Envelope Coding in the Lateral Superior Olive. III. Comparison With Afferent Pathways

Philip X. Joris1, 2 and Tom C. T. Yin2

1 Department of Neurophysiology, University of Wisconsin Medical School, Madison, Wisconsin 53706; and 2 Division of Neurophysiology, Medical School, University of Leuven, B-3000 Leuven, Belgium

    ABSTRACT
Abstract
Introduction
Methods
Results
Discussion
References

Joris, Philip X. and Tom C. T. Yin. Envelope coding in the lateral superior olive. III. Comparison with afferent pathways. J. Neurophysiol. 79: 253-269, 1998. Binaural cues for spatial localization of complex high-frequency sounds are interaural level and time differences (ILDs and ITDs). We previously showed that cells in the lateral superior olive (LSO) are sensitive to ITDs in the envelope of sinusoidally amplitude-modulated (AM) signals up to a modulation frequency of only ~800 Hz. To understand the limitations in this ITD-sensitivity, we here compare responses to monaural modulation in LSO and its input pathways, derived from cochlear nucleus and medial nucleus of the trapezoid body. These pathways have marked functional and morphological specializations, suggestive of adaptations for timing. Afferent cell populations were identified on the basis of electrophysiological signatures, and for each population, average firing rate and synchronization to AM tones were compared with auditory-nerve fibers and LSO cells. Except for an increase in modulation gain in some subpopulations, synchronization of LSO afferents was very similar to that in auditory nerve fibers in its dependency on sound pressure level (SPL), modulation depth, and modulation frequency. Distributions of cutoff frequencies of modulation transfer functions were largely coextensive with the distribution in auditory nerve. Group delays, measured from the phase of the response modulation as a function of modulation frequency, showed an orderly dependence on characteristic frequency and cell type and little dependence on SPL. Similar responses were obtained to a modulated broadband carrier. Compared with their afferents, LSO cells synchronized to monaurally modulated stimuli with a higher gain but often over a narrower range of modulation frequencies. Considering the scatter in afferent and LSO cell populations, ipsi- and contralateral responses were well matched in cutoff frequency and magnitude of delays. In contrast to their afferents, LSO cells show a decrease in average firing rate at high modulation frequencies. We conclude that the restricted modulation frequency range over which LSO cells show ITD-sensitivity does not result from loss of envelope information along the afferent pathway but is due to convergence or postsynaptic effects at the level of the LSO. The faithful transmission of envelope phase-locking in LSO afferents is consistent with their physiological and morphological adaptations, but these adaptations are not commensurate with the rather small effects of physiological ITDs reported previously, especially when compared with effects of ILDs. We suggest that these adaptations have evolved to allow a comparison of instantaneous amplitude fluctuations at the two ears rather than to extract interaural timing information per se.

    INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References

The auditory nerve (AN) provides the CNS with ample timing information about the envelope of amplitude-modulated (AM) signals (Cooper et al. 1993; Javel 1980; Joris and Yin 1992; Palmer 1982; Wang and Sachs 1993). Following early studies by Møller (1974), there has been a renewed interest in temporal envelope coding at the first level of synaptic integration: the cochlear nucleus (CN) (Frisina et al. 1990; Rhode and Greenberg 1994; Wang and Sachs 1994). Different cell types in the CN transform the envelope information supplied by the AN in different ways, but it is unclear whether and how this information is used behaviorally. The most convincing case for the use of envelope cues, as for temporal cues in general, comes from binaural studies. We describe the sequential transformations of envelope information in an anatomically and physiologically well-characterized binaural circuit, in which we previously demonstrated sensitivity to envelope interaural time differences (ITDs) of high-frequency AM signals (Joris and Yin 1995).

Disparities between the acoustic signals to the two ears provide cues for azimuthal sound localization. Interaural level differences (ILDs) are disparities in sound pressure level (SPL) for corresponding frequencies at the two eardrums, created by the interference of pinna and head with the sound field. Temporal disparities arise from path-length differences between the sound source and the two ears and occur in both carrier and envelope components of free-field acoustic stimuli (Middlebrooks and Green 1990; Roth et al. 1980). Human subjects can detect ITDs of the carrier at very small values (<20 µs), but only at frequencies below ~1.5 kHz. ITDs of the envelope are detected most easily for high-frequency carriers (Henning 1974). However, acoustically, physiologically, and psychophysically, ILDs are the most prominent localization cue at high frequencies. Physiological sensitivity to ITDs of high-frequency AM stimuli was first described in the inferior colliculus (Batra et al. 1989, 1993; Yin et al. 1984).

The lateral and medial superior olive (LSO and MSO) are the nuclei in the ascending auditory system where the primary binaural interactions subserving localization of sounds occur. Cells in LSO (Fig. 1) are excited by ipsilateral and inhibited by contralateral sounds. Ipsilateral input is provided by the spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN). Inhibitory input derives from the homolateral medial nucleus of the trapezoid body (MNTB), which receives its excitatory input from globular bushy cells (GBCs) in the contralateral AVCN. We use the term "LSO circuit" to denote LSO cells and their first- through third-order afferents (henceforth referred to collectively as LSO afferents), comprising an ipsilateral limb of AN fibers and SBCs ("ipsilateral afferents"), and a contralateral limb of AN fibers, GBCs, and MNTB cells ("contralateral afferents").


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FIG. 1. Cartoon of the lateral superior olive (LSO) circuit and the physiological signatures used to identify afferent responses. Recordings from excitatory spherical bushy cells (SBCs) in the anteroventral cochlear nucleus (AVCN) show a primary-like response to short tonebursts at their characteristic frequency (CF) and a prepotential in the extracellular spike waveform. Recordings from axons of excitatory globular bushy cells (GBCs) in the ipsilateral trapezoid body (TB) show a primary-like-with-notch pattern to short (25 ms) tonebursts. Inhibitory cells in the medial nucleus of the trapezoid body (MNTB) also show a prepotential and a primary-like-with-notch pattern as in GBCs to stimulation of the contralateral ear. AN, auditory nerve.

Study of AM responses of LSO afferents is of interest for two reasons. First, the range of envelope frequencies over which ITD-sensitivity in LSO was observed paralleled psychophysical data for human listeners but was more limited than expected on the basis of envelope information supplied to the CNS by the auditory nerve (Joris 1996; Joris and Yin 1992; Palmer 1982). This limit could be imposed by the afferents. Second, the LSO circuit has marked functional and morphological specializations in both of its limbs, suggestive of adaptations for timing. The three cell types involved in the transmission of signals from auditory nerve to LSO all have a "bushy" morphology, i.e., sparse and short dendrites (Brawer et al. 1974), and derive most of their input from a limited number of afferents through large axosomatic endings. AN fibers give large terminals to bushy cells: a single or few (2-5) so-called endbulbs of Held to SBCs (Melcher 1993; Ryugo and Sento 1991), and modified endbulbs from an estimated 10-25 auditory nerve fibers to GBCs (Liberman 1991; Spirou et al. 1990). The GBC input to MNTB cells is in the form of a single large calyx of Held (Smith et al. 1991; Spirou et al. 1990; Tolbert et al. 1982), apparently the first described and largest terminal in the mammalian brain (Jean-Baptiste and Morest 1975). SBCs give excitatory endings on the distal dendrites of ipsilateral LSO cells, whereas the inhibitory input to LSO from MNTB is placed on the proximal dendrites and soma (Cant 1984; Glendenning et al. 1991; Helfert et al. 1989; Zook and DiCaprio 1988; P. H. Smith, P. X. Joris, and T.C.T. Yin, unpublished results). Axon diameters of contralateral afferents are larger than those of ipsilateral afferents (see DISCUSSION). The bushy cells in AVCN and MNTB also share membrane properties that allow for a precise transmission of the temporal information available in their afferents (Banks and Smith 1992; Banks et al. 1993; Borst et al. 1995; Forsythe and Barnes-Davis 1993; Manis and Marx 1991; Oertel 1983; Smith and Rhode 1987; Wu and Kelly 1993; Wu and Oertel 1984).

