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The medial nucleus of the trapezoid body (MNTB) is a distinct nucleus in the superior olivary complex that transforms excitatory input from the cochlear nucleus into a widespread inhibitory output to distinct auditory brainstem nuclei. Few studies have dealt with the response properties of MNTB neurons to sound stimulation using in vivo preparations. In order to have a better understanding of the functional significance of the MNTB in auditory processing we report the basic temporal and spectral response properties of its principal cells using single-unit extracellular recordings to acoustic stimulation with pure tones and amplitude-modulated stimuli in the rat. Ninety-seven per cent of units showed V-shaped frequency response areas. Rate level functions were mainly saturating (51%) or monotonic (45%) at high intensities. Post-stimulus time histograms typically were characterised as primary-like with notch (59%) or primary-like (33%). Units showed good phase-locking to sinusoidally amplitude-modulated signals with vector strength VS values up to 0.87. Modulation transfer functions had low-pass shapes at near-threshold levels, with cut-off frequencies ranging from 370 to 1270 Hz. Exploration of the relationship between the temporal and spectral properties and the location of the units in the MNTB yielded characteristic frequency (CF)-dependent response properties (latency, Q(10) and cut-off frequency) following a medio-lateral gradient, and CF-independent response features (maximum firing rate) following a dorso-ventral gradient.

At the initial stages in neuronal development, GABAergic and glycinergic neurotransmission exert depolarizing responses, assumed to be of importance for maturation, which in turn shift to hyperpolarizing in early postnatal life due to development of the chloride homeostasis system. Spherical bushy cells (SBC) of the mammalian cochlear nucleus integrate excitatory glutamatergic inputs with inhibitory (GABAergic and glycinergic) inputs to compute signals that contribute to sound localization based on interaural time differences. To provide a fundamental understanding of the properties of GABAergic neurotransmission in mammalian cochlear nucleus, we investigated the reversal potential of the GABA-evoked currents (E GABA) by means of gramicidin-perforated-patch recordings in developing SBC. The action of GABA switches from depolarizing to hyperpolarizing by the postnatal day 7 due to the negative shift in E GABA. Furthermore, we studied the expression pattern of the K+-Cl(-)-extruding cotransporter KCC2, previously shown to induce a switch from neonatal Cl(-) efflux to the mature Cl(-) influx in various neuron types, thereby causing a shift from depolarizing to hyperpolarizing GABA action. The KCC2 protein is expressed in SBC already at birth, yet its activity is attained toward the end of the first postnatal week as indicated by pharmacological inhibition. Interruption of the Cl(-) extrusion by [(dihydroindenyl)oxy] alkanoic acid or furosemide gradually shifted E(GABA) in positive direction with increasing maturity, suggesting that KCC2 could be involved in maintaining low [Cl(-)]i after the postnatal day 7 thereby providing the hyperpolarizing Cl(-)-mediated inhibition in SBC.

Recent studies utilizing the mismatch negativity (MMN) event-related potential (ERP) revealed that when a repetitive sequence of sinusoidal tones is presented, the occasional insertion of a short deviation into some of the tones leads to the elicitation of an MMN only if it occurs during the initial 300 ms, but not beyond. In contrast, deviations occurring in speech sounds elicit MMN even beyond 300 ms. We conducted two experiments to resolve this conflict. We hypothesised that an additional transient within an otherwise unstructured tone may overcome this limitation. First, we tested for MMN to a deviance at the terminal part of a 650 ms tone which did or did not contain a gap. Only when the tone included the gap, MMN was obtained. Second, we compared the gap condition with two noise conditions, in which the gap was replaced by modulated white noise. The noise conditions differed with respect to the saliency of the perceived interruption of the tone. In all three conditions, MMN was elicited. These results demonstrate that structuring a sinusoidal tone by a gap or a noise interval is sufficient to regain MMN. It is suggested that the introduction of an additional transient triggers a new integration window overcoming the temporal constraints of automatic tone representation. This resolves the seeming contradiction between MMN studies using tonal and speech sounds.

