Function Of Interaural Time Differences Research Articles

Experience-Dependent Plasticity in Nucleus Laminaris of the Barn Owl.

Interaural time differences (ITDs) are a major cue for sound localization and change with increasing head size. Since the barn owl's head width more than doubles in the month after hatching, we hypothesized that the development of their ITD detection circuit might be modified by experience. To test this, we raised owls with unilateral ear inserts that delayed and attenuated the acoustic signal, and then measured the ITD representation in the brainstem nucleus laminaris (NL) when they were adults. The ITD circuit is composed of delay line inputs to coincidence detectors, and we predicted that plastic changes would lead to shorter delays in the axons from the manipulated ear, and complementary shifts in ITD representation on the two sides. In owls that received ear inserts starting around P14, the maps of ITD shifted in the predicted direction, but only on the ipsilateral side, and only in those tonotopic regions that had not experienced auditory stimulation prior to insertion. The contralateral map did not change. Thus, experience-dependent plasticity of the ITD circuit occurs in NL, and our data suggest that ipsilateral and contralateral delays are independently regulated. As a result, altered auditory input during development leads to long-lasting changes in the representation of ITD.Significance Statement The early life of barn owls is marked by increasing sensitivity to sound, and by increasing ITDs. Their prolonged post-hatch development allowed us to examine the role of altered auditory experience in the development of ITD detection circuits. We raised owls with a unilateral ear insert and found that their maps of ITD were altered by experience, but only in those tonotopic regions ipsilateral to the occluded ear that had not experienced auditory stimulation prior to insertion. This experience-induced plasticity allows the sound localization circuits to be customized to individual characteristics, such as the size of the head, and potentially to compensate for imbalanced hearing sensitivities between the left and right ears.

Normative Study of the Binaural Interaction Component of the Human Auditory Brainstem Response as a Function of Interaural Time Differences.

The binaural interaction component (BIC) of the auditory brainstem response (ABR) is obtained by subtracting the sum of the monaural right and left ear ABRs from the binaurally evoked ABR. The result is a small but prominent negative peak (herein called "DN1"), indicating a smaller binaural than summed ABR, which occurs around the latency of wave V or its roll-off slope. The BIC has been proposed to have diagnostic value as a biomarker of binaural processing abilities; however, there have been conflicting reports regarding the reliability of BIC measures in human subjects. The objectives of the current study were to: (1) examine prevalence of BIC across a large group of normal-hearing young adults; (2) determine effects of interaural time differences (ITDs) on BIC; and (3) examine any relationship between BIC and behavioral ITD discrimination acuity. Subjects were 40 normal-hearing adults (20 males and 20 females), aged 21 to 48 years, with no history of otologic or neurologic disorders. Midline ABRs were recorded from electrodes at high forehead (Fz) referenced to the nape of the neck (near the seventh cervical vertebra), with Fpz (low forehead) as the ground. ABRs were also recorded with a conventional earlobe reference for comparison to midline results. Stimuli were 90 dB peSPL biphasic clicks. For BIC measurements, stimuli were presented in a block as interleaved right monaural, left monaural, and binaural stimuli with 2000+ presentations per condition. Four measurements were averaged for a total of 8000+ stimuli per analyzed waveform. BIC was measured for ITD = 0 (simultaneous bilateral) and for ITDs of ±500 and ±750 µs. Subjects separately performed a lateralization task, using the same stimuli, to determine ITD discrimination thresholds. An identifiable BIC DN1 was obtained in 39 of 40 subjects at ITD = 0 µs in at least one of two measurement sessions, but was seen in lesser numbers of subjects in a single session or as ITD increased. BIC was most often seen when a subject was relaxed or sleeping, and less often when they fidgeted or reported neck tension, suggesting myogenic activity as a possible factor in disrupting BIC measurements. Mean BIC latencies systematically increased with increasing ITD, and mean BIC amplitudes tended to decrease. However, across subjects, there was no significant relationship between the amplitude or latency of the BIC and behavioral ITD thresholds. Consistent with previous studies, measurement of the BIC was time consuming and a BIC was sometimes difficult to obtain in awake normal-hearing subjects. The BIC will thus continue to be of limited clinical utility unless stimulus parameters and measurement techniques can be identified that produce a more robust response. Nonetheless, modulation of BIC characteristics by ITD supports the concept that the ABR BIC indexes aspects of binaural brainstem processing and thus may prove useful in selected research applications, e.g. in the examination of populations expected to have aberrant binaural signal processing ability.

