Is there a medial nucleus of the trapezoid body in humans?

Characterization of the superior olivary complex of chimpanzees (Pan troglodytes) in comparison to humans

The superior olivary complex (SOC) is a collection of nuclei in the hindbrain of mammals with numerous roles in hearing, including localization of sound sources in the environment, encoding temporal and spectral elements of sound, and descending modulation of the cochlea. While there have been several investigations of the SOC in primates, there are discrepancies in the descriptions of nuclear borders and even the presence of certain cell groups among studies and species. Herein, we aimed to clarify some of these issues by characterizing the SOC from chimpanzees using Nissl staining, quantitative morphometry and immunohistochemistry. We found the medial superior olive (MSO) to be the largest of the SOC nuclei and the arrangement of its neurons and peri-MSO to be very similar to humans. Additionally, we found neurons in the medial nucleus of the trapezoid body (MNTB) to be immunopositive for the calcium binding protein calbindin. Further, most neurons in the MNTB, and some neurons in the lateral nucleus of the trapezoid body were associated with large, calretinin-immunoreactive calyx terminals. Together, these findings indicate the organization of the SOC of chimpanzees is organized very similar to the SOC in humans and suggests modifications to this region among species consistent with differences in head/body size, restricted hearing range and sensitivity to low frequency sounds.

The Calyx of Held: A Hypothesis on the Need for Reliable Timing in an Intensity-Difference Encoder

The calyx of Held is the preeminent model for the study of synaptic function in the mammalian CNS. Despite much work on the synapse and associated circuit, its role in hearing remains enigmatic. We propose that the calyx is one of the key adaptations that enables an animal to lateralize transient sounds. The calyx is part of a binaural circuit that is biased toward high sound frequencies and is sensitive to intensity differences between the ears. This circuit also shows marked sensitivity to interaural time differences, but only for brief sound transients ("clicks"). In a natural environment, such transients are rare except as adventitious sounds generated by other animals moving at close range. We argue that the calyx, and associated temporal specializations, evolved to enable spatial localization of sound transients, through a neural code congruent with the circuit's sensitivity to interaural intensity differences, thereby conferring a key benefit to survival.

Neuroanatomical characterization of perineuronal net components in the human cochlear nucleus and superior olivary complex

The human auditory brainstem, especially the cochlear nucleus (CN) and the superior olivary complex (SOC) are characterized by a high density of neurons associated with perineuronal nets (PNs). PNs build a specific form of extracellular matrix surrounding the neuronal somata, proximal dendrites and axon initial segments. They restrict synaptic plasticity and control high-frequency synaptic activity, a prominent characteristic of neurons of the auditory brainstem. The distribution of PNs within the auditory brainstem has been investigated in a number of mammalian species. However, much less is known regarding PNs in the human auditory brainstem. The present study aimed at the immunohistochemical identification of PNs in the cochlear nucleus (CN) and superior olivary complex (SOC) in the human brainstem. We focused on the complex nature and molecular variability of PNs in the CN and SOC by using specific antibodies against the main PN components (aggrecan, brevican, neurocan and hyaluronan and proteoglycan link protein 1). Virtually all subnuclei within the ventral CN and SOC were found to be associated with PNs. Direct comparison between gerbil and human yielded similar fine structure of PNs and confirmed the typical tight interdigitation of PNs with synaptic terminals in both species. Noticeably, an elaborate combination of immunohistochemical labelings clearly supports the still debated existence of the medial nucleus of trapezoid body (MNTB) in the human brain. In conclusion, the present study demonstrates that PNs form a prominent extracellular structure on CN and SOC neurons in the human brain, potentially stabilizing synaptic contacts, which is in agreement with many other mammalian species.

