Mesencephalicus Lateralis Pars Dorsalis Research Articles

The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata).

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

Heterogeneous organization and connectivity of the chicken auditory thalamus (Gallus gallus).

The auditory ascending system contains parallel pathways in vertebrate brains. In chickens (Gallus gallus), three pathways arise from nucleus laminaris (NL), nucleus angularis (NA), and regio intermedius (RI) in the brainstem, innervating three subdivisions of the nucleus mesencephalicus lateralis pars dorsalis (MLd) in the midbrain. The current study reveals the segregation of these pathways in their subsequent projections to the nucleus ovoidalis (Ov) in the thalamus. Based on cytoarchitecture and myelin distribution, we identified seven Ov subregions, including five neuronal clusters within the Ov proper, the nucleus semilunaris parovoidalis (SPO), and the circum-ovoidalis (cOv). Immunocytochemistry further revealed that a ventromedial cluster of the Ov proper (Ovvm) contains unique cell types expressing α8 subunit nicotinic acetylcholine receptor, while SPO and cOv are characterized with expression of calcitonin-gene-related peptide and cholecystokinin. Tract tracing studies demonstrated that Ovvm is a major target of the NL-recipient zone of MLd, while the RI-recipient zone of MLd predominantly projects to a ventrolateral cluster of the Ov proper. Afferent inputs to the remaining regions of the Ov proper mostly arise from the NA-recipient zone of MLd. SPO and cOv receive a projection from the surrounding areas of MLd, named the nucleus intercollicularis. Importantly, the Ov proper, SPO and cOv all project to the Field L2 in the forebrain, the avian auditory cortex. Taken together, these results demonstrate that the avian auditory thalamus is a structurally and functionally heterogeneous structure, implicating an important role in generating novel representations for specific acoustic features.

The mesencephalic GCt-ICo complex and tonic immobility in pigeons (Columba livia): a c-Fos study.

Tonic immobility (TI) is a response to a predator attack, or other inescapable danger, characterized by immobility, analgesia and unresponsiveness to external stimuli. In mammals, the periaqueductal gray (PAG) and deep tectal regions control the expression of TI as well as other defensive behaviors. In birds, little is known about the mesencephalic circuitry involved in the control of TI. Here, adult pigeons (both sex, n=4/group), randomly assigned to non-handled, handled or TI groups, were killed 90min after manipulations and the brains processed for detection of c-Fos immunoreactive cells (c-Fos-ir, marker for neural activity) in the mesencephalic central gray (GCt) and the adjacent nucleus intercollicularis (ICo). The NADPH-diaphorase staining delineated the boundaries of the sub nuclei in the ICo-GCt complex. Compared to non-handled, TI (but not handling) induced c-Fos-ir in NADPH-diaphorase-rich and -poor regions. After TI, the number of c-Fos-ir increased in the caudal and intermediate areas of the ICo (but not in the GCt), throughout the rostrocaudal axis of the dorsal stratum griseum periventriculare (SGPd) of the optic tectum and in the n. mesencephalicus lateralis pars dorsalis (MLd), which is part of the ascending auditory pathway. These data suggest that inescapable threatening stimuli such as TI may recruit neurons in discrete areas of ICo-GCt complex, deep tectal layer and in ascending auditory circuits that may control the expression of defensive behaviors in pigeons. Additionally, data indicate that the contiguous deep tectal SCPd (but not GCt) in birds may be functionally comparable to the mammalian dorsal PAG.

Neuronal morphology in subdivisions of the inferior colliculus of chicken (Gallus gallus)

The avian inferior colliculus (IC), also referred to as the nucleus mesencephalicus lateralis pars dorsalis (MLd), is an auditory midbrain nucleus that converges auditory cues from tonotopically organized brainstem nuclei. This information is relayed onto the optic tectum on the one hand and to nucleus ovoidalis on the other hand. Morphologically, there has been considerable debate about the number and nomenclature of the subnuclei within the IC. Here, we provide morphological characteristics of single cells in five IC subnuclei in chicken.The cellular structure within the IC was studied by whole-cell patch technique and biocytin iontophoresis. In addition, histological staining was performed, to delineate the borders between subnuclei of the IC. We were able to discriminate between 5 subnuclei: the core of the central nucleus (ICCc), the medial and lateral shell of the central nucleus (ICCms and ICCls), the external nucleus (ICX) and the superficial nucleus (ICS) of the IC. Our findings suggest the existence of at least two different morphologies of neurons with two subtypes each.The IC in chicken is a largely homogenous nucleus in terms of neuronal anatomy on a cellular level. However, its compartmentation into diversified subnuclei with different neurophysiological characteristics suggests a complex system to process auditory information. The auditory system in chicken is not as hypertrophied as in specialists such as the barn owl, but appears to have comparable connectivity and cellular morphology.