Neural Maps of Interaural Time Difference in the American Alligator: A Stable Feature in Modern Archosaurs.

Detection of interaural time differences (ITDs) is crucial for sound localization in most vertebrates. The current view is that optimal computational strategies of ITD detection depend mainly on head size and available frequencies, although evolutionary history should also be taken into consideration. In archosaurs, which include birds and crocodiles, the brainstem nucleus laminaris (NL) developed into the critical structure for ITD detection. In birds, ITDs are mapped in an orderly array or place code, whereas in the mammalian medial superior olive, the analog of NL, maps are not found. As yet, in crocodilians, topographical representations have not been identified. However, nontopographic representations of ITD cannot be excluded due to different anatomical and ethological features of birds and crocodiles. Therefore, we measured ITD-dependent responses in the NL of anesthetized American alligators of either sex and identified the location of the recording sites by lesions made after recording. The measured extracellular field potentials, or neurophonics, were strongly ITD tuned, and their preferred ITDs correlated with the position in NL. As in birds, delay lines, which compensate for external time differences, formed maps of ITD. The broad distributions of best ITDs within narrow frequency bands were not consistent with an optimal coding model. We conclude that the available acoustic cues and the architecture of the acoustic system in early archosaurs led to a stable and similar organization in today's birds and crocodiles, although physical features, such as internally coupled ears, head size, or shape, and audible frequency range, vary among the two groups.SIGNIFICANCE STATEMENT Interaural time difference (ITD) is an important cue for sound localization, and the optimal strategies for encoding ITD in neuronal populations are the subject of ongoing debate. We show that alligators form maps of ITD very similar to birds, suggesting that their common archosaur ancestor reached a stable coding solution different from mammals. Mammals and diapsids evolved tympanic hearing independently, and local optima can be reached in evolution that are not considered by global optimal coding models. Thus, the presence of ITD maps in the brainstem may reflect a local optimum in evolutionary development. Our results underline the importance of comparative animal studies and show that optimal models must be viewed in the light of evolutionary processes.

Internally coupled middle ears enhance the range of interaural time differences heard by the chicken.

Interaural time differences (ITDs) are one of several principal cues for localizing sounds. However, ITDs are in the sub-millisecond range for most animals. Because the neural processing of such small ITDs pushes the limit of temporal resolution, the precise ITD range for a given species and its usefulness - relative to other localization cues - has been a powerful selective force in the evolution of the neural circuits involved. Birds and other non-mammals have internally coupled middle ears working as pressure-difference receivers that may significantly enhance ITDs, depending on the precise properties of the interaural connection. Here, the extent of this internal coupling was investigated in chickens, specifically under the same experimental conditions as typically used in investigations of the neurophysiology of ITD-coding circuits, i.e. with headphone stimulation and skull openings. Cochlear microphonics (CM) were recorded simultaneously from both ears of anesthetized chickens under monaural and binaural stimulation, using pure tones from 0.1 to 3 kHz. Interaural transmission peaked at 1.5 kHz at a loss of only -5.5 dB; the mean interaural delay was 264 µs. CM amplitude was strongly modulated as a function of ITD, confirming significant interaural coupling. The 'ITD heard' derived from the CM phases in both ears showed enhancement, compared with the acoustic stimuli, by a factor of up to 1.8. However, the experimental conditions impaired interaural transmission at low frequencies (<1 kHz). I identify factors that need to be considered when interpreting neurophysiological data obtained under these conditions and relating them to the natural free-field condition.

Commissural Gain Control Enhances the Midbrain Representation of Sound Location.

Sound localization, a fundamental process in hearing, is dependent on bilateral computations in the brainstem. How this information is transmitted from the brainstem to the auditory cortex, through several stages of processing, without loss of signal fidelity, is not clear. We show that the ability of neurons in the auditory midbrain to encode azimuthal sound location is dependent on gain control mediated by the commissure of the inferior colliculi. This finding demonstrates that commissural processing between homologous auditory nuclei, on either side of the midline, enhances the precision of sound localization.