Yes, there is a medial nucleus of the trapezoid body in humans

The medial nucleus of the trapezoid body (MNTB) is a collection of brainstem neurons that function within the ascending auditory pathway. MNTB neurons are associated with a number of anatomical and physiological specializations which make these cells especially well-equipped to provide extremely fast and precise glycinergic inhibition to its target neurons in the superior olivary complex and ventral nucleus of the lateral lemniscus. The inhibitory influence of MNTB neurons plays essentials roles in the localization of sound sources and encoding temporal features of complex sounds. The morphology, afferent and efferent connections and physiological response properties of MNTB neurons have been well-characterized in a number of laboratory rodents and some carnivores. Furthermore, the MNTB has been positively identified in all mammals examined, ranging from opossum and mice to chimpanzees. From the early 1970s through 2009, a number of studies denied the existence of the MNTB in humans and consequentially, the existence of this nucleus in the human brain has been debated for nearly 50 years. The absence of the MNTB from the human brain would negate current principles of sound localization and would require a number of novel adaptations, entirely unique to humans. However, a number of recent studies of human post-mortem tissue have provided evidence supporting the existence of the MNTB in humans. It therefore seems timely to review the structure and function of the MNTB, critically review the literature which led to the denial of the human MNTB and then review recent investigations supporting the existence of the MNTB in the human brain.

Hearing disorders in multiple sclerosis

Multiple sclerosis (MS) is a disease that is both a focal inflammatory and a chronic neurodegenerative disease. The focal inflammatory component is characterized by destruction of central nervous system myelin, including the spinal cord; as such it can impair any central neural system, including the auditory system. While on the one hand auditory complaints in MS patients are rare compared to other senses, such as vision and proprioception, on the other hand auditory tests of precise neural timing are never "silent." Whenever focal MS lesions are detected involving the pontine auditory pathway, auditory tests requiring precise neural timing are always abnormal, while auditory functions not requiring such precise timing are often normal. Azimuth sound localization is accomplished by comparing the timing and loudness of the sound at the two ears. Hence tests of azimuth sound localization must obligatorily involve the central nervous system and particularly the brainstem. Whenever a focal lesion was localized to the pontine auditory pathway, timing tests were always abnormal, but loudness tests were not. Moreover, a timing test that included only high-frequency sounds was very often abnormal, even when there was no detectable focal MS lesion involving the pontine auditory pathway. This test may be a marker for the chronic neurodegenerative aspect of MS, and, as such could be used to complement the magnetic resonance imaging scan in monitoring the neurodegenerative aspect of MS. Studies of MS brainstem lesion location and auditory function have led to advances in understanding how the human brain processes sound. The brain processes binaural sounds independently for time and level in a two-stage process. The first stage is at the level of the superior olivary complex (SOC) and the second at a level rostral to the SOC.

On the limit of neural phase locking to fine structure in humans

The frequency extent over which temporal fine structure is available in the human auditory system has recently become a topic of much discussion. It is common, in both the physiological and psychophysical literature, to encounter the assumption that fine structure is available to humans up to about 5 kHz or even higher. We argue from existing physiological, anatomical, and behavioral data in animals, combined with behavioral and anatomical data in humans, that it is unlikely that the human central nervous system has access to fine structure above a few kHz.

Brainstem auditory evoked potentials suggest a role for the ventral cochlear nucleus in tinnitus

Numerous studies have demonstrated elevated spontaneous and sound-evoked brainstem activity in animal models of tinnitus, but data on brainstem function in people with this common clinical condition are sparse. Here, auditory nerve and brainstem function in response to sound was assessed via auditory brainstem responses (ABR) in humans with tinnitus and without. Tinnitus subjects showed reduced wave I amplitude (indicating reduced auditory nerve activity) but enhanced wave V (reflecting elevated input to the inferior colliculi) compared with non-tinnitus subjects matched in age, sex, and pure-tone threshold. The transformation from reduced peripheral activity to central hyperactivity in the tinnitus group was especially apparent in the V/I and III/I amplitude ratios. Compared with a third cohort of younger, non-tinnitus subjects, both tinnitus, and matched, non-tinnitus groups showed elevated thresholds above 4 kHz and reduced wave I amplitude, indicating that the differences between tinnitus and matched non-tinnitus subjects occurred against a backdrop of shared peripheral dysfunction that, while not tinnitus specific, cannot be discounted as a factor in tinnitus development. Animal lesion and human neuroanatomical data combine to indicate that waves III and V in humans reflect activity in a pathway originating in the ventral cochlear nucleus (VCN) and with spherical bushy cells (SBC) in particular. We conclude that the elevated III/I and V/I amplitude ratios in tinnitus subjects reflect disproportionately high activity in the SBC pathway for a given amount of peripheral input. The results imply a role for the VCN in tinnitus and suggest the SBC pathway as a target for tinnitus treatment.

