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1.  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.
PMCID: PMC2041891  PMID: 15306644
auditory; binaural; hearing; inferior colliculus; localization; psychophysics
2.  Noise Reduction of Coincidence Detector Output by the Inferior Colliculus of the Barn Owl 
A recurring theme in theoretical work is that integration over populations of similarly tuned neurons can reduce neural noise. However, there are relatively few demonstrations of an explicit noise reduction mechanism in a neural network. Here we demonstrate that the brainstem of the barn owl includes a stage of processing apparently devoted to increasing the signal-to-noise ratio in the encoding of the interaural time difference (ITD), one of two primary binaural cues used to compute the position of a sound source in space. In the barn owl, the ITD is processed in a dedicated neural pathway that terminates at the core of the inferior colliculus (ICcc). The actual locus of the computation of the ITD is before ICcc in the nucleus laminaris (NL), and ICcc receives no inputs carrying information that did not originate in NL. Unlike in NL, the rate-ITD functions of ICcc neurons require as little as a single stimulus presentation per ITD to show coherent ITD tuning. ICcc neurons also displayed a greater dynamic range with a maximal difference in ITD response rates approximately double that seen in NL. These results indicate that ICcc neurons perform a computation functionally analogous to averaging across a population of similarly tuned NL neurons.
PMCID: PMC2492673  PMID: 16738236
interaural time difference; sound localization; inferior colliculus; nucleus laminaris; barn owl; pooling
3.  Preservation of Spectrotemporal Tuning Between the Nucleus Laminaris and the Inferior Colliculus of the Barn Owl 
Journal of neurophysiology  2007;97(5):3544-3553.
Performing sound recognition is a task that requires an encoding of the time-varying spectral structure of the auditory stimulus. Similarly, computation of the interaural time difference (ITD) requires knowledge of the precise timing of the stimulus. Consistent with this, low-level nuclei of birds and mammals implicated in ITD processing encode the ongoing phase of a stimulus. However, the brain areas that follow the binaural convergence for the computation of ITD show a reduced capacity for phase locking. In addition, we have shown that in the barn owl there is a pooling of ITD-responsive neurons to improve the reliability of ITD coding. Here we demonstrate that despite two stages of convergence and an effective loss of phase information, the auditory system of the anesthetized barn owl displays a graceful transition to an envelope coding that preserves the spectrotemporal information throughout the ITD pathway to the neurons of the core of the central nucleus of the inferior colliculus.
PMCID: PMC2532515  PMID: 17314241
4.  Sensitivity of Inferior Colliculus Neurons to Interaural Time Differences in the Envelope Versus the Fine Structure With Bilateral Cochlear Implants 
Journal of neurophysiology  2008;99(5):2390-2407.
Bilateral cochlear implantation seeks to improve hearing by taking advantage of the binaural processing of the central auditory system. Cochlear implants typically encode sound in each spectral channel by amplitude modulating (AM) a fixed-rate pulse train, thus interaural time differences (ITD) are only delivered in the envelope. We investigated the ITD sensitivity of inferior colliculus (IC) neurons with sinusoidally AM pulse trains. ITD was introduced independently to the AM and/or carrier pulses to measure the relative efficacy of envelope and fine structure for delivering ITD information. We found that many IC cells are sensitive to ITD in both the envelope (ITDenv) and fine structure (ITDfs) for appropriate modulation frequencies and carrier rates. ITDenv sensitivity was generally similar to that seen in normal-hearing animals with AM tones. ITDenv tuning generally improved with increasing modulation frequency up to the maximum modulation frequency that elicited a sustained response in a neuron (tested ≤Hz). ITDfs sensitivity was present in about half the neurons for 1,000 pulse/s (pps) carriers and was nonexistent at 5,000 pps. The neurons that were sensitive to ITDfs at 1,000 pps were those that showed the best ITD sensitivity to low-rate pulse trains. Overall, the best ITD sensitivity was found for ITD contained in the fine structure of a moderate rate AM pulse train (1,000 pps). These results suggest that the interaural timing of current pulses should be accurately controlled in a bilateral cochlear implant processing strategy that provides salient ITD cues.