Extent of lateralization of click trains elicited by interaural time differences—A comparison of measurement paradigms

Extent of lateralization measurements have previously been performed for various stimulus types, mainly using two measurement paradigms: (1) an acoustic pointer paradigm where the interaural level difference of a pointer stimulus is adjusted to match the intracranial position of a target stimulus and (2) a visual pointer paradigm where the intracranial position of a target stimulus is indicated on a linear scale. Both measurement paradigms are widely used, but rarely within the same study. In this study, the extent of lateralization elicited by unfiltered as well as 4 kHz bandpass-filtered click trains carrying interaural time differences (ITDs) up to 3 ms was measured using an acoustic as well as a visual pointer paradigm. Data from six young normal-hearing listeners were compared. Using the visual pointer paradigm, the extent of lateralization behavior as a function of stimulus ITD for unfiltered as well as bandpass filtered click trains coincided. The extent of lateralization as determined by the acous...

The Role of Conduction Delay in Creating Sensitivity to Interaural Time Differences.

Axons from the nucleus magnocellularis (NM) and their targets in nucleus laminaris (NL) form the circuit responsible for encoding interaural time difference (ITD). In barn owls, NL receives bilateral inputs from NM, such that axons from the ipsilateral NM enter NL dorsally, while contralateral axons enter from the ventral side. These afferents act as delay lines to create maps of ITD in NL. Since delay-line inputs are characterized by a precise latency to auditory stimulation, but the postsynaptic coincidence detectors respond to ongoing phase difference, we asked whether the latencies of a local group of axons were identical, or varied by multiples of the inverse of the frequency they respond to, i.e., to multiples of 2π phase. Intracellular recordings from NM axons were used to measure delay-line latencies in NL. Systematic shifts in conduction delay within NL accounted for the maps of ITD, but recorded latencies of individual inputs at nearby locations could vary by 2π or 4π. Therefore microsecond precision is achieved through sensitivity to phase delays, rather than absolute latencies. We propose that the auditory system "coarsely" matches ipsilateral and contralateral latencies using physical delay lines, so that inputs arrive at NL at about the same time, and then "finely" matches latency modulo 2π to achieve microsecond ITD precision.

Maps of interaural delay in the owl's nucleus laminaris.

Axons from the nucleus magnocellularis form a presynaptic map of interaural time differences (ITDs) in the nucleus laminaris (NL). These inputs generate a field potential that varies systematically with recording position and can be used to measure the map of ITDs. In the barn owl, the representation of best ITD shifts with mediolateral position in NL, so as to form continuous, smoothly overlapping maps of ITD with iso-ITD contours that are not parallel to the NL border. Frontal space (0°) is, however, represented throughout and thus overrepresented with respect to the periphery. Measurements of presynaptic conduction delay, combined with a model of delay line conduction velocity, reveal that conduction delays can account for the mediolateral shifts in the map of ITD.

A functional circuit model of interaural time difference processing.

Inputs from the two sides of the brain interact to create maps of interaural time difference (ITD) in the nucleus laminaris of birds. How inputs from each side are matched with high temporal precision in ITD-sensitive circuits is unknown, given the differences in input path lengths from each side. To understand this problem in birds, we modeled the geometry of the input axons and their corresponding conduction velocities and latencies. Consistent with existing physiological data, we assumed a common latency up to the border of nucleus laminaris. We analyzed two biological implementations of the model, the single ITD map in chickens and the multiple maps of ITD in barn owls. For binaural inputs, since ipsi- and contralateral initial common latencies were very similar, we could restrict adaptive regulation of conduction velocity to within the nucleus. Other model applications include the simultaneous derivation of multiple conduction velocities from one set of measurements and the demonstration that contours with the same ITD cannot be parallel to the border of nucleus laminaris in the owl. Physiological tests of the predictions of the model demonstrate its validity and robustness. This model may have relevance not only for auditory processing but also for other computational tasks that require adaptive regulation of conduction velocity.

