Connections of the tectum of the rattlesnake Crotalus viridis: an HRP study.

We have studied the connections of the tectum of the rattlesnake by tectal application of horseradish peroxidase. The tectum receives bilateral input from nucleus lentiformis mesencephali, posterolateral tegmental nuclei, anterior tegmental nuclei and periventricular nuclei; ipsilateral input from nucleus geniculatus pretectalis, and lateral geniculate nucleus pars dorsalis; and contralateral input from dorso-lateral posterior tegmental nucleus and the previously undescribed nucleus reticularis caloris (RC). RC is located on the ventro-lateral surface of the medulla and consists of large cells 25--45 micrometer in diameter. Efferent projections from the tectum can be traced to the ipsilateral nucleus lentiformis mesencephali, the ipsilateral lateral geniculate region, anterior tegmental region and a wide bilateral area of the neuropil of the ventral tegmentum and ventral medualla. We have not found any direct tectal projections from the sensory trigeminal nuclei including the nucleus of the lateral descending trigeminal tract (LTTD). We suggest that in the rattlesnake, RC is the intermediate link connecting LTTD to the tectum.     

Projections of the retinorecipient pretectal nuclei in the pigeon (Columba livia)

We have used anterograde autoradiographic and retrograde HRP techniques to investigate the efferent connections of the retinorecipient pretectal nuclei in the pigeon. In the accompanying paper we identified these nuclei in the pigeon as the nucleus lentiformis mesencephali--pars lateralis and pars medialis, the tectal gray, the area pretectalis, and pretectalis diffusus. Although there are reports of a few of the projections of these nuclei, they had not previously been the subject of a detailed study. We found that different cell types in the lentiformis mesencephali, pars medialis and the lentiformis mesencephali, pars lateralis have descending projections to different targets. These targets include the inferior olive, the cerebellum, the lateral pontine nucleus, the nucleus papillioformis, the nucleus of the basal optic root, the nucleus mesencephalicus profundus, pars ventralis, the nucleus principalis precommissuralis, and the stratum cellulare externum. We found that a few cells in the lentiformis mesencephali project to the medial pontine nucleus, but that a much heavier projection arises from the nucleus laminaris precommissuralis, which is medial to the nucleus lentiformis mesencephali, pars medialis. The tectal gray has predominantly ascending projections to the diencephalon. The nuclei that it projects to are the nucleus intercalatus thalami, the nucleus of the ventral supraoptic decussation, the nucleus posteroventralis, the ventral lateral geniculate nucleus, the nucleus dorsolateralis medialis, and the nucleus dorsolateralis anterior. The tectal gray also projects topographically to layers 4 and 8-13 of the optic tectum. Area pretectalis has both ascending and descending projections. It has ipsilateral ascending projections to the nucleus dorsolateralis anterior, pars magnocellularis, the nucleus lateralis anterior, and the nucleus ventrolateralis thalami. It has ipsilateral descending projections to the central gray, the nucleus of the basal optic root, pars dorsalis, the lateral pontine nucleus, and the deep layers of the optic tectum. It has contralateral projections to the area pretectalis, the nucleus Campi Foreli, the interstitial nucleus of Cajal, the nucleus of Darkschewitsch, the cerebellum, and the Edinger-Westphal nucleus. The efferent projections of pretectalis diffusus are limited. It projects contralaterally to the pretectalis diffusus, and ipsilaterally to the nucleus of the ventral supraoptic decussation, the lateral pons, and the cerebellum.4     

Efferent tectal pathways of the opossum (Didelphis virginiana)

