivity to ITDs with the approximate time difference required for complete lateralization being 1,200 micros on an average. The one subject not showing a sensitivity to ITDs performed worst in the localization test. CONCLUSIONS: Bilateral cochlear implantation can restore spatial hearing in cochlear implant users. Both ILDs and ITDs are used by bilateral cochlear implant users in sound localization with ILDs appearing to be the dominant cue.     

The Conductive Hearing Loss Due to an Experimentally Induced Middle Ear Effusion Alters the Interaural Level and Time Difference Cues to Sound Location

Otitis media with effusion (OME) is a pathologic condition of the middle ear that leads to a mild to moderate conductive hearing loss as a result of fluid in the middle ear. Recurring OME in children during the first few years of life has been shown to be associated with poor detection and recognition of sounds in noisy environments, hypothesized to result due to altered sound localization cues. To explore this hypothesis, we simulated a middle ear effusion by filling the middle ear space of chinchillas with different viscosities and volumes of silicone oil to simulate varying degrees of OME. While the effects of middle ear effusions on the interaural level difference (ILD) cue to location are known, little is known about whether and how middle ear effusions affect interaural time differences (ITDs). Cochlear microphonic amplitudes and phases were measured in response to sounds delivered from several locations in azimuth before and after filling the middle ear with fluid. Significant attenuations (20–40 dB) of sound were observed when the middle ear was filled with at least 1.0 ml of fluid with a viscosity of 3.5 Poise (P) or greater. As expected, ILDs were altered by ~30 dB. Additionally, ITDs were shifted by ~600 μs for low frequency stimuli (<4 kHz) due to a delay in the transmission of sound to the inner ear. The data show that in an experimental model of OME, ILDs and ITDs are shifted in the spatial direction of the ear without the experimental effusion.     

Neural and Behavioral Sensitivity to Interaural Time Differences Using Amplitude Modulated Tones with Mismatched Carrier Frequencies

Bilateral cochlear implantation is intended to provide the advantages of binaural hearing, including sound localization and better speech recognition in noise. In most modern implants, temporal information is carried by the envelope of pulsatile stimulation, and thresholds to interaural time differences (ITDs) are generally high compared to those obtained in normal hearing observers. One factor thought to influence ITD sensitivity is the overlap of neural populations stimulated on each side. The present study investigated the effects of acoustically stimulating bilaterally mismatched neural populations in two related paradigms: rabbit neural recordings and human psychophysical testing. The neural coding of interaural envelope timing information was measured in recordings from neurons in the inferior colliculus of the unanesthetized rabbit. Binaural beat stimuli with a 1-Hz difference in modulation frequency were presented at the best modulation frequency and intensity as the carrier frequencies at each ear were varied. Some neurons encoded envelope ITDs with carrier frequency mismatches as great as several octaves. The synchronization strength was typically nonmonotonically related to intensity. Psychophysical data showed that human listeners could also make use of binaural envelope cues for carrier mismatches of up to 2–3 octaves. Thus, the physiological and psychophysical data were broadly consistent, and suggest that bilateral cochlear implants should provide information sufficient to detect envelope ITDs even in the face of bilateral mismatch in the neural populations responding to stimulation. However, the strongly nonmonotonic synchronization to envelope ITDs suggests that the limited dynamic range with electrical stimulation may be an important consideration for ITD encoding.     

Congenital and Prolonged Adult-Onset Deafness Cause Distinct Degradations in Neural ITD Coding with Bilateral Cochlear Implants