Subdivisions of the auditory midbrain (n. mesencephalicus lateralis, pars dorsalis) in zebra finches using calcium-binding protein immunocytochemistry.

The midbrain nucleus mesencephalicus lateralis pars dorsalis (MLd) is thought to be the avian homologue of the central nucleus of the mammalian inferior colliculus. As such, it is a major relay in the ascending auditory pathway of all birds and in songbirds mediates the auditory feedback necessary for the learning and maintenance of song. To clarify the organization of MLd, we applied three calcium binding protein antibodies to tissue sections from the brains of adult male and female zebra finches. The staining patterns resulting from the application of parvalbumin, calbindin and calretinin antibodies differed from each other and in different parts of the nucleus. Parvalbumin-like immunoreactivity was distributed throughout the whole nucleus, as defined by the totality of the terminations of brainstem auditory afferents; in other words parvalbumin-like immunoreactivity defines the boundaries of MLd. Staining patterns of parvalbumin, calbindin and calretinin defined two regions of MLd: inner (MLd.I) and outer (MLd.O). MLd.O largely surrounds MLd.I and is distinct from the surrounding intercollicular nucleus. Unlike the case in some non-songbirds, however, the two MLd regions do not correspond to the terminal zones of the projections of the brainstem auditory nuclei angularis and laminaris, which have been found to overlap substantially throughout the nucleus in zebra finches.

Connections of the auditory brainstem in a Songbird, Taeniopygia guttata. I. Projections of nucleus angularis and nucleus laminaris to the auditory torus

Auditory information is important for social and reproductive behaviors in birds generally, but is crucial for oscine species (songbirds), in particular because in these species auditory feedback ensures the learning and accurate maintenance of song. While there is considerable information on the auditory projections through the forebrain of songbirds, there is no information available for projections through the brainstem. At the latter levels the prevalent model of auditory processing in birds derives from an auditory specialist, the barn owl, which uses time and intensity parameters to compute the location of sounds in space, but whether the auditory brainstem of songbirds is similarly functionally organized is unknown. To examine the songbird auditory brainstem we charted the projections of the cochlear nuclei angularis (NA) and magnocellularis (NM) and the third-order nucleus laminaris (NL) in zebra finches using standard tract-tracing techniques. As in other avian species, the projections of NM were found to be confined to NL, and NL and NA provided the ascending projections. Here we report on differential projections of NA and NL to the torus semicircularis, known in birds as nucleus mesencephalicus lateralis, pars dorsalis (MLd), and in mammals as the central nucleus of the inferior colliculus (ICc). Unlike the case in nonsongbirds, the projections of NA and NL to MLd in the zebra finch showed substantial overlap, in agreement with the projections of the cochlear nuclei to the ICc in mammals. This organization could suggest that the "what" of auditory stimuli is as important as "where."

Three subdivisions of the auditory midbrain in chicks (Gallus gallus) identified by their afferent and commissural projections

The auditory midbrain is a site of convergence of multiple auditory channels from the brainstem. In birds, two separate ascending channels have been identified, through which time and intensity information is sent to the nucleus mesencephalicus lateralis, pars dorsalis (MLd), the homologue of the central nucleus of the mammalian inferior colliculus. Using in vivo anterograde and retrograde tracing techniques, the current study provides two lines of anatomical evidence supporting the presence of a third ascending channel to the chick MLd. First, three non-overlapping zones of the MLd receive inputs from three distinct cell groups in the caudodorsal brainstem. The projections from the nucleus angularis (NA) and nucleus laminaris (NL) are predominantly contralateral and may correspond to the time and intensity channels. A rostromedial portion of the MLd receives bilateral projections mainly from the regio intermedius, an interposed region of cells lying at a caudal level between the NL and NA, as well as scattered neurons embedded in the 8th nerve tract, and probably a very ventral region of the NA. Second, the bilateral zones of the MLd on two sides of the brain are reciprocally connected and do not interact with other zones of the MLd via commissural connections. In contrast, the NL-recipient zone projects contralaterally upon the NA-recipient zone. The structural separation of the third pathway from the NA and NL projections suggests a third information-processing channel, in parallel with the time and intensity channels. Neurons in the third channel appear to process very low frequency information including infrasound, probably utilizing different mechanisms than that underlying higher frequency processing.