PMCID: PMC2570106  PMID: 18287556
5.  Asymmetric Excitatory Synaptic Dynamics Underlie Interaural Time Difference Processing in the Auditory System 
PLoS Biology  2010;8(6):e1000406.
In order to localize sounds in the environment, the auditory system detects and encodes differences in signals between each ear. The exquisite sensitivity of auditory brain stem neurons to the differences in rise time of the excitation signals from the two ears allows for neuronal encoding of microsecond interaural time differences.
Low-frequency sound localization depends on the neural computation of interaural time differences (ITD) and relies on neurons in the auditory brain stem that integrate synaptic inputs delivered by the ipsi- and contralateral auditory pathways that start at the two ears. The first auditory neurons that respond selectively to ITD are found in the medial superior olivary nucleus (MSO). We identified a new mechanism for ITD coding using a brain slice preparation that preserves the binaural inputs to the MSO. There was an internal latency difference for the two excitatory pathways that would, if left uncompensated, position the ITD response function too far outside the physiological range to be useful for estimating ITD. We demonstrate, and support using a biophysically based computational model, that a bilateral asymmetry in excitatory post-synaptic potential (EPSP) slopes provides a robust compensatory delay mechanism due to differential activation of low threshold potassium conductance on these inputs and permits MSO neurons to encode physiological ITDs. We suggest, more generally, that the dependence of spike probability on rate of depolarization, as in these auditory neurons, provides a mechanism for temporal order discrimination between EPSPs.
Author Summary
Animals can locate the source of a sound by detecting microsecond differences in the arrival time of sound at the two ears. Neurons encoding these interaural time differences (ITDs) receive an excitatory synaptic input from each ear. They can perform a microsecond computation with excitatory synapses that have millisecond time scale because they are extremely sensitive to the input's “rise time,” the time taken to reach the peak of the synaptic input. Current theories assume that the biophysical properties of the two inputs are identical. We challenge this assumption by showing that the rise times of excitatory synaptic potentials driven by the ipsilateral ear are faster than those driven by the contralateral ear. Further, we present a computational model demonstrating that this disparity in rise times, together with the neurons' sensitivity to excitation's rise time, can endow ITD-encoding with microsecond resolution in the biologically relevant range. Our analysis also resolves a timing mismatch. The difference between contralateral and ipsilateral latencies is substantially larger than the relevant ITD range. We show how the rise time disparity compensates for this mismatch. Generalizing, we suggest that phasic-firing neurons—those that respond to rapidly, but not to slowly, changing stimuli—are selective to the temporal ordering of brief inputs. In a coincidence-detection computation the neuron will respond more robustly when a faster input leads a slower one, even if the inputs are brief and have similar amplitudes.
PMCID: PMC2893945  PMID: 20613857
6.  Effects of reverberation on the directional sensitivity of auditory neurons across the tonotopic axis: Influences of ITD and ILD 
In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener’s ears, distorting the binaural cues for sound localization. We investigated the effects of reverberation on the directional sensitivity of single neurons in the inferior colliculus (IC) of unanesthetized rabbits. We find that reverberation degrades the directional sensitivity of single neurons, although the amount of degradation depends on the characteristic frequency (CF) and the type of binaural cues available. When interaural time differences (ITD) are the only available directional cue, low-CF cells sensitive to ITD in the waveform fine time structure maintain better directional sensitivity in reverberation than high-CF cells sensitive to ITD in the envelope induced by cochlear filtering. On the other hand, when both ITD and interaural level difference (ILD) cues are available, directional sensitivity in reverberation is comparable throughout the tonotopic axis of the IC. This result suggests that, at high frequencies, ILDs provide better directional information than envelope ITDs, emphasizing the importance of the ILD-processing pathway for sound localization in reverberation.
PMCID: PMC2896784  PMID: 20534831
directional sensitivity; interaural level difference; inferior colliculus; interaural time difference; reverberation; sound localization
7.  Responses to Interaural Time Delay in Human Cortex 
Journal of Neurophysiology  2008;100(5):2712-2718.
