The topography of aberrant ipsilateral retinogeniculate projections following neonatal ablation of the superior colliculus in rats

The topography of aberrant ipsilateral retinogeniculate projections following ablation of the superior colliculus in infancy has been examined. Rat pups received a unilateral ablation of the superior colliculus in infancy, which is known to produce an aberrant ipsilateral retinogeniculate projection in the caudal quarter of this nucleus, ipsilateral to the ablation. When adult, each rat received a retinal lesion in the eye ipsilateral to the ablation at varying locations along the temporal crescent and the brains were subsequently processed for anterograde degeneration. The topographic relationship between the retina's temporal crescent and the ipsilateral dorsal lateral geniculate nucleus appeared normal in the rostral three quarters of the nucleus, but an aberrant projection from the far temporal retina (the upper temporal crescent) was demonstrated in the caudal quarter of the nucleus, residing dorsolaterally beneath the optic tract. This location within the dorsal lateral geniculate nucleus normally receives its retinal input from the contralateral temporal retina at reduced eccentricity. As these two retinal regions are likely to be binocularly conjugate, it is proposed that these rearrangements in retinal terminal fields following early collicular ablation produce an ocularly aberrant yet visuotopically appropriate retinogeniculate projection.     

Ventral lateral geniculate nucleus projects to the dorsal lateral geniculate nucleus in the cat

The lamina C3 of the dorsal lateral geniculate nucleus of the cat does not receive retinal projections but instead receives visual information from the small subpopulation of W-type ganglion cells via the upper substratum of the stratum griseum superficiale of the superior colliculus. We herein report a projection from the lateral division of the ventral lateral geniculate nucleus into the lamina C3 of the dorsal lateral geniculate nucleus. As the lateral division receives projections from the contralateral retina and the ipsilateral upper stratum griseum superficiale of the superior colliculus, we suggest that these regions make up a small cell type W-cell neuronal network that provides visual information to layer I of the striate cortex via the lamina C3.     

Effects of very early monocular and binocular enucleation on primary visual centers in the tammar wallaby (Macropus eugenii)

The role of retinal afferents and their binocular interactions in the development of mammalian primary visual centers has been studied in the marsupial wallaby. Monocular and binocular enucleation was performed prior to any retinal innervation of the visual centers. After monocular enucleation retinal projections were traced by horseradish peroxidase histochemistry and compared with those in normal animals and those during development. The topography of retinal projections to the superior colliculus and the dorsal lateral geniculate nucleus after monocular enucleation was determined by making retinal lesions and tracing the remaining projections with horseradish peroxidase. The position and nature of the filling defects in terminal label were compared with controls with similarly placed lesions. The superior colliculus and dorsal lateral geniculate nucleus ipsilateral to the remaining eye were shrunken. Projections to the ipsilateral superior colliculus, ipsilateral accessory optic nuclei, and ipsilateral suprachiasmatic nucleus, although enlarged, never approached the density contralaterally, as was also the case during normal development. The expanded projection in the ipsilateral superior colliculus came primarily from temporal and ventral retina. In the dorsal lateral geniculate nucleus, terminal bands and cellular laminae, although not identical to normal, did develop. During normal development overlap of afferents from the two eyes occurs in the binocular region. The decrease in volume of the nucleus ipsilateral to the remaining eye after monocular enucleation suggests that the monocular region disappears in the absence of appropriate input and the binocular region survives. Contralaterally there was no decrease in volume, compatible with this idea. The topography of retinal projections supports this interpretation. It was normal contralaterally while ipsilaterally it was appropriate for the normal binocular region. There was an expansion of the projection along the lines of projection in what would normally be binocular regions of the nucleus, where retinal afferents failed to segregate in the absence of binocular competition. After binocular enucleation the alpha and beta segments of the dorsal lateral geniculate nucleus were still recognizable but cell-sparse zones were absent, as was the characteristic orientation of primary dendrites of geniculocortical cells. There are rigid developmental constraints operating on the innervation of territory by retinal afferents from the two eyes, and many features of the mature pattern arise without binocular interactions during development.     

