Interaural Level Difference Discrimination Thresholds for Single Neurons in the Lateral Superior Olive

The lateral superior olive (LSO) is one of the earliest sites in the auditory pathway that is involved in processing acoustical cues to sound location. Here, we tested the hypothesis that LSO neurons can signal small changes in interaural level differences (ILDs), a cue to horizontal sound location, of pure tones based on discharge rate consistent with psychophysical performance in the discrimination of ILDs. Neural thresholds for ILD discrimination were determined from the discharge rates and associated response variability of single units in response to 300 ms tones in the LSO of barbiturate-anesthetized cats using detection theory. Neural response variability was well described by a power function of the mean rate, both in individual neurons and collectively; LSO neurons were less variable than expected from a Poisson process. Compared with psychophysical data, the best-threshold ILDs of single LSO neurons were comparable with or better than behavior over the full range of frequencies (0.3–35 kHz) and pedestal ILDs (±25 dB) explored in this study. With a pedestal ILD of 0 dB, ILD increments of 1 dB could be discriminated by some neurons, with a median of 4.35 dB across neurons. For pedestal ILDs away from 0 dB, the best-threshold ILDs were as low as 0.5 dB, with a median of 2.3 dB. These findings support the hypothesis that the LSO plays a role in the extraction of ILD, and that the representation of ILD by LSO neurons may set a lower bound on the behavioral sensitivity to ILDs.

Related papers:

title: Auditory lateralization in monkeys: an examination of two cues serving directional hearing.
abstract: In assigning binaural ongoing time differences (phase) as the cue for localization of low frequencies, and binaural intensity differences as the cue for localization of high frequencies, the duplex theory has successfully accounted for human directional hearing of tones. Sensitivity of monkeys to these cues was examined in two experiments. The dependencies on frequency of interaural intensity difference thresholds (lateralization experiment I) and time difference thresholds (lateralization experiment II) were determined behaviorally on three monkeys (M. nemestrina). The range of frequencies was from 125 Hz to 8 kHz in experiment I and from 250 Hz to 2 kHz in experiment II. The results indicate that the duplex theory is applicable to monkeys. However, monkeys are less sensitive than man to both binaural cues. The shortest time disparity monkeys discriminate is 42 microseconds at 1.5 kHz and the smallest intensity difference is 3.5 dB at 500 Hz. Good agreement between the present findings and localization measurements [C. H. Brown et al., 1492 (1978)] suggests: (a) that monkeys utilize time disparity cues through higher frequencies than man; and (b) that inaccurate localization by monkeys at high frequencies reflects decreasing sensitivity to interaural intensity cues.

title: Localization of pure tones.
abstract: Eight cats were trained to localize the source of tonal signals, presented in a free field. Tones of six different frequencies, in octave steps from 250 Hz to 8 kHz, were used. The angle at which an animal's performance was 75% correct was defined as the threshold for minimal detectable angle. This localization threshold was obtained for each cat at each frequency. Across frequencies the localization thresholds changed little from 250 Hz to 2 kHz, then increased greatly at 4 kHz and decreased again at 8 kHz. Similarities and differences in the performance of man and cat in localizing pure tones are discussed in relation to the possibility that the cat, like man, uses differences in binaural time and in binaural intensity as the principle cues for sound localization.

title: Sound orientation behavior in cats. I. Localization of broadband noise.
abstract: Behavioral experiments measured the accuracy of the cat's voluntary head orientation responses to bursts of broadband noise at 104 locations in the frontal sound field. Cats were presented sound stimuli at randomly selected locations and received a food reward for releasing a lever when a light-emitting diode (LED) flashed at the same location. Head movements to the perceived location of the acoustic stimulus, and therefore expected location of the LED cue, were tracked by an electromagnetic sensor. Orientation responses to single sound bursts were directed to within 5 degrees of the azimuth and elevation of sounds within 15 degrees of the immediate frontal field and did not change for burst durations of 40, 100, and 200 ms. An increasing underestimation of target location was observed as the sound stimulus moved to more lateral azimuths and higher elevations. The "undershoot" phenomenon was reduced by allowing subjects to track paired stimulus bursts that repeated from the same location. These effects of sound location on the accuracy of orientation responses are predicted by the availability of mid-frequency spectral cues for sound localization.

title: Interaural sound pressure level differences associated with sound-source locations in the frontal hemifield of the domestic cat
abstract: Interaural sound pressure level differences (ILDs) associated with a range of sound-source azimuths and elevations in the frontal hemifields of four cats were measured for each of seven pure-tone stimuli ranging in frequency from 2-32 kHz. The overall pattern of ILD across location at each frequency was remarkably similar in all cats. At 2, 4 and 8 kHz the relationships of ILD to azimuth and elevation were generally monotonic with ILD increasing with increasing azimuth. At 12 kHz and above, however, non-monotonic relationships developed and circumscribed regions of particularly large, positive and negative ILDs were apparent. That ILDs associated with many sound-source locations on the near side of the head are negative in sign has not been widely noted in the previous literature. The data obtained in this study were compared with those from previous studies and the acoustical mechanisms likely to have contributed to production of the observed ILDs were considered. Some implications of the data for auditory localization in three-dimensional space were discussed.

title: Headphone simulation of free-field listening. I: Stimulus synthesis.
abstract: This article describes techniques used to synthesize headphone-presented stimuli that simulate the ear-canal waveforms produced by free-field sources. The stimulus synthesis techniques involve measurement of each subject's free-field-to-eardrum transfer functions for sources at a large number of locations in free field, and measurement of headphone-to-eardrum transfer functions with the subject wearing headphones. Digital filters are then constructed from the transfer function measurements, and stimuli are passed through these digital filters. Transfer function data from ten subjects and 144 source positions are described in this article, along with estimates of the various sources of error in the measurements. The free-field-to-eardrum transfer function data are consistent with comparable data reported elsewhere in the literature. A comparison of ear-canal waveforms produced by free-field sources with ear-canal waveforms produced by headphone-presented simulations shows that the simulations duplicate free-field waveforms within a few dB of magnitude and a few degrees of phase at frequencies up to 14 kHz.

title: Head-related transfer functions of the Rhesus monkey
abstract: Head-related transfer functions (HRTFs) are direction-specific acoustic filters formed by the head, the pinnae and the ear canals. They can be used to assess acoustical cues available for sound localization and to construct virtual auditory environments. We measured the HRTFs of three anesthetized Rhesus monkeys (Macaca mulatta) from 591 locations in the frontal hemisphere ranging from -90 degrees (left) to 90 degrees (right) in azimuth and -60 degrees (down) to 90 degrees (up) in elevation for frequencies between 0.5 and 15 kHz. Acoustic validation of the HRTFs shows good agreement between free field and virtual sound sources. Monaural spectra exhibit deep notches at frequencies above 9 kHz, providing putative cues for elevation discrimination. Interaural level differences (ILDs) and interaural time differences (ITDs) generally vary monotonically with azimuth between 0.5 and 8 kHz, suggesting that these two cues can be used to discriminate azimuthal position. Comparison with published subsets of HRTFs from squirrel monkeys (Saimiri sciureus) shows good agreement. Comparison with published human HRTFs from the frontal hemisphere demonstrates overall similarity in the patterns of ILD and ITD, suggesting that the Rhesus monkey is a good acoustic model for these two sound localization cues in humans. Finally, the measured ITDs in the horizontal plane agree well between -40 degrees and 40 degrees in azimuth with those calculated from a spherical head model with a radius of 52 mm, one-half the interaural distance of the monkey.