Humans use differences in the timing of sounds at the two ears to determine the location of a sound source. Various models have been posited for the neural representation of these interaural time differences (ITDs). These models make opposing predictions about the lateralization of ITD processing in the human brain. The weighted-image model predicts that sounds leading in time at one ear activate maximally the opposite brain hemisphere for all values of ITD. In contrast, the π-limit model assumes that ITDs beyond half the period of the stimulus center frequency are not explicitly encoded in the brain and that such “long” ITDs activate maximally the side of the brain to which the sound is heard. A previous neuroimaging study revealed activity in the human inferior colliculus consistent with the π-limit. Here we show that cortical responses to sounds with ITDs within the π-limit are in line with the predictions of both models. However, contrary to the immediate predictions of both models, neural activation is bilateral for “long” ITDs, despite these being perceived as clearly lateralized. Furthermore, processing of long ITDs leads to higher activation in cortex than processing of short ITDs. These data show that coding of ITD in cortex is fundamentally different from coding of ITD in the brain stem. We discuss these results in the context of the two models.
PMCID: PMC2585401  PMID: 18799604
8.  The effects of interaural time difference and intensity on the coding of low frequency sounds in the mammalian midbrain 
We examined how changes in intensity and interaural time difference (ITD) influenced the coding of low frequency sounds in the inferior colliculus (IC) of male gerbils at both the single neuron and population levels. We found that changes in intensity along the positive slope of the rate-level function (RLF) evoked changes in spectrotemporal filtering that influenced in the overall timing of spike events, but preserved their precision across trials such that the decoding of single neuron responses was not affected. In contrast, changes in ITD did not trigger changes in spectrotemporal filtering, but had strong effects on the precision of spike events and, consequently, on decoder performance. However, changes in ITD had opposing effects in the two brain hemispheres, and, thus, canceled out at the population level. These results were similar with and without the addition of background noise. We also found that the effects of changes in intensity along the negative slope of the rate-level function (RLF) were different from the effects of changes in intensity along the positive slope in that they evoked changes in both spectrotemporal filtering and in the precision of spike events across trials, as well as in decoder performance. These results demonstrate that, at least at moderate intensities, the auditory system employs different strategies at the single neuron and population levels simultaneously to ensure that the coding of sounds is robust to changes in other stimulus features.
PMCID: PMC3083843  PMID: 21389237
auditory; coding; decoding; spectrotemporal filtering; ITD; midbrain
9.  Biophysical basis of the sound analog membrane potential that underlies coincidence detection in the barn owl 
Interaural time difference (ITD), or the difference in timing of a sound wave arriving at the two ears, is a fundamental cue for sound localization. A wide variety of animals have specialized neural circuits dedicated to the computation of ITDs. In the avian auditory brainstem, ITDs are encoded as the spike rates in the coincidence detector neurons of the nucleus laminaris (NL). NL neurons compare the binaural phase-locked inputs from the axons of ipsi- and contralateral nucleus magnocellularis (NM) neurons. Intracellular recordings from the barn owl's NL in vivo showed that tonal stimuli induce oscillations in the membrane potential. Since this oscillatory potential resembled the stimulus sound waveform, it was named the sound analog potential (Funabiki et al., 2011). Previous modeling studies suggested that a convergence of phase-locked spikes from NM leads to an oscillatory membrane potential in NL, but how presynaptic, synaptic, and postsynaptic factors affect the formation of the sound analog potential remains to be investigated. In the accompanying paper, we derive analytical relations between these parameters and the signal and noise components of the oscillation. In this paper, we focus on the effects of the number of presynaptic NM fibers, the mean firing rate of these fibers, their average degree of phase-locking, and the synaptic time scale. Theoretical analyses and numerical simulations show that, provided the total synaptic input is kept constant, changes in the number and spike rate of NM fibers alter the ITD-independent noise whereas the degree of phase-locking is linearly converted to the ITD-dependent signal component of the sound analog potential. The synaptic time constant affects the signal more prominently than the noise, making faster synaptic input more suitable for effective ITD computation.
PMCID: PMC3821004  PMID: 24265615
phase-locking; sound localization; auditory brainstem; periodic signals; oscillation; owl
10.  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.
PMCID: PMC2504630  PMID: 17053864
binaural; discrimination; guinea pig; inferior colliculus; interaural correlation; jnd; interaural time difference (ITD); intrinsic variability; stimulus variability
11.  Maps of interaural time difference in the chicken’s brainstem nucleus laminaris 
Biological cybernetics  2008;98(6):541-559.
