Auditory pathways of caudal telencephalon and their relation to the song system of adult male zebra finches (Taenopygia guttata)

Auditory information is critical for vocal imitation and other elements of social life in songbirds. In zebra finches, neural centers that are necessary for the acquisition and production of learned vocalizations are known, and they all respond to acoustic stimulation. However, the circuits by which conspecific auditory signals are perceived, processed, and stored in long-term memory have not been well documented. In particular, no evidence exists of direct connections between auditory and vocal motor pathways, and two newly identified centers for auditory processing, caudomedial neostriatum (Ncm) and caudomedial hyperstriatum ventrale (cmHV), have no documented place among known auditory circuits. Our goal was to describe anatomically the auditory pathways in adult zebra finch males and, specifically, to show the projections by which Ncm and vocal motor centers may receive auditory input. By using injections of different kinds of neuroanatomical tracers (biotinylated dextran amines, rhodamine-linked dextran amines, biocytin, fluorogold, and rhodamine-linked latex beads), we have shown that, as in other avian groups, the neostriatal field L complex in caudal telencephalon is the primary forebrain relay for pathways originating in the auditory thalamus, i.e., the nucleus ovoidalis complex (Ov). In addition, Ncm and cmHV also receive input from the Ov complex. Ov has been broken down into two parts, the Ov “core” and “shell,” which project in parallel to different targets in the caudal telencephalon. Parts of the field L complex are connected among themselves and to Ncm, cmHV, and caudolateral HV (cIHV) through a complex web of largely reciprocal pathways. In addition, cIHV and parts of the field L complex project strongly to the “shelf” of neostriatum underneath the song control nucleus high vocal center (HVC) and to the “cup” of archistriatum rostrodorsal to another song-control nucleus, the robust nucleus of the archistriatum (RA). We have documented two points at which the vocal motor pathway may pick up auditory signals: the HVC-shelf interface and a projection from cIHV to the nucleus interfacialis (NIf), which projects to HVC. These data represent the most complete survey to date of auditory pathways in the adult male zebra finch brain, and of their projections to motor stations of the song system. © 1996 Wiley-Liss, Inc.     

Auditory pathways in the budgerigar. I. Thalamo-telencephalic projections

Thalamo-telencephalic auditory pathways in the budgerigar (Melopsittacus undulatus) were studied using horseradish peroxidase (HRP) histochemistry and amino acids autoradiography. The results indicate that in this species the thalamic auditory relay nucleus, n. ovoidalis, projects upon a circumscribed region of the caudal and caudomedial neostriatum including field 'L' and immediately adjacent portions of the neostriatum intermedium, pars dorsolateralis (NIDL). This region of NIDL also receives inputs from another thalamic nucleus, n. dorsolateralis posterior (DLP). In the DLP is in receipt of tectal inputs. Projections of DLP upon NIDL were confirmed with amino acids autoradiography. The results of the HRP experiments indicate that different portions of n. ovoidalis project upon different portions of field 'L' and NIDL. Neurons in the dorsal and lateral portions of the n. ovoidalis project upon more medial portions of field 'L'. Neurons located centrally in the n. ovoidalis project upon central and lateral portions of field 'L'. Neurons in the ventromedial portion of the n. ovoidalis are labeled in all cases in which HRP is placed in either field 'L' or in the DLP projection field immediately adjacent to field 'L' proper. HRP injections placed in NIDL lateral to the projection fields of the n. ovoidalis and DLP label neurons within other diencephalic nuclei including the n. subrotundus. The caudal and intermediate levels of the neostriatum intermedium apparently serve as a complex processing area for many thalamic inputs in this species. The existence of multiple ascending thalamo-telencephalic projections from portions of the thalamus receiving inputs from both the visual (i.e., tectal) and auditory (i.e., n. mesencephalicus lateralis pars dorsalis) portions of the midbrain roof (i.e., from DLP and from n. ovoidalis) suggests the possibility that intermodal associations may take place in these telencephalic fields. Such partially converging pathways may provide a basics for intermodal associations which are important in individual recognition and social signalling systems in this species.     