Mechanisms of Sound Localization in Mammals

The ability to determine the location of a sound source is fundamental to hearing. However, auditory space is not represented in any systematic manner on the basilar membrane of the cochlea, the sensory surface of the receptor organ for hearing. Understanding the means by which sensitivity to spatial cues is computed in central neurons can therefore contribute to our understanding of the basic nature of complex neural representations. We review recent evidence concerning the nature of the neural representation of auditory space in the mammalian brain and elaborate on recent advances in the understanding of mammalian subcortical processing of auditory spatial cues that challenge the “textbook” version of sound localization, in particular brain mechanisms contributing to binaural hearing.

Distribution of perineuronal nets in the human superior olivary complex

Perineuronal nets (PNNs) are specialized assemblies of chondroitin sulfate proteoglycans (CSPGs) in the central nervous system that form a lattice-like covering over the cell body, primary dendrites and initial axon segment of select neuronal populations. PNNs appear to play significant roles in development of the central nervous system, neuronal protection, synaptic plasticity and local ion homeostasis. In seven human brainstems (average age=81 years), we have utilized Wisteria floribunda (WFA) histochemistry and immunocytochemistry for CSPG to map the distribution of PNNs within the nuclei of the human superior olivary complex (SOC). Within the SOC, the majority of net-bearing neurons are situated in the most medially situated nuclei, especially the superior paraolivary nucleus and medial nucleus of the trapezoid body. Net-bearing neurons are consistently found in the ventral nucleus of the trapezoid body and posterior periolivary nucleus, but to a lesser extent in the lateral nucleus of the trapezoid body. Finally, perineuronal nets are typically absent from the lateral and medial superior olives.

Assessment of the role of the cochlear latency effect in lateralization of click sounds in humans

Interaural time and intensity disparities (ITD and IID) are the two cues to sound lateralization. "Time-only" hypothesis claims that an IID is first converted to an interaural afferent delay (Delta t), and is then processed by the central ITD mechanism, rendering a separate IID processor unnecessary. We tested this hypothesis by assessing the contribution of the cochlear latency effect to the psychophysical ITD/IID trading ratio. Auditory brainstem responses (ABRs) were used to measure the interaural afferent delays (Delta ts) that developed with a 20/sec dichotic click train used in the trading experiment. Except for small IIDs at low loudness levels, the physiological Delta t delay produced by an IID was significantly smaller than the ITD psychophysically traded for the same IID. We concluded that the cochlear latency effect alone cannot explain the psychophysical ITD/IID trading ratios and a separate IID mechanism must be involved.

Topographical and cellular distribution of perineuronal nets in the human cochlear nucleus

Specialized constructs of the extracellular matrix termed perineuronal nets surround the soma, primary dendrites and initial axon segment of some but not all neuronal populations in the central nervous system. In an effort to determine the cellular localization of perineuronal nets in the human cochlear nucleus (CN), we first performed a quantitative morphometric study of the human CN. We provide evidence for a laminar organization in the human dorsal cochlear nucleus (DCN; including molecular, granular and deep layers) as in other laboratory animals. Additionally, we find that the human ventral cochlear nucleus (VCN) contains distinct octopus, stellate, globular and spherical bushy cell populations, as described in other species. Using Wisteria floribunda histochemistry in five human brainstems, we identified perineuronal nets in the human cochlear nucleus. Perineuronal nets are associated with the vast majority of octopus and stellate cells in the caudal VCN. In the rostral VCN, dense perineuronal nets are associated with globular bushy cells and faint nets are associated with some spherical bushy cells and stellate cells. Few perineuronal nets are found in the DCN.