Responses to sound are consistent with these observations in several respects. Of all cell types in the CN, the temporal responses of SBCs and GBCs most resemble the AN input (Bourk 1976). Similarly, responses of MNTB principal cells are nearly identical to those of GBCs (Guinan et al. 1972a,b; P. H. Smith, P. X. Joris, and T.C.T. Yin, unpublished results). Despite the longer path length and extra synapse in the contralateral branch of the LSO circuit, the latencies of LSO responses to ipsi- and contralateral stimulation appear similar (Boudreau and Tsuchitani 1968; Tsuchitani 1988).

Remarkably, despite extensive documentation of the specializations in the LSO circuit, there is no unifying hypothesis to explain their need. They usually are interpreted as subserving monaural and interaural timing information, yet the textbook role for LSO is the extraction of ILDs. To address the relevance of these specializations for binaural temporal processing, we examine synchronization and rate behavior of the different components of the LSO circuit.

    METHODS
Abstract
Introduction
Methods
Results
Discussion
References

Most of our methods are as described in previous papers (Joris 1996; Joris and Yin 1995). We obtained extracellular recordings from cats anesthetized with pentobarbital. The surgical approach and recording techniques were identical to those in our previous reports, and LSO and afferent responses often were obtained within the same penetration. Responses can be recognized as being derived from LSO afferents with the aid of various electrophysiological signatures (Bourk 1976; Guinan et al. 1972a,b; Smith et al. 1991; Spirou et al. 1990), examples of which are shown in Fig. 1. Cells in AVCN with spikes preceded by prepotentials (PPs) and with primary-like (PL) responses to tones at characteristic frequency (CF) were considered SBCs. Primary-like-with-notch (PLN) responses recorded in the trapezoid body ipsilateral to the excitatory ear were considered to be from GBC axons. PLN responses, likely also from GBCs, also were encountered in the contralateral trapezoid body but are not included here because of the possible confusion with MNTB responses. PP units in the ventral brain stem, driven exclusively by the contralateral ear, were considered MNTB cells. Spikes were monitored on-line for PPs with an averaging oscilloscope. When a PP or complex waveform was present, the spike complex was averaged at a sampling rate of 40 kHz and a bandwidth of 0.1-10 kHz for >= 200 occurrences. In the vast majority of cases, the PP was also visible in the ongoing neural signal without averaging. LSO cells were identified histologically and on the basis of binaural contralateral inhibition, ipsilateral excitation (IE)-sensitivity and chopper responses to ipsilateral tone bursts at CF as described in Joris and Yin (1995). The "LSO-EE" cells described by Joris (1996) are not considered here. We also recorded AM data from monaural AVCN cells that were classified as choppers based on the multimodal poststimulus time histogram (PSTH) in response to short CF tone bursts as well as on response irregularity, quantified by the coefficient of variation (CV) calculated over the full range of SPLs (Young et al. 1988). Finally, we make use of 186 AN fibers from a previous study (Joris and Yin 1992) and 9 additional AN fibers (see Fig. 15) recorded using the same methods.


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FIG. 15. Comparison of 3 measures of envelope synchronization with wideband (ordinate) and tonal (abscissa) carriers. Each symbol represents data for 1 cell or axon, and different symbols are used for different afferent categories (inset). Three categories of cells did not fullfil all the criteria stated in METHODS and were classified on the basis of poststimulus time histogram: PHL, PL, and PLN refer to cells with phase-locked, primary-like, or primary-like-with-notch response to carrier, respectively. A: maximum synchronization values, derived from a level series. B: 3-dB cutoff frequency of modulation transfer functions. C: delay measured from phase-frequency functions. - - -, diagonals of equality. Slopes and y intercepts of linear regressions were A: 0.96 and -0.016, B: 0.96 and 60 Hz, and C: 1.01 and -0.03 ms.

The search stimulus consisted of binaurally beating tones (monaural tones in AN and AVCN experiments) logarithmically stepped in frequency from ~300 Hz to 30 kHz. For each unit, a threshold tuning curve was obtained, followed by a rate-level function for short (25 ms, 200 repetitions) tone bursts at CF, usually in 5- or 10-dB steps. In initial studies, each fiber encountered was studied, but in later experiments, only responses consistent with the afferent criteria stated above were further studied with AM stimuli. Stimulus parameters and response analyses were identical to our previous studies to enable comparison with AN (Joris and Yin 1992) and LSO (Joris 1996; Joris and Yin 1995). The general approach was to optimize stimulus parameters for maximal monaural and/or binaural phase-locking in each cell, while still keeping a high average spike rate. AM stimuli usually were modulated fully (modulation depth m = 100%), and the carrier frequency always was placed at the frequency to which the cell was most sensitive (= CF). Duration was 600 ms, repeated every second for typically 20 or 40 times. Magnitude and phase of synchronization at the envelope frequency were quantified with the vector strength R, tested for significance (P < 0.001) with the Rayleigh test and expressed as a gain measure defined by 20 log (2R/m).

We began by measuring a rate level function to AM stimuli with the carrier frequency at CF and modulation frequency at 100 Hz. We noted the SPL with maximal synchronization to the modulation frequency and subsequently obtained a modulation transfer function (MTF) at that SPL, in which modulation frequency was changed in linear steps, usually from 50 to 2,000 Hz. In LSO cells, data to contralateral modulation were obtained by stimulating the ipsilateral ear with an unmodulated tone at CF while presenting an AM tone contralaterally. This same stimulus sometimes was reversed in laterality to obtain ipsilateral modulation data. The ILD (= SPL at contralateral ear minus SPL at ipsilateral ear) at which modulation transfer functions for contralateral modulation were obtained was on average (for all LSO cells reported here) 0 dB, but in most cells, several runs were needed to find the ILD that resulted in robust synchronization to contralateral modulation combined with a high enough average firing rate, and that ILD was sometimes large (between 30 and -35 dB). Binaural sensitivity of LSO cells was assessed with a binaural AM beat stimulus, typically consisting of a 5-s-long AM stimulus delivered with a 1-Hz difference in modulation frequency to the two ears.

    RESULTS
Abstract
Introduction
Methods
Results
Discussion
References

Data were obtained for 653 single units (not including AN fibers), of which we selected 14 SBCs, 79 GBCs, 37 MNTB, and 24 LSO cells according to the physiological criteria stated in METHODS. MNTB and LSO cells also were localized histologically to the appropriate region, except for four MNTB cells from one animal, for which histology was not available, and five presumed LSO recordings that were localized rostral to LSO (see Joris 1996). To complement our small sample of SBCs, we report on 47 units that did not fulfill all the SBC criteria: 6 PP units recorded in AVCN but with undetermined PSTH, 17 PL units recorded in AVCN but lacking a PP, and 24 PL units recorded in the trapezoid body (TB). This sample may be "contaminated" with GBCs, which can show PL responses (Smith et al. 1991) and PPs (Bourk 1976). For comparative purposes, AM data also were obtained in 20 AVCN choppers.

Changing SPL

In the AN, envelope synchronization is nonmonotonically dependent on SPL (Cooper et al. 1993; Joris and Yin 1992; Smith and Brachman 1980; Wang and Sachs 1993), and the same was true in LSO afferents. Figure 2 shows average rate (*, left ordinate) and synchronization at the modulation frequency (open circle , right ordinate) as a function of SPL for one cell of each afferent class. In every case, the rate-level function is sigmoidal, and the synchronization-level function nonmonotonic. R reaches a maximum slightly above rate threshold, on average (in dB re. tuning curve threshold, ± SD) at 2.4 ± 6.6, 6.0 ± 7.0, and 5.5 ± 4.8 for SBCs, GBCs, and MNTB cells, respectively. The average in AN is 10.0 ± 4.95 (Joris and Yin 1992).


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FIG. 2. Synchronization-level functions for an afferent fiber of each category. Average rate (left ordinate, *) showed a sigmoidal, monotonic increase in all fibers. Synchronization at the envelope frequency (right ordinate, open circle ) was always nonmonotonic. bullet , nonsignificant synchronization (P < 0.001). CFs were as indicated, modulation frequency was 100 Hz.

