Connections of the pretectum in the cat.

The connections of the pretectal complex in the cat have been examined by anatomical methods which utilize the anterograde axonal transport of tritiated proteins or the retrograde axonal transport of the enzyme horseradish peroxidase. Following injections of tritiated amino acids into the eye, label can be seen in the contralateral and ipsilateral nucleus of the optic tract and olivary nucleus where it appears as two or three finger-like strips. Following large injections of tritiated amino acids into the pretectal complex transported label accumulates ipsilaterally in a region dorsolateral to the red nucleus, the central and pericentral divisions of the tegmental reticular nucleus, the intermediate layers of the superior colliculus, the nucleus of Darkschewitch, the thalamic reticular nucleus, zona incerta and fields of Forel, the central lateral nucleus, the pulvinar nucleus and the ventral lateral geniculate nucleus. Contralaterally label accumulates in the nucleus of the posterior commissure, the interstitial nucleus of Cajal, the anterior, posterior and medial pretectal nuclei, and the ventral lateral geniculate nucleus From smaller injections, more or less well confined to single nuclei, the following patterns of connections are demonstrated. The nucleus of the optic tract projects to the ipsilateral ventral lateral geniculate nucleus and pulvinar nucleus and to the contralateral nucleus of the posterior commissure. The anterior pretectal nucleus projects to the ipsilateral central lateral nucleus, the reticular nucleus, zona incerta, fields of Forel, the region dorsolateral to the red nucleus and to the contralateral anterior pretectal nucleus. The posterior pretectal nucleus seems to project only to the ipsilateral reticular nucleus and zona incerta. The central tegmental fields deep to the pretectum project to the tegmental reticular nucleus of the brainstem. When the injection involves the nucleus of the posterior commissure label is seen in the ipsilateral nucleus of Darkschewitch, and in the contralateral nucleus of the posterior commissure and interstitial nucleus of Cajal but no nucleus of the pretectum could be positively identified as projecting to any of the motor nuclei of cranial nerves III, IV, and VI. Following large injections of horseradish peroxidase into the pretectal complex, labeled cells are seen in the superficial layers of the ipsilateral superior colliculus, in the ipsilateral ventral lateral geniculate nucleus, reticular nucleus and zona incerta and in the contralateral anterior, medial and posterior pretectal nuclei, nucleus of the optic tract and ventral lateral geniculate nucleus.     

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.     

Visual projections to the pontine nuclei in the rabbit: orthograde and retrograde tracing studies with WGA-HRP

Visual projections to the pontine nuclei in the rabbit were examined by means of both orthograde and retrograde tracing of WGA-HRP. The tecto-pontine projection was examined following microinjections of WGA-HRP in the right superior colliculus. The projection to the pontine nuclei is strictly ipsilateral and terminates at middle and caudal levels of the pons. The projection is absent in rostral pontine nuclei. The strongest projection is to the dorsal border of the dorsolateral pontine nuclei and is the only projection seen when the primary injection site is confined to superficial laminae. When the primary injection site also includes intermediate and deep laminae, patches of labelled terminals are also seen within dorsolateral, lateral, peduncular, paramedian, and ventral pontine nuclei as well as in the contralateral nucleus reticularis tegmenti pontis. The striate corticopontine projection was also examined with orthograde tracing of WGA-HRP. The striate corticopontine projection is ipsilateral. Most labelled terminals were seen in dorsolateral and lateral pontine nuclei throughout the rostral half of pons with some additional terminal labelling in paramedian and peduncular nuclei. Labelled terminals were also seen in ventral pontine nuclei throughout the middle and caudal levels of the pons. In a retrograde tracing study, visual projections to the pontine nuclei were examined following microinjections of WGA-HRP into the pontine nuclei. Labelled cells were seen ipsilaterally in superficial and deep laminae of the superior colliculus and in layer V of striate and surrounding occipital cortex. The pontine nuclei also receive ipsilateral projections from the ventral lateral geniculate, the nucleus of the optic tract, anterior and posterior pretectal nuclei, and the dorsal and medial terminal nuclei of the accessory optic system. These pathways are potential sources of visual input to the cerebellum.     