title: Encoding of Interaural Level Differences for Sound Localization
abstract: One of the primary acoustical cues to the spatial location of a sound source in azimuth is the interaural level difference (ILD). The ILD cue results from the frequency- and direction-dependent modifications of the sound waves by the head and pinnae and is defined as the difference in sound pressure level of the signals arriving at the two ears as a function of frequency. The neurons comprising the lateral superior olive (LSO) are thought to be the most peripheral neurons in the ascending auditory pathway to encode the ILD cue. Although it has been known for decades that LSO neurons are sensitive to ILDs, the specific mechanisms for ILD selectivity have been elucidated only recently. Here we review the mechanisms of ILD processing by first discussing the physical bases of the ILD cue itself. Then we examine the anatomical, physiological, and biophysical specializations present in the neural circuitry through the neurons of the LSO that enable the encoding of ILDs. The specializations in the input neurons to the LSO permit the spectral and temporal attributes of the sound arriving at the two ears to be represented accurately while the specializations at the LSO allow ILDs to be encoded based on these representations on nearly a frequency-by-frequency and time-by-time basis. Comparative studies that have examined the functional role of the LSO through combined anatomical, physiological, and behavioral observations confirm that the LSO is necessary for sound source localization based on ILDs.

title: Midline and lateral field sound localization in the ferret (Mustela putorius): contribution of the superior olivary complex.
abstract: 1. The ability of ferrets to localize sound in space was determined before and after unilateral or bilateral lesions of the superior olivary complex (SOC). Lesions were made by pressure injection of kainic acid into the SOC through a stereotaxically positioned glass micropipette. The lesions destroyed the cell bodies in the superior olive without disrupting fibers of passage in the trapezoid body or other pathways in the auditory brain stem. The integrity of fibers was demonstrated by protargol staining of axonal processes and by the retrograde transport of horseradish peroxidase (HRP) from the inferior colliculus to other auditory brain stem nuclei. Behavioral tests were carried out separately for sound localization at midline and lateral field positions. Minimum audible angles were determined for single 45-ms noise bursts presented through paired loudspeakers positioned symmetrically around 0, -60, and +60 degrees azimuth. 2. Four ferrets received complete lesions of the left SOC, and two received complete lesions of the right SOC. In general, unilateral destruction of the superior olive resulted in impairments in sound localization in both left and right lateral fields. In some cases, deficits were also apparent on midline. Four additional animals received unilateral lesions that spared cells within the SOC. In most cases, deficits were apparent despite incomplete lesions of the SOC. The pattern of deficits was generally consistent with that found in animals with complete lesions. Most animals had difficulty localizing sounds in the lateral fields. 3. Four animals received bilateral lesions of the SOC. Three had complete or near-complete destruction of the superior olive on one side of the brain with relatively minor damage on the other side. Each of these animals exhibited behavioral deficits that were particularly severe ipsilateral to the more extensively damaged superior olive. One animal with complete bilateral destruction of the SOC was incapable of sound localization, even with 2-s noise bursts. This animal, however, suffered severe motor impairments after surgery that might have contributed to the apparent inability to localize sound. 4. Two animals with kainic acid lesions that caused little or no damage to the SOC were still capable of high levels of performance in tests of sound localization and had no elevation in minimum audible angles. These cases served as controls for the possible effects of nonspecific brain damage and demonstrated that kainic acid injections per se resulted in no obvious deficits in our test situation.(ABSTRACT TRUNCATED AT 400

title: Comparative Morphometry of Mammalian Central Auditory Systems: Variation in Nuclei and Form of the Ascending System
abstract: The volumes of the ten largest subcortical auditory nuclei were measured individually in a sample of 53 mammals, including 16 Australian and four American marsupials. The nuclear sizes relative to the total of subcortical auditory tissue were normalized and then analyzed individually for statistically reliable deviations. The overall form of the entire system of ten nuclei and two nuclear subsystems (cochlear nuclei, superior olives) were also analyzed for similarities and notable deviations among the animals. The results show that the absolute size of the auditory system varies more than 139-fold among the 53 mammals (with moles the smallest and humans the largest). Log auditory system volume and log brain weight are closely correlated (r = 0.903, p <0.0001). Bats, kangaroo rats, marmosa opossums, and Norway rats have the largest auditory systems relative to their brain size, while humans have the smallest by far. The other primates also have auditory system/brain size ratios smaller than the sample average, suggesting that the condition in humans is one result of an expansion of non-auditory brain parts rather than a reduction of the auditory system over geological time. The relative sizes of the ten nuclei are well ordered, with the inferior colliculus the largest nucleus by far and medial superior olive the smallest. Because the size of the superior olives, collectively, is reliably related to the size of anteroventral cochlear nucleus (r = 0.744, p <0.001), and not to the size of dorsal cochlear nucleus, the interconnectivity of the subcortical auditory system is probably a factor in the size of the nuclei. In its overall form, the subcortical auditory system is highly similar among mammals, with an average correlation across nuclei of 0.923. This high value means that the overall form of the system has been relatively stable over geological time. The animals with least deviation from the average form are ring-tailed possums, bandicoots, and yellow-bellied gliders, all marsupials. Those with the most unusual forms are mice, bats, and kangaroo rats, all placentals.

title: Lateralization of High‐Frequency Tones
abstract: Thresholds for interaural difference between the intensities of dichotic tone pulses were measured on five subjects by the method of constant stimuli, at frequencies between 250 and 10 000 cps and at a sensation level of 50 db. The just noticeable dichotic difference in intensity is about 1 db at 1000 cps, a little smaller at lower frequencies, and still smaller (0.5 db) at higher frequencies. This function is compared with the interaural difference in intensity produced by the just noticeable deviation from the median plane of an actual source of tone pulses. At low frequencies, where phase or time differences are generally considered more important than intensity differences for auditory localization, these two functions differ greatly. At frequencies between 1500 and 6000 cps, the threshold for a dichotic difference in intensity matches the interaural difference in intensity that is produced by the just noticeable deviation from the median plane of an actual source. The relation between the discriminat...