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 pre-requisites 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, below400 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.
PMCID: PMC3170859  PMID: 18491165
Auditory; Hearing; Sound localization; Sensory
12.  Cross-Correlation in the Auditory Coincidence Detectors of Owls 
Interaural time difference (ITD) plays a central role in many auditory functions, most importantly in sound localization. The classic model for how ITD is computed was put forth by Jeffress (1948). One of the predictions of the Jeffress model is that the neurons that compute ITD should behave as cross-correlators. Whereas cross-correlation-like properties of the ITD-computing neurons have been reported, attempts to show that the shape of the ITD response function is determined by the spectral tuning of the neuron, a core prediction of cross-correlation, have been unsuccessful. Using reverse correlation analysis, we demonstrate in the barn owl that the relationship between the spectral tuning and the ITD response of the ITD-computing neurons is that predicted by cross-correlation. Moreover, we show that a model of coincidence detector responses derived from responses to binaurally uncorrelated noise is consistent with binaural interaction based on cross-correlation. These results are thus consistent with one of the key tenets of the Jeffress model. Our work sets forth both the methodology to answer whether cross-correlation describes coincidence detector responses and a demonstration that in the barn owl, the result is that expected by theory.
PMCID: PMC2637928  PMID: 18685035
barn owl; interaural time difference; cross-correlation; coincidence detection; sound localization; nucleus laminaris
13.  Binaural Gain Modulation of Spectrotemporal Tuning in the Interaural Level Difference-Coding Pathway 
The Journal of Neuroscience  2013;33(27):11089-11099.
In the brainstem, the auditory system diverges into two pathways that process different sound localization cues, interaural time differences (ITDs) and level differences (ILDs). We investigated the site where ILD is detected in the auditory system of barn owls, the posterior part of the lateral lemniscus (LLDp). This structure is equivalent to the lateral superior olive in mammals. The LLDp is unique in that it is the first place of binaural convergence in the brainstem where monaural excitatory and inhibitory inputs converge. Using binaurally uncorrelated noise and a generalized linear model, we were able to estimate the spectrotemporal tuning of excitatory and inhibitory inputs to these cells. We show that the response of LLDp neurons is highly locked to the stimulus envelope. Our data demonstrate that spectrotemporally tuned, temporally delayed inhibition enhances the reliability of envelope locking by modulating the gain of LLDp neurons' responses. The dependence of gain modulation on ILD shown here constitutes a means for space-dependent coding of stimulus identity by the initial stages of the auditory pathway.
PMCID: PMC3718367  PMID: 23825414
14.  Difference in response reliability predicted by spectrotemporal tuning in the cochlear nuclei of barn owls 
The brainstem auditory pathway is obligatory for all aural information. Brainstem auditory neurons must encode the level and timing of sounds, as well as their time-dependent spectral properties, the fine structure and envelope, which are essential for sound discrimination. This study focused on envelope coding in the two cochlear nuclei of the barn owl, nucleus angularis (NA) and nucleus magnocellularis (NM). NA and NM receive input from bifurcating auditory nerve fibers and initiate processing pathways specialized in encoding interaural time (ITD) and level (ILD) differences, respectively. We found that NA neurons, though unable to accurately encode stimulus phase, lock more strongly to the stimulus envelope than NM units. The spectrotemporal receptive fields (STRFs) of NA neurons exhibit a pre-excitatory suppressive field. Using multilinear regression analysis and computational modeling, we show that this feature of STRFs can account for enhanced across-trial response reliability, by locking spikes to the stimulus envelope. Our findings indicate a dichotomy in envelope coding between the time and intensity processing pathways as early as at the level of the cochlear nuclei. This allows the ILD processing pathway to encode envelope information with greater fidelity than the ITD processing pathway. Furthermore, we demonstrate that the properties of the neurons’ STRFs can be quantitatively related to spike timing reliability.
PMCID: PMC3059808  PMID: 21368035
Nucleus angularis; STRF; spectrotemporal tuning; cochlear nuclei; barn owl; response reliability
15.  Behavioral Sensitivity to Interaural Time Differences in the Rabbit 
Hearing research  2007;235(1-2):134-142.