The organization of the pretectal nuclei in the trout: a revision based on experimental holodogical studies

The neuronal tracer DiI was applied to different brain centers of the rainbow trout in order to study the connections of pretectal nuclei. Our results showed that some pretectal nuclei receive a direct projection from the contralateral retina: the parvocellular superficial pretectal nucleus, the central pretectal nucleus, the intermediate pretectal nucleus and the ventral accessory optic nucleus. In turn, the central pretectal, the intermediate pretectal and the ventral accessory optic nuclei, together with the paracommissural nucleus, project to the cerebellum and the torus longitudinalis. The magnocellular superficial pretectal nucleus does not receive retinal projections, but receives ipsilateral projections from the optic tectum and the mesencephalic tegmentum. In turn, it projects to the ipsilateral oculomotor nucleus and lateral nucleus of the valvula. The posterior pretectal nucleus and the parvocellular superficial pretectal nucleus receive afferents from the ipsilateral nucleus isthmi. The posterior pretectal nucleus projects to the inferior hypothalamic lobe. Our results reveal a conspicuous projection from the ipsilateral parvocellular superficial pretectal nucleus to the contralateral one and also to the contralateral posterior prectectal nucleus, not reported in previous experimental studies of teleosts. Pretectal centers appear to integrate visual/optic-related centers mainly with the hypothalamus and the cerebellum. The organization of the trout pretectum was compared with the pretectal organization patterns proposed in various teleosts.     

Retinal Projections to the Pretectum, Accessory Optic System and Superior Colliculus in Pigmented and Albino Ferrets

Retinal projections to the pretectal nuclei, accessory optic system and superior colliculus in pigmented and albino ferrets were studied using anterograde tracing techniques. Both Nissl- and myelin-stained material was used to identify the pretectal nuclei, nuclei of the accessory optic system and the layers of the superior colliculus. Following monocular injection of either horseradish peroxidase or rhodamine-B-isothiocyanate, four pretectal nuclei, including the nucleus of the optic tract, posterior pretectal nucleus, anterior pretectal nucleus and the olivary pretectal nucleus, could be identified to receive direct retinal input in both pigmented and albino strains. In the accessory optic system, retinal terminals were observed in the dorsal, lateral and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. The retinal projection to the superior colliculus was found to innervate the three superficial layers. The retinal projections to the pretectal nuclei and nuclei of the accessory optic system in the pigmented animals were bilateral, although the label was most dense contralateral to the injected eye. Ipsilateral retinal projections to the pretectal nuclei and nuclei of the accessory optic system appeared to be absent in albino ferrets, i.e. they were invisible with our methods. In both pigmented and albino ferrets retinal terminals in the contralateral superior colliculus densely innervated the three superficial layers. In both strains the ipsilateral projection appeared as clusters which were absent in rostral and caudal poles. In pigmented animals the ipsilateral projection was much denser and more extensive than in albinos. Following injection of retrograde tracers into the brainstem at the level of the dorsal cap of the inferior olive, retrogradely labelled neurons in the pretectum were found in the ipsilateral nucleus of the optic tract. Their somata overlapped mainly with scattered retinal terminals close to the pretectal surface and rarely or not all with the deeper prominent terminal clusters. In the accessory optic system, inferior olive projecting neurons were observed in all four ipsilateral nuclei and fully coincided with the retino-recipient zones. In the superior colliculus, retrogradely labelled neurons were found contralateral to the injection site in the deep layers.     

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.     

Retinal projections in gymnotid fishes

Retinal projections were studied in four species of gymnotid fishes, Gymnotus carapo, Hypopomus artedi, Eigenmannia virescens and Sternopygus sp. with the aid of cobalt or HRP labelling and autoradiographic techniques. The optic tract gives off a small branch, the axial optic tract and then, after crossing in the midline, splits into a dorsomedial, dorsal and ventral fascicle. E. virescens and Sternopygus sp. display in addition an accessory optic tract. In all four species retinal projections are bilateral; ipsilateral projections, however, are extremely sparse. In all four species, the retinal fibres terminate bilaterally in the suprachiasmatic nucleus, dorsolateral optic nucleus of the thalamus and the optic nucleus of the posterior commissure; a bilateral retinotectal projection was only found in E. virescens and G. carapo. Retinal projections are only contralateral to the ventromedial nucleus of the thalamus, the central pretectal nucleus and the accessory optic nucleus. The contralateral retinotectal fibres terminate in the stratum fibrosum and griseum superficiale, and in the stratum album centrale and stratum periventriculare. A small accessory optic tract and nucleus were detected in E. virescens and Sternopygus sp. but not in G. carapo and H. artedi. The results indicate that the visual system of gymnotid fish is as simple as that of mormyrids. The poor visibility in the environment where these animals live and the additional sensory system which these animals possess may explain the poor development of the visual system.     

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.     