Localization of sounds by the auditory system is based on the analysis of three sources of information: interaural level differences (ILD, caused by an attenuation of the sound as it travels to the more distant ear), interaural time differences (ITD, caused by the additional amount of time it takes for the sound to arrive at the more distant ear), and spectral cues (caused by direction-specific spectral filter properties of the pinnae). Although in a number of psychophysiological studies cortical processes of ITD and ILD analysis were investigated, there is hitherto no evidence on the cortical processing of spectral cues for sound localization. The objective of the present experiment was to test whether it is possible to observe electrophysiological correlates of sound localization based on spectral cues. In an auditory oddball experiment, 80 ms of broadband noise from varying free field locations were presented to inattentive participants. Mismatch negativities (MMNs) were observed for pairs of standards and location deviants located symmetrically with respect to the interaural axis. As interaural time and level differences are identical for such pairs of sounds, the observed MMNs most likely reflect cognitive processes of sound localization utilizing the spectral filter properties of the pinnae. MMN latencies suggest that sound localization based on spectral cues is slower than ITD- or ILD-based localization.

We report evidence for a context- and not stimulus-dependent functional asymmetry in the left and right human auditory midbrain, thalamus, and cortex in response to monaural sounds. Neural activity elicited by left- and right-ear stimulation was measured simultaneously in the cochlear nuclei, inferior colliculi (ICs), medial geniculate bodies (MGBs), and auditory cortices (ACs) in 2 functional magnetic resonance imaging experiments. In experiment 1, pulsed noise was presented monaurally to either ear, or binaurally, simulating a moving sound source. In experiment 2, only monaural sounds were presented. The results show a modulation of the neural responses to monaural sounds by the presence of binaural sounds at a time scale of tens of seconds: In the absence of binaural stimulation, the left and right ICs, MGBs, and ACs responded stronger to stimulation of the contralateral ear. When blocks of binaural stimuli were interspersed in the sound sequence, the contralateral preference vanished in those structures in the right hemisphere. The resulting hemispheric asymmetry was similar to the asymmetry demonstrated for spatial sound processing. Taken together, the data demonstrate that functional asymmetries in auditory processing are modulated by context. The observed long time constant suggests that this effect results from a "top-down" mechanism.

We use novel noise-like sound stimuli to identify cortical areas in which the functional magnetic resonance signal covaries with spectral and temporal acoustic complexity. The results support a model of hemispheric functional asymmetry for fine-grained spectral and fast temporal processing.

The present study investigates the acoustic basis of the hemispheric asymmetry for the processing of speech and music. Experiments on this question ideally involve stimuli that are perceptually unrelated to speech and music, but contain acoustic characteristics of both. Stimuli in previous studies were derived from speech samples or tonal sequences. Here we introduce a new class of noise-like sound stimuli with no resemblance of speech or music that permit independent parametric variation of spectral and temporal acoustic complexity. Using these stimuli in a functional MRI experiment, we test the hypothesis of a hemispheric asymmetry for the processing of spectral and temporal sound structure by seeking cortical areas in which the blood oxygen level dependent (BOLD) signal covaries with the number of simultaneous spectral components (spectral complexity) or the temporal modulation rate (temporal complexity) of the stimuli. BOLD-responses from the left and right Heschl's gyrus (HG) and part of the right superior temporal gyrus covaried with the spectral parameter, whereas covariation analysis for the temporal parameter highlighted an area on the left superior temporal gyrus. The portion of superior temporal gyrus in which asymmetrical responses are apparent corresponds to the antero-lateral auditory belt cortex, which has been implicated with spectral integration in animal studies. Our results support a similar function of the anterior auditory belt in humans. The findings indicate that asymmetrical processing of complex sounds in the cerebral hemispheres does not depend on semantic, but rather on acoustic stimulus characteristics.