title: Effect of Behavioral Context on Representation of a Spatial Cue in Core Auditory Cortex of Awake Macaques
abstract: Primary auditory cortex plays a crucial role in spatially directed behavior, but little is known about the effect of behavioral state on the neural representation of spatial cues. Macaques were trained to discriminate binaural cues to sound localization, eventually allowing measurement of thresholds comparable to human hearing. During behavior and passive listening, single units in low-frequency auditory cortex showed robust and consistent tuning to interaural phase difference (IPD). In most neurons, behavior exerted an effect on peak discharge rate (58% increased, 13% decreased), but this was not accompanied by a detectable shift in the best IPD of any cell. Neurometric analysis revealed a difference in discriminability between the behaving and passive condition in half of the sample (52%), but steepening of the neurometric function (29%) was only slightly more common than flattening (23%). This suggests that performance of a discrimination task does not necessarily confer an advantage in understanding the representation of the spatial cue in primary auditory cortex but nevertheless revealed some physiological effects. These results suggest that responses observed during passive listening provide a valid representation of neuronal response properties in core auditory cortex.

title: Lateralization of tonal stimuli by the cat.
abstract: The ability of the cat to “lateralize” tonal signals having interaural intensive or interaural temporal disparities was measured. Interaural intensive differences were studied at 0.5, 1.0, and 3.0 kHz. Interaural temporal differences were studied at 0.5, 1.0, 2.0, and 3.0 kHz. Miniature audio transducers were employed to present the stimuli to the animals. The transducers were held in a fixed spatial relation to the auditory canal by means of “pinna inserts” and leather helmets. An avoidance response in a shuttle box was used as the dependent variable. The animals' task involved the detection of a right‐left reversal in a gated sequence of tone bursts. The cat appears to be about as sensitive as the human to both interaural intensive and temporal disparities at each of the frequencies studied.

title: Some implications of the stochastic behavior of primary auditory neurons
abstract: SummaryRecent electrophysiological studies of the activity in response to acoustic stimuli of single primary afferent neurons in the VIIIth nerve of mammals strongly suggest that the spike activity is inherently stochastic in thata)the detailed pattern of activity varies unpredictably from repetition to repetition of an identical acoustic wave-form, butb)appropriate averages of this activity show stability or statistical regularity.
Since the only way in which auditory information can reach the more central parts of the nervous system is via the VIIIth nerve, the effective “neural noise” implied by such stochastic “coding” of auditory information must set some sort of limits on auditory discriminations. As shown in this paper, these limits can be calculated if an appropriate statistical model of the neural activity is accepted. Of course, no more than a bound on discrimination performance can be determined in this way, since there is no reason to suppose that the central processing in the nervous system is effectively ideal — particularly in the artificial situations of auditory psychophysics. But if these bounds closely approximate performance (and there is some reason to believe they may, at least for the very simple auditory discriminations treated in this paper) then it seems reasonable to maintain that the observed stochastic “coding” in a sense accounts for or explains these behavioral limits.The key to such an analytical effort as proposed here is the availability of sufficient data on the activity in single primary afferent fibers to permit one to infer a reasonable statistical model describing the total (spike) activity in all the tens of thousands of acoustic fibers of the VIIIth nerve in response to the signals of interest. In spite of extensive studies of single-fiber activity by a number of observers, highfrequency tone bursts (several kilocycles or more) are one of the few classes of signals for which an even roughly adequate amount of data exists at present. For such signals the data suggest that the firing pattern of each neuron can be modeled as a sample function from a simple stationary random process whose parameters depend on stimulus frequency, tensity, cochlear location, neuron sensitivity, etc. The desired overall model can then be obtained by assuming reasonable distributions for these parameters over the population of neurons.Given such a model for the pattern of activity in the auditory nerve, discriminations between tone bursts of different frequencies or different intensities can be studied. These differences will be reflected in the properties of the random processes characterizing the auditory nerve. Using the theory of statistical hypothesis testing, it is possible to determine the smallest difference which could be reliably discriminated by an ideal computer examining the total activity of all fibers. As will be shown, the performance of this ideal computer parallels in a number of ways the performance of human observers. The general trends of this performance, moreover, (including an effect similar to Weber's Law) are largely insensitive to the details of the random process model for the neuron activity. However, it is interesting that the functioning of the ideal computer is particularly and uniquely simple for precisely that class of random process models which seems best to represent the physiological data.

title: Frequency discrimination in the auditory system: Place or periodicity mechanisms?
abstract: The pattern of all-or-none firings in many sensory nerves can usefully be modeled as a set of sample functions from non-stationary random processes whose parameters are determined by the sensory stimulus. Given such a description, one can compare the parameter estimation, discrimination, or detection performance of a human observer (as measured behaviorally) with the computed limiting performance of an ideal observer operating on the same neural input. Such a comparison is carried out in detail for the discrimination in frequency of an auditory sinusoidal tone burst. The results suggest that the human listener behaves as if he ignored (at least for this class of sounds) the periodicity information in the auditory nerve pattern (resulting from the phase-locking of the firings to the stimulus), but made full effective use of the place information (resulting primarily, although not entirely, from the mechanical tuning of the inner ear). On the other hand, it is known that the auditory system can discriminate certain more complicated stimuli on the basis of periodicity information if that is the only cue available. A modification of the structure of an optimum sinusoidal frequency discrimination system is conjectured that seems to be, at least qualitatively, consistent with the available evidence.

title: Optimal binaural processing based on point process models of preprocessed cues
abstract: Toward localizing high-frequency tonal signals in the horizontal plane, the lateral superior olive (LSO) encodes the interaural level differences, the crucial cue for azimuthal angle extraction by more central nuclei in the binaural pathway. This paper derives optimal processors for localization using inputs from the LSO and anteroventral cochlear nucleus. Both Poisson and non-Poisson point processes are developed to model cellular responses. The LSO projection is shown to be the only input needed in maximum likelihood angle estimation, and it is processed similarly under different LSO response models. However, localization performance depends strongly on the model’s regularity, with increasing regularity improving localization accuracy and precision.

title: Regularity and latency of units in ventral cochlear nucleus: implications for unit classification and generation of response properties.
abstract: 1. The responses of neurons in the ventral cochlear nucleus (VCN) of decerebrate cats are described with regard to their regularity of discharge and latency. Regularity is measured by estimating the mean and standard deviation of interspike intervals as a function of time during responses to short tone bursts (25 ms). This method extends the usual interspike-interval analysis based on interval histograms by allowing the study of temporal changes in regularity during transient responses. The coefficient of variation (CV), equal to the ratio of standard deviation to mean interspike interval, is used as a measure of irregularity. Latency is measured as the mean and standard deviation of the latency of the first spike in response to short tone bursts, with 1.6-ms rise times. 2. The regularity and latency properties of the usual PST histogram response types are shown. Five major PST response type classes are used: chopper, primary-like, onset, onset-C, and unusual. The presence of a prepotential in a unit's action potentials is also noted; a prepotential implies that the unit is recorded from a bushy cell. 3. Units with chopper PST histograms give the most regular discharge. Three varieties of choppers are found. Chop-S units (regular choppers) have CVs less than 0.35 that are approximately constant during the response; chop-S units show no adaptation of instantaneous rate, as measured by the inverse of the mean interspike interval. Chop-T units have CVs greater than 0.35, show an increase in irregularity during the response and show substantial rate adaptation. Chop-U units have CVs greater than 0.35, show a decrease in irregularity during the response, and show a variety of rate adaptation behaviors, including negative adaptation (an increase in rate during a short-tone response). Irregular choppers (chop-T and chop-U units) rarely have CVs greater than 0.5. Choppers have the longest latencies of VCN units; all three groups have mean latencies at least 1 ms longer than the shortest auditory nerve (AN) fiber mean latencies. 4. Chopper units are recorded from stellate cells in VCN (35, 42). Our results for chopper units suggest a model for stellate cells in which a regularly firing action potential generator is driven by the summation of the AN inputs to the cell, where the summation is low-pass filtered by the membrane capacitance of the cell.(ABSTRACT TRUNCATED AT 400