Bilateral cochlear implant (CI) users perform poorly on tasks involving interaural time differences (ITD), which are critical for sound localization and speech reception in noise by normal-hearing listeners. ITD perception with bilateral CI is influenced by age at onset of deafness and duration of deafness. We previously showed that ITD coding in the auditory midbrain is degraded in congenitally deaf white cats (DWC) compared to acutely deafened cats (ADC) with normal auditory development (Hancock et al., J. Neurosci, 30:14068). To determine the relative importance of early onset of deafness and prolonged duration of deafness for abnormal ITD coding in DWC, we recorded from single units in the inferior colliculus of cats deafened as adults 6 months prior to experimentation (long-term deafened cats, LTDC) and compared neural ITD coding between the three deafness models. The incidence of ITD-sensitive neurons was similar in both groups with normal auditory development (LTDC and ADC), but significantly diminished in DWC. In contrast, both groups that experienced prolonged deafness (LTDC and DWC) had broad distributions of best ITDs around the midline, unlike the more focused distributions biased toward contralateral-leading ITDs present in both ADC and normal-hearing animals. The lack of contralateral bias in LTDC and DWC results in reduced sensitivity to changes in ITD within the natural range. The finding that early onset of deafness more severely degrades neural ITD coding than prolonged duration of deafness argues for the importance of fitting deaf children with sound processors that provide reliable ITD cues at an early age.     

Responses of Auditory Nerve and Anteroventral Cochlear Nucleus Fibers to Broadband and Narrowband Noise: Implications for the Sensitivity to Interaural Delays

The quality of temporal coding of sound waveforms in the monaural afferents that converge on binaural neurons in the brainstem limits the sensitivity to temporal differences at the two ears. The anteroventral cochlear nucleus (AVCN) houses the cells that project to the binaural nuclei, which are known to have enhanced temporal coding of low-frequency sounds relative to auditory nerve (AN) fibers. We applied a coincidence analysis within the framework of detection theory to investigate the extent to which AVCN processing affects interaural time delay (ITD) sensitivity. Using monaural spike trains to a 1-s broadband or narrowband noise token, we emulated the binaural task of ITD discrimination and calculated just noticeable differences (jnds). The ITD jnds derived from AVCN neurons were lower than those derived from AN fibers, showing that the enhanced temporal coding in the AVCN improves binaural sensitivity to ITDs. AVCN processing also increased the dynamic range of ITD sensitivity and changed the shape of the frequency dependence of ITD sensitivity. Bandwidth dependence of ITD jnds from AN as well as AVCN fibers agreed with psychophysical data. These findings demonstrate that monaural preprocessing in the AVCN improves the temporal code in a way that is beneficial for binaural processing and may be crucial in achieving the exquisite sensitivity to ITDs observed in binaural pathways.     

Interaural time coincidence detectors are present at birth: evidence from binaural interaction

Binaural processing of sounds in mammals is presumably initiated within the auditory nuclei of the caudal pons. The binaural difference waveform (BD) can be derived from the sum of the waveforms evoked by right monaural clicks plus left monaural clicks minus the waveform evoked by binaural clicks. In adults, the BD's first positive peak (beta) is large only for stimuli with interaural time differences (ITDs) that produce a fused acoustic percept. Humans at birth can localize and discriminate sound sources, but their head circumference is about two-thirds of an adult head. In order to test whether beta is related to head circumference, we recorded beta in human neonates as a function of ITD. Binaural clicks with ITDs ranging between 0 and 1000 micros were used to derive BD waveforms in 34 neonates. For ITD=0, beta was detectable in 56% of newborns. The incidence of beta detection then decreased as ITD increased. Only 9% of the babies had detectable beta for all ITDs. No correlation was found between the existence of beta and other properties of the monaural or binaural auditory brainstem response. The finding that for some infants beta was present for all ITDs up to 1.0 ms suggests that there is no recalibration of brainstem delay lines with head growth. Our data suggest that the brainstem auditory pathway for detecting interaural time differences in the adult is probably present at birth. Maturational factors such as increased myelination and greater firing synchrony probably improve the detectability of beta with age. The second peak in the BD waveform (delta) was highly correlated with the existence of wave VI in the binaural and monaural waveforms.     

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.     

Improving Interaural Time Difference Sensitivity Using Short Inter-pulse Intervals with Amplitude-Modulated Pulse Trains in Bilateral Cochlear Implants

Journal of the Association for Research in Otolaryngology - Interaural time differences (ITDs) at low frequencies are important for sound localization and spatial speech unmasking. These ITD cues...