Comparative analysis of neurogenesis between the core and shell regions of auditory areas in the chick ( Gallus gallus domesticus)

Early embryogenesis can reflect constituting organizations and evolutionary origins of brain areas. To determine whether a clear core-versus-shell distinction of neurogenesis that occurs from the auditory midbrain to the telencephalon in the reptile also appears in the bird, a single dose of [ 3H]-thymidine was injected into chick ( Gallus gallus domesticus) eggs at some successive embryonic days (E) (from E3 to E10). Towards the end of hatching, [ 3H]-thymidine labeling was examined, and the results were as follows: 1) Neuronal generation in the nucleus intercollicularis (ICo) (shell region) began at E3, whereas neurogenesis began at E4 in the nucleus mesencephalicus lateralis pars dorsalis (MLd) (core region); 2) Neurogenesis initiated at E3 in the nucleus ovoidalis (Ov) shell, but initiated at E4 in the rostral Ov core. In the medial or caudal Ov core, the percentage of heavily-labeled neurons with [ 3H]-thymidine was significantly lower at E3 age group than that in the Ov shell; 3) In field L1 and L3, two flanking regions of the primary telencephalic auditory area (field L2a), neurogenesis started at E5, but started at E6 in field L2a. These data indicate that the onset of embryogenesis began earlier in the auditory shell areas than in the core areas from the midbrain to the telencephalon. These findings provide insight into the organization of auditory nuclei and their evolution in amniotes.

Echolocation, vocal learning, auditory localization and the relative size of the avian auditory midbrain nucleus (MLd)

The avian nucleus mesencephalicus lateralis, pars dorsalis (MLd) is an auditory midbrain nucleus that plays a significant role in a variety of acoustically mediated behaviours. We tested whether MLd is hypertrophied in species with auditory specializations: owls, the vocal learners and echolocaters. Using both conventional and phylogenetically corrected statistics, we find that the echolocating species have a marginally enlarged MLd, but it does not differ significantly from auditory generalists, such as pigeons, raptors and chickens. Similarly, all of the vocal learners tend to have relatively small MLds. Finally, MLd is significantly larger in owls compared to all other birds regardless of how the size of MLd is scaled. This enlargement is far more marked in asymmetrically eared owls than symmetrically eared owls. Variation in MLd size therefore appears to be correlated with some auditory specializations, but not others. Whether an auditory specialist possesses a hypertrophied MLd appears to be depend upon their hearing range and sensitivity as well as the ability to resolve small azimuthal and elevational angles when determining the location of a sound. As a result, the only group to possess a significantly large MLd consistently across our analyses is the owls. Unlike other birds surveyed, owls have a battery of peripheral and other central auditory system specializations that correlate well with their hearing abilities. The lack of differences among the generalists, vocal learners and echolocaters therefore reflects an overall similarity in hearing abilities, despite the specific life history requirements of each specialization and species. This correlation between the size of a neural structure and the sensitivity of a perceptual domain parallels a similar pattern in mammals.

ZENK protein regulation by song in the brain of songbirds.

When songbirds hear the song of another individual of the same species or when they sing, the mRNA levels of the ZENK gene increase rapidly in forebrain areas involved in vocal communication. This gene induction is thought to be related to long-term neuronal change and possibly the formation of song-related memories. We used immunocytochemistry to study the levels and distribution of ZENK protein in the brain of zebra finches and canaries after presentation of song playbacks. Birds that heard the playbacks and did not sing in response showed increased ZENK protein levels in auditory brain areas, including the caudomedial neostriatum and hyperstriatum ventrale, fields L1 and L3, the shelf adjacent to the high vocal center (HVC), the cup adjacent to the nucleus robustus archistriatalis (RA), and the nucleus mesencephalicus lateralis pars dorsalis (MLd). No ZENK expression was seen in song nuclei in these birds. Males that sang in response to the playbacks showed, in addition to auditory areas, increased ZENK protein in several song control nuclei, most prominently in HVC, RA, area X, and the dorsomedial nucleus (DN) of the intercollicular complex. The rise in ZENK protein followed that described previously for ZENK mRNA by a short lag, and the distribution of ZENK-labeled cells was in agreement with previous analysis of mRNA distribution. Thus, ZENK protein regulation can be used to assess activation of brain areas involved in perceptual and motor aspects of song. Possible implications of ZENK induction in these areas are discussed.