Calcitonin gene-related peptide immunoreactive cells and fibers in forebrain vocal and auditory nuclei of the budgerigar (Melopsittacus undulatus).

The distributions of calcitonin gene-related peptide (CGRP) immunoreactive neurons and fibers were mapped within forebrain vocal control and auditory nuclei of a vocal learning psittacine species, the budgerigar (Melopsittacus undulatus). Immunoreactivity was exhibited by telencephalic nuclei previously associated with vocal control pathways on the basis of both tract tracing studies and gene mapping: the central nucleus of the anterior archistriatum (AAc), central nucleus of the lateral neostriatum (NLc), magnocellular nucleus the lobus parolfactorius (LPOm), the oval nucleus of the ventral hyperstiratum (HVo) and the medial division of the oval nucleus of the anterior neostriatum (NAom). The main body of NAo also contained an exceptionally high density of immunoreactive fibers. In contrast to the condition in oscine songbirds, CGRP-positive neuronal somata were not present in any telencephalic vocal control nucleus. CGRP-positive somata were present, however, in diencephalic cell groups that included the shell region of the nucleus ovoidalis (Ov), the nucleus dorsolateralis posterior (DLP) and a region of the ventral thalamus that was retrogradely labeled by tracer deposits into HVo and AAc. CGRP immunoreactive fibers were observed within auditory areas of the telencephalon including Field L and the neostriatum intermedium pars dorsolateralis. The likely sources of these fibers are CGRP-positive neurons within the Ov shell and DLP.     

Projections of the oval nucleus of the hyperstriatum ventrale in the budgerigar: relationships with the auditory system

The afferent and efferent projections of a vocal control nucleus, the oval nucleus of the hyperstriatum ventrale (HVo), were mapped out in a parrot, the budgerigar (Melopsittacus undulatus) to determine the relationships of this nucleus to the auditory system. In budgerigars, HVo is connected to both the anterior forebrain pathway as well as to nuclei forming the descending projection system to the brainstem (Durand et al. [1997] J. Comp. Neurol. 377:179-206). Previous studies (Brauth et al. [1997] Proc. N. Y. Acad. Sci. 807:368-385; Durand and Brauth [1998] Neurosci Abstr 24:78.9) indicate that HVo lesions disrupt vocal performance and that HVo neurons show long latency electrophysiologic auditory responses. HVo has also been shown to receive input from neurons in the immediately adjacent HV (Durand et al. [1997] J. Comp. Neurol. 377:179-206). Thus, the focus of the present study was to elucidate relationships between HVo, its immediately adjacent surround and telencephalic auditory nuclei. The results show that, although the lateral and medial portions of HVo are interconnected with one another, inputs to these areas and their surrounds are distinctively different. The most substantial auditory system inputs are derived from the frontal lateral neostriatum (NFl) and supracentral nucleus of the lateral neostriatum (NLs); these project primarily to the lateral HVo and lateral HVo surround. The medial HVo and surround receive only sparse or modest input from auditory nuclei, including the caudomedial neostriatum (NCM), neostriatum intermedium pars lateralis (NIL), Fields L1 and L3, and the neostriatum intermedium pars ventrolateralis (NIVL). Other sources of input to the HVo surround include the hyperstriatum accessorium (HA), the supralaminar area of the frontal neostriatum (NAs), the ventral anterior archistriatum (AAv), the medial archistriatum (Am) and the medial HV. Neurons in the HV immediately medial to HVo project to a shell region around the entire nucleus. Both the ventral paleostriatum (VP) and ventral part of the central nucleus of the lateral neostriatum (NLc) project to HVo but not to the surround. Previously described projections (Durand et al., 1997) from HVo to the NAom, NLc, and the magnicellular nucleus of the lobus parolfactorius (LPOm) were confirmed.     