The localization of low-frequency sounds mainly relies on the processing of microsecond temporal disparities between the ears, since low frequencies produce little or no interaural energy differences. The overall auditory cortical response to low-frequency sounds is largely symmetrical between the two hemispheres, even when the sounds are lateralized. However, the effects of unilateral lesions in the superior temporal cortex suggest that the spatial information mediated by lateralized sounds is distributed asymmetrically across the hemispheres. This paper describes a functional magnetic resonance imaging experiment, which shows that the interaural temporal processing of lateralized sounds produces an enhanced response in the contralateral planum temporale (PT). The response is stronger and extends further into adjacent regions of the inferior parietal lobe (IPL) when the sound is moving than when it is stationary. This suggests that the interaural temporal information mediated by lateralized sounds is projected along a posterior pathway comprising the PT and IPL of the respective contralateral hemisphere. The differential responses to moving sounds further revealed that the left hemisphere responded predominantly to sound movement within the right hemifield, whereas the right hemisphere responded to sound movement in both hemifields. This rightward asymmetry parallels the asymmetry associated with the allocation of visuo-spatial attention and may underlie unilateral auditory neglect phenomena.

Horizontal sound localization relies on the extraction of binaural acoustic cues by integration of the signals from the two ears at the level of the brainstem. The present experiment was aimed at detecting the sites of binaural integration in the human brainstem using functional magnetic resonance imaging and a binaural difference paradigm, in which the responses to binaural sounds were compared with the sum of the responses to the corresponding monaural sounds. The experiment also included a moving sound condition, which was contrasted against a spectrally and energetically matched stationary sound condition to assess which of the structures that are involved in general binaural processing are specifically specialized in motion processing. The binaural difference contrast revealed a substantial binaural response suppression in the inferior colliculus in the midbrain, the medial geniculate body in the thalamus and the primary auditory cortex. The effect appears to reflect an actual reduction of the underlying activity, probably brought about by binaural inhibition or refractoriness at the level of the superior olivary complex. Whereas all structures up to and including the primary auditory cortex were activated as strongly by the stationary as by the moving sounds, non-primary auditory fields in the planum temporale responded selectively to the moving sounds. These results suggest a hierarchical organization of auditory spatial processing in which the general analysis of binaural information begins as early as the brainstem, while the representation of dynamic binaural cues relies on non-primary auditory fields in the planum temporale.

Detection of mistuned partials in otherwise harmonic complex tones was investigated in naïve subjects of three different age groups. Signals were presented at constant sensation level to compensate for differences in hearing sensitivity and to specifically examine age-related changes in inharmonicity perception. Performance was measured under two conditions, monaural signal presentation and dichotic signal-noise presentation, with the latter aiming at the influence of contralateral distractor sounds. Stimuli were complex tones with ten harmonics and 125-Hz fundamental frequency. Mistuning detection was measured for the first, second, fourth, and eighth harmonic. In a three-interval, three-alternative forced-choice procedure, subjects were required to distinguish a complex tone containing one mistuned partial from two reference tones, with all partials at their harmonic frequencies. Thresholds were measured as the amount of frequency shift necessary for the mistuning to be detected. Performance deteriorated moderately with age for the two higher partials tested, but not for the lower ones. Thresholds for dichotic signal/noise presentation did not differ significantly from monaural ones in any of the age groups. Results are discussed in relation to hypotheses of harmonicity perception in auditory scene analysis and with respect to the investigation of patients suffering form respective deficits due to acquired brain lesions.