Experimental anatomic study of the topography of the retinofugal projections in Discoglossus pictus (Amphibia, Anura). I. The thalamus: re-examination of the contralateral retinotopy and the origin of the uncrossed optic fibers

The topography of retinofugal projections on the thalamus of an Anuran Amphibia was studied by Fink-Heimer technique. After small lesions in retina, degeneration was found in all contralateral neuropils and in one or several ipsilateral neuropils according to retinal location: A lesion in temporal retina is followed by large axonal degeneration in neuropil of Bellonci; a dorsal lesion invaded whole geniculate neuropil while within it, contralateral ventral and nasal projections are scarce. Ipsilaterally, excepted for the nasal retina, all territories project to anterior neuropils; but, only temporal fibres ended in ipsilateral pretectal neuropil. The incinate neuropil receives bilateral projections from the whole retina. This study shows that, rather than a strict segregation, an overlap of quadratic projections into the same neuropil and a divisional repartition of these projections between the two anterior neuropils seem to be the rule.     

Retinal projections in the native cat, Dasyurus viverrinus.

Retinal projections were examined in the native cat, Dasyurus viverrinus using Fink-Heimer material and autoradiography. We found six regions in the brain which receive retinal projections. These are (1) the dorsal lateral geniculate nucleus (2) the ventral lateral geniculate nucleus (3) the lateral posterior nucleus (4) the pretectum (5) the superior colliculus, and (6) the accessory optic system. We did not examine the hypothalamus. The accessory optic system and the lateral posterior nucleus receive a contralateral retinal projection only and the other four regions receive a bilateral retinal projection. There is extensive binocular overlap in the dorsal lateral geniculate nucleus. On the side contralateral to an eye injection of 3H leucine our autoradiographs show four contralateral layers which fill most of the nucleus. Three of these layers, 3, 4 and 5, also receive input from the opsilateral eye. Layer 1 which lies adjacent to the optic tract receives only contralateral retinal input. Layer 2 receives a direct retinal input only from the ipsilateral eye. The ipsilateral projection to the dorsal lateral geniculate nucleus forms a fairly continuous patch which is not divided into separate layers. The ipsilateral retinal input is located in the dorsal part of the lateral geniculate nucleus. The ventral quarter of the nucleus only receives a contralateral retinal input and therefore represents the monocular part of the visual field.     

Organization of retinogeniculate projections in turtles of the genera Pseudemys and Chrysemys

Organization of retinal projections to the dorsal lateral geniculate complex in turtles has been studied by means of light and electron microscopic axon tracing techniques. Orthograde degeneration studies with Fink-Heimer methods following restricted retinal lesions show the entire retina has a topologically organized projection to the contralateral dorsal lateral geniculate complex. The nasotemporal axis of the retina projects along the rostrocaudal axis of the geniculate complex; the dorsoventral axis of the retina projects along the dorsoventral axis of the geniculate complex. The projection to the ipsilateral dorsal lateral geniculate complex originates from the ventral, temporal and nasal edges of the retina. The nasotemporal axis of the ipsilateral retina projects along the rostrocaudal axis of the geniculate complex. It was not possible to determine the orientation of the dorsoventral axis of the ipsilateral retina on the geniculate complex. Light microscopic autoradiographic tracing experiments and electron microscopic degeneration experiments show the retinogeniculate projection has a laminar organization. Retinogeniculate terminals are found in both the neuropile and cell plate throughout all three subnuclei of the dorsal lateral geniculate complex but have a distinctive distribution in each subnucleus. In the subnucleus ovalis, they are frequent in both the neuropile and cell plate which forms the rostral pole of the complex. In the dorsal subnucleus, they are most prevalent in the outer part of the neuropile layer, less frequent in the inner part of the neuropile, and rare in the cell plate. In the ventral subnucleus, they are frequent in the outer part of the neuropile but are also common in the inner part of the neuropile and cell plate. These observations point to several principles of geniculate organization in turtles. First, the complex receives projections from the entire contralateral retina and a segment of the ipsilateral retina. It thus has monocular and binocular segments that together receive a topologically organized representation of the binocular visual space and the contralateral monocular visual space. Second, the three geniculate subnuclei receive information from different, specialized regions of the retina and visual space. Subnucleus ovalis receives information from the frontal binocular visual field. The ventral subnucleus receives information from the caudal binocular field. The dorsal subnucleus receives input from the contralateral monocular field. Third, there is a lamination of retinal inputs in the geniculate complex which differs in character within the three subnuclei.(ABSTRACT TRUNCATED AT 400 WORDS)     