The avian somatosensory system: connections of regions of body representation in the forebrain of the pigeon

In order to establish the basic connectivity of physiologically identified somatosensory regions of the thalamus and telencephalon in the pigeon, injections of wheatgerm agglutinin-horseradish peroxidase were made under electrophysiological control and the projections were charted following conventional neurohistochemistry. The physiological recordings generally confirmed the findings of Delius and Bennetto (Brain Research, 37 (1972) 205-221) of somatosensory sites within the dorsal thalamus, anterior hyperstriatum and caudomedial neostriatum, and the anatomical results show that the thalamic cells of origin of the projections to the two telencephalic regions are largely separate: a rostral cell group comprising nucleus dorsalis intermedius ventralis anterior projects to the anterior hyperstriatum accessorium (HA), whilst a caudal cell group comprising caudal regions of nucleus dorsolateralis posterior (DLP) projects to the medial neostriatum intermedium and caudale (NI/NC). Caudal DLP is also the origin of a visual projection to NI/NC, and its terminal field also approximates that of the thalamic auditory nucleus ovoidalis. Since the anterior HA and NI/NC were here shown to be reciprocally connected, there is a possibility of multimodal input to both telencephalic regions. HA was also further defined as the origin of the basal branch of the septomesencephalic tract, and hence potentially provides an outlet for both telencephalic somatosensory regions. The results are discussed within a comparative context.     

Axonal connections of the High Vocal Center and surrounding cortical regions in juvenile and adult male zebra finches

Neuronal connections of the High Vocal Center (HVC), a cortical nucleus of songbirds necessary for learned vocal behavior, and the region adjacent to HVC called paraHVC (pHVC), were studied in adult and juvenile male zebra finches. Extremely small injections of fluorescent dextran amines or biocytin were made within subregions of HVC and pHVC to define the precise nature and development of these pathways. In adults, all HVC injections produced an even, nontopographic distribution of retrograde label throughout the medial magnocellular nucleus of the anterior neostriatum (mMAN), the interfacial nucleus (NIf), and the uvaeform nucleus of the thalamus (Uva) and an even distribution of anterograde label within area X of the striatum and the robust nucleus of the archistriatum (RA). These same patterns of projections were present in juvenile birds 20–23 days of age, including the projection from HVC to RA, which has previously been reported to develop only after 25–30 days of age. Results also establish a novel efferent projection from HVC to pHVC in both juvenile and adult birds. Injections into pHVC indicate that this region receives afferent input from song control areas HVC, mMAN, medial regions of the parvicellular shell of lateral MAN, NIf, and Uva and projects to Area X, caudomedial regions of striatum, and regions of the caudomedial neostriatum (NCM). Thus, neuronal connections of pHVC are highly integrated with circuitry important for vocal behavior and are distinct from those of HVC. Such differences establish HVC and pHVC as separate brain areas and suggest that each may serve a different function in vocal behavior. Control injections in both juveniles and adults produced specific patterns of projections from areas outside of HVC to areas outside of RA, illustrating an overall spatial organization of projections from HVC and neighboring cortical areas. Further, although neuronal connections of HVC are not topographic, projections of HVC, pHVC, and surrounding areas demonstrate a broad spatial organization of efferents to striatum and regions surrounding RA, thus defining a level of organization beyond that of individual song control nuclei. J. Comp. Neurol. 397:118–138, 1998. © 1998 Wiley-Liss, Inc.     

Auditory pathways in the budgerigar. II. Intratelencephalic pathways

The projections of two telencephalic areas in receipt of projections from the auditory relay nucleus of the thalamus (nucleus ovoidalis) were studied in the budgerigar (Melopsittacus undulatus) with autoradiographic methods. These nuclei are called field 'L' and neostriatum intermedium pars dorsolateralis (NIDL). The results show that neurons in both fields project laterally to a portion of the neostriatum intermedium pars ventrolateralis (NIVL) and rostrally to the rostromedial archistriatum. Horseradish peroxidase experiments confirm these projections and indicate that field 'L', NIDL and NIVL also receive projections from neurons in the hyperstriatum ventrale (HV). The projections of field 'L' and NIDL neurons to the rostromedial archistriatum may act as pathways subserving auditory feedback. Neurons in this portion of the archistriatum project to the contralateral field 'L', NIDL and NIVL. Furthermore, the medial archistriatal projection field of neurons within field 'L', NIDL and NIVL (i.e. rostromedial archistriatum) is located adjacent to a large archistriatal neuronal field projecting to the medulla, including the lateral reticular formation of the medulla. This large archistriatal field includes the nucleus archistriatalis robustus, the telencephalic nucleus identified as the source of projections to the lower motoneurons of the syrinx. Thus, projections from auditory telencephalic areas to the rostromedial archistriatum may serve functions related to processes associated with learning and vocal motor control.     