title: Signal detection theory and psychophysics
abstract: Book on statistical decision theory and sensory processes in signal detection theory and psychophysics

title: Detectability Index Measures of Binaural Masking Level Difference Across Populations of Inferior Colliculus Neurons
abstract: In everyday life we continually need to detect signals against a background of interfering noise (the “cocktail party effect”): a task that is much easier to accomplish using two ears. The binaural masking level difference (BMLD) measures the ability of listeners to use a difference in binaural attributes to segregate sound sources and thus improve their discriminability against interfering noises. By computing the detectability of tones from rate-versus-level functions in the presence of a suprathreshold noise, we previously demonstrated that individual low-frequency delay-sensitive neurons in the inferior colliculus are able to show BMLDs. Here we consider the responses of a population of such neurons when the noise level is held constant (as conventionally in psychophysical paradigms). We have sampled the responses of 121 units in the inferior colliculi of five guinea pigs to identical noise and 500 Hz tones at both ears (NoSo) and to identical noise but with the 500 Hz tone at one ear inverted (NoSπ). The result suggests that the neurons subserving detection of So tones in No (identical noise at the two ears) noise are those neurons with best frequencies (BFs) close to 500 Hz that respond to So tones with an increase in their discharge rate from that attributable to the noise. The detection of the inverted (Sπ) signal is also attributable to neurons with BFs close to 500 Hz. However, among these neurons, the presence of the Sπ tone was indicated by an increased discharge rate in some neurons and by a decreased discharge rate in others.

title: Interaural Time Difference Discrimination Thresholds for Single Neurons in the Inferior Colliculus of Guinea Pigs
abstract: Sensitivity to changes in the interaural time difference (ITD) of 50 msec tones was measured in single units in the inferior colliculus of urethane-anesthetized guinea pigs. ITD functions were measured with 100 repeats and fine spacing (100 points per cycle). The just noticeable difference (jnd) for ITD was determined using receiver operating characteristic (ROC) analysis of the spike-count distribution at each ITD. The jnd became progressively smaller as the signal frequency increased from 50 to 800 Hz but became unmeasurable above 1 kHz. The lowest jnds (30 μsec) were comparable with human jnds, indicating that there is sufficient information in the firings of individual neurons to permit discrimination without obligatory pooling. ROC analysis requires the choice of a reference ITD from which the jnd may be found by stepping the target ITD through the ITD function. For each neuron the reference was chosen to minimize the jnd. The lowest jnd was usually for ipsilateral leading references, near the minimum of the ITD function where the variance was also low, but where the slope was nearing its steepest. This was despite the peak of the ITD function occurring for contralateral leading stimuli. When the reference ITD was on midline, a jnd could be obtained by looking for firing rates either greater or smaller than the firing rate at midline. The lower jnd was usually obtained by looking for a decrease in firing rate. As duration increased, jnds either decreased or increased, depending on unit type, whereas when level increased, jnds generally increased.

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

title: Visual cortex neurons in monkeys and cats: detection, discrimination, and identification.
abstract: A descriptive function method was used to measure the detection, discrimination, and identification performance of a large population of single neurons recorded from within the primary visual cortex of the monkey and the cat, along six stimulus dimensions: contrast, spatial position, orientation, spatial frequency, temporal frequency, and direction of motion. First, the responses of single neurons were measured along each stimulus dimension, using analysis intervals comparable to a normal fixation interval (200 ms). Second, the measured responses of each neuron were fitted with simple descriptive functions, containing a few free parameters, for each stimulus dimension. These functions were found to account for approximately 90% of the variance in the measured response means and response standard deviations. (A detailed analysis of the relationship between the mean and the variance showed that the variance is proportional to the mean.) Third, the parameters of the best-fitting descriptive functions were utilized in conjunction with Bayesian (optimal) decision theory to determine the detection, discrimination, and identification performance for each neuron, along each stimulus dimension. For some of the cells in monkey, discrimination performance was comparable to behavioral performance; for most of the cells in cat, discrimination performance was better than behavioral performance. The behavioral contrast and spatial-frequency discrimination functions were similar in shape to the envelope of the most sensitive cells; they were also similar to the discrimination functions obtained by optimal pooling of the entire population of cells. The statistics which summarize the parameters of the descriptive functions were used to estimate the response of the visual cortex as a whole to a complex natural image. The analysis suggests that individual cortical neurons can reliably signal precise information about the location, size, and orientation of local image features.

title: Single Auditory Units in the Superior Olivary Complex: II: Locations of Unit Categories and Tonotopic Organization
abstract: The locations of various categories of units are described and correlations are found between unit properties and unit locations. Units on the edges of the medial superior olive (MSO) and the lateral superior olive (LSO) appear to have properties different from units inside these nuclei. Many units in the dorsomedial periolivary nucleus had wide tuning curves and some had very long latencies. Almost all units in the medial nucleus of the trapezoid body (MNTB) had irregular firing patterns, but most units in the LSO and the dorsolateral periolivary nucleus had regular firing patterns. Class 2 (off) units may correspond to ‘stellate’ cells of the MNTB.With few exceptions, units dorsolateral to the MSO were excited by ipsilateral sounds, and units ventromedial to the MSO, or in the ventral nucleus of the lateral lemniscus (VNLL), were excited by contralateral sounds.Best frequencies were arranged from high to low (1) from ventromedial to dorsolateral in the MSO, (2) from the medial limb to the lateral limb i...