Superior olivary complex organization and cytoarchitecture may be correlated with function and catarrhine primate phylogeny

In the mammalian auditory system, the medial nucleus of the trapezoid body and the lateral superior olive (MNTB-LSO system) contribute to binaural intensity processing and lateralization. Localization precision varies with the sound frequencies. As recency of common ancestry with human beings increases, primates have improved low-frequency sensitivity and reduced sensitivity to higher frequencies. The medial part of the MNTB is devoted to higher frequency processing. Thus, its high-frequency-dependent function is nearly lost in humans and its role in binaural processing as part of the contralateral pathway to the LSO remains questionable. Here, Nissl-stained sections of the superior olivary complex of man (Homo sapiens), bonobo (Pan paniscus), chimpanzee (Pan troglodytes), gorilla (Gorilla gorilla), orangutan (Pongo pygmaeus), gibbon (Hylobates lar), and macaque (Macaca fascicularis) were compared to reveal differences and coincidences. From chimpanzees to humans, the size of the LSO decreased, and the MNTB as a compact nucleus nearly disappears. From chimpanzees to humans, the LSO/MNTB ratio increases dramatically too, whereas the LSO/MSO ratio remains 1.1; a finding that probably corresponds to the phylogenetic proximity between the species.

Interaural level difference discrimination thresholds for single neurons in the lateral superior olive

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

The medial nucleus of the trapezoid body: comparative physiology

Principal cells of the medial nucleus of the trapezoid body (MNTB) receive their excitatory input through large somatic terminals, the calyces of Held, which arise from axons of globular bushy cells located in the contralateral ventral cochlear nucleus. Discharges of MNTB neurons are characterized by high stimulus evoked firing rates, temporally precise onset responses, and a high degree of phase-locking to either pure tones or stimulus envelopes. Since the calyx of Held synapse is accessible to in vitro and to in vivo recordings, it serves as one of the most elaborate models for studying synaptic transmission in the mammalian brain. Although in such studies, the major emphasis is on synaptic physiology, the interpretation of the data will benefit from an understanding of the MNTB's contribution to auditory signal processing, including possible functional differences in different species. This implies the consideration of possible functional differences in different species. Here, we compare single unit recordings from MNTB principal cells in vivo in three different rodent species: gerbil, mouse and rat. Because of their good low-frequency hearing gerbils are often used in in vivo preparations, while mice and rats are predominantly used in slice preparations. We show that MNTB units in all three species exhibit high firing rates and precise onset-timing. Still there are species-specific specializations that might suggest the preferential use of one species over the others, depending on the scope of the respective investigation.

Characterization of neuronal subsets surrounded by perineuronal nets in the rhesus auditory brainstem

The distribution of perineuronal nets and the potassium channel subunit Kv3.1b was studied in the subdivisions of the cochlear nucleus, the medial nucleus of the trapezoid body, the medial and lateral superior olivary nuclei, the lateral lemniscal nucleus and the inferior colliculus of the rhesus monkey. Additional sections were used for receptor autoradiography to visualize the patterns of GABAA and GABAB receptor distribution. The Kv3.1b protein and perineuronal nets [visualized as Wisteria floribunda agglutinin (WFA) binding] were revealed, showing corresponding region-specific patterns of distribution. There was a gradient of labelled perineuronal nets which corresponded to that seen for the intensity of Kv3.1b expression. In the cochlear nucleus intensely and faintly stained perineuronal nets were intermingled, whereas in the medial nucleus of the trapezoid body the pattern changed to intensely stained perineuronal nets in the medial part and weakly labelled nets in its lateral part. In the inferior colliculus, intensely labelled perineuronal nets were arranged in clusters and faintly labelled nets were arranged in sheets. Using receptor autoradiography, GABAB receptor expression in the anterior ventral cochlear nucleus was revealed. The medial part of the medial nucleus of the trapezoid body showed a high number of GABAA binding sites whereas the lateral part exhibited more binding sites for GABAB. In the inferior colliculus, we found moderate GABAB receptor expression. In conclusion, intensely WFA-labelled structures are those known to be functionally involved in high-frequency processing.