Projections of the lateral terminal accessory optic nucleus of the common marmoset (Callithrix jacchus)

The connections of the lateral terminal nucleus (LTN) of the accessory optic system (AOS) of the marmoset monkey were studied with anterograde ³H-amino acid light autoradiography and horseradish peroxidase retrograde labeling techniques. Results show a first and largest LTN projection to the pretectal and AOS nuclei including the ipsilateral nucleus of the optic tract, dorsal terminal nucleus, and interstitial nucleus of the superior fasciculus (posterior fibers); smaller contralateral projections are to the olivary pretectal nucleus, dorsal terminal nucleus, and LTN. A second, mejor bundle produces moderate-to-heavy labeling in all ipsilateral, accessory oculornotor nuclei (nucleus of posterior commissure, interstitial nucleus of Cajal, nucleus of Darkschewitsch) and nucleus of Bechterew; some of the fibers are distributed above the caudal oculomotor complex within the supraoculornotor periaqueductal gray. A third projection is ipsilateral to the pontine and mesencephalic reticular formations, nucleus reticularis tegmenti pontis and basilar pontine complex (dorsolateral nucleus only), dorsal parts of the medial terminal accessory optic nucleus, ventral tegmental area of Tsai, and rostral interstitial nucleus of the medial longitudinal fasciculus. Lastly, there are two long descending bundles: (1) one travels within the medial longitudinal fasciculus to terminate in the dorsal cap (ipsilateral > > contralateral) and medial accessory olive (ipsilateral only) of the inferior olivary complex. (2) The second soon splits, sending axons within the ipsilateral and contralateral brachium conjunctivum and is distributed to the superior and medial vestibular nuclei. The present findings are in general agreement with the documented connections of LTN with brainstem oculomotor centers in other species. In addition, there are unique connections in marmoset monkey that may have developed to serve the more complex oculomotor behavior of nonhuman primates. © 1995 Wiley-Liss, Inc.     

Retinofugal and retinopetal projections in the green sunfish, Lepomis cyanellus

The retinofugal and retinopetal connections in the green sunfish were studied by autoradiographic and horseradish peroxidase methods. All retinofugal fibers decussate in the optic chiasm. Some fibers project to contralateral preoptic and hypothalamic nuclei while others recross to project to the comparable ipsilateral nuclei. Contralaterally, the medial optic tract projects to the periventricular thalamic and pretectal nuclei and, sparsely, to the rostral optic tectum. The dorsal optic tract projects to the parvocellular portion of the superficial pretectal nucleus, the central pretectal nucleus, nucleus corticalis, and the rostral portion of the optic tectum. The ventral optic tract primarily projects to the caudal portion of the optic tectum, giving off fibers in route to innervate various nuclei, including the parvocellular superficial pretectal nucleus and the dorsal and ventral accessory optic nuclei. The axial optic tract projects to the dorsal accessory optic nucleus, the central pretectal nucleus, and the caudal optic tectum. Retinal fibers reach the ipsilateral thalamus, pretectum and other sites via a redecussation through the posterior commissure. From outgroup analysis it is concluded that such redecussating fibers are an independently derived character within actinopterygians and are homoplasous to nondecussating ipsilateral retinal projections in other vertebrates. Neurons retrogradely labeled with horseradish peroxidase were found to form a rostrocaudal column from the olfactory bulb and nerve through the ventral telencephalon to caudal diencephalic levels along the medial aspect of the optic tract. It is possible that all these neurons consist of one population of migrated ganglion cells of the nervus terminalis.     

Retinopretectal and accessory optic projections of normal mice and the OKN-defective mutant mice beige, beige-J, and pearl