Efferent tectal pathways have been determined for the opossum, Didelphis virginiana, by employing the Nauta-Gygax technique ('54) on animals with tectal lesions of varying sizes. The superior colliculus projected tectothalamic fascicles to the suprageniculate nucleus, the central nucleus of the medial geniculate body, the lateral posterior thalamus, the pretectal nucleus, the ventral lateral geniculate nucleus, the fields of Forel and zona incerta, the parafascicular complex, the paracentral thalamic nucleus and in some cases to restricted areas of the anterior thalamus. Degenerating fibers from superior collicular lesions showed profuse distribution to the deeper layers of the superior colliculus on both sides and to the midbrain tegmentum, but only minimally to the red nucleus and substantia nigra. Fibers of tectal origin did not distribute to the motor nuclei of the oculomotor or trochlear nerves. At pontine levels, efferent fascicles from the superior colliculus were present as an ipsilateral tectopontine and tectobulbar tract and as a crossed predorsal bundle. The tectopontine tract ended mostly within the lateral and ventral basal pontine nuclei, whereas the ipsilateral tectobulbar tract distributed to certain specific areas of the reticular formation throughout the pons and medulla, minimally to the most medial portion of the motor nucleus of the facial nerve and to the nucleus of the inferior olive. The predorsal tract contributed fascicles to certain nuclei of the pontine raphe, extensively to the medial reticular formation of the pons, to the central and ventral motor tegmental nuclei of the reticular formation within the pons and medulla, to the paraabducens region, minimally to cells within restricted portions of the motor nucleus of the facial nerve, to certail specific regions of the caudal medulla and to the cervical cord as far caudally as the fourth segment. The tectospinal fascicles were few but some ended related to the spinal accessory nucleus and the ventral medial nucleus of the ventral horn. Lesions of the inferior colliculus resulted in degenerating fibers which distributed rostrally to the rostral nucleus of the lateral lemniscus and parabrachial region, to the suprageniculate nucleus, the parabigeminal nucleus and to the central nucleus of the medial geniculate body. The inferior colliculus also contributed fibers to the ipsilateral tectopontine and tectobulbar tracts. The latter bundle was traced as far caudally as the medulla and may arise from cells of the superior colliculus which are situated dorsal to the nucleus of the inferior colliculus.     

Retinal recipient nuclei in the painted turtle,Chrysemys picta: An autoradiographic and HRP study

Retinofugal pathways in the painted turtle were examined with autoradiographic and HRP methods. The majority of the retinal fibers decussate at the optic chiasm and course caudally to terminate in 12 regions of the diencephalon and mesencephalon. The pars dorsalis of the lateral geniculate nucleus is the densest target in the thalamus. Two nuclei dorsal to pars dorsalis—the dorsal optic and dorsal central nuclei—receive optic input. Three nuclei ventral to pars dorsalis are retinal targets—the ventral geniculate nucleus, nucleus ventrolateralis pars dorsalis, and nucleus ventrolateralis pars ventralis. Contralateral fibers course through the pretectum where they terminate in nucleus geniculatis pretectalis, nucleus lentiformis mesencephali, nucleus posterodorsalis, and the external pretectal nucleus. Retinal fibers also terminate within the superficial zone of the optic tectum. HRP material demonstrates three optic fiber layers—laminae 9, 12, and 14. Optic fibers leave the main optic tract as a distinct accessory tegmental optic pathway and terminate in the basal optic nucleus. Ipsilateral retinal terminals occur in a pars dorsalis and a pars ventralis of the lateral geniculate nucleus, the dorsal optic nucleus, nucleus posterodorsalis, the basal optic nucleus, and in laminae 9 and 12 of the optic tectum. Rostrally, the ipsilateral tectal fibers occupy two zones along the medial and lateral tectal roof; these zones converge caudally and are continuous along the caudal wall of the tectum.     