Alterations of the crossed parabigeminotectal projection induced by neonatal eye removal in rats

Projections of the parabigeminal nucleus to the contralateral superior colliculus and dorsal lateral geniculate nucleus were examined in normal adult pigmented rats and in adult rats from which one or both eyes had been removed at birth. In normal rats the crossed parabigeminotectal projection is restricted to the superficial layers in anterior and medial areas of colliculus, regions innervated also by the lower temporal portion of the ipsilateral retina. In unilaterally enucleated animals the crossed parabigeminotectal projection to the “denervated” colliculus is expanded, as is the retinal projection from the ipsilateral eye. In addition, there is acrossed parabigeminal projection to the “denervated” dorsal lateral geniculate nucleus in these rats. In bilaterally enucleated animals the parabigeminotectal projection is expanded, but not as greatly as in unilateral enucleation cases; there is a crossed parabigeminothalamic projectionin these animals as well. The corresponding termination patterns of the contralateral parabigeminal nucleus and the ipsilateral retina in the normal superior colliculus mayindicate a functional and/or developmental interdependence between the projections from these two regions. The existence of an expanded parabigeminotectal projection in bilaterally enucleated rats shows that a sustained ipsilateral retinotectal projection is not necessary for the establishment of a crossed parabigeminotectal projection, but points to the possibility that ipsilateral retinal input may constrain the parabigeminal projection to terminate within certain boundaries. The even greater expansion of the projection from the parabigeminal nucleus to the colliculus which receives an expanded projection from the ipsilateral retina of unilaterally enucleated rats suggests that the functional organization of the ipsilateral retinotectal projection may be capable of restricting the size of the terminal field of the crossed parabigeminotectal projection.     

An autoradiographic study of the efferent connections of the superior colliculus in the cat

The differential projections of the three main cellular strata of the superior colliculus have been examined in the cat by the autoradiographic method. The stratum griseum superficiale projects caudally to the parabigeminal nucleus and rostrally to several known visual centers: the nucleus of the optic tract and the olivary pretectal nucleus in the pretectum; the deepest C laminae of the dorsal lateral geniculate nucleus; the large-celled part of the ventral lateral geniculate nucleus; the posteromedial, large-celled part of the lateral posterior nucleus of the thalamus. Several of these projections are topographically organized. The stratum griseum profundum gives rise to most of the descending projections of the superior colliculus. Ipsilateral projections pass to both the dorsolateral and lateral divisions of the pontine nuclei, the cuneiform nucleus, and the raphe nuclei, and to extensive parts of the brainstem reticular formation: the tegmental reticular nucleus, and the paralemniscal, lateral, magnocellular, and gigantocellular tegmental fields. Contralateral projections descending in the predorsal bundle pass to the medial parts of the tegmental reticular nucleus and of some of the tegmental fields, the dorsal part of the medial accessory nucleus of the inferior olivary complex, and to the ventral horn of the cervical spinal cord. Ascending projections of the stratum griseum profundum terminate in several nuclei of the pretectum, the magnocellular nucleus of the medial geniculate complex and several intralaminar nuclei of the thalamus, and in the fields of Forel and zona incerta in the subthalamus. The strata grisea profundum and intermediale each have projections to homotopic areas of the contralateral superior colliculus, to the pretectum, and to the central lateral and suprageniculate nuclei of the thalamus. However, the stratum griseum intermediale has few or no descending projections.     