Using Tweedie distributions for fitting spike count data

The nature of spike count distributions is of great practical concern for the analysis of neural data. These distributions often have a tendency for 'failures' and a long tail of large counts, and may show a strong dependence of variance on the mean. Furthermore, spike count distributions often show multiplicative rather than additive effects of covariates. We analyzed the responses of neurons in primary auditory cortex to transposed stimuli as a function of interaural time differences (ITD). In more than half of the cases, the variance of neuronal responses showed a supralinear dependence on the mean spike count. We explored the use of the Tweedie family of distributions, which has a supralinear dependence of means on variances. To quantify the effects of ITD on neuronal responses, we used generalized linear models (GLMs), and developed methods for significance testing under the Tweedie assumption. We found the Tweedie distribution to be generally a better fit to the data than the Poisson distribution for over-dispersed responses. Standard analysis of variance wrongly assumes Gaussian distributions with fixed variance and additive effects, but even generalized models under Poisson assumptions may be hampered by the over-dispersion of spike counts. The use of GLMs assuming Tweedie distributions increased the reliability of tests of sensitivity to ITD in our data. When spike count variance depends strongly on the mean, the use of Tweedie distributions for analyzing the data is advised.

Spectral integration of interaural time differences in auditory localization

This study investigates how the auditory system integrates spatial information across frequency. In experiment 1, discrimination thresholds for interaural time differences (ITDs) were measured as a function of both reference ITD and center frequency (CF) of noises with bandwidth of one ERB. In addition, discrimination thresholds were also measured as a function of CF for different values of interaural coherence (IC) typical of sounds in realistic acoustic environments. For both high ICs and small reference ITDs, discrimination thresholds were lowest for CFs between 700 and 1000 Hz. For smaller ICs and larger reference ITDs, this dominance region shifted towards lower CFs. A conceptual localization model was developed that used the variance of the ITD thresholds to optimally weight the contribution of the individual frequency bands before spectral integration. In experiment 2, the model was tested by asking listeners to align a broadband noise signal with an ITD that was fixed across frequency onto a broadband noise target with different ITDs in individual 1 ERB-wide subbands. The results were consistent with both the model predictions and the shift of dominance range observed in experiment one.

Neural ITD coding with bilateral cochlear implants: effect of binaurally coherent jitter

Poor sensitivity to the interaural time difference (ITD) constrains the ability of human bilateral cochlear implant users to listen in everyday noisy acoustic environments. ITD sensitivity to periodic pulse trains degrades sharply with increasing pulse rate but can be restored at high pulse rates by jittering the interpulse intervals in a binaurally coherent manner (Laback and Majdak. Binaural jitter improves interaural time-difference sensitivity of cochlear implantees at high pulse rates. Proc Natl Acad Sci USA 105: 814-817, 2008). We investigated the neural basis of the jitter effect by recording from single inferior colliculus (IC) neurons in bilaterally implanted, anesthetized cats. Neural responses to trains of biphasic pulses were measured as a function of pulse rate, jitter, and ITD. An effect of jitter on neural responses was most prominent for pulse rates above 300 pulses/s. High-rate periodic trains evoked only an onset response in most IC neurons, but introducing jitter increased ongoing firing rates in about half of these neurons. Neurons that had sustained responses to jittered high-rate pulse trains showed ITD tuning comparable with that produced by low-rate periodic pulse trains. Thus, jitter appears to improve neural ITD sensitivity by restoring sustained firing in many IC neurons. The effect of jitter on IC responses is qualitatively consistent with human psychophysics. Action potentials tended to occur reproducibly at sparse, preferred times across repeated presentations of high-rate jittered pulse trains. Spike triggered averaging of responses to jittered pulse trains revealed that firing was triggered by very short interpulse intervals. This suggests it may be possible to restore ITD sensitivity to periodic carriers by simply inserting short interpulse intervals at select times.