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.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways

Extracellular, iontophoretic injections of horseradish peroxidase were used to anterogradely fill axons efferent from the optic tectum in garter snakes. The tectal efferent pathways consist of six axon types with distinct projections and terminal morphologies. Tectogeniculate axons pass into the diencephalon via the optic tract, bearing collaterals that form spatially restricted, rodlike arbors in the pretectum, the ventral lateral geniculate nucleus, and the ventrolateral nucleus. Tectoisthmi axons exit the tectum as a thin-caliber component of the ventral tectobulbar tract. They form spatially restricted, spherical arbors within nucleus isthmi. Tectoisthmobulbar axons also give rise to small, spherical arbors within nucleus isthmi, but the parent axons continue caudally into the pontine and medullary reticular formation issuing many short collateral branches. Tectorotundal axons reach the diencephalon via the tectothalamic tract and give rise to fine terminal collaterals in the nucleus of the tectothalamic tract ipsilaterally and in nucleus rotundus bilaterally. Single axons form sheetlike terminal fields that span the rostrocaudal extent of nucleus rotundus. Ipsilateral tectobulbar axons descend into the midbrain tegmentum where they issue several thick collaterals that terminate widely throughout the nucleus lateralis profundus mesencephali. The parent axon continues caudally giving off several widely spreading collaterals within the pontine and medullary reticular formation. Crossed tectobulbar axons enter the dorsal tectobulbar tract and cross the midline to form the predorsal bundle. Single axons give rise to terminal collaterals in the nucleus lateralis profundus mesencephali bilaterally, the contralateral pontine and medullary reticular formation, and the intermediate gray of the cervical spinal cord.     

Tectal efferents in the banded water snake,Natrix sipedon

Visual information reaches the dorsal thalamus by two distinct routes in most reptiles. Retinal efferents terminate directly in the dorsal lateral geniculate nucleus (DLGN). Retinal information is also channeled indirectly through the tectum to nucleus rotundus. Retinal projections to DLGN and tectum are also well esablished in snakes, but the status of the tecto-rotundal link of the indirect visual pathway is uncertain. Thus, tectal efferents were studied with Fink-Heimer methods in banded water snakes (Natrix sipedon). The tectum gives rise to crossed and uncrossed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and osterior commissures. tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mesencephali, lateral habenular nuclei, and posterodorsal nuclei. Ascending projections reach the ventral lateral geniculate and suprapeduncular nuclei. there is a diffuse projection to the central part of the caudal thalamus and a dense, bilaternal projection to the DLGN. These results indicate that the relation of the tectum to the dorsal thalamus is different in snakes than in other reptiles. Nucleus rotundus is either absent or poorly differentiated and there is a strong convergence of the direct and indirect visual pathways at DLGN.     

Efferent projections of the superior colliculus in the opossum

The Fink and Heimer technique was used to study the ascending and descending fiber degenerations resulting from unilateral electrolytic lesions in the superior colliculus of sixteen adult opossums. The ascending fiber degeneration left the colliculus by way of a small parabrachial bundle or with the brachium of the superior colliculus. The former bundle contributed degenerating terminals to the suprageniculate, parabrachial, and magnocellular medial geniculate nuclei and appeared to terminate in the lateral terminal nucleus of Hayhow (Hayhow, '66). The latter bundle contributed fibers to the pretectal and posterolateral nuclei as well as discrete projections to the ventral lateral geniculate nucleus and ventral thalamus. Descending degenerating fibers in the brainstem were destributed along three tracts: (1) a medial predorsal bundle, (2) an intermediate tectoreticular, and (3) a lateral tectopontine. These tracts were seen to terminate mainly in tegmental and reticular centers as well as in the lateral basilar nucleus of the pons. Degenerating fibers of the predorsal bundle were followed to lower medullary levels but could not be traced to spinal levels. The present findings are descussed in relation to data reported for more developed mammalian species.     