Cytoarchitecture of the human superior olivary complex: Medial and lateral superior olive

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

The lateral superior olive: a functional role in sound source localization

Sound location in azimuth is signaled by differences in the times of arrival (interaural time difference, ITDs) and the amplitudes (interaural level differences, ILDs) of the stimuli at the ears. Psychophysical studies have shown that low- and high-frequency sounds are localized based on ITDs and ILDs, respectively, suggesting that dual mechanisms mediate localization. The anatomical and physiological bases for this "duplex theory" of localization are found in the medial (MSO) and lateral (LSO) superior olives, two of the most peripheral sites in the ascending auditory pathway receiving inputs from both ears. The MSO and LSO are believed to be responsible for the initial encoding of ITDs and ILDs, respectively. Here the author focuses on ILDs as a cue to location and the role of the LSO in encoding ILDs. Evidence from disparate fields of study supports the hypothesis that the LSO is the initial ILD processor in the mammalian auditory system.

Characterization of the human superior olivary complex by calcium binding proteins and neurofilament H (SMI-32)

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.

Functional anatomy of auditory brainstem nuclei: application to the anatomical basis of brainstem auditory evoked potentials

Brainstem auditory evoked potentials (BAEP) are used routinely in clinical practice to evaluate the normality of the lower auditory system. The objective of this review is to describe the functional anatomy of the structures implicated in BAEP generation (cochlear nerve and the auditory brainstem nuclei). Indications and results of BAEP in clinical practice are presented and correlated with auditory structures, which generate each waveform of BAEP.

Sound lateralization and interaural discrimination. Effects of brainstem infarcts and multiple sclerosis lesions

Subjects with brainstem lesions due to either an infarct or multiple sclerosis (MS) underwent two types of binaural testing (lateralization testing and interaural discrimination) for three types of sounds (clicks and high and low frequency narrow-band noise) with two kinds of interaural differences (level and time). Two major types of abnormalities were revealed in the lateralization performances: perception of all stimuli, regardless of interaural differences (time and/or level) in the center of the head (center-oriented), or lateralization of all stimuli to one side or the other of the head (side-oriented). Similar patterns of abnormal lateralization (center-oriented and side-oriented) occurred for MS and stroke patients. A subject's pattern of abnormal lateralization testing was the same regardless of the type of stimulus or type of interaural disparity. Lateralization testing was a more sensitive test than interaural discrimination testing for both types of subjects. Magnetic resonance image (MRI) scanning in three orthogonal planes of the brainstem was used to detect lesions. A semi-automated algorithm superimposed the auditory pathway onto each MRI section. Whenever a lesion overlapped the auditory pathway, some binaural performance was abnormal and vice versa. Given a lateralization test abnormality, whether the pattern was center-oriented or side-oriented was mainly determined by lesion site. Center-oriented performance was principally associated with caudal pontine lesions and side-oriented performance with lesions rostral to the superior olivary complex. For lesions restricted to the lateral lemniscus and/or inferior colliculus, whether unilateral or bilateral, just noticeable differences (JNDs) were nearly always abnormal, but for caudal pontine lesions JNDs could be normal or abnormal. MS subjects were more sensitive to interaural time delays than interaural level differences particularly for caudal pontine lesions, while stroke patients showed no differential sensitivity to the two kinds of interaural differences. These results suggest that neural processing of binaural stimuli is multilevel and begins with independent interaural time and level analyzers in the caudal pons.

Synaptophysin staining in normal brain: importance for diagnosis of ganglioglioma