Figure 3 shows responses of a LSO cell to a stimulus that was binaural but modulated and varied in SPL on only one side: ipsilateral (left) and contralateral (right). The level functions in those conditions are basically complementary. To ipsilateral modulation, there is a sigmoidal increase in rate and a decrease in synchronization at high SPLs, as was the case for afferents (Fig. 2). Conversely, with contralateral modulation, average rate decreases and synchronization increases at increasing SPL. The decrease in rate with contralateral modulation is simply a reflection of the IE interaction and has been studied extensively with unmodulated tones. The increase in synchronization may seem unexpected but also is understood easily from this IE interaction. At low SPLs, the MNTB provides a well-synchronized inhibitory input (Fig. 2) against the sustained excitation from the unmodulated ipsilateral ear. A simple subtraction of these two inputs predicts "notched" rather than "peaked" histograms in LSO and thus low synchronization values. Conversely, at higher SPLs, the period histograms of MNTB cells broaden and their R values decrease. Subtraction of these broad histograms from the sustained ipsilateral excitatory drive results in narrow period histograms, and thus high synchronization values, in LSO cells.


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FIG. 3. Synchronization-level functions in a LSO cell to ipsilateral (left) and contralateral (right) modulation. Abscissa indicates the sound pressure level (SPL) of the modulated waveform: the stimulus at the other ear was unmodulated and held at 44 dB. Modulation frequency was 100 Hz. Only significant synchronization values are graphed.

Several studies have indicated that CN cells generally show a decrease in R for increasing SPL but that response classes differ in the steepness of the downslope above the point of maximum synchronization (Frisina et al. 1990; Kim et al. 1990; Rhode and Greenberg 1994; Wang and Sachs 1994). We therefore compared the slope of a linear regression through this part of the function with measurements previously obtained in the AN with the same procedure (Joris and Yin 1992). These slopes were very similar in AN fibers, bushy cells, and MNTB cells [sample size (dB-1); AN: 186, -0.012 ± 0.003; SBC: 9, -0.010 ± 0.0013; GBC: 41, -0.011 ± 0.002; MNTB: 21, -0.011 ± 0.002]. However, the slopes for ipsilateral LSO responses (n = 21, -0.007 ± 0.0039) were significantly more shallow than in the AN (P < 10-7, Mann-Whitney U), indicating less of a decrease in R at high SPLs.

SBCs and GBCs show stronger synchronization to low-frequency pure tones than AN fibers (Carney 1990; Joris et al. 1994a,b) and also have been reported to have improved synchronization to envelopes (Frisina et al. 1990; Joris et al. 1994a; Rhode and Greenberg 1994; Wang and Sachs 1994). For each fiber, we obtained the value of maximal significant synchronization from the synchronization-level function. These values are shown at each cell's CF in the scatter diagram of Fig. 4 (see legend for means and SD). A first striking feature is the general coextensiveness of AN and bushy cell values. For the small sample of SBCs, this seems to be the case over the entire CF range that was sampled. GBCs and MNTB cells diverge from AN fibers below ~7 kHz because of opposite tendencies: whereas AN fibers tend to show lower synchronization values at low CFs (Joris and Yin 1992; Wang and Sachs 1993), GBCs and MNTB cells tend to show higher values at low CFs than at high CFs. A second feature is that LSO cells typically reach higher maximal R values than their afferents, at all CFs and for both ipsi- and contralateral monaural modulation. As reported earlier (Joris and Yin 1995) and as expected for an IE-type interaction (Joris 1996), R values to binaural stimulation are high (Fig. 4; star ) when the envelopes to the two ears are out of phase.


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FIG. 4. Maximum synchronization values (Rm) for different cell classes, derived from synchronization-level functions. For LSO afferents, 1 datapoint is shown per cell. Up to 3 kinds of measure are shown for LSO cells: maximal synchronization to monaural ipsi- or contralateral modulation (filled triangles), and to binaural modulation (star ). For contralateral modulation, a binaural stimulus was used with an unmodulated CF tone of fixed SPL at the ipsilateral ear. For binaural modulation, the value at the interaural time difference (ITD) giving the largest R was taken. Sample sizes, mean values ± SD: AN 316, 0.63 ± 0.12; SBC 11, 0.65 ± 0.11; GBC 47,0.74 ± 0.10; MNTB 24, 0.74 ± 0.08; LSO ipsi 22, 0.88 ± 0.05; LSO contra 10, 0.87 ± 0.11.

Modulation depth functions were obtained in a limited sample of afferents (not shown). Keeping SPL fixed at the level at which maximum synchronization was obtained, we varied m between 0 and 1 in steps of 0.1. For 27 responses so obtained (in SBCs, GBCs, and MNTB cells), R always showed a monotonic and saturating increase with m, as in AN and LSO (Joris and Yin 1992, 1995).

Changing modulation frequency

SYNCHRONIZATION MAGNITUDE. Envelope ITD-sensitivity requires temporal envelope information at the site of binaural interaction. This information, supplied to the CNS via the AN, deteriorates at high modulation frequencies: modulation transfer functions, which are graphs of synchronization as a function of modulation frequency, are low-pass (Joris and Yin 1992; Møller 1976; Palmer 1982). Surprisingly, envelope ITD-sensitivity in LSO becomes unmeasurable at frequencies more than an octave below the highest envelope frequencies found in the AN (Joris 1996). To investigate whether this limitation arises in the LSO afferents, we obtained MTFs for the different types of afferents, examples of which are shown in Fig. 5. For these examples, and in all other afferents examined, the basic shape of the MTF was low-pass. We determined the 3-dB cutoff frequency of each function, which is the modulation frequency at which the gain is 3 dB down from the maximum. The cutoffs of the afferents illustrated (Fig. 5, arrows; for values see legend) are within the range measured in AN fibers of corresponding CF.


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FIG. 5. Modulation transfer functions for 4 cells in the LSO circuit. Ordinate values are R converted to gain. Positive gain values signify higher modulation depth in the response than in the stimulus. Filled symbols indicate nonsignificant synchronization, arrows indicate 3-dB cutoff point. Insets: averaged spike waveforms with prepotential and the primary-like-with-notch (PLN) histogram used for cell classification. Cells were chosen so that their CFs are approximately equal (within the limits of our sample). Stimulus level (dB) was at SPL giving maximal synchronization at 100 Hz. Cutoff frequency, SPL and CF were: AN: 984 Hz, 4 dB, 19.4 kHz; SBC: 864 Hz, 24 dB, 10.6 kHz; GBC: 1122 Hz, 19 dB, 17 kHz; MNTB: 992 Hz, 29 dB, 18.3 kHz.

Modulation transfer functions of LSO cells to monaural modulation of either ear were also low-pass. Figure 6 shows examples of ipsi- and contralateral MTFs for two LSO cells. Several features are of note. First, within a cell the functions for ipsilateral and contralateral stimulation were similar in shape with well-matched cutoff frequencies. The examples of Fig. 6 show the most similar (right) and dissimilar (left) data pairs. Second, maximal gains to monaural modulation are higher than in afferents, as noted earlier (Fig. 4). Third, whereas 3-dB cutoffs in afferents show an orderly dependence on CF that largely overlaps with the distribution found in the AN, this relationship was less systematic for LSO cells (Fig. 7). The cutoffs in LSO never exceeded 1 kHz and, in about one-half of the cells, were lower than cutoffs of afferents of the same CF. The LSO and afferent distributions differ especially at CFs above ~10 kHz. The differences between cutoffs to ipsi- and contralateral modulation were smaller than the range of cutoffs seen within the afferent populations and were not significant (Mann-Whitney U and Wilcoxon matched pairs tests).


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FIG. 6. Modulation transfer functions to ipsi- and contralateral modulation for 2 LSO cells with the least (left) and most (right) similar cutoff frequencies in the population. Insets: poststimulus time histogram to unmodulated ipsilateral CF tone of 25 ms. Cutoff frequencies for contra- and ipsilateral modulation were 920 and 626 Hz (left) and 541 and 558 Hz (right). SPLs (dB) for ipsilateral modulation were 39 (left) and 14 (right), SPLs for contralateral modulation were; left: 39 (contra)/74 (ipsi), right: 44 (contra)/24 (ipsi). CFs were 20.9 (left) and 18.3 kHz (right).