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.     

Convergence of bilateral auditory midbrain inputs on neurons in the auditory thalamus of chicken

In the avian ascending auditory pathway, the nucleus mesencephalicus lateralis pars dorsalis (MLd; the auditory midbrain center) receives inputs from virtually all lower brainstem auditory nuclei and sends outputs bilaterally to the nucleus ovoidalis (Ov; the auditory thalamic nucleus). Axons from part of the MLd terminate in a particular domain of Ov, thereby suggesting a formation of segregated pathways point-to-point from lower brainstem nuclei via MLd to the thalamus. However, it has not yet been demonstrated whether any spatial clustering of thalamic neurons that receive inputs from specific domains of MLd exists. Ov neurons receive input from bilateral MLds; however, the degree of laterality has not been reported yet. In this study, we injected a recombinant avian adeno-associated virus, a transsynaptic anterograde vector into the MLd of the chick, and analyzed the distribution of labeled postsynaptic neurons on both sides of the Ov. We found that fluorescent protein-labeled neurons on both sides of the Ov were clustered in domains corresponding to subregions of the MLd. The laterality of projections was calculated as the ratio of neurons labeled by comparing ipsilateral to contralateral projections from the MLd, and it was 1.86 on average, thereby indicating a slight ipsilateral projection dominance. Bilateral inputs from different subdomains of the MLd converged on several single Ov neurons, thereby implying a possibility of a de novo binaural processing of the auditory information in the Ov.     

Immunohistochemistry and neural connectivity of the Ov shell in the songbird and their evolutionary implications

The neuropeptide immunohistochemistry and neural connectivity of areas surrounding the thalamic auditory nucleus (the nucleus ovoidalis [Ov]), as well as the areas to which it is connected, were investigated in a songbird, the Bengalese finch. The results showed that met-enkephalin was present in the Ov shell and most of the areas connected to it, but not in the Ov core. Anterograde and retrograde tracing studies showed that the Ov shell was more widely connected than the Ov core. The Ov shell was mainly connected to: 1). areas flanking the primary telencephalic auditory field (i.e., fields L2b, L1, and L3) and areas surrounding the robust nucleus of the archistriatum (RA); 2). several hypothalamic areas such as the nucleus ventromedialis hypothalami (VMN) and the nucleus anterior medialis hypothalami (AM). Some of these areas connected to the Ov shell are thought to be involved in auditory mediated neurosecretory activities. These results, which are similar to those reported previously in non-songbirds, suggest that the Ov shell and other surrounding areas of auditory and song-control nuclei are conserved in birds. These findings are discussed in terms of the evolution of the core-and-surround organization of auditory and song-control nuclei.     

鸣禽中脑听觉核团边缘区神经投射的研究

对鸣禽白腰文鸟（Ｌｏｎｃｈｕｒａ ｓｔｒｉａｔａ）中脑听觉核团－背外侧核（ｎｕｃｌｅｕｓ ｍｅｓｅｎｃｅｐｈａｌｉｃｕｓ ｌａｔｅｒａｌｉｓ,ｐａｒｓ ｄｏｒｓａｌｉｓ,ＭＬｄ）和丘间核（ｎｕｃｌｅｕｓ ｉｎｔｅｒｃｏｌｌｉｃｕｌａｒｉｓ,ＩＣｏ）的脑啡肽免疫化学特异和神经联系进行了研究,结果发现：脑啡肽能标记纤维或细胞主要分布于中脑听觉核团的痛外侧核边缘区、丘间核,而背外侧核中央几乎无分布。双向     

Rostral wulst of passerine birds: II. Intratelencephalic projections to nuclei associated with the auditory and song systems.