Neuronal and mixed glioneuronal tumors traditionally have comprised a very small percentage of intrinsic central nervous system neoplasms, although they are somewhat more common among juvenile brain tumors and in the temporal lobe. Neuronal differentiation increasingly is recognized in pleomorphic xanthoastrocytoma, intraventricular neurocytoma, and subependymal giant cell astrocytoma. However, the diagnostic distinctions between subtle ganglioglioma (with rare neurons) and infiltrating glioma with entrapped neurons and between infiltrating oligodendroglioma and parenchymal neurocytoma are problematic but may be clinically important. Recently, it was proposed that perisomatic synaptophysin immunostaining in the human central nervous system reliably and selectively discriminates neoplastic from nonneoplastic neurons. Using this criterion, the number of brain stem and spinal cord gangliogliomas could be increased substantially. We canvassed synaptophysin immunostaining patterns in the normal brain stem, cerebellum, and forebrain, and found that synaptophysin-positive neurons are distributed broadly in the normal human brain. In disturbed neocortical tissue, such as near vascular malformations, synaptophysin-positive neurons and irregular white-matter synaptophysin immunostaining are visualized. Although synaptophysin-positive neurons are found in gangliogliomas and archipelagos of synaptophysin reactivity are found in neurocytomas, these patterns clearly are not pathognomonic for glioneuronal tumors and must be interpreted with caution whenever other histologic or ultrastructural evidence of neuronal differentiation is lacking.

Generators of the brainstem auditory evoked potential in cat. III: Identified cell populations

This paper examines the relationship between different brainstem cell populations and the brainstem auditory evoked potential (BAEP). First, we present a mathematical model relating the BAEP to underlying cellular activity. Then, we identify specific cellular generators of the click-evoked BAEP in cats by combining model-derived insights with key experimental data. These data include (a) a correspondence between particular brainstem regions and specific extrema in the BAEP waveform, determined from lesion experiments, and (b) values for model parameters derived from published physiological and anatomical information. Ultimately, we conclude (with varying degrees of confidence) that: (1) the earliest extrema in the BAEP are generated by spiral ganglion cells, (2) P2 is mainly generated by cochlear nucleus (CN) globular cells, (3) P3 is partly generated by CN spherical cells and partly by cells receiving inputs from globular cells, (4) P4 is predominantly generated by medial superior olive (MSO) principal cells, which are driven by spherical cells, (5) the generators of P5 are driven by MSO principal cells, and (6) the BAEP, as a whole, is generated mainly by cells with characteristic frequencies above 2 kHz. Thus, the BAEP in cats mainly reflects cellular activity in two parallel pathways, one originating with globular cells and the other with spherical cells. Since the globular cell pathway is poorly represented in humans, we suggest that the human BAEP is largely generated by brainstem cells in the spherical cell pathway. Given our conclusions, it should now be possible to relate activity in specific cell populations to psychophysical performance since the BAEP can be recorded in behaving humans and animals.

Ultrastructural and immunocytochemical observations on the superior olivary complex of the mustached bat

This study investigates the functional organization of the superior olivary complex of the mustached bat with classical transmission electron microscopy and postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glycine antisera in semithin serial sections. The ultrastructure and distribution of terminal types in the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) closely resemble that of other mammals; the organization within the medial superior olive (MSO) differs significantly. The differences concern the relative proportion of putatively inhibitory boutons, which appear as symmetrical synapses with flattened vesicles on MSO somata. In the bat, inhibitory boutons comprised 75-100% of perisomatic boutons, a value identical to that observed in the LSO. These terminals most likely arise from the MNTB. In other species, putatively inhibitory terminals form a much smaller proportion of perisomatic boutons in MSO. This difference suggests that in the bat MSO excitatory input to cell somata is considerably reduced and outweighed by inhibitory input. This suggestion is corroborated by immunocytochemical data. Glycine-immunoreactive puncta encrust somata of LSO and MSO cells to a similar degree and in rather homogeneous patterns throughout these nuclei. Putatively GABAergic terminals are located mainly on distal dendrites of MSO and LSO cells. Regional variations in the density of GABA-immunoreactive puncta in LSO suggest that different tonotopic zones are under differential modulatory influence. Both the LSO and MSO of the mustached bat contain significant amounts of putatively inhibitory projection cells. Coexistence of both antigens was commonly observed in subsets of cells. Quantitative analyses of labeling patterns and comparisons with other mammals suggest that the mix of neurotransmitters in projection cells of LSO and MSO is phylogenetically flexible, and thus the details of the functions of ascending pathways are species specific. In contrast to other mammals, the bat MSO forms parallel output pathways with excitatory and inhibitory components. Data are discussed in relation to specialized physiological response features.