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FIG. 7. Cutoff frequencies of modulation transfer functions to monaural modulaton in LSO cells and their afferents. Sample size: AN (138), SBC (5), GBC (35), MNTB (23), LSO ipsi (13), and LSO contra (12).

As mentioned in INTRODUCTION, the range of modulation frequencies over which LSO cells show ITD-sensitivity is also lower than expected from the phase-locking range in LSO afferents (Joris 1996). A comparison of LSO cutoff frequencies for binaural and monaural modulation is shown in Fig. 8. The monaural values are those of Fig. 7, for the LSO cells that also were studied binaurally. The binaural values are 3-dB cutoff frequencies from synchronization functions to a binaural AM beat stimulus (see Fig. 7B in Joris 1996). The two sets of measures are well correlated (r = 0.82) but virtually all data points lie above the diagonal of equality, i.e., cutoffs to monaural modulation are systematically and significantly higher than the binaural ones (Wilcoxon matched pairs test: P < 10-4).


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FIG. 8. Comparison of binaural and monaural 3-dB cutoff frequencies in LSO cells. Monaural values are cutoffs for ipsi- (down-triangle) and contralateral (triangle ) monaural modulation of Fig. 7, for the subset of cells (n = 13) that also were studied with a binaural AM beat. Binaural values were obtained from synchronization functions to a 1-Hz binaural AM beat stimulus, presented over a range of modulation frequencies. For 2 cells (black-triangle and black-down-triangle ) in which phase-locking to the beat became insignificant before a 3-dB value was reached, the highest modulation frequency with significant phase-locking is used as cutoff frequency.

Despite the higher gains and lower cutoffs in LSO, the modulation transfer functions of afferents and LSO cells were similar in shape. This is illustrated in Fig. 9, which shows functions with the ordinate normalized to maximum gain and the abscissa normalized to cutoff frequency. The functions of the three classes of afferents fall within the bounds of AN envelope phase-locking (Fig. 11 in Joris and Yin 1992). The functions of LSO cells are more varied and some have more high-pass slope at low modulation frequencies, but overall they are remarkably similar to the afferent responses. Also, there are no indications in our sample of marked differences in shape in the responses to ipsi- and contralateral modulation.


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FIG. 9. Modulation transfer functions to monaural modulation in LSO cells and their afferents. Ordinate values are normalized to maximum gain; abscissa values are normalized to the 3-dB cutoff frequency at which all functions intersect. To avoid overlap between populations, data of each population are offset by 10 dB, and some functions are clipped at the highest modulation frequencies. For SBCs and LSO cells, all functions reaching a 3-dB cutoff are shown. For GBCs and MNTB cells, a sample of size and CF distribution similar to LSO is shown. Sample size: SBC (5), GBC (14), MNTB (13), LSO ipsi (14), and LSO contra (11). Only data points with statistically significant envelope phase-locking are shown.


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FIG. 11. A: delays, measured from phase-frequency functions as shown in previous figure. Sample size: AN (153), SBC (7), GBC (40), MNTB (23), LSO ipsi (17), and LSO contra (15). B: ipsi- and contralateral delays, pairwise measured on the same LSO cells, were correlated (r = 0.72). - - -, equality. All data points are corrected for acoustic delay between the driver and a calibration probe close to the eardrum, calculated for each experiment from the phase values of the acoustic calibration (average was 0.4 ms for the AN experiments and 0.28 ms in the CNS recordings, for which different driver assemblies were used).

SYNCHRONIZATION PHASE. As in AN fibers, LSO cells and their afferents show a linear accumulation of phase lag with modulation frequency. The examples in Fig. 10 show that the slope of the cumulated phase-frequency functions is related to cell type, being largest for LSO cells and shortest for AN fibers. Moreover, the slopes for ipsilateral and contralateral modulation in LSO are well matched, but the functions are offset by ~0.5 cycle due to the different sign of the input (excitatory or inhibitory) (Joris 1996). The steepness of each function, which provides an estimate of the total delay accumulated between acoustic driver and recording site, was quantified by the slope of a linear regression fitted (all with r2 >=  0.98) to the data points with significant phase-locking. Figure 11A shows the slopes of the linear regressions for the different cell classes as a function of CF. Most striking in these data are the similarity of delays in LSO to ipsi- and contralateral modulation, despite the longer pathway for the contralateral signal including an extra synapse. The mean difference between the ipsi- and contralateral delays (for LSO cells in which both measures were available, n = 15) was 182 µs, indicating that on average, the contralateral inhibition arrived 182 µs later than the ipsilateral excitation. Moreover, the good matching extends to the single cell level because ipsi- and contralateral delays were correlated (r = 0.72), i.e., even though delays differed in magnitude among cells, they tended to be matched within cells (Fig. 11B).


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FIG. 10. Examples of phase-frequency plots to monaural modulation. These data are the phase portions of the synchronization measurements of the cells shown in Figs. 5 and 6 (right). Data for ipsilateral modulation all converge to a y intercept ~0.25 cycles, which is the stimulus envelope phase measured without time delay (the stimulus envelope started in sine phase). Phase values are uncorrected for acoustic delay. Slopes of linear regressions, corrected for acoustic delay, were: AN, 1.55 ms; SBC, 2.50 ms; GBC, 2.32 ms; MNTB, 3.16 ms; LSO ipsi, 4.75 ms; and LSO contra, 5.32 ms.

Within each cell class delay showed an orderly curvilinear dependence on CF, which can be viewed as the sum of a CF-dependent part arising in the cochlea and a CF-independent DC term composed of various fixed delays (Ruggero and Rich 1987). To the extent that differences in delay between populations of the LSO circuit are determined by a fixed conduction delay, the delay values for each population should be DC-shifted relative to its afferents. A least-square fit of the delays in auditory nerve with a power functiontau  = aCFb + d (Anderson et al. 1971; Smolders and Klinke 1986) yielded the parameters (r = 0.93, delay tau  in ms, CF in kHz):
τ = 3.08 CF<SUP>−0.53</SUP>+ 1.04 (1)
We fit the same function to the other populations of Fig. 11 with d as a free parameter, which provides an estimate of the CF-independent high-frequency asymptote, and obtained 1.8 ms for GBC, 2.2 ms for MNTB, 4.1 ms for LSO responses to contralateral modulation, and 3.9 ms for LSO responses to ipsilateral modulation (1 outlying data point at 519 Hz excluded). (Note that values at 40 kHz have not reached the asymptote but are ~0.5 ms higher).

As mentioned earlier, we obtained AM data of cells presumed to be SBCs on the basis of their PL response pattern or presence of a PP. To complement the small sample of SBCs, a summary of these "presumed SBCs" is shown in Fig. 12. Different symbols are used for the various subpopulations (see legend). Values of maximal synchronization (Fig. 12A) and cutoff frequency (Fig. 12B) are within the range of values observed in the AN and follow the same trends with CF, consistent with our findings for SBCs. Delays of cells recorded in the AVCN (Fig. 12C; open circle  or odot ) are short and overlap with delays measured in the AN. Delays of PL fibers recorded in the TB (Fig. 12, bullet ) are longer and similar to delays measured on MNTB cells (Fig. 11, square ).


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FIG. 12. Summary scatterplots for measurements on presumed SBCs. Symbols differentiate cells with primary-like (PL) response pattern, recorded in AVCN (PL-CN) or TB (PL-TB), and AVCN cells with PP. Most (8/11) PL-TB fibers were recorded contralateral to the excitatory ear.