Retinal projections to the pretectal and terminal accessory optic nuclei were studied in normal wild-type mice and mutant mice with abnormal optokinetic nystagmus (OKN, Mangini, Vanable, Williams, and Pinto: J. Comp. Neurol. 241:191-209, '85). The mutants used were pearl, which exhibits an inverted OKN in response to stimulation of only the temporal retina, and beige and beige-J, which show inverted OKN in response to stimulation of only the temporal retina and, in addition, exhibit eye movements with a vertical component in response to horizontally moving, full-field stimuli. These projections were studied following intraocular injections of 3H-proline or horseradish peroxidase (HRP) with, respectively, light microscopic autoradiography or HRP histochemistry. In wild-type mice, strong contralateral retinal projections covered the entire nucleus of the optic tract, the anterior and posterior divisions of the olivary pretectal nucleus, and the posterior pretectal nucleus. Similar heavy contralateral projections were distributed over the dorsal and medial terminal nuclei of the accessory optic system. Also, terminals sparsely covered the entire neuropil of the contralateral lateral terminal nucleus in some but not all wild-type mice. The most prominent accessory optic input was to the medial terminal nucleus and was provided by the inferior fasciculus of the accessory optic tract. A typical mammalian superior fasciculus of the accessory optic system with anterior, middle, and posterior components was present. Ipsilateral label was found in anterior and posterior olivary pretectal nuclei in all of the wild-type animals, but was found inconsistently in the ipsilateral terminal accessory optic nuclei. The pattern of contralateral retinal projection to the nucleus of the optic tract and posterior pretectal nucleus in mutants was indistinguishable from that seen in the normal wild-type mice. However, retinal inputs to the ipsilateral anterior and posterior olivary pretectal nuclei were significantly reduced in pearl mutants and were exceedingly sparse in the beige and beige-J mutant mice, while the contralateral inputs to these nuclei were increased in a complementary fashion in the mutants. The labeling of the accessory optic input to the contralateral dorsal terminal nucleus appeared to be substantially reduced in all of the mutant mice. The size of the principal accessory optic fascicle, the inferior fasciculus, was significantly smaller in beige, beige-J, and pearl mice; this reduction was greater in the beige and beige-J than in the pearl mice.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projection from the accommodation-related area in the superior colliculus of the cat

Our previous study has indicated that accommodative responses can be evoked with weak currents applied to a circumscribed area of the superior colliculus in the cat. We investigated efferent projections from this area with biocytin in the present study. The accommodation area in the superior colliculus was identified by systematic microstimulation in each of five anesthetized cats. Accommodative responses were detected by an infrared optometer. After mapping the superior colliculus, biocytin was injected through a glass micropipette into the accommodation area, where accommodative responses were elicited with low-intensity microstimulation. In addition, accommodative responses to stimulation of the superior colliculus were compared before and after an injection of muscimol, an agonist of inhibitory neurotransmitter, into the pretectum. Following the injection of biocytin, in the ascending projections, labeled terminals were seen mainly in the caudal portion of the nucleus of the optic tract, the nucleus of the posterior commissure, the posterior pretectal nucleus, the olivary pretectal nucleus, the mesencephalic reticular formation at the level of the oculomotor nucleus, and the lateral posterior nucleus of the thalamus on the ipsilateral side. Less dense terminals were seen in the anterior pretectal nucleus, the zona incerta, and the centromedian nucleus of the thalamus. In the descending projections, labeled terminals were observed mainly in the paramedian pontine reticular formation, the nucleus raphe interpositus, and the dorsomedial portion of the nucleus reticularis tegmenti pontis on the contralateral side. Less dense terminals were also seen in the nucleus of the brachium of the inferior colliculus, the cuneiform nucleus, the medial part of the paralemniscal tegmental field, and the dorsolateral division of the pontine nuclei on the ipsilateral side. Following the injection of muscimol into the pretectum, including the nucleus of the optic tract, the posterior pretectal nucleus, and the nucleus of the posterior commissure, accommodative responses evoked by microstimulation of the superior colliculus were reduced to 33–55% of the value before the injections. These findings suggest that the accommodation area in the superior colliculus projects to the oculomotor nucleus through the ipsilateral pretectal area, especially the nucleus of the optic tract, the nucleus of posterior commissure, and the posterior pretectal nucleus, and also projects to the pupilloconstriction area (the olivary pretectal nucleus), the vergence-related area (the mesencephalic reticular formation), and the active visual fixation-related area (the nucleus raphe interpositus). © 1996 Wiley-Liss, Inc.     

Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

The tecto-thalamic connections in the brain of the rabbit

We have correlated the tectal connections and cytoarchitecture of regions in the rabbit's midbrain and caudal thalamus. The inferior colliculus projects ipsilaterally to the central gray, superior colliculus, and via the brachium of the inferior colliculus to its interstitial nucleus and the parabrachial region of the midbrain tegmentum. From the brachium, fibers fan out to the principal and internal divisions of the medial geniculate. A smaller contralateral pathway sweeps into the contralateral inferior colliculus and in its brachium to the interstitial nucleus, the parabrachial region, and the internal and principal divisions of the medial geniculate. The superior collicular projection is mainly ipsilateral. Medially, fibers terminate in the central gray and pretectal area. Laterally, fibers ascend in the superior brachium to parabrachial region, suprageniculate pretectal nucleus, posterior complex, caudodorsal internal division of the medial geniculate, and to a discrete part of the ventral nucleus of later geniculate. A component of the commissure of Gudden originates in the rostral superior colliculus and terminates in the contralateral ventral lateral geniculate, posterior complex, pretectal area and midbrain tegmentum. Interconnections between the colliculi and overlap of their projections in the parabrachial region, the central gray, and the internal division of the medial geniculate are described.     