We have previously shown that the hyperstriatum accessorium (HA) of the rostral wulst in zebra finches and green finches is the origin of a pyramidal-like tract with substantial projections to the brainstem and cervical spinal cord. Here, we show that the HA also is the origin of a set of intratelencephalic projections with terminal fields in the lateral part of the frontal neostriatum, the shell surrounding the lateral magnocellular nucleus of the anterior neostriatum, the lobus parolfactorius surrounding area X, the nucleus interface, auditory fields L1 and L3, the shelf underlying the high vocal center, the dorsolateral caudal neostriatum, the dorsocaudal part of the nucleus robustus archistriatalis, and the ventral archistriatum. The cells of origin of these projections are located predominantly laterally in the HA, close to and sometimes within the intercalated HA, which receives somatosensory projections from the dorsal thalamus. The specific implications of these findings for auditory and vocal function are unclear, but the apparent overlap of auditory and somatosensory inputs in several of these regions suggests the possibility of mechanisms for stimulus enhancement or depression, depending on the congruence of stimuli within a cell's "in-register" multiple receptive fields.     

Parallel pathways and convergence onto HVc and adjacent neostriatum of adult zebra finches (Taeniopygia guttata)

The structure and connectivity of the forebrain nucleus HVc, a site of sensorimotor integration in the song control system of oscine birds, were investigated in adult zebra finches. HVc in males comprises three cytoarchitectonic subdivisions: the commonly recognized central region with large and medium-sized darkly staining cells, a ventral caudomedial region with densely packed small and medium-sized cells, and a dorsolateral region with oblong cells and rows of cells. All three subdivisions project to area X and the robust nucleus of the archistriatum, with more complexity in the classes and distribution of cells than previously reported. In females, HVc is very small and has a cytoarchitecture distinct from that of the three male subdivisions. The structure of HVc in females treated with estradiol at 15 days of age is similar to male HVc. Tracer studies in males with fluorescent and biotinylated dextrans demonstrate non-topographic projections onto HVc that may carry auditory information, including type 1 and type 2 neurons in subdivisions L1 and L3 of the field L complex, a class of neurons in nucleus interface, nucleus uvaeformis, the caudal neostriatum ventral to HVc, and intrinsic HVc connections. These data are interpreted in terms of HVc's functional properties. Additionally, the neostriatum immediately ventral to HVc receives projections from field L, ventral hyperstriatum, and caudal neostriatum, and projects to a region surrounding RA and near to or into area X. The similarity of the connectivity of HVc and adjacent neostriatum suggests the possibility that they share a common origin. © 1995 Wiley-Liss, Inc.     

Connections of a motor cortical region in zebra finches: relation to pathways for vocal learning

The lateral magnocellular nucleus of the anterior neostriatum (lMAN) is necessary for both initial learning of vocal patterns in developing zebra finches, as well as for modification of adult song under some circumstances. Lateral MAN is composed of two subregions: a core of magnocellular neurons and a surrounding shell composed primarily of parvocellular neurons. Neurons in lMAN(core) project to a region of motor cortex known as robust nucleus of the archistriatum (RA), whereas neurons in lMAN(shell) project to a region adjacent to RA known as dorsal archistriatum (Ad). We studied the axonal connections of Ad in adult male zebra finches. In contrast to RA, Ad neurons make a large number of efferent projections, which do not include direct inputs to vocal or respiratory motor neurons. The major efferent projections of Ad are to: (1) the striatum of avian basal ganglia; (2) a dorsal thalamic zone (including the song-control nuclei dorsomedial nucleus of the posterior thalamus [DMP] and dorsolateral nucleus of the medial thalamus [DLM]); (3) restricted regions within the lateral hypothalamus (stratum cellulare externum [SCE]), which may also relay information to the same dorsal thalamic zone; (4) a nucleus in the caudal thalamus (medial spiriform nucleus [SpM]); (5) deep layers of the tectum, which project to the thalamic song-control nucleus Uva; (6) broad regions of pontine and midbrain reticular formation; and (7) areas within the ventral tegmental area and substantia nigra (ventral tegmental area [AVT], substantia nigra [SN]), which overlap with regions that project to Area X, a song-control nucleus of avian striatum. Inputs to Ad derive not only from lMAN(shell), but also from a large area of dorsolateral caudal neostriatum (dNCL), which also receives input from lMAN(shell). That is, lMAN(shell) neurons project directly to Ad, and also multisynaptically to Ad via dNCL. Double-labeling studies show that lMAN(shell) contains two different populations of projection neurons: one that projects to Ad and another to dNCL. These results are exciting for two main reasons. The first is that some of these projections represent potential closed-loop circuits that could relay information back to song-control nuclei of the telencephalon, possibly allowing diverse types of song-related information to be both integrated between loops and compared during the period of auditory-motor integration. Because both auditory experience with an adult (tutor) song pattern and auditory feedback are essential to vocal learning, closed-loop pathways could serve as comparator circuits in which efferent commands, auditory feedback, and the memory of the tutor song are compared in an iterative fashion to achieve a gradual refinement of vocal production until it matches the tutor song. In addition, these circuits seem to have a strong integrative and limbic flavor. That is, the axonal connections of Ad neurons clearly include regions that receive inputs not only from somatosensory, visual, and auditory areas of cortex, but also from limbic regions, suggesting that they may be involved in higher order sensory processing, arousal, and motivation.     