AVERAGE RATE. With increases of modulation frequency >300 Hz, ITDs are decreasingly effective in modulating the firing rate of LSO cells (Joris 1996; Joris and Yin 1995). Moreover, the set point around which rate is modulated decreases, so that at high modulation frequencies, firing rate often drops near zero at all ITDs. We made two kinds of observations concerning the origin of this drop in rate. In Fig. 13, we graph the average firing rate counterpart of the modulation transfer functions of Fig. 9, with the addition of a number of presumed SBCs to supplement the small SBC sample. The drop in firing rate at high modulation frequencies also is present in LSO responses to monaural modulation and is strong for contralateral modulation (Fig. 13E) and more varied for ipsilateral stimulation (Fig. 13B). Possible reasons for this drop with increasing modulation frequency would include changes in average firing rate of afferents: a decrease in rate on the excitatory (ipsilateral) side or an increase in rate on the inhibitory (contralateral) side. However, responses from LSO afferents indicate that neither is the case. There is little change in average rate in SBCs (Fig. 13A), consistent with our findings in AN (Joris and Yin 1992). Similarly, rate changes in the contralateral LSO afferents (GBCs in Fig. 13C, MNTB cells in Fig. 13D) are smaller than in LSO and moreover are in the wrong direction. The decrease in rate above ~300 Hz in many MNTB cells should cause decreased inhibition and therefore increased firing rate in LSO cells, contrary to what is observed(Fig. 13E).


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FIG. 13. Average firing rate for modulation transfer functions shown in Fig. 9. All available data points are shown (up to modulation frequency <= 2 kHz), rather than only those with significant synchronization, and different symbols are used for different cells or axons. In addition to the 5 SBCs shown in Fig. 9 (which fullfilled all criteria and are identified here with solid lines and symbols), A includes 9 "presumed SBCs" (see text). Responses to ipsi- (B) and contralateral modulation (E) derived from the same LSO cells are shown with same symbols.

Although rate changes in the LSO afferents were small, we noted a systematic trend in GBCs and MNTB cells when the rate curves were graphed with modulation frequency normalized to cutoff frequency (as in Fig. 9): rate tended to peak about an octave below the cutoff frequency (not shown). This form of rate dependency has been reported for Onset-choppers in CN (Rhode and Greenberg 1994) and may reflect the stronger sensitivity of GBCs and MNTB cells to intensity changes when compared with AN fibersor SBCs.

ADDITIONAL FINDINGS. The data presented so far give a coherent picture of envelope phase-locking in the LSO circuit: LSO afferents show strong phase-locking over a wide range of modulation frequencies; this is translated by LSO cells into ITD-sensitivity over a more limited frequency range. To put these findings in perspective, we report some additional observations, for smaller samples of cells, with regard to the effect of SPL on modulation transfer functions, the envelope phase-locking when a broadband carrier is used, and finally how bushy cells compare with the other major AVCN cell type: the chopper/stellate cells.

To assess the functional role of envelope ITD-sensitivity, it is important to consider how encoding of the phase of the stimulus envelope is affected by SPL. We performed two analyses bearing on this question. First, we examined the phase of synchronization-level functions obtained at 100 Hz. All afferents and the response of LSO cells to ipsilateral modulation (LSO-ipsi) showed a moderate, linear decrease in phase with increasing SPL (not shown: see Joris and Yin 1992, 1995 for examples in AN and LSO, respectively). The slope of a linear fit was on average only a few thousandths of a cycle per dB. Expressed in the time dimension (µs/dB), averages were -11.3 (AN, n = 285), -11.4 (SBC, n = 10), -16.7 (GBC, n = 39), -20.3 (MNTB, n = 19), and -27.7 (LSO to ipsilateral modulation, n = 13). In contrast to the negative slope of afferents and LSO-ipsi, the phase of the LSO response to contralateral modulation at 100 Hz increased with SPL: 7.8 µs/dB (n = 6).

Second, in a limited number of cells, we obtained monaural modulation transfer functions over a range of SPLs. Synchronization magnitude is illustrated for two cells, a SBC and GBC in Fig. 14, A and B, respectively. There is a gain change with SPL, also observed in the level-synchronization functions (Fig. 2), but little overall change in shape of the functions. The delays measured from the slopes of the phase-frequency function of five cells are shown as a function of SPL in Fig. 14C. Clearly, delays do not show the systematic decrease with SPL seen with more traditional latency measures such as first spike or first peak latency.


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FIG. 14. A and B: modulation transfer functions at multiple SPLs for a SBC (A, CF = 3.8 kHz) and GBC (B, CF = 28.3 kHz) recorded in CN. Different line types are used to demarcate different functions, and SPL is indicated on each function. C: delays measured from the slope of phase-frequency functions obtained at multiple SPLs in 5 cells. A and B identify delay functions for cells of top panels. Solid symbols indicate cells for which cell class was tentative: the cell with shortest delay values did not show a prepotential but had a PL response (CF: 11.4 kHz), the cell with the longest delay values had a PLN reponse and was recorded in the contralateral TB (CF: 17.2 kHz). CF of the remaining SBC (solid line) was 2.5 kHz. Delay values are corrected for acoustic delay.

Most of the envelope synchronization data in this series of papers were obtained with a tonal carrier, placed at the CF of the cell under study. To examine the generality of our findings with regard to the spectral extent of the carrier used, we studied envelope phase-locking to a sinusoidally modulated broadband noise (40 kHz wide; modulation depth 100%) in 28 monaural cells. The form of the dependence of envelope synchronization on AM parameters was identical for tonal and noise carriers: a monotonic increase in R with increasing modulation depth, a nonmonotonic relationship (increase then decrease) with SPL, and a low-pass modulation transfer function, but there were quantitative differences. Figure 15 shows a comparison of synchronization measures obtained within each cell for a CF carrier (abscissa) and noise carrier (ordinate). Synchronization magnitudes (Fig. 15A), obtained from the maximum of a level series as in Fig. 2, were correlated moderately (r2 = 0.64) and were generally lower with the noise carrier. Delays (Fig. 15C) and 3-dB cutoff frequencies (Fig. 15B) measured from modulation transfer functions were well correlated (r2 values of 0.97 and 0.90, respectively), with points clustered around the diagonal of equality.

The behavior of LSO afferents to modulated noise carriers is consistent with ITD-sensitivity to such stimuli, tested over a range of modulation frequencies in two LSO cells. The cell in Fig. 16 shows ITD-sensitivity over a similar range of modulation frequencies for both types of carrier, consistent with the correlation in cutoffs in Fig. 16B. However, ITD-sensitivity is weaker for the noise carrier (Fig. 16A) than for the CF carrier (Fig. 16B), in that spike rate differences between peaks and troughs are smaller for the noise carrier. This is consistent with the poorer synchronization to envelopes of such carriers in the afferents (Fig. 15A). Finally, the good agreement in delays found in LSO afferents for the two types of stimuli (Fig. 15C) is consistent with the similarity in characteristic delay (Yin and Kuwada 1983) measured in this cell for the two types of carrier (arrows).


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FIG. 16. ITD-sensitivity in a LSO cell (CF = 6.7 kHz) to a modulated wideband carrier (A) and CF carrier (B). ILD was set to 0 for both types of carrier, and modulation frequency was increased from 50 Hz in 50-Hz steps until modulation became insignificant. Responses are shown over the same range of modulation frequencies (50-450 Hz) and are set apart with alternating line types. Characteristic delay measured (as in Joris 1996) over this range was 391 µs for A and 500 µs for B. Responses were obtained for a binaural AM stimulus with an envelope frequency difference (beat) of 1 Hz and are graphed as a function of ITD, calculated from the interaural phase differences in the envelopes of that stimulus (see Joris and Yin 1995). Positive ITD is defined as the contralateral ear leading in time.

Chopper responses were encountered regularly in recordings in AVCN and trapezoid body. These responses are associated with stellate cells (Smith and Rhode 1989), which are of interest in the context of this study because they constitute the other major cell type, besides the bushy cells, of the AVCN and also because they show some structural and physiological resemblances to LSO cells. Both transient and sustained choppers (for definition see legend Fig. 17) had nonmonotonic synchronization-level functions at 100 Hz with higher maximal R values (mean = 0.76 ± 0.071, n = 18) than AN fibers, but their MTFs were less stereotyped with SPL than those of AN fibers and bushy cells and show more of a band-pass characteristic at high SPL, as reported previously (Frisina et al. 1990; Rhode and Greenberg 1994). Cutoff frequencies, measured at the SPL giving the highest gain at 100 Hz, did not show an increase with CF (Fig. 17) and were much lower than in AN or LSO afferents (Fig. 8) particularly at CFs > 10 kHz. The lack of CF dependence suggests a temporal ceiling that is similar for choppers of all CFs.