title: Interaural Phase and Level Difference Sensitivity in Low-Frequency Neurons in the Lateral Superior Olive
abstract: The lateral superior olive (LSO) is believed to encode differences in sound level at the two ears, a cue for azimuthal sound location. Most high-frequency-sensitive LSO neurons are binaural, receiving inputs from both ears. An inhibitory input from the contralateral ear, via the medial nucleus of the trapezoid body (MNTB), and excitatory input from the ipsilateral ear enable level differences to be encoded. However, the classical descriptions of low-frequency-sensitive neurons report primarily monaural cells with no contralateral inhibition. Anatomical and physiological evidence, however, shows that low-frequency LSO neurons receive low-frequency inhibitory input from ipsilateral MNTB, which in turn receives excitatory input from the contralateral cochlear nucleus and low-frequency excitatory input from the ipsilateral cochlear nucleus. Therefore, these neurons would be expected to be binaural with contralateral inhibition. Here, we re-examined binaural interaction in low-frequency (less than ∼3 kHz) LSO neurons and phase locking in the MNTB. Phase locking to low-frequency tones in MNTB and ipsilaterally driven LSO neurons with frequency sensitivities <1.2 kHz was enhanced relative to the auditory nerve. Moreover, most low-frequency LSO neurons exhibited contralateral inhibition: ipsilaterally driven responses were suppressed by raising the level of the contralateral stimulus; most neurons were sensitive to interaural time delays in pure tone and noise stimuli such that inhibition was nearly maximal when the stimuli were presented to the ears in-phase. The data demonstrate that low-frequency LSO neurons of cat are not monaural and can exhibit contralateral inhibition like their high-frequency counterparts.

title: The Coding of Spatial Location by Single Units in the Lateral Superior Olive of the Cat. I. Spatial Receptive Fields in Azimuth
abstract: The lateral superior olive (LSO) is one of the most peripheral auditory nuclei receiving inputs from both ears, and LSO neurons are sensitive to interaural level differences (ILDs), one of the primary acoustical cues for sound location. We used the virtual space (VS) technique to present over earphones broadband stimuli containing natural combinations of localization cues as a function of azimuth while recording extracellular responses from single LSO cells. The responses of LSO cells exhibited spatial receptive fields (SRFs) in azimuth consonant with their sensitivity to ILDs of stimuli presented dichotically: high discharge rates for ipsilateral azimuths where stimulus amplitude to the excitatory ear exceeded that to the inhibitory ear, rapidly declining rates near the midline, and low rates for contralateral azimuths where the amplitude to the inhibitory ear exceeded that to the excitatory ear. Relative to binaural stimulation, presentations of the VS stimuli to the ipsilateral ear alone yielded increased rates, particularly in the contralateral field, confirming that the binaural SRFs were shaped by contralateral inhibition. Our finding that LSO neurons respond to azimuth consistent with their ILD sensitivity supports the long-held hypothesis that LSO neurons compute a correlate of the ILD present in free-field stimuli. Only weak correlations between the properties of pure-tone ILD functions and the SRFs were found, indicating that ILD sensitivity measured at only one sound level is not sufficient to predict sensitivity to azimuth. Sensitivity to spatial location was also retained over a wide range of stimulus levels under binaural, but not monaural, conditions.

title: Interaural level difference processing in the lateral superior olive and the inferior colliculus.
abstract: Interaural level differences (ILDs) provide salient cues for localizing high-frequency sounds in space, and populations of neurons that are sensitive to ILDs are found at almost every synaptic level from brain stem to cortex. These cells are predominantly excited by stimulation of one ear and predominantly inhibited by stimulation of the other ear, such that the magnitude of their response is determined in large part by the intensities at the 2 ears. However, in many cases ILD sensitivity is also influenced by overall intensity, which challenges the idea of unambiguous ILD coding. We investigated whether ambiguity is reduced from one synaptic level to another for 2 centers in the so-called ILD processing pathway. We recorded from single cells in the free-tailed bat lateral superior olive (LSO), the first station where ILDs are coded, and the central nucleus of the inferior colliculus (ICC), which receives a strong projection from the LSO, as well as convergent projections from many other auditory centers. We assessed effects of overall intensity by comparing ILD functions generated with different fixed intensities to the excitatory ear. LSO cells were characterized by functions that shifted in a systematic manner with increasing intensity to the excitatory ear. In contrast, significantly more ICC cells had functions that were stable across overall sound intensity, indicating that hierarchical transformations increase stability. Furthermore, a population analysis based on proportion of active cells indicated that stability in the ICC was greatly enhanced when overall population activity was considered.

title: Monaural and interaural intensity discrimination: level effects and the "binaural advantage".
abstract: This study examined whether the level effects seen in monaural intensity discrimination (Weber's law and the "near miss") in a two-interval task are also observed in discrimination of interaural intensity differences (IIDs) in a single-interval task. Both tasks were performed for various standard levels of 4-kHz pure tones and broadband noise. The Weber functions (10 log deltaI/I versus I in dB) in the monaural and binaural conditions were parallel. For noise, the Weber functions had slopes close to zero (Weber's law) while the Weber functions for the tones had a mean slope of -0.089 (near miss). The near miss for the monaural and binaural tasks with tones was eliminated when a high-pass masker was gated with the listening intervals. The near-miss was also observed for 250- and 1000-Hz tones in the binaural task despite overall decreased sensitivity to changes in IID at 1000 Hz. The binaural thresholds showed a small (about 2-dB) advantage over monaural thresholds only in the broadband noise conditions. More important, however, is the fact that the level effects seen monaurally are also seen binaurally. This suggests that the basic mechanisms responsible for Weber's law and the near miss are common to monaural and binaural processing.

title: The statistical reliability of signals in single neurons in cat and monkey visual cortex
abstract: The variability of the discharge of visual cortical neurons in cats and macaque monkeys limits the reliability with which such neurons can relay signals about weak visual stimuli. In general, the variance of a neuron's firing rate is directly proportional to its mean firing rate. The probability that a neuron will fire a criterion number of impulses on a stimulus trial grows monotonically with the contrast of a sinusoidal grating stimulus. Neural probability functions prepared either by computing the probability of criterion response or by integrating receiver operating characteristics to yield the probability of correct choice in a two-alternative forced-choice situation resemble psychometric functions obtained in psychophysical and behavioral experiments on humans and animals, but are shallower in slope. The slopes of neuronal probability functions are slightly higher when they are estimated over short time periods, but even so do not equal the slopes measured psychophysically in human and monkey observers. This discrepancy in slope could be explained if the whole observer responded only when about four neurons were active together.

title: Spectral cues for sound localization in cats: a model for discharge rate representations in the auditory nerve.
abstract: Neural representations of pinna-based spectral cues for sound localization were modeled by simulating auditory nerve discharge rates to noise bursts that had been shaped by filtering properties of the cat's head-related transfer functions (HRTFs) at 179 locations in the frontal field. The auditory nerve model transformed spectral differences between HRTFs into simulated neural rate differences. Linear equations for this transformation were developed from actual auditory nerve responses to a limited subset of HRTF-filtered noise bursts [Rice et al., 1776 (1995)]. Signal detection methods were used to investigate simulated neural responses to pairwise changes between HRTFs. The quality of neural representation for these changes, in terms of d' values, declined when the reference HRTF was moved from a central location (0 degree AZ, 0 degree EL) to a large positive azimuth in the horizontal plane (75 degrees AZ, 0 degree EL) or a high elevation in the median plane (0 degree AZ, 75 degrees EL). Most simulated responses exhibited large d' values for comparisons of contralateral versus ipsilateral azimuths, or eccentric versus frontal elevations. This rate information resulted from directionally dependent changes in the overall gain of HRTFs. In addition, fibers with best frequency (BF: the frequency of greatest sensitivity for individual fibers) between 5 and 18 kHz showed large d' values for HRTF contrasts in the immediate frontal field because of the effects of spectral notches (i.e., sharp drops in gain over a narrow frequency range). Spectral notches also played a prominent role in simulations that required identification of HRTF location in the absence of a fixed reference stimulus. These modeling results correspond well with previously described patterns in the cat's localization behaviors.