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° and can detect very small changes in the ITD (10–20 μs). In contrast, many animals localize sounds with less precision than humans. Rabbits, for example, have sound localization thresholds of ~22°. 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 ± 300 μs. The behavioral task was conditioned avoidance and the stimulus was band-limited noise (500–1500 Hz). Across animals, the average discrimination threshold was 50–60 μs for reference ITDs of 0 to ± 200 μs. There was no trend in the thresholds across this range of reference ITDs. For a reference ITD of ± 300 μs, which is near the limit of the physiological window defined by the head width in this species, the discrimination threshold increased to ~100 μs. 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 Suppl 532:46–53, 1997) is determined by factors other than head size.
PMCID: PMC2692955  PMID: 18093767
Sound localization; animal psychoacoustics; neural discrimination
16.  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.
PMCID: PMC3123442  PMID: 21567250
coincidence detection; interaural time difference; discrimination; binaural; sound localization
17.  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.
PMCID: PMC3123442  PMID: 21567250
coincidence detection; interaural time difference; discrimination; binaural; sound localization
18.  Theoretical foundations of the sound analog membrane potential that underlies coincidence detection in the barn owl 
A wide variety of neurons encode temporal information via phase-locked spikes. In the avian auditory brainstem, neurons in the cochlear nucleus magnocellularis (NM) send phase-locked synaptic inputs to coincidence detector neurons in the nucleus laminaris (NL) that mediate sound localization. Previous modeling studies suggested that converging phase-locked synaptic inputs may give rise to a periodic oscillation in the membrane potential of their target neuron. Recent physiological recordings in vivo revealed that owl NL neurons changed their spike rates almost linearly with the amplitude of this oscillatory potential. The oscillatory potential was termed the sound analog potential, because of its resemblance to the waveform of the stimulus tone. The amplitude of the sound analog potential recorded in NL varied systematically with the interaural time difference (ITD), which is one of the most important cues for sound localization. In order to investigate the mechanisms underlying ITD computation in the NM-NL circuit, we provide detailed theoretical descriptions of how phase-locked inputs form oscillating membrane potentials. We derive analytical expressions that relate presynaptic, synaptic, and postsynaptic factors to the signal and noise components of the oscillation in both the synaptic conductance and the membrane potential. Numerical simulations demonstrate the validity of the theoretical formulations for the entire frequency ranges tested (1–8 kHz) and potential effects of higher harmonics on NL neurons with low best frequencies (<2 kHz).
PMCID: PMC3821005  PMID: 24265616
phase-locking; sound localization; auditory brainstem; periodic signals; oscillation; owl
19.  Phase Locking of Auditory-Nerve Fibers to the Envelopes of High-Frequency Sounds: Implications for Sound Localization 
Journal of neurophysiology  2006;96(5):2327-2341.
Although listeners are sensitive to interaural time differences (ITDs) in the envelope of high-frequency sounds, both ITD discrimination performance and the extent of lateralization are poorer for high-frequency sinusoidally amplitude-modulated (SAM) tones than for low-frequency pure tones. Psychophysical studies have shown that ITD discrimination at high frequencies can be improved by using novel transposed-tone stimuli, formed by modulating a high-frequency carrier by a half-wave–rectified sinusoid. Transposed tones are designed to produce the same temporal discharge patterns in high-characteristic frequency (CF) neurons as occur in low-CF neurons for pure-tone stimuli. To directly test this hypothesis, we compared responses of auditory-nerve fibers in anesthetized cats to pure tones, SAM tones, and transposed tones. Phase locking was characterized using both the synchronization index and autocorrelograms. With both measures, phase locking was better for transposed tones than for SAM tones, consistent with the rationale for using transposed tones. However, phase locking to transposed tones and that to pure tones were comparable only when all three conditions were met: stimulus levels near thresholds, low modulation frequencies (<250 Hz), and low spontaneous discharge rates. In particular, phase locking to both SAM tones and transposed tones substantially degraded with increasing stimulus level, while remaining more stable for pure tones. These results suggest caution in assuming a close similarity between temporal patterns of peripheral activity produced by transposed tones and pure tones in both psychophysical studies and neurophysiological studies of central neurons.