Changes in catecholamine levels and turnover rates in hypothalamic, vocal control, and auditory nuclei in male zebra finches during development

The catecholamines norepinephrine (NE) and dopamine (DA) have been implicated in the sexual differentiation of brain and behavior and in species-specific learning in several species. To determine if these neurotransmitters might be involved in sexual differentiation of the vocal control system and song learning in male zebra finches, NE and DA levels and turnover rates were quantified in 10 behaviorally relevant brain nuclei [6 vocal control (VCN), 2 auditory (AN), and 2 hypothalamic (HN)] at four critical points during sexual differentiation of the VCN and the period of song learning, 25, 35, 55, and 90 days of age. Some birds were pretreated with α-methyl-para-tyrosine (αMPT) to allow estimation of NE and DA turnover rates. NE and DA levels in microdissected nuclei were quantified using high-performance liquid chromatography with electrochemical detection. αMPT treatment suppressed catecholamine synthesis just as effectively in juveniles as it does in adults and proved an effective method for estimating NE and DA turnover rates. Patterns of NE and DA function in most VCN and AN over development were quite different from those in HN in which NE and DA function changed gradually and showed no striking peaks. NE turnover rates changed significantly over development in all six VCN [nucleus interfacialis (Nlf), high vocal center (HVC), nucleus robustus of the archistriatum (RA), dorsomedial portion of the intercollicular nucleus (DM), Area X of the parolfactory lobe, and lateral portion of the magnocellular nucleus of the anterior neostriatum (IMAN)]; one AN [nucleus mesencephalicus lateralis pars dorsalis (MLd)], and one HN [preopticus anterior (POA)]. NE levels changed significantly in two VCN (Nlf and Area X). In Nlf, RA, Area X, IMAN, and MLd, NE levels and/or turnover rates showed a striking peak at day 25, which was not seen in HN. Both DA levels and turnover rates changed profoundly over development in 5 of 6 VCN (Nlf, RA, DM, Area X, and IMAN) and both AN (MLd and Field L). These nuclei showed striking peaks in DA levels and turnover rates, primarily on day 35 and/or 55, which then declined profoundly by day 90. This contrasted with the minimal change in DA turnover rates seen in one HN (POA) and the sixth VCN, HVC. In several VCN and AN, NE and DA levels and turnover rates during development reached levels never seen in adult males. Previous research has shown that catecholamine function is heightened in VCN during development compared to surrounding tissues. Our data demonstrate that NE and DA function during development shows pronounced peaks in most VCN not seen in HN. This is interesting because both VCN and HN are hormone sensitive, and both show hormone-modulated NE and DA function in adult males. The timing of these peaks suggests that increased catecholaminergic function may be involved in sexual differentiation of the VCN and song learning in finches. © 1998 John Wiley & Sons, Inc. J Neurobiol 34: 329–346, 1998

Changes in catecholamine levels and turnover rates in hypothalamic, vocal control, and auditory nuclei in male zebra finches during development

The catecholamines norepinephrine (NE) and dopamine (DA) have been implicated in the sexual differentiation of brain and behavior and in species-specific learning in several species. To determine if these neurotransmitters might be involved in sexual differentiation of the vocal control system and song learning in male zebra finches, NE and DA levels and turnover rates were quantified in 10 behaviorally relevant brain nuclei [6 vocal control (VCN), 2 auditory (AN), and 2 hypothalamic (HN)] at four critical points during sexual differentiation of the VCN and the period of song learning, 25, 35, 55, and 90 days of age. Some birds were pretreated with alpha-methyl-para-tyrosine (alphaMPT) to allow estimation of NE and DA turnover rates. NE and DA levels in microdissected nuclei were quantified using high-performance liquid chromatography with electrochemical detection. AlphaMPT treatment suppressed catecholamine synthesis just as effectively in juveniles as it does in adults and proved an effective method for estimating NE and DA turnover rates. Patterns of NE and DA function in most VCN and AN over development were quite different from those in HN in which NE and DA function changed gradually and showed no striking peaks. NE turnover rates changed significantly over development in all six VCN [nucleus interfacialis (Nlf), high vocal center (HVC), nucleus robustus of the archistriatum (RA), dorsomedial portion of the intercollicular nucleus (DM), Area X of the parolfactory lobe, and lateral portion of the magnocellular nucleus of the anterior neostriatum (IMAN)]; one AN [nucleus mesencephalicus lateralis pars dorsalis (MLd)], and one HN [preopticus anterior (POA)]. NE levels changed significantly in two VCN (Nlf and Area X). In Nlf, RA, Area X, IMAN, and MLd, NE levels and/or turnover rates showed a striking peak at day 25, which was not seen in HN. Both DA levels and turnover rates changed profoundly over development in 5 of 6 VCN (Nlf, RA, DM, Area X, and IMAN) and both AN (MLd and Field L). These nuclei showed striking peaks in DA levels and turnover rates, primarily on day 35 and/or 55, which then declined profoundly by day 90. This contrasted with the minimal change in DA turnover rates seen in one HN (POA) and the sixth VCN, HVC. In several VCN and AN, NE and DA levels and turnover rates during development reached levels never seen in adult males. Previous research has shown that catecholamine function is heightened in VCN during development compared to surrounding tissues. Our data demonstrate that NE and DA function during development shows pronounced peaks in most VCN not seen in HN. This is interesting because both VCN and HN are hormone sensitive, and both show hormone-modulated NE and DA function in adult males. The timing of these peaks suggests that increased catecholaminergic function may be involved in sexual differentiation of the VCN and song learning in finches.