title: Neural model of stereoacuity and depth interpolation based on a distributed representation of stereo disparity [published erratum appears in J Neurosci 1991 Mar;11(3):following Table of Contents]
abstract: We have developed a model for the representation of stereo disparity by a population of neurons that is based on tuning curves similar in shape to those measured physiologically (Poggio and Fischer, 1977). Signal detection analysis was applied to the model to generate predictions of depth discrimination thresholds. Agreement between the model and human psychophysical data was possible in this model only when the population size representing disparity in a small patch of visual field was in the range of about 20-200 units. Interval encoding and rate encoding were found to be inconsistent with these data. Psychophysical data on stereo interpolation (Westheimer, 1986a) suggest that there are short-range excitatory and long-range inhibitory interactions between disparity- tuned units at nearby spatial locations. We extended our population model of disparity coding at a single spatial location to include such lateral interactions. When there was a small disparity gradient between stimuli at 2 locations, units in the intermediate, unstimulated position developed a pattern of activity corresponding to the average of the 2 lateral disparities. When there was a large disparity gradient, units at the intermediate position developed a pattern of activity corresponding to an independent superposition of the 2 lateral disparities, so that both disparities were represented simultaneously. This mixed population pattern may underlie the perception of depth discontinuities and transparent surfaces. Similar types of distributed representations may be applicable to other parameters, such as orientation, motion, stimulus size, and motor coordinates.

title: Pulse‐number distribution for the neural spike train in the cat’s auditory nerve
abstract: Pulse‐number distributions (PNDs) were recorded from primary afferent fibers in the auditory nerve of the cat, using standard extracellular microelectrode recording techniques. Pure‐tone and broadband‐noise stimuli were used. The number of neural spikes (pulses) n was measured in a set of contiguous intervals, each of duration T seconds. The quantity n varies from one interval to another. These data were then used to determine the PND, which is the probability  p(n,T) of occurrence of n spikes in the time T, versus the number n. The estimated mean and variance of  p(n,T) were obtained. Two different values of T were used. An unexpected observation was that the count mean‐to‐variance ratio R is relatively constant and independent of the stimulus intensity. Use of the PND as a statistical measure of the underlying neural point process has a number of virtues. For example, the PND readily exhibits the existence of spike clusters (e.g., pairs) for some units. The PND is essentially unaffected by time jitter and time quantization and provides a statistically significant measure for units firing at low rates. A study of the scaled and unscaled pulse‐interval distributions (PIDs), under conditions of spontaneous firing, demonstrates that the occurrences of neural events are generally not describable by a renewal process. Our investigation shows that none of the point processes customarily used to model the auditory neural spike train is consistent with all of the data. It appears that the encoding of acoustic information into nerve spikes in the peripheral auditory system takes the form of a cluster point process similar to the Neyman–Scott type. For pure‐tone excitation, the PND will be well represented as a multinomial distribution in this case.

title: Interaural intensity discrimination: insensitivity at 1000 Hz.
abstract: Recent data from three laboratories have replicated Mills' [ (1960)] finding that interaural intensity discrimination is relatively poorer for tones of 1000 Hz than for tones of either higher or lower frequencies. To get a finer look at this frequency effect, interaural intensity difference thresholds were obtained from four subjects for tones of several frequencies around 1000 Hz. An adaptive two-interval forced-choice procedure was employed, in which the overall intensity of the signals was varied randomly in order to prevent subjects from listening to monaural loudness changes. Despite large intersubject differences in overall sensitivity to interaural intensity differences, all four subjects showed a local peak in their threshold functions at or near 1000 Hz. This curious "1000-Hz effect" might be explained by imagining that an interaural intensity comparator operates more efficiently as frequency increases, but that a peripheral interaural intensity difference to interaural-time difference conversion contributes to laterality judgments for low-frequency tones, thus acting to lower thresholds again for frequencies below 1000 Hz.

title: Discrimination of interaural differences of level as a function of frequency.
abstract: Discrimination of interaural differences of level (IDLs) was measured for pure tones as a function of frequency and as a function of the interaural difference of phase or level of a standard. Varying the interaural difference of the standard was assumed to change the lateral position of its intracranial image. Threshold IDLs were approximately constant over a frequency range from 200-5000 Hz, except in a region near 1000 Hz where they were slightly elevated. Thresholds increased as the value of the standard interaural differences of phase or level increased, implying that interaural resolution declines as the lateral image moves away from midline. The results are generally consistent with the predictions of current models of lateralization, but additions to these models are required in order for them to account for the slight frequency dependence of threshold IDLs.

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

title: Difference thresholds for interaural intensity.
abstract: In an earlier paper, wqe examined the distribution of binaural resolvins power by measuring threshold increments of interaural delay as a function of overall dealy [E. R. Hafter and J. Demaio,  (1975)]. In the current study, similar measures were made for interaural increments of intensity. As before, the stimuli were bandpass clicks of either low (0. 1–2 kHz) or hish (3–4 kHz) frequency. For overall interaural differences as sreat as 24 dB, it seems that performance was based on the interaural differences, and not on the monaural increments/decrements that are concommitant with a binaural change in level. As was the case with time, sensitivity to interaural intensity was reasonably constant across the range tested, indicating that, unlike the case for vision, spatial resolution in the auditory system is not concentrated in the center. A simple binaural trading ratio which converts intensity time can be shown to fit the data for low‐frequency clicks quite well. However, the ...

title: Sense and the single neuron: probing the physiology of perception.
abstract: The newly defined field of cognitive neuroscience attempts to draw together the study of all brain mechanisms that underlie our mental life. Historically, the major sensory pathways have provided the most trustworthy insights into how the brain supports cognitive functions such as perception, attention, and short-term memory. The links between neural activity and perception, in particular, have been studied revealingly in recent decades. Here we review the striking progress in this area, giving particular emphasis to the kinds of neural events that underlie the perceptual judgments of conscious observers.