Nuclei of the lateral lemniscus project directly to the thalamic auditory nuclei in the pigeon

Non-mesencephalic origins of ascending afferents to the thalamic auditory nuclei, nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO), were identified in the pigeon in retrograde tracing experiments. These origins comprise dorsal and ventral lateral lemniscal nuclei. Injections of anterograde tracers into these nuclei produced terminal labelling in SPO in particular. These experiments show for the first time that the inferior colliculus is not an obligatory relay to the auditory thalamus.     

Direct comparison of projections from the central amygdaloid region and nucleus accumbens shell

Certain neurochemical and connectional characteristics common to extended amygdala and the nucleus accumbens shell suggest that the two represent a single functional-anatomical continuum. If this is so, it follows that the outputs of the two structures should be substantially similar. To address this, projections from the caudomedial shell and central nucleus of the amygdala, a key extended amygdala structure, were demonstrated in Sprague-Dawley rats with different anterograde axonal tracers processed separately to exhibit distinguishable brown and blue-black precipitates. The caudomedial shell projection is strong in the ventral pallidum and along the medial forebrain bundle through the lateral preopticohypothalamic continuum into the ventral tegmental area, distal to which it thins abruptly. The central nucleus projects strongly to the bed nucleus of the stria terminalis and the sublenticular extended amygdala, but substantially to the lateral hypothalamus only at levels behind the rostral part of the entopeduncular nucleus. Innervation of the ventral tegmental area by the central amygdala is minimal, but the lateral one-third of the substantia nigra, pars compacta and an adjacent lateral part of the retrorubral field receive substantial central amygdala input. Central amygdaloid projections are robust in caudal brainstem sites, such as the reticular formation, parabrachial nucleus, nucleus of the solitary tract and dorsal vagal complex, all of which receive little input from the accumbens. The substantial differences in the output systems of the caudomedial shell of accumbens and central amygdala suggest that the two represent distinct functional-anatomical systems.     

Efferent projections of the medial preoptic nucleus and medial hypothalamus in the pigeon

The efferent projections of the medial preoptic nucleus (POM), anterior-medial hypothalamic area (AM), and the posteromedial hypothalamic nucleus (PMH) in the pigeon were traced by the autoradiographic technique. Similar and differential connections were noted from these regions. Projections from POM and AM-PMH were traced to nucleus septalis lateralis, nucleus dorsomedialis thalami, nucleus dorsolateralis anterior thalami (pars ventralis), posterior hypothalamic and medial mammillary areas, area ventralis tegmenti (Tsai), central gray of midbrain and nucleus intercollicularis and substantia grisea periventricularis of the midbrain. The density of silver grains in these regions differed with POM and AM-PMH injections. Other projections were observed exclusively from only one or two of the nuclear regions injected. Connections from POM and the rostral part of AM were seen to the median eminence, neurohypophysis, and the nucleus of anterior pallial commissure. Only cells of the anterior part of AM project fibers to nucleus septalis medialis. In the hypothalamus, projections from POM are concentrated in the periventricular region and in the preoptic-hypophyseal tract in the extreme lateral hypothalamus, while AM-PMH projections are heaviest in the medial hypothalamus and lateral preoptic area. A major difference in the connections of PMH from POM is the more substantial PMH projection to the midbrain. A prominent projection courses dorsolaterally and posteriorly from PMH toward nucleus ovoidalis and splits into two pathways: a lateral pathway which heavily innervates n. intercollicularis and the periventricular gray and a ventrolateral projection to the midbrain tegmentum. The projections described above provide anatomical substrates for neuroendocrine, autonomic, and behavioral functions.     