Connections of the bullfrog striatum: Efferent projections

Autoradiographic and degeneration techniques were used to describe striatal efferents in the bullfrog (Rana catesbeiana). Horseradish peroxidase (HRP) was then placed in the major terminal fields to reveal the striatal cells responsible for these projections. Except for the small ventral eminence of the lateral pallium immediately adjacent to the dorsal striatum, no pallial region receives a striatal projection. Most striatal efferents descend in the lateral forebrain bundle (LFB), passing through the anterior entopedun-cular nucleus, where one large fascicle decussates in the anterior commissure and innervates the contralateral anterior entopeduncular nucleus and caudal ventral striatum. A smaller fascicle exits the LFB to terminate in the ipsilateral lateral amygdala. The remaining efferents continue caudal in the LFB through the posterior entopeduncular nucleus, with sparse projections to the ventral thalamus, the adjacent preoptic areas, and the posterior tuberculum leaving the bundle at various points. At pretectal levels, some efferents leave the LFB to run dorsally, through the caudal pole of the central thalamic nucleus and into the posterior division of the lateral nucleus and the lateral portion of the posterior thalamic nucleus. Efferents also continue caudal, through the superficial tegmental cell groups (nucleus profundus mesencephali and superficial isthmal reticular nucleus) before turning dorsomedially into the ventral anterodorsal, lateral anteroventral, and rostral pole of the posterodorsal tegmental fields. A small superficial projection continues to isthmal levels but cannot be traced beyond. Tegmental HRP injections retrogradely fill cells in the dorsal and ventral striatum as well as nucleus accumbens and the anterior entopeduncular nucleus. Pretectal HRP injections fill cells only in the caudal ventral striatum and anterior entopeduncular nucleus. Anterior entopeduncular nucleus HRP injections fill numerous cells in all striatal divisions, but some of this filling may be due to interrupted fibers of passage. Thus the anuran striatum, which receives its major input from thalamic nuclei relaying tectal and toral input, can in turn influence the midbrain roof via several disynaptic pathways: through the anterior entopeduncular nucleus, pretectum, and tegmentum, all of which project directly to the tectum and torus (Wilczynski and Northcutt, ′77; Wilczynski, ′81). Additional trisynaptic routes through the anterior entopeduncular nucleus and its pretectal and tegmental connections parallel the striatal routes.     

Efferent tectal pathways in two chondrichthyans,the shark Scyliorhinus canicula and the ray Raja clavata

The efferent connections of the tectum mesencephali in the shark Scyliorhinus canicula and the ray Raja clavata have been studied by using the silver impregnation methods of Nauta-Gygax ('54) and Fink-Heimer ('67). After a unilateral lesion made through all six tectal layers, three distinct pathways could be observed: (1) an ascending projection both ipsi- and contralateral to the pretectal area, the dorsomedial region of the thalamus, and the lateral geniculate body, (2) a commissural projection to the contralateral tectum and intercollicular nucleus, and (3) a descending projection to the rhombencephalic reticular formation. The last mentioned tract can be subdivided into (a) the ipsilateral tractus tectobulbaris ventralis and intermedius, giving off fibers to the intercollicular nucleus, the nucleus reticularis isthmi, and the medial and median reticular formation of the rhombencephalon and (b) the contralateral tractus tectobulbaris dorsalis, which connects the tectum with the contralateral medial reticular formation. Contrary to what has been found in other vertebrates there is no distinct segregation with respect to laterality of tectoreticular connections. Neither an ipsilateral projection to the nucleus isthmi nor a direct tectospinal pathway could be demonstrated with the techniques used.     

Optic tectum of the eastern garter snake, Thamnophis sirtalis. V. Morphology of brainstem afferents and general discussion