Enhancement and suppression in the auditory midbrain nucleus (MLD) of the pigeon

Auditory evoked potentials (AEPs) and spike responses were recorded from the same recording site in the nucleus mesencephalicus lateralis pars dorsalis (MLD) in pigeons with a tungsten microelectrode. Depending on the recording sites within the MLD, enhancement and suppression of the AEPs in response to clicks were observed at particular frequencies of a background continuous pure tone. Post stimulus time histograms (PSTs) of the spike responses, if available in such cases, were recorded from the same position by the same electrode. Suppression of the AEPs always occurred but enhancement occurred in only 21% of the trials. The frequencies of tone bursts that caused maximum AEP were vaguely related to the frequencies of continuous pure tones that elicited maximum suppression of the AEPs in response to clicks. However, enhancement was produced by a continuous pure tone of approximately 1.5 kHz, independent of the frequencies of tone bursts that produced maximum AEPs. Most of the PSTs in such instances showed parallel relations between the spike responses and the amplitudes of the AEPs. The nature of the enhancement and suppression of the click evoked AEPs during continuous pure tones was clearly different from those in recordings from the nucleus magnocellularis, nucleus angularis and Field L in respect to the probability of occurrence of enhancement and suppression.

Effects of unilateral and bilateral cochlea removal on 2-deoxyglucose patterns in the chick auditory system.

The 2-deoxyglucose (2DG) method was used to map functional activity in the auditory system of chicks that had been subjected to unilateral or bilateral cochlea removal. Following survival times of 1 day to 4 weeks, chicks were exposed to continuous white noise in the 2DG experiments. In monaural subjects nucleus angularis and nucleus magnocellularis showed faint 2DG uptake on the side contralateral to the intact ear. In the binaural nucleus laminaris, the asymmetrical and almost mirror-imaged labeling pattern (Lippe, Stewart, and Rubel: Brain Res. 196:43-58, '80) was produced. The superior olive (OS) was strongly labeled on the ipsilateral side, whereas the contralateral OS showed only a slight 2DG uptake at its medial border. The lateral lemniscus and nucleus lemnisci lateralis, pars ventralis (LLv) showed stronger activation on the contralateral side. Both Nissl stains and 2DG patterns provide evidence that nucleus ventralis lemnisci lateralis (VLV) can be subdivided into an anterior (VLVa) and a posterior (VLVp) part. Whereas VLVp is labeled only contralaterally, VLVa is labeled on both sides with similar intensity. Nucleus mesencephalicus lateralis, pars dorsalis (MLD) is strongly labeled throughout contralaterally. The ipsilateral MLD shows a defined ventral portion of high 2DG uptake. Intensity of labeling here is symmetrical to the corresponding area of the contralateral MLD. These symmetrical patterns were related to the tonotopic organization of MLD, which was mapped in intact animals by using tone stimuli. Assuming that symmetrical 2DG uptake in monaural animals indicates excitatory input from both ears (EE-cells), it appears that these EE-cells occupy a sector of each isofrequency plane in MLD. Nucleus ovoidalis (Ov) generally was stronger labeled on the contralateral side. The columnar organization of field L as seen in monaural chicks has already been described (Scheich, Exp. Brain Res. 51:199-205, '83). In bilaterally deafened chicks, MLD, Ov, and layer L2 of field L showed strong but spatially restricted 2DG accumulation in contrast to absence of labeling in peripheral nuclei. The 2DG patterns in monaural chicks are likely to reflect excitatory input within the auditory system. In addition they reveal new insights into the functional organization of some of its nuclei. In particular, they support the notion that MLD contains maps of several interaural integration mechanisms similar to field L. Labeling in the auditory system of bilaterally deafened chicks may result from descending projections or from other than auditory inputs.