Organization of afferent and efferent projections of the nucleus basalis prosencephali in a passerine,Taeniopygia guttata

The connections of nucleus basalis (NB) of the rostral forebrain of the zebra finch were investigated electrophysiologically and with anterograde and retrograde tracing methods to determine their functional organization, the sources of their pontine afferents, and the targets of their telencephalic efferents. The nucleus was found to be partitioned into three major components, a rostral lingual part that received a hypoglossal projection via a lateral subnucleus of the principal sensory trigeminal nucleus (PrV), a middle beak part that received a trigeminal projection via a medial subnucleus of PrV, and a caudal auditory part that received a short latency auditory projection via the intermediate nucleus of the lateral lemniscus. Beak NB also received a projection from a paralateral lemniscal nucleus, and the dorsocaudal part of auditory NB and the medially adjacent neostriatum also received a projection from a lateral subnucleus of the superior vestibular nucleus (VS). The efferent projections of each of the three major parts of NB were mainly to the adjacent neostriatum frontale (NF), which then provided projections to the lobus parolfactorius (exclusive of area X), the lateral archistriatum intermedium (Ail), and the lateral neostriatum caudale (NCl). Ail received a projection from NCl and provided terminal fields to the contralateral NCl and the NF. The major projections of Ail, however, descended bilaterally through the brainstem via the occipitomesencephalic tracts, with dense terminations in the medial spiriform nucleus and with extensive bilateral terminations throughout the lateral reticular formation of the pons and medulla. For the most part, jaw, tongue, and tracheosyringeal motor nuclei did not receive terminations. The results suggest that NB in zebra finch, like NB in pigeon and duck, is likely to be a major component of trigeminal sensorimotor circuitry involved in feeding and in other oral-manipulative behaviors. Results also show that the auditory component of NB is not directly linked to the vocal control system at telencephalic levels, but the possibility remains that the lingual, beak, and auditory parts of NB play a role in vocalization by multisynaptic influences on cranial nerve motor nuclei innervating various parts of the vocal tract. © 1996 Wiley-Liss, Inc.     

Thalamic connections of the auditory cortex in marmoset monkeys: core and medial belt regions

In this study and its companion, the cortical and subcortical connections of the medial belt region of the marmoset monkey auditory cortex were compared with the core region. The main objective was to document anatomical features that account for functional differences observed between areas. Injections of retrograde and bi-directional anatomical tracers targeted two core areas (A1 and R), and two medial belt areas (rostromedial [RM] and caudomedial [CM]). Topographically distinct patterns of connections were revealed among subdivisions of the medial geniculate complex (MGC) and multisensory thalamic nuclei, including the suprageniculate (Sg), limitans (Lim), medial pulvinar (PM), and posterior nucleus (Po). The dominant thalamic projection to the CM was the anterior dorsal division (MGad) of the MGC, whereas the posterior dorsal division (MGpd) targeted RM. CM also had substantial input from multisensory nuclei, especially the magnocellular division (MGm) of the MGC. RM had weak multisensory connections. Corticotectal projections of both RM and CM targeted the dorsomedial quadrant of the inferior colliculus, whereas the CM projection also included a pericentral extension around the ventromedial and lateral portion of the central nucleus. Areas A1 and R were characterized by focal topographic connections within the ventral division (MGv) of the MGC, reflecting the tonotopic organization of both core areas. The results indicate that parallel subcortical pathways target the core and medial belt regions and that RM and CM represent functionally distinct areas within the medial belt auditory cortex.