title: A computational analysis of the relationship between neuronal and behavioral responses to visual motion
abstract: We have documented previously a close relationship between neuronal activity in the middle temporal visual area (MT or V5) and behavioral judgments of motion (Newsome et al., 1989; Salzman et al., 1990; Britten et al., 1992; Britten et al., 1996). We have now used numerical simulations to try to understand how neural signals in area MT support psychophysical decisions. We developed a model that pools neuronal responses drawn from our physiological data set and compares average responses in different pools to produce psychophysical decisions. The structure of the model allows us to assess the relationship between “neuronal” input signals and simulated psychophysical performance using the same methods we have applied to real experimental data. We sought to reconcile three experimental observations: psychophysical performance (threshold sensitivity to motion stimuli embedded in noise), a trial-by-trial covariation between the neural response and the monkey's choices, and a modest correlation between pairs of MT neurons in their variable responses to identical visual stimuli. Our results can be most accurately simulated if psychophysical decisions are based on pools of at least 100 weakly correlated sensory neurons. The neurons composing the pools must include a broader range of sensitivities than we encountered in our MT recordings, presumably because of the inclusion of neurons whose optimal stimulus is different from the one being discriminated. Central sources of noise degrade the signal-to-noise ratio of the pooled signal, but this degradation is relatively small compared with the noise typically carried by single cortical neurons. This suggests that our monkeys base near-threshold psychophysical judgments on signals carried by populations of weakly interacting neurons; these populations include many neurons that are not tuned optimally for the particular stimuli being discriminated.

title: A neuronal population code for sound localization
abstract: The accuracy with which listeners can locate sounds is much greater than the spatial sensitivity of single neurons. The broad spatial tuning of auditory neurons indicates that a code based on the responses of ensembles of neurons, a population code, must be used to determine the position of a sound in space. Here we show that the tuning of neurons to the most potent localization cue, the interaural time difference in low-frequency signals (<∼2 kHz; refs 4, 5), becomes sharper as the information ascends through the auditory system. We also show that this sharper tuning increases the efficiency of the population code, in the sense that fewer neurons are required to achieve a given acuity.

title: Prediction of auditory spatial acuity from neural images on the owl's auditory space map
abstract: The owl can discriminate changes in the location of sound sources as small as 3° and can aim its head to within 2° of a source. A typical neuron in its midbrain space map has a spatial receptive field that spans 40°—a width that is many times the behavioural threshold. Here we have quantitatively examined the relationship between neuronal activity and perceptual acuity in the auditory space map in the barn owl midbrain. By analysing changes in firing rate resulting from small changes of stimulus azimuth, we show that most neurons can reliably signal changes in source location that are smaller than the behavioural threshold. Each source is represented in the space map by a focus of activity in a population of neurons. Displacement of the source causes the pattern of activity in this population to change. We show that this change predicts the owl's ability to detect a change in source location.

title: Neural population code for fine perceptual decisions in area MT
abstract: In the middle temporal (MT) area of primates, many motion-sensitive neurons with a wide range of preferred directions respond to a stimulus moving in a single direction. These neurons are involved in direction perception, but it is not clear how perceptual decisions are related to the population response. We recorded the activities of MT neurons in rhesus monkeys while they discriminated closely related directions, and examined the relationship between the activities of neurons tuned to different directions and the monkeys' choices. Perceptual decisions were significantly correlated with the activities of the highest-precision neurons but not with those of the lowest-precision neurons. The combined performance of the high-precision neurons matched the monkeys' behavior, whereas the ability to predict behavior based on the entire active population was poor. These results suggest that fine discrimination decisions are crucially dependent on the activities of the most informative neurons.

title: Influence of Response Variability on the Coding Performance of Central Gustatory Neurons
abstract: We explored how variability in responding to taste stimuli could impact the signaling of taste quality information by neuron types and individual cells in the nucleus of the solitary tract. One hundred sixty-two neurons recorded from anesthetized rats were grouped using multivariate analysis of taste responses to the following (in m): 0.5 sucrose, 0.1 NaCl, 0.01 HCl, and 0.01 quinine-HCl. Neurons fell into one of three groups corresponding to cell types that responded optimally to sucrose, NaCl, or HCl. A statistical model was used to examine whether responses observed among neurons within each group could be correctly attributed to the optimal stimulus or another tastant on the basis of spike count. Results revealed poor classification performance in some cases attributable to wide variations in the sensitivities of neurons that compose a cell type. This outcome leads us to question whether neuron types could faithfully encode a single taste quality. We then theoretically explored whether a hypothetical observer of individual neurons could discriminate between spiking rates to different tastants during the first second of stimulus processing. Spike rate was found to be an unreliable predictor of stimulus quality for each neuron tested. However, additional analyses suggested that taste stimuli could be identified by a reader that attends to the relative spiking activities of different kinds of neurons in parallel. Rather than assigning meaning to individual neurons or categories of them, central gustatory circuits may signal quality information using a strategy that involves the relative activities of neurons with different sensitivities to tastants.

title: Neural sensitivity to interaural envelope delays in the inferior colliculus of the guinea pig.
abstract: 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.

title: Interaural level differences and the level-meter model.
abstract: The interaural level difference (ILD) plays a significant role in sound localization. However, the definition of ILD for noise is open to some interpretation because it is not obvious how to deal with the inevitable level fluctuations. In this article, the ILD is interpreted as an energylike (time-integrated) measure of stimulus level, independent of other stimulus details-particularly interaural correlation. This concept is called the "level-meter model." The model was tested by measuring human ILD thresholds for noise stimuli that were interaurally correlated, or anticorrelated, or uncorrelated. An additional test (not involving lateralization) measured the threshold for level discrimination based on loudness. According to the level-meter model, all four thresholds should be the same. The experimental results showed that the predictions of the level-meter model held good to within about half a dB, although thresholds for level discrimination were systematically higher than ILDs. Among the ILDs themselves, thresholds were slightly higher for uncorrelated noise. The latter result could be explained by replacing the level-meter model with a loudness-meter model, incorporating temporal integration. The same model accounted for the bandwidth dependence of the threshold.

title: Sensitivity to interaural intensitive disparities: listeners' use of potential cues.
abstract: Thresholds for interaural intensitive disparities (IIDs) for a 500-Hz tone were measured in several stimulus conditions including those in which the use of intracranial position as a cue was effectively eliminated by roving the interaural temporal disparity of the stimuli. Removing position as a cue resulted in substantial degradation of sensitivity to IID. The overall patterning of the data suggests that threshold-IIDs measured in standard binaural paradigms that yield fused intracranial images reflect the use of changes in intracranial position. That is, comparisons among the data suggest that listeners' judgments depend upon binaural spatial cues and not on comparisons of the concomitant monaural increments and decrements in level, per se, that inevitably result from the imposition of IIDs.

title: Acoustic chiasm: efferent projections of the lateral superior olive
abstract: The efferent connections of the cat's lateral superior olive (LSO) were examined first with kainic acid-induced anterograde degeneration and tritiated leucine autoradiography and then by systematic repetition of HRP and fluorescent dye retrograde tract-tracing techniques. The results show that virtually all LSO cells have axons ascending either contralaterally or ipsilaterally to high pontine and midbrain levels of the brainstem. Most terminate in the ventrolateral division of either the ipsilateral or contralateral central nucleus of the inferior colliculus, some terminate in the ipsilateral or contralateral dorsal nucleus of the lateral lemniscus, and a small number terminate in the ipsilateral intermediate nucleus of the lateral lemniscus. Only a small proportion (less than 5%) of LSO cells project to both sides via axon collaterals. The ipsilateral, contralateral, and bilateral projections arise from three overlapping subpopulations of cells within LSO: Those projecting ipsilaterally are concentrated in its lateral limb; those projecting contralaterally are concentrated in its medial limb; the few projecting bilaterally are thinly scattered throughout. Therefore, a lateral-medial gradient is present across LSO based on the laterality of its cell's efferent targets. This gradient parallels LSO's tonotopic gradient: The higher the characteristic frequency of an LSO cell, the more likely it is to project contralaterally. This arrangement of LSO's ascending projections, with most of its lateral cells projecting ipsilaterally and most of its medial cells projecting contralaterally, is similar to the arrangement of the optic chiasm in animals with overlapping eye-fields. Its presence seems to provide an anatomical basis for some recent electrophysiological and behavioral reports of chiasm-like properties of the superior olivary complex.