The brain stem evoked response and medial nucleus of the trapezoid body

Single-unit responses of cat superior olivary complex neurons to acoustic stimuli were examined to determine whether the units' action potentials were sufficiently synchronized to contribute to the brain stem evoked response. The medial nucleus of the trapezoid body and lateral superior olive are two major nuclei within the cat superior olivary complex. The first-spike discharge latencies of medial nucleus of the trapezoid body and lateral superior olivary neurons to monaural presentations of tone burst stimuli were measured as a function of stimulus level. Evidence is provided to support the hypotheses that in cat the medial nucleus of the trapezoid body may contribute directly to the monaural brain stem evoked response by producing action potentials synchronized to stimulus onset and may also contribute indirectly to the brain stem evoked response binaural difference wave bc by inhibiting the lateral superior olive unit excitatory responses synchronized to stimulus onset.

Prediction of binaural click lateralization by brainstem auditory evoked potentials

A previous study by Furst et al. (1985) has shown that in healthy subjects brainstem responses evoked by binaural auditory stimuli with interaural time difference (ITD) and interaural level difference (ILD) include information about the integration of data received by both ears. A correlation was found between the first major peak of the binaural difference waveform and perception of click lateralization and fusion. We have now tested whether a similar correlation exists in patients with multiple sclerosis (MS). The ability to lateralize dichotic clicks was tested in MS patients with normal audiograms. Two kinds of psychoacoustical experiments were employed: (1) A matching experiment in which the subject was asked to match the perceived positions of two click trains, one of which consisted of dichotic clicks with ILD and the other dichotic clicks with ITD; and (2) A positional JND experiment in which the subject was asked to determine the difference in perceived position of two successive click trains. Two reference positions were tested, the head center and the side of the head near the ear, while the control was either on ITD or on ILD. According to the psychoacoustical performances, three groups of patients were identified. Group I consisted of patients who performed normally in all the psychoacoustical experiments. Group II patients were able to lateralize binaural clicks but performed abnormally in the matching experiment and in the position discrimination experiment when the control was on ITD and the reference position was the head center. The patients in Group II performed normally in the discrimination experiments when the control was on ILD, and when the control was on ITD but the reference position was the head side. Group III consisted of those who were not able to perform either one of the psychoacoustical experiments. They perceived the same binaural clicks in different positions in different times. Brainstem auditory potentials evoked by dichotic clicks with different ILDs and ITDs were measured in all the MS patients, and the corresponding binaural difference (BD) waveforms were calculated. Whenever beta, the first major peak of BD, was identified it was used to obtain a physiological matching curve. It was derived by matching an ILD on the basis of similar beta latencies. For every patient, in either Group I or II, the physiological matching curve was very similar to his psychoacoustical matching curve.

The effect of brainstem lesions on brainstem auditory evoked potentials in the cat

Brainstem auditory evoked potentials (BAEPs) were recorded before and after cuts were made in either the midline trapezoid body (TB), the lateral lemniscus (LL), or the combined dorsal and intermediate acoustic striae (DAS/IAS) in 23 anesthetized cats. Monaural and binaural rarefaction clicks were presented at a rate of 10 per s, and the potentials recorded from a vertex electrode referenced to either earbar or to the neck. The potentials were filtered so that fast and slow components could be examined separately and special efforts were exerted to obtain stable conditions so that small changes in waveforms could be significant. Lesions of the DAS/IAS produced negligible changes in either the fast or slow waves. Lesions of the midline TB reduced the amplitudes of peaks P3 through P5, while greatly reducing the amplitude of the slow wave. Complete lesions of the LL always reduced the amplitude of the slow wave. Lesions of the ventral part of the LL were more likely to reduce the amplitude of P4-P5. Our interpretations of these lesion experiments are based on the idea that individual fast peaks of the BAEP represent compound action potentials of fiber pathways. According to this view, only synchronized activity generated in populations of neurons that are both favorably oriented in space and significant in number, will contribute to the fast peak.