Autoradiographic study of the retinal projections in the Chinese pangolin, Manis pentadactyla

Autoradiography was used to investigate the optic system of the Chinese pangolin, Manis pentadactyla. The pattern of retinal projections in the Chinese pangolin is similar to that described in other mammals. Each retina projects bilaterally to the suprachiasmatic nucleus, dorsal and ventral lateral geniculate nuclei, pretectal area, and superior colliculus (SC). Only contralateral projections are found to the medial, lateral, and dorsal accessory optic nuclei. The large lateral nucleus receives a dense projection from the retina and forms a compact mass on the dorsolateral area of the cerebral peduncle. The lamination of the SC could not be clearly demonstrated in the brain of the Chinese pangolin.     

The projection of optic fibers to the visual centers in the cat

Retinal projections to the thalamus and midbrain were studied by the Nauta-Laidlaw technique. After unilateral enucleation, degeneration was found in five areas: dorsal lateral geniculate nucleus (LGN_d, thalamus), ventral lateral geniculate nucleus (LGN_v, subthalamus), nucleus of optic tract (NOT, pretectum), superior colliculus (S.C., tectum), and accessory optic nuclei (AON, tegmentum). Degeneration after focal lesions in nasal retina made by photocoagulation is found in four areas of the contralateral LGN_d: Laminae A and B, nucleus interlaminaris centralis (NIC), and the medial part of nucleus interlaminaris medialis (NIM). Temporal lesions lead to degeneration ipsilaterally in three areas: lamina A₁, NIC, and the lateral part of NIM. Central lesions are followed by bilateral projections; the area of degeneration on each side is greater than that of peripheral lesions of comparable size. Focal degeneration is found in NIM after either central or peripheral retinal lesions. Each retina projects to both NOT and both S.C. In NOT, degeneration following nasal lesions is strictly contralateral while after temporal lesions is mainly ipsilateral. A bilateral projection of temporal retina to NOT constitutes an exception to theories of chiasmal decussation. Both nasal and temporal projections to AON are wholly crossed, thus constituting another example of projection of temporal retina to the contralateral side, Nasal fibers project contralaterally and nearly all temporal fibers ipsilaterally to S.C.     

Retinal ganglion cell size groups projecting to the superior colliculus and the dorsal lateral geniculate nucleus in the North American opossum

The retinal ganglion cells projecting to the superior colliculus (SC) and dorsal lateral geniculate nucleus (LGNd) of the North American opossum (Didelphis virginiana) were studied by using the retrograde transport of horseradish peroxidase (HRP). The four ganglion cell size groups recognized previously were found to project in systematically different ways. After injections of HRP into the superior colliculus, labeled cells were seen in nasal retina contralateral to the injection and in temporal retina both ipsilateral and contralateral to the injection. In contralateral nasal retina cells of all size classes were labeled, while in contralateral temporal retina small (8-14 micrometers diameter), small-medium (15-19 micrometers diameter), and large (greater than 24 micrometers diameter) cells were labeled but few, if any, large-medium (20-24 micrometers diameter) cells were labeled. In ipsilateral temporal retina, soma size groups labeled included small-medium, large-medium, and large cells, but very few small cells. A nasal-temporal difference in the soma size of ganglion cells projecting to the SC was found: Labeled cells in temporal retina were 1.7-4.2 micrometers larger than their counterparts in nasal retina. Following injection of HRP into the LGNd, label was seen in contralateral nasal and ipsilateral temporal retina with no label seen in contralateral temporal retina. The labeled cells were small-medium, large-medium, and large. No small ganglion cells were labeled from the LGNd. A small nasal-temporal soma size difference in retinal ganglion cells projecting to the LGNd was seen: labeled cells in temporal retina were 1.0-2.1 micrometers larger than in nasal. It is concluded that all four ganglion cell size groups in the opossum project to the SC, but that only the three largest project to the LGNd.     