The voltage-gated potassium (Kv) channel subunit Kv1.1, encoded by the Kcna1 gene, is expressed strongly in the ventral cochlear nucleus (VCN) and the medial nucleus of the trapezoid body (MNTB) of the auditory pathway. To examine the contribution of the Kv1.1 subunit to the processing of auditory information, in vivo single-unit recordings were made from VCN neurons (bushy cells), axonal endings of bushy cells at MNTB cells (calyces of Held), and MNTB neurons of Kcna1-null (-/-) mice and littermate control (+/+) mice. Thresholds and spontaneous firing rates of VCN and MNTB neurons were not different between genotypes. At higher sound intensities, however, evoked firing rates of VCN and MNTB neurons were significantly lower in -/- mice than +/+ mice. The SD of the first-spike latency (jitter) was increased in VCN neurons, calyces, and MNTB neurons of -/- mice compared with +/+ controls. Comparison along the ascending pathway suggests that the increased jitter found in -/- MNTB responses arises mostly in the axons of VCN bushy cells and/or their calyceal terminals rather than in the MNTB neurons themselves. At high rates of sinusoidal amplitude modulations, -/- MNTB neurons maintained high vector strength values but discharged on significantly fewer cycles of the amplitude-modulated stimulus than +/+ MNTB neurons. These results indicate that in Kcna1-null mice the absence of the Kv1.1 subunit results in a loss of temporal fidelity (increased jitter) and the failure to follow high-frequency amplitude-modulated sound stimulation in vivo.

The medial nucleus of the trapezoid body (MNTB) plays an important role in the processing of interaural intensity differences, a feature that is critical for the localization of sound sources. It is generally believed that the MNTB functions primarily as a passive relay in converting excitatory input originating from the contralateral cochlear nucleus (CN) into an inhibitory input to the ipsilateral lateral superior olive. However, studies showing that the MNTB itself is also the target of inhibitory input suggest that the MNTB may serve more than a sign-converting function. To examine the fidelity of signal transmission at the CN-MNTB synapse, presynaptic calyceal potentials ("prepotentials"), reflecting the excitatory input to the MNTB neuron, and postsynaptic action potentials were simultaneously monitored with the same electrode during in vivo extracellular recordings from the gerbil's MNTB. Presynaptic activity differed from postsynaptic activity in several respects: (1) Spontaneous and sound-evoked discharge rates were greater presynaptically than postsynaptically. (2) Frequency tuning was sharper postsynaptically than presynaptically. (3) Calyceal terminals and MNTB neurons both showed phasic-tonic response patterns to tonal stimulation, but the duration of the onset response and the level of the tonic component were reduced postsynaptically. (4) Phase-locking to sound frequencies up to 1 kHz was greater postsynaptically than presynaptically. (5) The rate-intensity characteristics of pre- and postsynaptic activities differed significantly from each other in half of the MNTB neurons. To test the hypothesis that acoustically evoked inhibition of MNTB neurons contributed to the relatively lower levels of postsynaptic discharge, two-tone stimulation was applied, wherein the response to one tone-burst, set at the neuron's characteristic frequency, can be reduced by addition of a second "inhibitory" tone. The inhibitory tone caused a much larger reduction in post- than in presynaptic activity, indicating an acoustically evoked inhibitory influence directly on MNTB units. These findings show that transmission at the CN-MNTB synapse does not occur in a fixed one-to-one manner and that the response of MNTB neurons reflects the integration of their excitatory and inhibitory inputs.

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., (Copyright 2003 Wiley-Liss, Inc.)

Spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their main excitatory input from auditory nerve fibers (ANFs) through large synapses, endbulbs of Held. These cells are also the target of inhibitory inputs whose function is not well understood. The present study examines the role of inhibition in the encoding of low-frequency sounds in the gerbil's AVCN. The presynaptic action potentials of endbulb terminals and postsynaptic action potentials of SBCs were monitored simultaneously in extracellular single-unit recordings in vivo. An input-output analysis of presynaptic and postsynaptic activity was performed for both spontaneous and acoustically driven activity. Two-tone stimulation and neuropharmacological experiments allowed the effects of neuronal inhibition and cochlear suppression on SBC activity to be distinguished. Ninety-one percent of SBCs showed significant neuronal inhibition. Inhibitory sidebands enclosed the high- or low-frequency, or both, sides of the excitatory areas of these units; this was reflected as a presynaptic to postsynaptic increase in frequency selectivity of up to one octave. Inhibition also affected the level-dependent responses at the characteristic frequency. Although in all units the presynaptic recordings showed monotonic rate-level functions, this was the case in only half of the postsynaptic recordings. In the other half of SBCs, postsynaptic inhibitory areas overlapped the excitatory areas, resulting in nonmonotonic rate-level functions. The results demonstrate that the sound-evoked spike activity of SBCs reflects the integration of acoustically driven excitatory and inhibitory input. The inhibition specifically affects the processing of the spectral, temporal, and intensity cues of acoustic signals.