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FIG. 17. Modulation transfer function 3-dB cutoff frequencies of choppers (diamonds) compared with AN (+). Cells were classed as choppers on the basis of pattern and regularity of responses to short (25 ms) CF tonebursts. Coefficient of variation (CV: calculated with analysis window of 12-18 ms and bindwidth of 1 ms) was < 0.3 for sustained (Chop S) and 0.5 > CV >=  0.3 for transient (Chop T) choppers.

    DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References

Two generalizations can be made from the current results. 1) There is a high degree of conservation in the magnitude of envelope phase-locking in both ipsi- and contralateral afferent channels of the LSO circuit for all stimulus parameters, which ensures that the LSO is supplied with envelope information over the widest possible range of modulation frequencies and well matched for the two sides in terms of amplitude and phase. 2) Because cells interposed between the AN and LSO have AM responses that resemble the AN rather than the LSO, the previously noted differences between envelope phase-locking in LSO and AN reflect input convergence or intrinsic properties at the level of LSO.

Limits in LSO afferents

All LSO afferents showed low-pass modulation transfer functions very similar to those of AN fibers (Figs. 5 and 9), with 3-dB cutoff frequencies that largely overlapped with the AN distribution (Figs. 7 and 12B). Only at CFs > 20 kHz, where measurements were sparse, were GBC and MNTB cutoffs in the lower part of the AN range. Dependence of synchronization on SPL in LSO afferents was nonmonotonic as in the AN (Fig. 2). Maximum synchronization values (Fig. 4) of GBCs and MNTB cells were higher than AN fibers at CFs below ~7 kHz but covered the same range at higher CFs. Neither in synchronization magnitude nor3-dB cutoff frequency range was there any indication of a difference between SBCs and AN fibers (Figs. 4, 7, and 12) although our sample of SBCs >10 kHz is very limited.

Many cell types in CN show higher gains than bushy cells, but over restricted ranges of modulation frequencies or SPLs (Frisina et al. 1990; Kim et al. 1990; Rhode and Greenberg 1994). For example, CN choppers had high gains but their cutoff frequencies were well below AN values (Fig. 17). In contrast, SBCs and GBCs preserve envelope information over a wide range of modulation frequencies and this range is little affected by SPL (Figs. 7 and 14). These properties, combined with sharp frequency tuning to tones and sustained responsiveness, make these cells ideally suited to supply the binaural system with information regarding amplitude fluctuations over a certain bandwidth of carrier frequencies.

Several studies reported an enhancement of envelope synchronization in cells with PLN responses relative to AN fibers (Frisina et al. 1990; Rhode and Greenberg 1994; Wang and Sachs 1994) although these studies diverge on the extent of gain increase. Our data indicate the importance of considering CF in such comparisons as populations diverged only clearly below ~7 kHz (Fig. 4). A similar effect can be seen in the data of Wang and Sachs (1994). Comparison of the measurements for AN, PL, PLN, and choppers by Rhode and Greenberg (1994) with our results (legend Fig. 4) for AN, SBCs, GBCs, and choppers shows higher maximal synchronization values in our data (except for GBCs). The average slopes for the synchronization-level functions in the two studies are very similar.

Monaural and binaural limits in LSO cells

In contrast to the general similarities in envelope synchronization between AN and LSO afferents, responses of cells in LSO to monaural modulation deviated in several ways from their afferents, including higher maximum synchronization values (Fig. 4) and a reduced modulation frequency range (Fig. 7). Phenomenologically, these transformations parallel findings on pure tone synchronization in CN bushy cells, which also is enhanced in magnitude but restricted in frequency range relative to the AN (Joris et al. 1994a).

MAXIMUM SYNCHRONIZATION. High synchronization values of LSO or presumed LSO cells to ipsilateral modulation were reported earlier (Batra et al. 1997b; Joris 1996) and are consistent with their good timing properties to pure tone onset and fine-structure (Finlayson and Caspary 1991; Joris and Yin 1995; Tsuchitani 1997). High synchronization values to contralateral stimulation at first may appear more surprising but are qualitatively consistent with a subtraction scheme (see RESULTS). Although other factors, such as postsynaptic temporal smearing of phase-locked inhibitory events (Sanes 1990) or poor synchronization among the MNTB cells converging on the LSO cell, could contribute to a longer inhibitatory postsynaptic potential (IPSP) and consequently narrow period histograms with high synchronization values, our results using ITDs of click stimuli (Joris and Yin 1995) suggest that the effective IPSPs from the contralateral side are short, on the order of 1 ms.

PHASE-LOCKING FREQUENCY RANGE. Synchronization of LSOcells to the envelope difference frequency in a binaural AM beat stimulus of increasing modulation frequency is restricted to modulation frequencies <= 800 Hz (Joris 1996). Such synchronization functions can be viewed as binaural MTFs: responses only can be modulated by envelope interaural phase differences if both ipsi- and contralateral input pathways carry envelope phase information. We therefore compared the phase-locking range in three steps of the LSO circuit: in LSO afferents, in LSO cells to monaural modulation, and finally in LSO cells to binaural modulation. The phase-locking ranges of LSO afferents are essentiallyAN-like (Fig. 7); the limited range for ITD-sensitivity in LSO therefore must derive from postsynaptic limitations such as poor synchronization across converging MNTB or SBC afferents, dendritic filtering, or temporal summation of subthreshold events. Monaural MTFs measured on LSO cells did show a more limited range of phase-locking than LSO afferents (Fig. 7), but, surprisingly, not as limited as the range for ITD-sensitivity (Fig. 8). This discrepancy may originate partly from our experimental and analytic procedures. R values to a binaural beat stimulus are lower (<0.6, see Fig. 7A in Joris 1996) and noisier than monaural R values at the modulation frequency (>0.6, Fig. 4), which tends to reduce the cutoff value (see also Batra et al. 1997b). Also, the decrease in average rate with modulation frequency was stronger with binaural than with monaural modulation, probably reflecting the smaller ILD range used binaurally (±20 dB) than monaurally (30 to -35 dB for contralateral modulation and usually no contralateral stimulus during ipsilateral modulation). Nevertheless, there are clear examples of cells that phase-lock monaurally to modulation frequencies at which there is no detectable ITD-sensitivity. Thus not all LSO cells transform the full-frequency range of temporal information supplied by its afferents into ITD-sensitivity.

Another surprise was the large range of cutoffs in LSO cells; this contrasts with the well-behaved relationship with CF observed in LSO afferents. Most striking is the similarity in phase-locking range for ipsi- and contralateral modulation despite differences in the two sets of afferent inputs in terms of 1) sign and time course of the postsynaptic potential (Sanes 1990; Wu and Kelly 1991) and 2) distal (SBC) versus proximal (MNTB) placement of the synaptic terminals (Cant 1984; P. H. Smith, P. X. Joris, and T.C.T. Yin, unpublished data). The observation that cutoff frequencies vary widely in different cells, but within a cell, tend to be similar for ipsi- and contralateral modulation and are correlated with the cutoff for binaural modulation (Fig. 8) suggest that the frequency limitation is at a postsynaptic stage that influences both monaural channels to the same degree.

The envelope phase-locking range is more limited in LSO than in its afferents, but this range is nevertheless still quite extensive, considering that LSO cells are third-order ipsilaterally and fourth-order contralaterally. AVCN choppers offer an illustrative contrast because they are second order yet on average have lower cutoff frequencies than LSO cells (Fig. 17).

AVERAGE RATE. Average firing rate of LSO cells decreased in most cells for increasing frequency of either contra- or ipsilateral modulation, as we found previously for binaural modulation (Fig. 13). Because this decrease was stronger or even in the opposite direction than would be predicted from average rate changes in afferents, it must be caused by temporal factors in the binaural interaction. The rate decrease for contralateral modulation is presumably due to loss of envelope phase-locking at high modulation frequencies. With increasing modulation frequency, the MNTB input to LSO becomes effectively demodulated or sustained. As discussed previously (Joris 1996), a sustained, unmodulated MNTB input inhibits the LSO cell more effectively than a modulated MNTB input. The rate decrease for ipsilateral modulation is harder to explain but also more variable. A possible explanation is complementary to the one just offered for contralateral modulation. MNTB cells provide a base level of inhibitory input (their average spontaneous rate was 29 spikes/s) to LSO. This input likely blocks modulated excitatory input (at low ipsilateral modulation frequencies) less effectively than sustained excitatory input (at high modulation frequencies).