PMCID: PMC2013745  PMID: 16807349
20.  Detection of Interaural Time Differences in the Alligator 
The auditory systems of birds and mammals use timing information from each ear to detect interaural time difference (ITD). To determine whether the Jeffress-type algorithms that underlie sensitivity to ITD in birds are an evolutionarily stable strategy, we recorded from the auditory nuclei of crocodilians, who are the sister group to the birds. In alligators, precisely timed spikes in the first-order nucleus magnocellularis (NM) encode the timing of sounds, and NM neurons project to neurons in the nucleus laminaris (NL) that detect interaural time differences. In vivo recordings from NL neurons show that the arrival time of phase-locked spikes differs between the ipsilateral and contralateral inputs. When this disparity is nullified by their best ITD, the neurons respond maximally. Thus NL neurons act as coincidence detectors. A biologically detailed model of NL with alligator parameters discriminated ITDs up to 1 kHz. The range of best ITDs represented in NL was much larger than in birds, however, and extended from 0 to 1000 μs contralateral, with a median ITD of 450 μs. Thus, crocodilians and birds employ similar algorithms for ITD detection, although crocodilians have larger heads.
PMCID: PMC3170862  PMID: 19553438
21.  Multiplicative Auditory Spatial Receptive Fields Created by a Hierarchy of Population Codes 
PLoS ONE  2009;4(11):e8015.
A multiplicative combination of tuning to interaural time difference (ITD) and interaural level difference (ILD) contributes to the generation of spatially selective auditory neurons in the owl's midbrain. Previous analyses of multiplicative responses in the owl have not taken into consideration the frequency-dependence of ITD and ILD cues that occur under natural listening conditions. Here, we present a model for the responses of ITD- and ILD-sensitive neurons in the barn owl's inferior colliculus which satisfies constraints raised by experimental data on frequency convergence, multiplicative interaction of ITD and ILD, and response properties of afferent neurons. We propose that multiplication between ITD- and ILD-dependent signals occurs only within frequency channels and that frequency integration occurs using a linear-threshold mechanism. The model reproduces the experimentally observed nonlinear responses to ITD and ILD in the inferior colliculus, with greater accuracy than previous models. We show that linear-threshold frequency integration allows the system to represent multiple sound sources with natural sound localization cues, whereas multiplicative frequency integration does not. Nonlinear responses in the owl's inferior colliculus can thus be generated using a combination of cellular and network mechanisms, showing that multiple elements of previous theories can be combined in a single system.
PMCID: PMC2776990  PMID: 19956693
22.  Mechanisms for Adjusting Interaural Time Differences to Achieve Binaural Coincidence Detection 
Understanding binaural perception requires detailed analyses of the neural circuitry responsible for the computation of interaural time differences (ITDs). In the avian brainstem, this circuit consists of internal axonal delay lines innervating an array of coincidence detector neurons that encode external ITDs. Nucleus magnocellularis (NM) neurons project to the dorsal dendritic field of the ipsilateral nucleus laminaris (NL) and to the ventral field of the contralateral NL. Contralateral-projecting axons form a delay line system along a band of NL neurons. Binaural acoustic signals in the form of phase-locked action potentials from NM cells arrive at NL and establish a topographic map of sound source location along the azimuth. These pathways are assumed to represent a circuit similar to the Jeffress model of sound localization, establishing a place code along an isofrequency contour of NL. Three-dimensional measurements of axon lengths reveal major discrepancies with the current model; the temporal offset based on conduction length alone makes encoding of physiological ITDs impossible. However, axon diameter and distances between Nodes of Ranvier also influence signal propagation times along an axon. Our measurements of these parameters reveal that diameter and internode distance can compensate for the temporal offset inferred from axon lengths alone. Together with other recent studies these unexpected results should inspire new thinking on the cellular biology, evolution and plasticity of the circuitry underlying low frequency sound localization in both birds and mammals.