Origin of ascending auditory projections to the nucleus mesencephalicus lateralis pars dorsalis in the chicken

Ascending auditory projections to the nucleus mesencephalicus lateralis pars dorsalis (MLd) were studied in white Leghorn chickens by means of unilateral injections of horseradish peroxidase into the MLd and by injections of tritiated leucine into nucleus angularis or the combined nucleus magnocellularis and nucleus laminaris. The experiments showed that nucleus angularis sends an extensive projection to the contralateral MLd and a smaller projection to the rostral pole of the ipsilateral MLd; the lagenar region contributes to these bilateral connections. Nucleus angularis also projects bilaterally to the superior olive and nucleus ventralis lemnisci lateralis and to the contralateral nucleus lemnisci lateralis pars ventralis and dorsal nucleus of the lateral lemniscus. Projections from nucleus laminaris were demonstrated to the ipsilateral superior olive, to the contralateral lemniscal nuclei and a small medial region in MLd bilaterally; the contralateral projection is much denser than the ipsilateral one. Other nuclei having ascending connections with MLd include the contralateral superior olive, the ipsilateral nucleus lemnisci lateralis pars ventralis, the contralateral nucleus ventralis lemnisci lateralis and the contralateral MLd. The ipsilateral superior olive and nucleus ventralis lemnisci lateralis also project to MLd but much more sparsely than in their contralateral projection. Although several of these findings correspond with auditory connections previously shown in the pigeon brainstem, they differ fundamentally in that we find both nucleus angularis and nucleus laminaris projecting to different areas of the MLd on both sides of the brain. In particular, our observation that the cochlear nucleus has bilateral connections with MLd demonstrates an important avian similarity with the brainstem auditory pathways of other terrestrial vertebrates.

Barn owls can use ongoing‐time disparities to determine the azimuth of a sound source

A class of auditory neurons in the owl's nucleus mesencephalicus lateralis pars dorsalis (MLD), a midbrain auditory nucleus, have small spatial receptive fields. We sought our relationships between the spatial receptive fields of these neurons (measured under free‐field conditions) and sensitivities to binaural disparities (presented dichotically). To switch easily between free‐field and dichotic stimulation we constructed threaded, miniature earphones, which could be inserted into threaded rings implanted into the owl's ear canals. Neurons with spatial receptive fields were found to be sensitive to a narrow range of ongoing‐time disparities in the biologically significant range of tens of microseconds. There was a strong correlation between the azimuth of the spatial receptive fields and the magnitude of the ongoing‐time disparities. Freely behaving owls were presented with similar ongoing‐time disparities through identical earphones. They reacted to the stimuli in a manner consistent with the conclusion that ongoing‐time disparities were used to ascertain the azimuth of a sound source.

Neuronal and behavioral sensitivity to binaural time differences in the owl

We demonstrated that ongoing time disparity (OTD) was a sufficient cue for the azimuthal component of receptive fields of auditory neurons in the owl (Tyto alba) midbrain and that OTDs were sufficient to mediate meaningful behavioral responses. We devised a technique which enabled us to change easily between free field and dichotic stimuli while recording from single auditory neurons in the owl mesencephalicus lateralis pars dorsalis (MLD). MLD neurons with restricted spatial receptive fields ("space-mapped neurons") showed marked sensitivity to specific ongoing time disparities. The magnitudes of these disparities were in the behaviorally significant range of tens of microseconds. The ongoing time disparities were correlated significantly with the azimuthal center of receptor fields. Space-mapped neurons were insensitive to transient disparities. MLD neurons which were not space-mapped, i.e., were omnidirectional, did not show any sensitivity to specific OTDs. We confirmed the behavioral relevance of OTD as a cue for localizing a sound in azimuth by presenting OTD differences to tame owls. Using head turning as an assay, we showed that OTD was a sufficient cue for the azimuth of a sound. The relationship between azimuth and OTD obtained from our neurophysiological experiments matched closely the relationship obtained from our behavioral experiments.