The brainstem auditory evoked potential asymmetry is replicable and reliable

Lateralization of neural function is generally thought to occur only at the level of the cerebral cortex and perhaps the thalamus. Levine and McGaffigan [EEG Clin. Neurophysiol. 55, 532-537, 1983] challenged this view by identifying a neural asymmetry at the level of the brainstem. They analyzed brainstem auditory evoked potentials (BAEPs) and found that peak III(+) amplitude (baseline-to-peak) was significantly larger in response to right than to left ear stimulation. The current paper demonstrates that this BAEP asymmetry is (a) reliable within and between subjects, (b) present for 33/sec and 10/sec click rates, and (c) more reliable when amplitude is measured peak-to-peak. This brainstem asymmetry may reflect the general tendency of humans to orient rightward and/or may be a precursor of higher level asymmetries.

Comparison of cat and human brain-stem auditory evoked potentials

Brain-stem auditory evoked potentials (BAEPs) elicited by clicks were recorded from both humans and cats. The responses of the two species were compared as functions of click level, click rate, ear stimulated, and electrode position. Since the BAEPs appear to have both high- and low-frequency components, the responses were filtered to analyze these components separately. The similarities and differences in the behavior of the peaks of the two species support the view that the first three (positive and negative) high-frequency peaks which are comparably numbered have similar generators, but the later comparably numbered peaks do not. The presence of binaural interaction beginning with P4 and PV suggests a correspondence between peaks P4 through P5 in cat with PV through PVI, respectively, in human. The similarity in behavior of these peaks also support this correspondence. Furthermore, when conduction times are estimated from interpeak latencies, this correspondence of peaks agrees more closely with the relative pathway lengths in the two species, than does the correspondence based on comparable numbering.

The human auditory brain stem: a comparative view

This study compares human brain stem auditory centers with those of the cat in terms of their topography and cytoarchitecture. Graphic reconstructions of the brain stem pathway illustrate differences in configuration of human auditory centers, such as mediolateral elongation of the cochlear nuclei and rostral prolongation of the superior olivary complex. Greater human brain stem size creates a considerably longer auditory pathway: the distance traversed by axons passing from the cochlear nuclei to the ipsilateral inferior colliculus is approximately 14 mm in the cat and 35 mm in man, while the distance to the contralateral colliculus is about 22 mm in the cat and 46 mm in man. Neuronal groups which are well developed in the human brain stem are the populations of large relay neurons in the cochlear nuclei, the medial olivary nucleus, periolivary region, dorsal nucleus of the lateral lemniscus, and inferior colliculus. In contrast, a number of nuclei and cell groups are very poorly developed or absent in the human auditory system: these include several types of small neurons in the cochlear nuclei, the lateral olivary nucleus, nucleus of the trapezoid body, and ventral nucleus of the lateral lemniscus. The functional implications of these changes are discussed.

The projections of principal cells of the medial nucleus of the trapezoid body in the cat

Previous studies suggest that the principal cells of the medial nucleus of the trapezoid body (MNTB) give rise to the projection from MNTB to the lateral superior olivary nucleus (LSO) of the same side, where they mediate rapid inhibitory effects of contralateral sound stimulation. In the present study, we explored certain morphological features of this connection as well as several other projections of the MNTB by using anterograde and retrograde axonal tracing methods. Following injections of tritiated leucine into MNTB, labeled axons reached LSO by passing ventral to, dorsal to, and through the medial superior olivary nucleus, and gave rise to labeling around the somata and proximal dendrites of LSO fusiform cells. As measured in autoradiograms of 2 micron plastic sections, these axons had a modal diameter of 5-6 micron. Terminal labeling, tentatively attributed to principal cell axons, was also seen in the ventral nucleus of the lateral lemniscus (VNLL) and the dorsomedial and ventromedial periolivary nuclei. HRP injections into the LSO and the VNLL showed that the principal cell projected to both of these nuclei and revealed a topographic arrangement of the projection to the LSO which is consistent with tonotopic maps determined electrophysiologically. Control HRP injections demonstrated that other minor projections of the MNTB arose from minor cell populations in this nucleus. The findings provide a morphological correlate of certain physiological findings and suggest a wider role for the MNTB in the ascending auditory system than previously has been supposed.