Brainstem neurons that project to the optic tectum of the eastern garter snake were identified by retrograde transport of horseradish peroxidase. The distribution and morphology of tectal afferent axons from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation were then studied by anterograde transport of horseradish peroxidase. Diencephalic projections to the tectum arise from the ventral lateral geniculate complex ipsilaterally and the ventrolateral nucleus, suprapeduncular nucleus, and nucleus of the ventral supraoptic decussation bilaterally. Three pretectal groups (the lentiform thalamic nucleus, the lentiform mesencephalic-pretectal complex and the geniculate pretectal nucleus) give rise to heavy, bilateral tectal projections. Small neurons in nucleus isthmi and large reticular neurons in nucleus lateralis profundus mesencephali also give rise to bilateral projections. Caudal to the tectum, projections arise bilaterally from the pontine and medullary tegmentum, nuclei of the lateral lemniscus, the posterior colliculus, and the sensory trigeminal nucleus. A small contralateral projection arises from the medial vestibular complex. Tectal afferents from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation had characteristic morphologies and laminar distributions within the tectum. However, these afferents fall into two groups based on their spatial organization. Afferents from the thalamus and nucleus isthmi arise from small neurons with spatially restricted, highly branched dendritic trees. Their axons terminate in single, highly branched and bouton-rich arbors about 100 micron in diameter. By contrast, afferents from the midbrain reticular formation and the pretectum arise from large neurons with long, radiate, and sparsely branched dendritic trees. Their axons course parallel to the tectal surface and emit numerous collateral branches that are distributed widely through the mediolateral and rostrocaudal extent of either the central or superficial gray layers. Each collateral bears several small, spatially disjunct clusters of boutons.     

Afferent and efferent connections of the optic tectum in the carp (Cyprinus carpio L.)

The afferent and efferent connections of the tectum opticum in the carp (Cyprinus carpio L.) were studied with the HRP method. Following iontophoretic peroxidase injections in several parts of the tectum anterograde transport of the enzyme revealed tectal projections to the lateral geniculate nucleus, dorsal tegmentum, pretectal nuclei, nucleus rotundus, torus longitudinalis, torus semicircularis, nucleus isthmi, contralateral tectum and to the mesencephalic and bulbar reticular formations.Tectal afferents were demonstrated by retrograde HRP transport in the area dorsalis pars centralis of the telencephalon, torus longitudinalis, torus semicircularis, nucleus isthmi, nucleus profundus mesencephali, several pretectal nuclei, dorsomedial and dorsolateral thalamic nuclei, nucleus of the posterior commissure, mesencephalic and bulbar reticular nuclei and nucleus ruber. Visuo-cerebellar circuitry was investigated by means of peroxidase injections in the various parts of the cerebellum. These experiments revealed indirect retino- and tecto-cerebellar pathways via the pretectal nuclei and the nucleus isthmi.     

Tectal connections in Python reticulatus

The origins of the axons terminating in the mesencephalic tectum in Python reticulatus were examined by unilateral tectal injections of horseradish peroxidase. Kutrogradely labeled cells were observed bilaterally throughout the spinal cord in all subdivisions of the trigeminal system, with the exception of nucleus principalis, which showed labeled cells only on the ipsilateral side. Labeling of the reticular formation occurred bilaterally in nucleus reticular is interiormagnocellularis, nucleus reticularis lateralis, nucleus reticularis medius and the mesencephalic reticular formation. The tectum also receives bilateral projections from the dorsal tegmentaJ field, the nucleus of the lateral lemniscus and nucleus isthmi, and ipsilateral projections from nucleus profundus mesencephali. A few labeled cells were found ipsilaterally in the locus coeruleus and in nuclei vestibulares ventrolateralis and centromedialis. In the diencephalon labeled cells were observed ipsilaterally in nucleus ventrolateralis thalami, nucleus ventromedialis thalami, nucleus suprapeduncularis, and in the dorsal and ventral lateral geniculate nuclei. Bilateral labeling was observed in nucleus periventricularis hypo-thalami. Furthermore, labeling was ipsilaterally present in the ventral telen-cephalic areas. The tectum in Python reticulatus receives a wide variety of afferent connections which confirm the role of the tectum as an integration center of visual and exteroceptive information.     