PMCID: PMC2822993  PMID: 20053889
Sound; Localization; Auditory; Brainstem; Axon; Conduction; Velocity
23.  Optical Imaging of Interaural Time Difference Representation in Rat Auditory Cortex 
We used in vivo voltage-sensitive dye optical imaging to examine the cortical representation of interaural time difference (ITD), which is believed to be involved in sound source localization. We found that acoustic stimuli with dissimilar ITD activate various localized domains in the auditory cortex. The main loci of the activation pattern shift up to 1 mm during the first 40 ms of the response period. We suppose that some of the neurons in each pool are sensitive to the definite ITD and involved in the transduction of information about sound source localization, based on the ITD. This assumption gives a reasonable fit to the Jeffress model in which the neural network calculates the ITD to define the direction of the sound source. Such calculation forms the basis for the cortex's ability to detect the azimuth of the sound source.
PMCID: PMC2654020  PMID: 19277218
auditory cortex; interaural time difference; optical imaging; voltage-sensitive dye
24.  Sensitivity to Interaural Time Differences in the Inferior Colliculus with Bilateral Cochlear Implants 
Bilateral cochlear implantation attempts to increase performance over a monaural prosthesis by harnessing the binaural processing of the auditory system. Although many bilaterally implanted human subjects discriminate interaural time differences (ITDs), a major cue for sound localization and signal detection in noise, their performance is typically poorer than that of normal-hearing listeners. We developed an animal model of bilateral cochlear implantation to study neural ITD sensitivity for trains of electric current pulses delivered via bilaterally implanted intracochlear electrodes. We found that a majority of single units in the inferior colliculus of acutely deafened, anesthetized cats are sensitive to ITD and that electric ITD tuning is as sharp as found for acoustic stimulation with broadband noise in normal-hearing animals. However, the sharpness and shape of ITD tuning often depended strongly on stimulus intensity; some neurons had dynamic ranges of ITD sensitivity as low as 1 dB. We also found that neural ITD sensitivity was best at pulse rates below 100 Hz and decreased with increasing pulse rate. This rate limitation parallels behavioral ITD discrimination in bilaterally implanted individuals. The sharp neural ITD sensitivity found with electric stimulation at the appropriate intensity is encouraging for the prospect of restoring the functional benefits of binaural hearing in bilaterally implanted human subjects and suggests that neural plasticity resulting from previous deafness and deprivation of binaural experience may play a role in the poor ITD discrimination with current bilateral implants.
PMCID: PMC2041852  PMID: 17581961
binaural hearing; electric stimulation; neural prosthesis; cochlear implant; inferior colliculus; ITD
25.  The representation of sound localization cues in the barn owl's inferior colliculus 
The barn owl is a well-known model system for studying auditory processing and sound localization. This article reviews the morphological and functional organization, as well as the role of the underlying microcircuits, of the barn owl's inferior colliculus (IC). We focus on the processing of frequency and interaural time (ITD) and level differences (ILD). We first summarize the morphology of the sub-nuclei belonging to the IC and their differentiation by antero- and retrograde labeling and by staining with various antibodies. We then focus on the response properties of neurons in the three major sub-nuclei of IC [core of the central nucleus of the IC (ICCc), lateral shell of the central nucleus of the IC (ICCls), and the external nucleus of the IC (ICX)]. ICCc projects to ICCls, which in turn sends its information to ICX. The responses of neurons in ICCc are sensitive to changes in ITD but not to changes in ILD. The distribution of ITD sensitivity with frequency in ICCc can only partly be explained by optimal coding. We continue with the tuning properties of ICCls neurons, the first station in the midbrain where the ITD and ILD pathways merge after they have split at the level of the cochlear nucleus. The ICCc and ICCls share similar ITD and frequency tuning. By contrast, ICCls shows sigmoidal ILD tuning which is absent in ICCc. Both ICCc and ICCls project to the forebrain, and ICCls also projects to ICX, where space-specific neurons are found. Space-specific neurons exhibit side peak suppression in ITD tuning, bell-shaped ILD tuning, and are broadly tuned to frequency. These neurons respond only to restricted positions of auditory space and form a map of two-dimensional auditory space. Finally, we briefly review major IC features, including multiplication-like computations, correlates of echo suppression, plasticity, and adaptation.
PMCID: PMC3394089  PMID: 22798945
sound localization; central nucleus of the inferior colliculus; auditory; plasticity; adaptation; interaural time difference; interaural level difference; frequency tuning

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