title: A model for the computation and encoding of azimuthal information by the lateral superior olive.
abstract: A structural model is proposed for the processing of interaural intensity differences by the lateral superior olive. One fundamental assumption is that the incoming excitatory projections from the ipsilateral anteroventral cochlear nucleus innervate columns of LSO neurons serially according to threshold. A second fundamental assumption is that the inhibitory innervation from the ipsilateral medial nucleus of the trapezoid body is also serially arranged according to threshold but in the opposite direction along the LSO column. Using neurophysiological and neuroanatomical data for neuronal response curves, connectional patterns, and cell and synapse numbers, the model was formulated quantitatively and implemented for machine computation. Azimuthal location is encoded by the position along the LSO column where LSO cell firing first goes to zero. Accuracy of coding was tested for three different connectional schemes, for variations in neuronal parameters, and for cell and synapse death. Encoding is shown to be independent of absolute sound level and to vary linearly with interaural intensity difference.

title: Sensitivity to Interaural Correlation of Single Neurons in the Inferior Colliculusof Guinea Pigs
abstract: Sensitivity to changes in the interaural correlation of 50-ms bursts of narrowband or broadband noise was measured in single neurons in the inferior colliculus of urethane-anaesthetized guinea pigs. Rate vs. interaural correlation functions (rICFs) were measured using two methods. These methods compensated in different ways for the inherent variance in interaural correlation between tokens with the same expected correlation. The shape of all rICFs could be best described by power functions allowing them to be summarized by two parameters. Most rICFs were best fit by a power below 2, indicating that they were only slightly nonlinear. However, there were a few fitted functions that had a power of 3–6, indicating marked curvature. Modeling results indicate that the nonlinearity of the majority of rICFs was explicable in terms of the monaural transduction stages; however, some of the rICFs with power greater than 2 require either multiple inputs to the coincidence detector or additional nonlinearities to be included in the model. Discrimination thresholds were estimated at reference correlations of −1, 0, and +1 using receiver operating characteristic (ROC) analysis of the spike-count distribution at each correlation. Thresholds spanned the full possible range, from a minimum of 0.1 to the maximum possible of 2. Thresholds were generally highest with a reference correlation of −1, intermediate with a reference of 0, and lowest with a reference correlation of +1. Thresholds were lowest for the most steeply sloped rICFs, but thresholds were not strongly correlated to the spike rate variance. The lowest thresholds occurred using narrowband noise that was compensated for internal delays, but they were still about three times larger than human psychophysical thresholds measured using similar stimuli. The data suggest that, unlike pure tone interaural time difference, discrimination of a population measure is required to account for behavioral interaural correlation discrimination performance.

title: The superior olivary complex and audition: A comparative study
abstract: The sizes of the three major nuclei of the mammalian superior olivary complex were examined in a number of different species and variations in size were related to the relative prominence of haring and vision in each species. Size was measured by counting neurons in the several nuclei and by measuring the diameter of the eyes.

title: Cytoarchitecture of the human superior olivary complex: Medial and lateral superior olive
abstract: The superior olivary complex is a group of brainstem nuclei involved in hearing and includes the medial superior olive (MSO) and the lateral superior olive (LSO), surrounded by periolivary cell groups. The structure and functional roles of the MSO and LSO have been the subject of many investigations in laboratory animals and it has largely been assumed that these findings are directly transferable to humans. However, little is known conclusively regarding the detailed organization of the human superior olivary complex. The goal of this study is to provide a detailed analysis of the cytoarchitecture of the human MSO and LSO. Results from the examination of eight human brainstems confirm the existence of a conserved MSO and provide evidence of a prominent and highly ordered LSO. Unbiased stereological estimates of neuronal number indicate approximately 15,500 neurons in the MSO and 5600 neurons in the LSO. Additionally, a three-dimensional model of the MSO and LSO was constructed and provides evidence that the human LSO is composed of medial and lateral segments. Finally, an analysis of neuronal morphology, in Nissl stained and Golgi impregnated tissue, provides evidence of multiple neuronal classes within each nucleus and further that these neurons demonstrate a precise geometric arrangement (depending on the nucleus) that is suggestive of isofrequency laminae.

title: Is there a medial nucleus of the trapezoid body in humans?
abstract: The medial nucleus of the trapezoid body (MNTB) appears to be a prominent auditory structure in many mammals. However, the presence of an MNTB in the human brain has not been clearly established. One of the most characteristic features of the cat MNTB is the presence of large somatic endings with multiple synaptic sites, the calyces of Held. We examined adult human brains at both light and electron microscopic levels and found neurons with unusually large endings in a location that is similar to that for the MNTB in other animals. Moreover, the sizes and shapes of some cells in this area are similar to the principal cells of the cat MNTB. These observations support the idea that humans have cells that resemble MNTB neurons in other species. It has been suggested that the cat MNTB may be involved in the generation of wave 3 of its brainstem auditory evoked potentials, so the presence of an MNTB in the human brain may have implications in interpreting brainstem potentials in man.

title: Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI‐32)
abstract: This study provides a morphologic characterization of the human superior olivary complex as revealed by immunohistochemistry by using antibodies against the calcium binding proteins parvalbumin, calbindin, calretinin, and the nonphosphorylated neurofilament H SMI‐32. By combining these markers, it was possible to establish the neuronal architecture and details of the morphologic organization (including axonal terminals) of the different nuclei. The medial superior olivary nucleus is formed by a sheet of parallel‐oriented cells. A clear segregation of axon terminals was noticed on the medially and laterally oriented dendrites of the mostly bipolar neurons. The lateral superior olivary nucleus lacked a distinct nuclear shape but was formed by several patches of rather irregularly arranged neurons. Calretinin or parvalbumin immunoreactive afferent terminals were observed which contacted somata or dendrites of these neurons. The immunolabeling also revealed the boundaries of the dorsal periolivary nucleus and morphologic detail of its neurons. A coherent nuclear structure that could be addressed as the medial nucleus of the trapezoid body was not identified by any single one or by combinations of the markers used. The data were also used to establish a three‐dimensional‐reconstruction of the three major subnuclei of the superior olivary complex. The results are discussed with respect to the possible role of the superior olivary complex in the processing of spatial acoustic information in the azimuthal plane.  2003.