Tectoreticular pathways in the turtle, Pseudemys scripta. I. Morphology of tectoreticular axons

Tectoreticular projections in turtles were examined by reconstructing from serial sections axons that were anterogradely filled with horseradish peroxidase after tectal injections. Three tectoreticular pathways each contain extensively collateralized axons. The crossed dorsal pathway (TBd) contains large and small caliber axons. After leaving the tectum, TBd axons emit collaterals into the ipsilateral profundus mesencephali rostralis and then give off a main rostral branch that bears secondary collaterals in the ipsilateral interstitial nucleus of the medial longitudinal fasciculus and the suprapeduncular nucleus. The main trunks cross the midline and descend in the predorsal bundle, generating collaterals at regular intervals. These terminate mostly in the medial half of the reticular core from the midbrain to the caudal medulla. Axons in the uncrossed intermediate pathway also emit collaterals into a midbrain reticular nucleus (profundus mesencephali caudalis) and often have a thick rostral branch. The main caudal trunks, however, remain ipsilateral and travel in a diffuse, laterally placed tract, where each emits a long series of collaterals into the lateral half of the reticular core. The uncrossed ventral pathway (TBv) contains medium and small caliber axons. TBv axons often have collaterals within the tectum and apparently lack main rostral branches. Their caudal trunks run in the tegmental neuropile below the TBi where they collateralize less exuberantly than do TBd and TBi axons. The morphology of axons in all three pathways suggests that projections from disjunct tectal loci converge at many rostrocaudal levels within the reticular formation. This point was examined explicitly in experiments in which two disjunct injections were placed in one tectal lobe. Intermediate pathway axons traced from the two loci initially formed two distinct bundles but then intermingled in the reticular formation.     

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.     

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.     

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.     

Intensity and pattern discrimination after lesions of the pretectal complex,accessory optic nucleus and ventral geniculate in pigeons

Pigeons were trained to discriminate between pairs of visual stimuli that differed in intensity or pattern. After completion of traianing, bilateral, stereotaxic lesions were made in various cell groups in the mesencephalon and diencephalon that receive terminals of the optic tract. The target regions were nucleus ectomamillaris (accessory optic nucleus), nucleus lentiformis mesencephali and area pretectalis (pretectal complex) and the nucleus geniculatus lateralis, pars ventralis (ventral geniculate). In some cases, combined lesions of nucleus lentiformis mesencephali and area pretectalis were made. Lesions of nucleus ectomamillaris, nucleus lentiformis mesencephali, area pretectalis, or ventral geniculate did not produce major impairments of discrimination performance nor did combined lesions of nucleus lentiformis mesencephali and area pretectalis. A number of cases of intended destruction of the ventral geniculate also had extensive damage to the overlying nucleus rotundus. In several of these cases of combined destruction of nucleus rotundus and ventral geniculate, the previously reported discrimination deficits following nucleus rotundus lesions did not appear. In those cases in which the nucleus rotundus deficit was observed, the lesions were found to include the nucleus subpretectalis, which, like nucleus rotundus, receives tectofugal fibers via the brachium of the superior colliculus. The data of the ventral geniculate + rotundus cases and ventral geniculate + rotundus + subpretectalis cases suggest that sensory deficits following a lesion in a particular cell group may not necessarily indicate that the sensory information is processed in that cell group, but rather that the lesion had deprived other cell groups of the appropriate input necessary for their proper functioning.     

Tectal projections in the goldfish (Carassius auratus): a degeneration study.

Efferents revealed by degeneration staining following tectal lesions in goldfish are presented. Four major projections were found. Ascending ipsilateral projections to pretectal-diencephalic areas exit the tectum rostrally and laterally and terminate in the area pretectalis (AP), lateral geniculate (LGN), nucleus pretectalis (NP), and nucleus rotundus (NR). Ascending contralateral projections exit rostrally and possibly laterally, enter the posterior and postoptic commissures and terminate in the contralateral AP, LGN, NP, NR, and rostral tectum. A medially directed projection enters the intertectal commissure, and some of these fibers may terminate sparsely in an area of the contralateral tectum homotopic to the lesion. A descending projection exits the tectum laterally and projects ipsilaterally to a dorsolateral tegmental nucleus (DLT) and the lateral reticular formation of the tegmentum and pons, and contralaterally to the medial reticular formation of the tegmentum and pons.