In this functional MRI study the frequency-dependent localization of acoustically evoked BOLD responses within the human auditory cortex was investigated. A blocked design was employed, consisting of periods of tonal stimulation (random frequency modulations with center frequencies 0.25, 0.5, 4.0, and 8.0 kHz) and resting periods during which only the ambient scanner noise was audible. Multiple frequency-dependent activation sites were reliably demonstrated on the surface of the auditory cortex. The individual gyral pattern of the superior temporal plane (STP), especially the anatomy of Heschl's gyrus (HG), was found to be the major source of interindividual variability. Considering this variability by tracking the frequency responsiveness to the four stimulus frequencies along individual Heschl's gyri yielded medio-lateral gradients of responsiveness to high frequencies medially and low frequencies laterally. It is, however, argued that with regard to the results of electrophysiological and cytoarchitectonical studies in humans and in nonhuman primates, the multiple frequency-dependent activation sites found in the present study as well as in other recent fMRI investigations are no direct indication of tonotopic organization of cytoarchitectonical areas. An alternative interpretation is that the activation sites correspond to different cortical fields, the topological organization of which cannot be resolved with the current spatial resolution of fMRI. In this notion, the detected frequency selectivity of different cortical areas arises from an excess of neurons engaged in the processing of different acoustic features, which are associated with different frequency bands. Differences in the response properties of medial compared to lateral and frontal compared to occipital portions of HG strongly support this notion.

The superior paraolivary nucleus (SPN) of the superior olivary complex (SOC) though morphologically well described, has not been characterized physiologically. Here we report the basic response properties of SPN units acquired with extracellular recording techniques under monaural acoustic stimulation in the Mongolian gerbil. Poststimulus-time histograms corresponded to those described earlier for the cat's cochlear nucleus (onset, chopper, primary-like), and partly to those previously acquired in other SOC nuclei (tonic, off/rebound). Two-thirds of the units responded solely to contralateral stimulation (40% excitatory [E], 19% inhibitory [I], 6% mixed [EI]). Most of the remainder responded equally to stimulation from either ear (18% I.I, 9% E.E). Overall, the monaural contralateral input was more effective than the ipsilateral and bilateral input. Characteristic frequencies and response areas covered the entire hearing range of the gerbil and the units mostly showed broad frequency-tuning. In combination, these properties suggest that the SPN might be a constituent of an afferent pathway encoding stimulus features across broad frequency ranges.

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.

The auditory nerve serves as the only excitatory input to neurons in the avian cochlear nucleus, nucleus magnocellularis (NM). NM neurons in immature animals are dependent upon auditory nerve signals; when deprived of them, many NM neurons die, and the rest atrophy. Auditory nerve terminals release glutamate, which can stimulate second messenger systems by activating a metabotropic glutamate receptor (mGluR). Therefore, it is possible that the effectors of mGluR-stimulated signal transduction systems are needed for NM neuronal survival. This study shows that mGluR activation in NM neurons attenuates voltage-dependent changes in [Ca2+]j. Voltage-dependent Ca2+ influx was also attenuated by increasing cAMP with forskolin, VIP, or 8-bromo-cAMP, indicating that mGluR activation may stimulate adenylate cyclase. The main results may be summarized as follows. NM neurons possess high voltage-activated Ca2+ channels that were modulated by quisqualate, glutamate, and (+/-)trans-ACPD, in that order of potency. Glutamatergic inhibition of Ca2+ influx was not blocked by L-AP3 or L-AP4, which antagonize the actions of mGluRs in other neural systems; it was blocked by serine-O-phosphate. Finally, the attenuation of voltage-dependent Ca2+ influx was duplicated by cAMP accumulators. Since NM neurons have high rates of spontaneous activity and higher rates of driven activity, the expression of this mGluR turns out to be very valuable: without it, [Ca2+]j could reach lethal concentrations. These results provide an important clue as to the identity of an intracellular signal that may play an important role in NM neuronal survival.