Topographic organization of the projections from cortical areas 17, 18, and 19 onto the thalamus,pretectum and superior colliculus in the cat

The distribution of cortical projections from areas 17, 18, and 19 to the lateral thalamus, pretectum, and superior colliculus was investigated with the autoradiographic tracing method. Cortical areas 17, 18 and 19 were demonstrated to project retinotopically and in register upon the dorsal lateral geniculate nucleus, medial interlaminar nucleus, lateral zone of the lateral posterior complex, nucleus of the optic tract and superior colliculus. Area 19 was shown to project retinotopically upon the pulvinar nucleus. Clear retinotopic organization was not demonstrable in the projections of areas 17, 18 and 19 to the reticular complex of the thalamus and ventral lateral geniculate nucleus, or in the projection of area 19 to the anterior pretectal nucleus. The cortical projections were employed to define the retinotopic organization of the nucleus of the optic tract, pulvinar nucleus, and lateral zone of the lateral posterior complex. The cortical projections show the vertical meridian to be represented caudally, with the lower visual field represented laterally, and the upper visual field medially, within the nucleus of the optic tract. The projections of area 19 to the pulvinar nucleus demonstrate the lower visual field to be represented rostrally and the upper visual field caudally in this nucleus; the vertical meridian to be represented at the lateral border and the visual field periphery to be represented at the medial border of the pulvinar nucleus. Cortical projections to the lateral zone of the lateral posterior complex demonstrate the lower visual field to be represented rostrally and the upper visual field caudally; the vertical meridian to be represented at the medial limit and the visual field periphery at the lateral border of the termination zones. On the basis of the experimental findings, a new terminology is introduced for the feline lateral posterior complex. Divisions are proposed which correspond to zones with demonstrably distinct afferent input. The pulvinar nucleus is defined by the distribution of projections from area 19. Three flanking divisions are defined within the lateral posterior complex; a lateral division recipient of projections from area 17, 18 and 19, an interjacent division recipient of projections of the superficial layers of the superior colliculus, and a medial division flanking the tectorecipient zone medially.     

Shifting retinal maps in the development of the lateral geniculate nucleus

During development, the bilateral projections from each eye to subcortical visual structures in the mammal initially overlap throughout the majority of the dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) before retracting to their separate territories. It has been shown in the ferret that during this period the larger contralateral retinal projection to both the dLGN and SC is retinotopically organised. By making small retinal lesions, and then anterogradely labelling the remaining retinofugal pathway from one eye, this study demonstrates that on the day of birth there is a superficial region of the dLGN in which the retinotopic map cannot be demonstrated. This region may be the presumptive C laminae. Further, by making small lesions in the temporal retina it has been shown that the smaller ipsilateral projection is also retinotopically organised before it retracts. Large lesions confined to the nasal retina had no effect on the pattern of label in the ipsilateral dLGN. Consequently, the ipsilateral projection which fills the nucleus at this stage must arise from the temporal retina. Because of this, the process of segregation requires that the retinotopic maps from each eye shift in relation to one another, and the borders of the nucleus to form the adult pattern.     

Retinal projections in the ringtailed possum Pseudocheirus peregrinus.

The retinal projections in the ringtailed possum, Pseudocheirus peregrinus were determined using Fink-Heimer material and autoradiography. At least seven regions in the brain receive retinal projections. These are (1) the suprachiasmatic nucleus of the hypothalamus (2) the dorsal lateral geniculate nucleus (3) the ventral lateral geniculate nucleus (4) the lateral posterior nucleus (5) the pretectum (6) the superior colliculus, and (7) the accessory optic system. The accessory optic system and lateral posterior nucleus receive a contralateral retinal projection only and the other five regions receive a bilateral retinal projection. The dorsal lateral geniculate nucleus consists of two parts: an outer alpha division of closely packed cells and an inner beta division containing loosely scattered cells. There are no cell layers apparent within the alpha division in Nissl sections. The autoradiographs and Fink-Heimer material reveal four concealed laminae within the alpha division. Lamina 1, which is adjacent to the optic tract and lamina 3 receive a predominantly contralateral input. Laminae 2 and 4 receive a predominantly ipsilateral input. The beta segment contains a fifth lamina which receives contralateral retinal input.     