Tonotopic Tuning in a Sound Localization Circuit

Nucleus laminaris (NL) neurons encode interaural time difference (ITD), the cue used to localize low-frequency sounds. A physiologically based model of NL input suggests that ITD information is contained in narrow frequency bands around harmonics of the sound frequency. This suggested a theory, which predicts that, for each tone frequency, there is an optimal time course for synaptic inputs to NL that will elicit the largest modulation of NL firing rate as a function of ITD. The theory also suggested that neurons in different tonotopic regions of NL require specialized tuning to take advantage of the input gradient. Tonotopic tuning in NL was investigated in brain slices by separating the nucleus into three regions based on its anatomical tonotopic map. Patch-clamp recordings in each region were used to measure both the synaptic and the intrinsic electrical properties. The data revealed a tonotopic gradient of synaptic time course that closely matched the theoretical predictions. We also found postsynaptic band-pass filtering. Analysis of the combined synaptic and postsynaptic filters revealed a frequency-dependent gradient of gain for the transformation of tone amplitude to NL firing rate modulation. Models constructed from the experimental data for each tonotopic region demonstrate that the tonotopic tuning measured in NL can improve ITD encoding across sound frequencies.

Maps of interaural time difference in the chicken’s brainstem nucleus laminaris

Animals, including humans, use interaural time differences (ITDs) that arise from different sound path lengths to the two ears as a cue of horizontal sound source location. The nature of the neural code for ITD is still controversial. Current models differentiate between two population codes: either a map-like rate-place code of ITD along an array of neurons, consistent with a large body of data in the barn owl, or a population rate code, consistent with data from small mammals. Recently, it was proposed that these different codes reflect optimal coding strategies that depend on head size and sound frequency. The chicken makes an excellent test case of this proposal because its physical prerequisites are similar to small mammals, yet it shares a more recent common ancestry with the owl. We show here that, like in the barn owl, the brainstem nucleus laminaris in mature chickens displayed the major features of a place code of ITD. ITD was topographically represented in the maximal responses of neurons along each isofrequency band, covering approximately the contralateral acoustic hemisphere. Furthermore, the represented ITD range appeared to change with frequency, consistent with a pressure gradient receiver mechanism in the avian middle ear. At very low frequencies, below 400 Hz, maximal neural responses were symmetrically distributed around zero ITD and it remained unclear whether there was a topographic representation. These findings do not agree with the above predictions for optimal coding and thus revive the discussion as to what determines the neural coding strategies for ITDs.

Temporal Damping in Response to Broadband Noise. II. Auditory Nerve

II. Auditory nerve. Low-frequency neurons in the inferior colliculus (IC) show a damped oscillatory response as a function of interaural time differences (ITDs) of broadband noise. It was previously shown that several features of such noise-delay functions are well predicted by the composite curve, generated by the linear summation of responses to tones with varying ITD. This indicates a surprising degree of linearity at the midbrain level of the auditory pathway. A similar comparison between responses to tones and to noise has not been made at a more peripheral, monaural level and it is therefore unclear to what extent this linearity reflects peripheral physiology. Here, we compare cat auditory nerve responses to broadband noise and to isolevel tones. We constructed shuffled autocorrelograms for responses to tones and summed across frequencies to obtain a monaural composite curve. We then compare this composite curve to the shuffled autocorrelogram of responses to broadband noise and find that the match between tonal and noise responses is poorer at the level of the auditory nerve than at the level of the IC. The apparent linearity of responses in the IC is thus even more surprising than was apparent from its original report because it results from mechanisms interposed between the auditory nerve and the IC.

Functional delay of myelination of auditory delay lines in the nucleus laminaris of the barn owl

In the barn owl, maps of interaural time difference (ITD) are created in the nucleus laminaris (NL) by interdigitating axons that act as delay lines. Adult delay line axons are myelinated, and this myelination is timely, coinciding with the attainment of adult head size, and stable ITD cues. The proximal portions of the axons become myelinated in late embryonic life, but the delay line portions of the axon in NL remain unmyelinated until the first postnatal week. Myelination of the delay lines peaks at the third week posthatch, and myelinating oligodendrocyte density approaches adult levels by one month, when the head reaches its adult width. Migration of oligodendrocyte progenitors into NL and the subsequent onset of myelination may be restricted by a glial barrier in late embryonic stages and the first posthatch week, since the loss of tenascin-C immunoreactivity in NL is correlated with oligodendrocyte progenitor migration into NL.

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.