This report describes the postnatal development of hearing range, auditory sensitivity and tonotopy within the inferior colliculus (IC) of a mammal specialized for ultrasonic hearing. The experimental animal, Carollia perspicillata, has an adult hearing range of 7-110 kHz (characteristic frequencies) but lack any significant overrepresentation of a limited frequency band as known for rhinolophoid bats and Pteronotus. The audiogram of the newborn Carollia includes characteristic frequencies from 8 to 76 kHz, which is about 65% of the adult hearing range. As in adults, low frequencies are represented in the dorsolateral portion of the IC. However, at birth the ventromedial IC is non-responsive to acoustic stimulation up to intensities of 90 dB SPL. During development there is a progressive conversion of non-responsive IC areas into acoustically responsive slabs with characteristic frequencies above 76 kHz along the dorsolateral to ventromedial (low-to-high frequency) IC axis. This development is superimposed by a non-uniform shift of characteristic frequency: a decrease of CFs in dorsolateral regions, and an increase of CFs in ventromedial areas. The results suggest a bidirectional shift of frequency representation along the cochlear tonotopic axis.

In the early postnatal period of many mammals and in the perihatching period of chicks the auditory ranges are restricted to the species-specific low- and mid-frequency ranges. During subsequent development, the high frequency hearing expands (depending on the species) by 1-4 octaves. Adult-like audition is established between the 4th and the 7th week. It is still discussed controversially, how the extension of the auditory ranges relates to the maturation of orderly frequency representation in the cochleae of the respective species. The present review summarizes investigations of the development of tonotopy in nuclei of the central auditory system, and discusses how the centrally acquired data might contribute to the understanding of the maturation of cochlear stimulus transduction and to the development of frequency maps.

In the auditory forebrain (field L) of the European starling (Sturnus vulgaris), single unit responses were recorded for a wide range of complex stimuli, comprising different forms of amplitude and frequency modulation. About two-third of the units locked to sinusoidal modulation regardless of whether frequency (SFM) or amplitude (SAM) was modulated. On average, however, frequency led to stronger synchronization. Both the proportion of phase locking and its mean strength showed a low-pass dependence on modulation frequency. The lower efficiency of amplitude modulation is also visible in unit responses when SAM is combined with (random) frequency modulation. For the assessment of response strength and its comparison across the tested repertoire of complex stimuli, a new index (REX) is introduced which primarily weighs similarity of the spike trains in identically repeated stimulus runs. Applied to a set of 311 field L neurons, also this approach discloses the two stimulus classes lacking frequency modulation (pure tone and SAM) as the least effective. A new measure for response latency, the Effective Response Delay (ERD), based on the spike-triggered analysis of responses to randomly frequency-modulated sounds (RFM), reflects physiological delays better than conventional latency. So, ERD correction of SAM and SFM Period Histograms allowed to disclose response effective stimulus ranges independent of modulation frequency.