Subcortical projections to lateral geniculate and thalamic reticular nuclei in the hooded rat

Restricted injections either of horseradish peroxidase conjugated with wheat germ agglutinin, or of unconjugated horseradish peroxidase were made into hooded rats in order to distinguish subcortical sources of afferents to dorsal lateral geniculate nucleus from those to the adjacent visually responsive thalamic reticular nucleus, which modulates geniculate activity. Five “nonvisual” brainstem regions project to the dorsal lateral geniculate nucleus: mesencephalic reticular formation, dorsal raphe nucleus, periaqueductal gray matter, dorsal tegmental nucleus, and locus coeruleus. Projections are generally bilateral, but ipsilateral projections dominate. Of these regions, three also project ipsilaterally to the thalamic reticular nucleus: mesencephalic reticular formation, periaqueductal gray matter, and dorsal tegmental nucleus. Similar discrete injections of horseradish peroxidase into ventral lateral geniculate nucleus allowed a comparison of afferents to dorsal and ventral lateral geniculate nuclei. In addition to the five nonvisual brainstem regions which project to the dorsal division, the ventral lateral geniculate nucleus receives afferents from the perirubral reticular formation and the central gray matter at the thalamic level. The dorsal and ventral lateral geniculate nuclei receive substantially different afferents from subcortical visual centres. The dorsal division receives projections from superior colliculus, pretectum, and parabigeminal nucleus whereas the ventral division receives afferents from superior colliculus, additional pretectal nuclei, lateral terminal nucleus of the accessory optic system, and the contralateral ventral lateral geniculate nucleus.     

A retinotopic analysis of the central connections of the optic nerve in the frog

The possibility of retinotopic organization in the optic nerve projections to the contralateral and ipsilateral diencephalon was studied by means of partial retinal lesions and staining for terminal degeneration by the Fink-Heimer technique. A retinotopic pattern of projection was observed in the nucleus of Bellonci, the corpus geniculatum thalamicum and the posterior thalamic nucleus. The temporal quadrant of the retina, and, to a lesser extent, the ventral quadrant projected to the ipsilateral side as well as to the contralateral side. In each diencephalic region noted above, the temporal and dorsal quadrants of the retina were represented more posteriorly (posteroventrally), and ventral and nasal quadrants projected more anteriorly (anterodorsally). The areas of representation for the temporal and ventral quadrants were located superior (superoposterior) to those for the dorsal and nasal quadrants. In their overall configuration and orientation, the retino-diencephalic maps show mirror-image reversal with respect to the retino-tectal projection. Since, in their areal extent, both the retino-diencephalic maps and the retino-tectal map are approximately parallel to the ventricular surface, their mirror-image reversal appears to indicate a reversal in the polarity of developmental processes across the di-mesencephalic junction. The retinotopic organization within the optic tract in the diencephalon and tectum was also analyzed. In the optic tract, the quadrants of the retina are reassembled such that the dorsal and nasal quadrants are widely separated in, respectively, the ventral and dorsal edges of the tract; the temporal and ventral quadrants are systematically represented in intermediate levels in the tract, the temporal quadrant above the dorsal, and the ventral quadrant below the nasal. When the optic tract bifurcates to encircle the tectum, the fibers from the ventral and nasal quadrants enter the dorsomedial arm and the fibers from the temporal and dorsal quadrants enter the ventrolateral arm of the optic tract. The paths taken by optic fibers in traversing the tectum to reach their areas of termination were reconstructed. Many optic fibers show an alignment parallel to an anteroventral posterodorsal axis as they cross the surface of the tectum, but the OS vs IS characterization of the fibroarchitecture of the tectum appears to be an oversimplification.     