GABAergic Inhibition Controls Neural Gain in Inferior Colliculus Neurons Sensitive to Interaural Time Differences

We investigated the role of GABAergic inhibition on the responses of inferior colliculus (IC) neurons sensitive to interaural time differences (ITDs) in anesthetized guinea pigs. Responses to static and dynamic ITDs were obtained before, during, and after recovery from ionotophoretic application of GABA, or antagonists to the GABA(A) receptor gabazine and bicuculline. For most neurons, a linear relationship was observed between discharge rates evoked by a particular ITD during drug application and control discharge rates. Blocking GABAergic inhibition, or adding exogenous GABA, scaled IC discharge rates in a multiplicative (divisive) and/or additive (subtractive) manner. When the influence of iontophoresed GABA antagonists or exogenous GABA on discharge rates was accounted for, GABAergic inhibition was found to have no effect on the ITD tuning properties of IC neurons. The tuning sharpness of ITD functions, the ITD that evoked 50% response magnitude, and the relative symmetry of ITD functions around their peak response were unaffected by blockade of inhibition or addition of tonic inhibition. However, the ability of neurons to discriminate between ITDs by virtue of differences in their discharge rate was altered by blocking or adding GABA. We propose that inhibition in the IC is involved in the control of the neural gain of the output of IC neurons rather than the regulation of ITD tuning. This gain control appears to arise from a combination of additive and multiplicative processes, and may involve mechanisms such as shunting inhibition or changes in the efficacy of inhibitory and excitatory inputs.

Neural Sensitivity to Interaural Envelope Delays in the Inferior Colliculus of the Guinea Pig

Interaural time differences (ITDs) are important cues for mammalian sound localization. At high frequencies, sensitivity to ITDs, which are conveyed only by the envelope of the waveforms, has been shown to be poorer than sensitivity to ITDs at low frequencies, which are conveyed primarily by the fine structure of the waveforms. Recently, human psychophysical experiments have demonstrated that sensitivity to envelope-based ITDs in high-frequency transposed tones can be equivalent to low-frequency fine-structure-based ITD sensitivity. Transposed tones are designed to provide high-frequency auditory nerve fibers (ANFs) with similar temporal information to that provided by low-frequency tones. We investigated neural sensitivity to ITDs in high-frequency transposed and sinusoidally amplitude modulated (SAM) tones, in the inferior colliculus of the guinea pig. Neural sensitivity to ITDs in transposed tones was found to be greater than that to ITDs in SAM tones; in response to transposed tones, neural firing rates were more modulated as a function of ITD and discrimination thresholds were found to be lower than those in response to SAM tones. Similar to psychophysical findings, ITD discrimination of single neurons in response to transposed tones for rates of modulation <250 Hz was comparable to neural discrimination of ITDs in low-frequency tones. This suggests that the neural mechanisms that mediate sensitivity to ITDs at high and low frequencies are functionally equivalent, provided that the stimuli result in appropriate temporal patterns of action potentials in ANFs.

A physiologically based model of interaural time difference discrimination.

Interaural time difference (ITD) is a cue to the location of sounds containing low frequencies and is represented in the inferior colliculus (IC) by cells that respond maximally at a particular best delay (BD). Previous studies have demonstrated that single ITD-sensitive cells contain sufficient information in their discharge patterns to account for ITD acuity on the midline (ITD = 0). If ITD discrimination were based on the activity of the most sensitive cell available ("lower envelope hypothesis"), then ITD acuity should be relatively constant as a function of ITD. In response to broadband noise, however, the ITD acuity of human listeners degrades as ITD increases. To account for these results, we hypothesize that pooling of information across neurons is an essential component of ITD discrimination. This report describes a neural pooling model of ITD discrimination based on the response properties of ITD-sensitive cells in the IC of anesthetized cats. Rate versus ITD curves were fit with a cross-correlation model of ITD sensitivity, and the parameters were used to constrain a population model of ITD discrimination. The model accurately predicts ITD acuity as a function of ITD for broadband noise stimuli when responses are pooled across best frequency (BF). Furthermore, ITD tuning based solely on a system of internal delays is not sufficient to predict ITD acuity in response to 500 Hz tones, suggesting that acuity is likely refined by additional mechanisms. The physiological data confirms evidence from the guinea pig that BD varies systematically with BF, generalizing the observation across species.