The response characteristic of auditory forebrain neurons in the European starling was established both with artificial stimuli (AS) and a conspecific territorial song as a natural stimulus (NS1). Applying experimenter-centred statistical methods for response detection and for scaling response strength, and spike-triggered analyses for the delimitation of the key sound parameters (spectrotemporal receptive field STRF, Aertsen et al. 1980) the study aimed at disclosing differences in the processing of the two stimulus classes, AS and NS. With the STRF as reference, we find congruence (1) in the best frequency with those determined under sweep and bandpass noise stimulation, (2) in response latency, and (3) in response-intensity dependence, further similarity in the overall frequency characteristic. Partitioning the song into 42 acoustically defined segments allowed to further delimit the response criteria under natural stimulation. They are easily understood from the AS response characteristics: (1) In the neuronal sample as a whole, long segments are more effective than short and, among the short, loud segments are more effective than faint; (2) Units showing their best excitatory response to AS in a certain frequency band are most probably excited by segments with a high proportion of their power concentrated upon or near this band; (3) Units with a slow (build-up) AS response react to a lower number of song segments than those dynamically following AS transients. Our data give no hint towards adaptive, feature detection properties of single neurons in field L. Instead, these neurons appear to base their response solely on the short-time spectrotemporal structure of the stimulus, irrespective of its natural or artificial origin.

The vocal motor control of the larynx was studied with single unit recordings from the efferent motor nucleus (nucleus ambiguus) in the CF-FM-bat Rhinolophus rouxi, spontaneously emitting echolocation sounds. The experiments were performed in a stereotaxic apparatus that allowed differentiation of activities in the recorded nucleus depending on the electrode position (Fig. 1). Echolocation calls and respiration activity were monitored simultaneously, thus it was possible to compare the time course of the motor control activity during respiration with and without concurrent vocalization. Unit discharges were classified as laryngeal motoneuron activity according to their correlation with the time course (onset and end) of echolocation calls and their discharge rate as: Pre-off-tonic, pre-off-phasic, off-pauser, off-tonic, on-chopper, on-tonic, prior-tonic and inhibitory (Fig. 4). The on-chopper and on-tonic discharge patterns were assigned to the motor activity of the lateral cricoarytenoid muscle and the off-pauser and off-tonic discharge patterns to the motor activity of the posterior cricoarytenoid muscle controlling the time course of vocal pulses. Motoneuron activities recorded under the condition of systematically shifted frequencies in the emitted echolocation calls were investigated in Doppler-shift compensating bats responding to electronically simulated echoes. Of all neurons classified as motor control, only units of the pre-off-tonic discharge type (cricothyroid muscle) changed their activity with frequency shifts in the vocalized pulses; they showed a positive linear correlation with the emitted sound frequency (Fig. 6). In addition, single unit activities in strict synchronization to vocalization were recorded, that by their low discharge rate were not valid as motor control, and were considered to represent activities of interneurons or internuclear neurons connecting the nucleus ambiguus with other vocalization- and respiration-centers (Fig. 3c). Electric lesions in the brain stem and iontophoretically applied horseradish peroxidase (HRP) served as references for localization and morphological identification of the recording sites in cell stained brain slices.

1. The development of vocalization and hearing was studied in Sri Lankan horseshoe bats (Rhinolophus rouxi) during the first postnatal month. The young bats were caught in a nursing colony of rhinolophids in which birth took place within a two week period. 2. The new-born bats emitted isolation calls through the mouth. At the beginning these calls consisted of pure tones with frequencies below 10 kHz (Fig. 1). During the first postnatal week the call frequency increased to about 15 kHz, and the fundamental was augmented by two to four harmonics. No evoked potentials to pure tone stimuli could be elicited in the inferior colliculus of this age group, i.e., auditory processing at the midbrain level was not demonstrable. 3. Evoked potentials were first recorded in the second week, broadly tuned to 15-45 kHz, with a maximum sensitivity between 15-25 kHz. In the course of the second week, however, higher frequencies up to 60 kHz became progressively incorporated into the audiogram (Fig. 3). The fundamental frequency of the multiharmonic isolation calls, emitted strictly through the mouth, increased to about 20 kHz. 4. In the bats' third postnatal week an increased hearing sensitivity (auditory filter) emerged, sharply tuned at frequencies between 57 and 60 kHz (Fig. 4e). The same individuals were also the first to emit long constant frequency echolocation calls through the nostrils (Fig. 4c). The energy of the calls was arranged in harmonic frequency bands with the second harmonic exactly tuned to the auditory filter. These young bats continued to emit isolation calls through the mouth, which were, however, not harmonically related to the echolocation calls (Fig. 4b, d). 5. During the fourth week, both the auditory filter and the matched echolocation pulses (the second harmonic) shifted towards higher frequencies (Fig. 5). During the fifth week the fundamental frequency of the calls was progressively attenuated, and both the second harmonic of the pulses and the auditory filter reached the frequency range typical for adult bats of 73-78 kHz (Fig. 6). 6. The development of audition and vocalization is discussed with regard to possible interactions of both subsystems, and their incorporation into the active orientation system of echolocation.