HRP study of the organization of auditory afferents ascending to central nucleus of inferior colliculus in cat

The ascending auditory projections to central nucleus of inferior colliculus its ventrolateral and dorsomedial subdivisions (IC_VI, and IC_DM) have been studied in cat using both pressure and electrophoretic injections of horseradish peroxidase (HRP). The results indicate that the predominant ascending projections to inferior colliculus orginate in (1) contralateral cochlear nucleus, (2) contralateral and ipsilateral lateral superior olive, (3) ipsilateral medial superior olive, (4) ipsilateral ventral nucleus of the lateral lemniscus, (5) ipsilateral and contralateral dorsal nucleus of the lateral lemniscus, and (6) contralateral inferior colliculus. In addition, ipsilateral cochlear nucleus, ipsilateral and contralateral intermediate nucleus of the lateral lemniscus, ipsilateral, and to a lesser extent contralateral, periolivary nuclei project to inferior colliculus. Of these nuclei, the lateral superior olive projects exclusively to IC_VL and ipsilateral cochlear nucleus and contralateral inferior colliculus project mostly, if not exclusively, to IC_DM. Many of these projections demonstrate a cochleotopic organization and frequently a nucleotopic organization as well. A cochleotopic organization of the projections is apparent for cochlear nucleus and superior olivary complex. A nucleotopic organization suggests that the heaviest terminations of contralateral inferior colliculus are medial and dorsal in inferior colliculus, of medial superior olive are dorsal and lateral, of superior olivary complex are rostral, of cochlear nucleus are caudal, and of ventral nucleus of the lateral leminiscus are caudal.     

Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the rat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (VLG) are ventral thalamic derivatives within the lateral geniculate complex. In this study, IGL and VLG efferent projections were compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the geniculate in four pathways. A dorsal pathway innervates the thalamic lateral dorsal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculohypothalamic tract to the suprachiasmatic nucleus and the anterior hypothalamus (IGL). A medial pathway innervates the zona incerta and dorsal hypothalamus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); and the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and subparaventricular zone (IGL). A caudal pathway projects medially to the posterior hypothalamic area and periaqueductal gray and caudally along the brachium of the superior colliculus to the medial pretectal area and the nucleus of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterior pretectal nucleus, the intermediate and deep gray of the superior colliculus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medial and lateral pontine gray, the parabrachial region, and the accessory inferior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives.     

Second order auditory pathways in the chimpanzee

Substantial portions of the dorsal, and almost the entire posteroventral and anteroventral (Av) cochlear nuclei were aspirated unilaterally in a chimpanzee. Axonal degeneration was studied by the Fink-Heimer method. The greatest amount of degeneration was followed medially from the region of Av into the lateral part of the trapezoid body. Degeneration also coursed around the superior surface of the restiform body and was traced into the dorsal and intermediate acoustic striae. Within the superior olivary complex, degeneration was distributed to: the ipsilateral lateral superior olive; laterally and medially oriented dendrites of the ipsilateral and contralateral medial superior olivary nuclei respectively (some periosomatic degeneration also was present bilaterally); the contralateral medial trapezoid nucleus; retro-olivary and preolivary cell groups bilaterally. Abundunt degeneration passed into the contralateral lateral lemniscus and was distributed largely to its ventral nucleus. The contralateral central nucleus of the inferior colliculus was a major site of termination of ascending second order auditory fibers. The caudal tip of the ipsilateral ventral nucleus of the lateral lemniscus received abundant degeneration, but this diminished rostrally. The ipsilateral inferior colliculus contained a moderate amount of degeneration. A fair number of degenerated second order auditory fibers ascended in the contralateral brachium of the inferior colliculus and were distributed both to the principal and magnocellular divisions of the medial geniculate body. This pathway appears to represent a phylogenetic advance in the brain of the great ape.