The Termination of Retinofugal Fibers in the Crab-Eating Monkey (Macaca irus): an Autoradiographic Study

Abstract: In the crab-eating monkey ( Macaca irus ), the subcortical projection from the retina was studied by means of the autoradiographic method. Retinofugal fibers terminated bilaterally in the suprachiasmatic, pregeniculate, lateral geniculate, olivary, pretectal, lateral terminal and dorsal terminal nuclei and the superior colliculi. In the ipsilateral lateral geniculate nucleus, the retinofugal fibers terminated on laminae 0, 2, 3, and 5; in the contralateral nucleus, they ended in laminae 1, 4, and 6. In the superior colliculi, retinal terminals were aggregated in the stratum griseum superficiale. The ipsilateral suprachiasmatic nucleus showed a heavier labeling than the contralateral nucleus.     

Retinal projections in lamprey (Lampetra fluviatilis)

Visual projections in lamprey were investigated using two methods,--one by revealing transport of horseradish peroxidase, and the other by silver impregnation of degenerating axons and terminals after enucleation of the eye. Both methods produced similar results. The chiasm showed incomplete crossing of retinal fibres, the major part of which, as an optic tract, proceed along the contralateral thalamus up to the entry into the optic tectum, while the smaller part takes the same course on the ipsilateral side. Besides, from the posterior part of the optic chiasm an axial optic tract branches off, which proceeds through the central part of the contralateral thalamus up to the pretectal nucleus, individual fibres of which enter the central grey layer of the optic tectum. On the contralateral side, the visual projections are localized in the lateral geniculate body, pretectal nucleus, in the three upper layers of the optic tectum, in the ventrolateral area of the optic tectum and as solitary diffuse projections in the mesencephalic tegmentum. Innervation of thalamic and pretectal nuclei are realized by two tracts--the tractus opticus proper, and tractus opticus axialis. On the ipsilateral side visual projections, excepting the optic tract, are scarce and in the thalamus appear as small areas of the lateral geniculate body and pretectum adjacent to the optic tract. Solitary visual projections were found in two upper layers of the rostral optic tectum and in larger numbers in the 3rd and 4th layers of the caudal part and in ventrolateral area of the optic tectum. Projections in mesencephalic tegmentum were single. Diffuse visual projections in the lateral part of hypothalamus could be revealed only by the silver impregnation method. Using the peroxidase method two types of cells were observed in mesencephalic tegmentum where, possibly, the centrifugal fibres proceeding to retina, originate. A comparison is made of central visual projections in lampreys and other representatives of nonmammalian vertebrates.     

Tectal projections of an infrared sensitive snake,Crotalus viridis

Crotaline snakes have detectors for infrared radiation and this information is projected to the optic tectum in a spatiotopic manner. The tectal projections were examined in Crotalus viridis with the use of silver methods for degenerating fibers and the autoradiographic and horseradish peroxidase tracing methods. Large lesions included all of the tectal layers but not the underlying structures. Projections to the thalamus include a sparse input to the ipsilateral ventral and dorsal lateral geniculate nuclei, the ventromedial nucleus, and nucleus lentiformis thalami. Nucleus rotundus was not detected. The projections to the pretectal nuclei are primarily ipsilateral to the nucleus lentiformis mesencehali and pretectal nucleus. At the level of the mesencephalon, tectal efferents are bilateral to nucleus profundus mesencephali and the tegmentum. There is minimal input to the contralateral deep tectal layers. There are ispilateral terminations in a nucleus identified as the posterolateral tegmental nucleus. Descending fibers include the two major tracts—the ventral tectobulbar tract that terminates in the ipsilateral lateral reticular formation and the predorsal bundle that distributes throughout the contralateral medial reticular formation. Two small descending tracts were noted—the intermediate and dorsal tectobulbar tracts. All of these descending tracts appear to terminate by the time they reach the caudal medulla. After superficial lesions terminals could be found in the ventral lateral geniculate nucleus, the nucleus profundus mesencephali, and the posterolateral tegmental nucleus; the two major descending tracts contained degenerated fibers as well. The areas receiving tectal input in Crotalus were compared to those of other reptiles and discussed.     

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.