Interaural timing

Previous studies of LSO have shown that ipsi- and contralateral responses are well matched for a variety of tonal suprathreshold response measures (Boudreau and Tsuchitani 1970). We found that this similarity extends to responses to envelope modulation in terms of gain, delay, shape, and cutoff frequency of modulation transfer functions. The response parameter that has attracted most attention in interaural comparisons is latency. Previous studies with tones and clicks (Boudreau and Tsuchitani 1968; Caird and Klinke 1983; Sanes and Rubel 1988; Tsuchitani 1988, 1997) suggested similar latencies for inhibition and excitation because even the first spike in response to ipsilateral stimulation can be inhibited by a simultaneously gated stimulus to the contralateral ear. However, direct measurement of latency for the inhibitory ear is difficult with such stimuli. We estimated delays by measuring the slope of the phase-frequency relationship in each cell (Figs. 10 and 11). Such delays do not show a decrease with increasing SPL (Fig. 14C), unlike traditional measures of onset latency, but consistent with the notion that they are determined mainly by fixed conduction delays. Similar observations using a different technique were made by Møller (1975).

The differences in delay for responses to ipsi- and contralateral modulation were surprisingly small (Fig. 11), not only in view of the path-length differences but also relative to the absolute magnitude of the delays involved. The good match of ipsi- and contralateral delays is consistent with a characteristic delay analysis of responses to binaural modulation, which also indicates that delays of both sides are matched within <800 µs (Joris 1996). The mean characteristic delay for all cells was 200 µs, which is identical to the difference in asymptotic delay tau  measured here (Fig. 18: 4.1 and 3.9 ms for contra- and ipsilateral modulation, respectively) and close to the mean difference in contra- and ipsilateral delays (182 µs). Thus on average, the contralateral signal is delayed slightly, by 0.2 ms, relative to the ipsilateral signal. Even more striking is the observation that the delays are correlated (Fig. 11B), suggesting that the matching extends even to the single cell level. Small characteristic delays, but with an opposite bias, also were found by Batra et al. (1997a) in cells with ITD-sensitivity similar to LSO cells.


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FIG. 18. Summary diagram of delay measurements (in ms) on a tracing of a coronal section through the superior olivary complex. Values are the asymptotic or CF-independent part of a power function fit to the data of Fig. 11A (see text). Italic numbers on bottom give differences in delay between cell classes. Long delay between AN and LSO to ipsilateral modulation (2.9 ms) probably derives from the small diameter and circuitous path of SBC axons, and their termination on distal LSO dendrites. Comparatively short delay between AN and LSO to contralateral modulation (3.1 ms in total) derives from fast axonal conduction and axosomatic placement of terminals along each relay point.

The delays incurred along the afferent chain are of the expected magnitude. Relative to each preceding stage the asymptotic delays on the contralateral side are: 1.0 ms for AN; AN + 0.8 ms for GBC; GBC + 0.4 ms for MNTB; MNTB + 1.9 ms for the LSO response to contralateral modulation. Data on SBC were too few and restricted in CF to adequately fit, and data on presumed SBC were inhomogeneous in terms of site of recording, but visual inspection of Figs. 11 and 12C clearly shows that the delay of LSO to ipsilateral modulation (AN + 2.9 ms) was derived largely from delay accrued after the AN-SBC synapse. The ranking of these afferent delays is consistent with axon diameters, which are largest for GBCs (Brownell 1975: 8-15 µm; Spangler et al. 1985: 8-15 µm; Spirou et al. 1990; Smith et al. 1991; van Noort 1969: 8-12 µm), smaller for MNTB (Spangler et al. 1985: 5-6 µm; P. H. Smith, P. X. Joris, and T.C.T. Yin, unpublished results: 4-6 µm), and apparently yet smaller for SBCs (Brownell 1975: 3-5 µm; Smith et al. 1993: 2.5-5 µm for low-CF SBC axons; Warr 1966: 1.8 µm; van Noort 1969: 4 µm) though we are not aware of quantitative measurements on identified high-CF SBC axons projecting to LSO. Another factor in the long ipsilateral delay may be path length. Labeled low-CF SBCs project to LSO via a variable and circuitous route (Smith et al. 1993). Moreover, the ipsilateral signals likely incur a substantial dendritic delay because of the distal placement of the SBC terminals on LSO cells (Cant 1984).

Of course, the site and nature of recording (extracellular or axonal) affect these absolute delay estimates because different amounts of axonal conduction delay are involved. Moreover, in the extracellular MNTB recordings, the trigger point of spike timing varied across cells because in about one-third of the cases the largest waveform amplitude and triggering occurred early in the complex waveform and possibly reflected the pre- rather than the postsynaptic potential, which would partly offset differences between MNTB and GBC populations. Comparisons of ipsi- and contralateral delays in recordings from a single LSO cell are not affected by such factors.

We have previously shown (Joris and Yin 1995) that average rate of LSO cells is affected by both ITDs and ILDs. It is well known that changes in SPL affect response onset latency, so that ILDs may affect firing rate not only by changes in firing rate in afferent pathways but also by changing time of arrival of afferent spikes at the site of convergence. This is the basis for the latency hypothesis (Jeffress 1948). Despite the absence of change with SPL in the slope of the phase-frequency functions, the response phase at a single modulation frequency decreases linearly with increasing SPL, as we previously described in AN and LSO. This is likely due to adaptation, which shifts spikes to earlier phase values of the modulation cycle. The change in response phase was small (~10-30 µs/dB) but possibly significant given the physiological ranges of ITDs (±400 µs for cat) (Roth et al. 1980) and ILDs (±30 dB) (Irvine 1987; Musicant et al. 1990). However, the functional importance of the latency-SPL interaction for AM responses in LSO is questionable because of the large "direct" effect of SPL and ILD on firing rate (Joris and Yin 1995). The interaction may be more important for transient responses (Joris and Yin 1995; Pollack 1988; Yin et al. 1985; Irvine et al. 1998).

Cell classification

We chose physiological criteria that optimized sorting of cells to the appropriate categories, but this procedure likely also biased our sample toward cells with pronounced characteristics. For example, one of our requirements for classification of a cell as GBC was a clear PLN response pattern, which may have biased the GBC sample toward cells with good timing properties. Indeed, a small number of GBCs identified anatomically by their calyceal ending in the MNTB maintain a PL response pattern at high SPLs (Smith et al. 1991) and would be categorized physiologically as presumed SBC if recorded in the TB, or SBC if recorded from CN [where GBCs also can display large PPs (Bourk 1976)]. Another possible bias between SBC and GBC populations may have been introduced by the somatic (SBC) versus axonal (GBC) recordings. For example, precision of phase-locking to pure tones was found to be significantly better in axonal recordings from bushy cells when compared with most values reported for extracellular recordings in CN (Joris et al. 1994a). However, the similarity in findings between MNTB cells and GBCs, as well as between the SBC and presumed SBC populations, argue against the significance of any such effects on envelope synchronization.