Mapping of auditory area in the cerebellar vermis and hemispheres of little brown bats, Myotis lucifugus, shows that a large area of the bat's cerebellum contains units responding to acoustic stimuli. These units had latencies between 4 and 34 msec and best frequencies between 33.0 and 92.5 kHZ. The Q10-dB values of their tuning curves ranged from 2.0 to 19.7. Most of the units studied fired only a few impulses during a stimulus with minimum thresholds between 22 and 90 dB SPL. Units in the cerebellar vermis tend to have higher best frequencies and shorter latencies than those in the cerebellar hemispheres. However, there is no evidence of clear tonotopic organization therein.

Horseradish peroxidase was applied by inotophoretic injections to physiologically identified regions of the laryngeal motor nucleus, the nucleus ambiguus in the CF/FM bat Rhinolophus rouxi. The connections of the nucleus ambiguus were analysed with regards to their possible functional significance in the vocal control system, in the respiration control system, and in mediating information from the central auditory system. The nucleus ambiguus is reciprocally interconnected with nuclei involved in the generation of the vocal motor pattern, i.e., the homonomous contralateral nucleus and the area of the lateral reticular formation. Similarly, reciprocal connections are found with the nuclei controlling the rhythm of respiration, i.e., medial parts of the medulla oblongata and the parabrachial nuclei. Afferents to the nucleus ambiguus derive from nuclei of the 'descending vocalization system' (periaqueductal gray and cuneiform nuclei) and from motor control centers (red nucleus and frontal cortex). Afferents to the nucleus ambiguus, possibly mediating auditory influence to the motor control of vocalization, come from the superior colliculus and from the pontine nuclei. The efferents from the pontine nuclei are restricted to rostral parts of the nucleus ambiguus, which hosts the motoneurons of the cricothyroid muscle controlling the call frequency.

1. The diencephalic auditory nucleus of the European starling, the nucleus ovoidalis, shows rostrocaudal and dorsoventral diameters of 500-800 microns and a mediolateral diameter of 800-1000 microns. This small and sharply delimited nucleus is composed of densely packed neurons. 2. Its tonotopic organization consists of evenly spaced isofrequency contours, with best frequencies decreasing ventrally. The frequency range was found to be 150 Hz to 7030 Hz. 3. Apart from tonotopic organization, other characteristics of single units demonstrate the uniformity of the neuronal population. Units have high spontaneous activities (mean 61 pps; Fig. 4a), and show mainly stimulus correlated tonic discharge patterns. In most cases, excitatory frequency bands are enclosed by inhibitory frequency bands. 4. Single units were tested, applying various stimulus classes differing in time structure (BPN, sine, FM up, FM down, SFM, SAM) but sharing a common frequency band. All neurons tested responded to all classes. Evaluation of stimulus class preference, however, revealed that BPN and SFM caused the strongest responses, whereas FM and SAM were less effective. 5. Comparison of the single unit responses in the ovoid nucleus with those known for avian auditory forebrain and midbrain centres strongly suggests a relay function for the diencephalic nucleus.