Relevance of binaural envelope information

We conclude this series of papers by posing a paradox. The afferent pathways to LSO are characterized by the presence of specializations in membrane properties, axon diameters, and size and placement of terminals. These features minimize differences between total ipsi- and contralateral conduction delay and limit spatial and temporal integration. Speculation on their purpose traditionally has been in terms of timing, which motivated the present series of studies of ITD-sensitivity in LSO. Humans have a well-documented ability to lateralize high-frequency sounds on the basis of envelope ITDs. The envelope ITD-sensitivity that we described in the preceding papers showed a number of parallels with human performance. Psychophysically, the smallest ITDs that can be discriminated approach those for low-frequency fine-structure (Henning 1974), but unphysiologically large ITDs are necessary to move the intracranial image away from midline (Bernstein and Trahiotis 1985). Physiologically, rate is much more dependent on ILD than on ITD if stimuli are restricted to the physiological range (Joris and Yin 1995). The weakness of the ITD-sensitivity is in stark contrast to the dramatic dimensions of the functional and morphological features, e.g., the endings of globular bushy cells on MNTB cells, the calyces of Held, are probably the largest terminals in the mammalian CNS (Jean-Baptiste and Morest 1975). On the other hand, these features seem unnecessary or even disadvantageous for ILD-sensitivity. It can be argued that an ILD-extracting circuit should optimally receive inputs that integrate over time and perhaps frequency because ILDs are a complex function of frequency at each spatial position (Musicant et al. 1990; Rice et al. 1992). Other cochlear nucleus cell types seem better suited to convey information on SPL needed to process ILDs (Rhode and Smith 1986; Shofner and Dye 1989). If not to enable extraction of ITDs or static ILDs, what then is the benefit of these specializations? We suggest that specializations in the LSO circuit optimize preservation of envelope information and comparison of instantaneous amplitude fluctuations at the two ears.

Before presenting our arguments, we wish to clearly distinguish two aspects in specializations for timing, which in some cases may be implemented by the same structural adaptation. First, a pathway may be optimized to transmit monaural temporal information. The limited amount of convergence, the dominant axosomatic terminals on bushy cells and MNTB cells, and the rectifying membrane properties of these cells usually are thought to subserve such optimization. These properties enable phase-locking in the successive components of each monaural channel over a wide range of carrier and modulation frequencies. Second, pathways may be optimized for conduction speed and binaural or interaural timing, so that envelope information is supplied to the binaural processor in the appropriate time frame. If the binaural operation performed is the extraction of ITDs, then the appropriate time frame is the range of ITDs that the animal is likely to encounter in a natural environment.

ITD-sensitivity in LSO can be viewed in two ways. A first extreme hypothesis is that the LSO is concerned really with detection of ITDs in complex stimuli. It generally is believed that timing information for low-frequency stimuli is distributed via delay lines to coincidence detectors in the MSO, which map the full range of physiological ITDs in neural space (Goldberg and Brown 1969; Jeffress 1948; Yin and Chan 1990). A similar view for LSO would hold that the specializations to match the input pathways in terms of timing and spectral integration, as well as the presence of IE-interaction, have evolved to allow calculation of ITDs and that ILD-sensitivity is a byproduct of this evolution. Given that physiologically and psychophysically at high frequencies ILDs are a much more potent cue than ITDs, this view is untenable.

An alternative hypothesis is that the input specializations and IE-interaction allow calculation of ILDs with a short integration time and that envelope ITDs are a problem to be dealt with rather than a cue. Many natural signals are modulated in frequency and amplitude. Correct calculation of ILD requires the comparison of corresponding parts of the waveform by the binaural comparator. This calculation would be complicated by the presence of large interaural delays, as would be created by millisecond differences in conduction time between ipsi- and contralateral inputs. One solution would be to have binaural convergence of inputs with a long integration time that would minimize the effect of large differences in conduction delay. However, this would limit envelope information to low modulation frequencies. Another solution is to match the timing of the inputs at thesite of binaural interaction, by delaying the ipsilateral inputand/or expediting the contralateral input via large axons and synaptic specializations. The small range of characteristic delays (Joris 1996) and of differences in monaural delays (Fig. 11) relative to the long delays required to carry the signals to the LSO argues that the LSO circuit embodies the latter solution. Through its specializations, this system is able to extract ILDs, and its output still contains information about temporal modulations in amplitude and frequency.

An analogy with disparity processing in the visual domain is illustrative. Because of light's speed there are of course no interocular time differences in the sense defined for acoustic ITDs. Fine stereopsis is thought to depend on disparity sensitivity in binocular cortical cells, derived from spatial phase and/or position differences in the receptive fields of the two eyes (Barlow et al. 1967; DeAngelis et al. 1991). Such detectors can track disparity of a time-varying (e.g., moving) stimulus only if the two eyes supply temporal information matched for the two eyes. Being a slower sensory system, the timing requirements in vision are less demanding than in audition, so that large temporal mismatches are tolerated before stereoscopic perception is lost (<= 50 ms) (review in Howard and Rogers 1995). An example of the effect of interocular timing is found in the Pulfrich illusion, in which motion in the fronto-parallel plane appears to be in depth if one eye is covered with a light-attenuating filter. The filter introduces a neural temporal delay, which shifts the interocular phase at which binocular cells discharge maximally (Carney et al. 1989). We surmise that this time sensitivity should be viewed as a necessary outcome of a spatial disparity detector with a time resolution scaled to the demands of visual processing. Similarly, we suggest that the good match of interaural delays in the LSO has evolved to enable the extraction of ILDs at corresponding points in frequency and time of the acoustic waveform in both ears. In a sense, there is a time correspondence problem for ILD computation, like there is a spatial correspondence problem for stereoscopic vision. If the monaural signals would have been delivered by sluggish channels preserving little timing information, the requirement for temporal matching of ipsi- and contralateral signals would have been less stringent, as for binocular cells. However, the afferents of the LSO contain envelope-related timing information over the full range available in the AN. Thus it is important that the interaural timing in this circuit is controlled precisely.

LSO as a multiplexer

This teleological argument by itself does not specify the benefit of maintaining the envelope in the extraction of ILD. That temporal and spectral accuracy are so well preserved at the input stage suggests that the LSO might have a broader role in spatial hearing than extracting ILD information for coding of azimuthal position of a single, static source. A first role may be temporal multiplexing of ILDs. By virtue of the preservation of temporal envelope information in the afferent inputs, LSO cells are able to encode quick alterations in ILD, e.g., as generated by multiple, modulated sound sources with overlapping spectra but differing spatial location. It is important to note here that the ILD coding in LSO is constrained by the absence of tuning for this cue. Cells do not show a characteristic or best ILD by which the cue value would be mapped as a focus of activity in a cell population graded for such property. Rather, the available evidence supports a scalar coding of ILD---by virtue of the sigmoidal ILD function and activity summed across cells---reinforcing the need for a multiplexing strategy.

Another need for temporal accuracy may be tracking of dynamic ILDs, e.g., as caused by moving sources or head or pinna movements (Young et al. 1996). Also, it is plausible that preservation of modulation information after binaural interaction is needed to allow integration of cues at a later stage. For example, envelopes correlated across frequency regions improve the lateralization of high-frequency complex sounds (Saberi 1995). Another example is the improved speech intelligibility in a noisy environment by integration of modulation and binaural cues, as exemplified by a computer algorithm (Kollmeier and Koch 1994). Third, this series of papers has emphasized responses to ongoing ITDs in AM stimuli, but accuracy in localizing high-frequency transients ("snapping twigs") or detection of onset time differences also may require the specializations for time found in the LSO circuit (Caird and Klinke 1983; Joris and Yin 1995; Irvine et al. 1998). Finally, it has been suggested that binaural cues may be coded by the timing patterns of LSO discharges rather than by rate of firing (Tsuchitani 1997).

To conclude, we wish to reemphasize (see also Joris 1996) that our interpretation does not preclude the usefulness of high-frequency ITDs for localization of sounds. The core of our argument is that this capability is too weak and occurs under too stringent conditions to warrant the powerful morphological and physiological specializations that are found in this circuit. Whether computational demands along the lines suggested necessitate these specializations remains to be demonstrated behaviorally and physiologically.

    ACKNOWLEDGEMENTS

  The authors thank R. Batra, S. Kuwada, A. R. Palmer, and P. H. Smith for comments on the manuscript. We gratefully acknowledge the assistance of I. Siggelkow, J. Meister, J. A. Ekleberry (for histology), R. Kochhar, J. Sekulski (for software), and G. Meulemans (for photography).

  This work was supported by National Institute of Deafness and Other Communications Disorders Grant DC-00116.

    FOOTNOTES

  Address for reprint requests: P. X. Joris, Division of Neurophysiology, Medical School, University of Leuven, Campus Gasthuisberg, B-3000 Leuven, Belgium.

  Received 7 May 1997; accepted in final form 5 August 1997.

    REFERENCES
Abstract
Introduction
Methods
Results
Discussion
References

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