Interhemispheric connections in the visual cortex of the squirrel monkey (Saimiri sciureus)

The callosal connections within the posterior parietal and occipital cortices were studied in the squirrel monkey with horseradish peroxidase tracing techniques. The data were evaluated with particular emphasis on the relationship of major callosal connections along the 17-18 border. The overall pattern of callosal connections in the squirrel monkey also was compared with callosal patterns in other New World simians. Our results show that the dense band of callosal connections along the 17-18 border in the squirrel monkey differs from the connections observed in other New World monkeys in that it is virtually confined to area 18 and avoids area 17. In addition to a continuous band of callosal connections in area 18 that parallels the 17-18 border, rostral extensions of the band are oriented perpendicular to the 17-18 border and present an obvious periodicity. The remaining parieto-occipital cortex contains a complex pattern of callosal connections that is strikingly similar to patterns reported for other New World monkeys. Thus, it is likely that the dorsolateral extrastriate visual cortex in the squirrel monkey is organized in a manner similar to that found within other New World monkeys.

The projection of frontal cortical oculomotor areas to the superior colliculus in the domestic cat

The projection of frontal cortical oculomotor areas to separate laminae of the superior colliculus was examined in adult domestic cats. Following the placement of WGA-HRP/HRP injections within the superficial, intermediate, and deep layers of the superior colliculus, several observations may be made regarding the frontal corticotectal projection in the cat. Large injections including all of the collicular layers result in the retrograde labeling of neurons in frontal cortical areas that have been suggested previously to be analogous to the frontal eye fields in monkeys. These areas are a portion of the ventral bank and lip of the cruciate sulcus, including a small portion of the medial wall of the hemisphere, and the medial and lateral banks and fundus of the presylvian sulcus. The same pattern of retrograde labeling in frontal cortical areas is seen when the injection sites are restricted to either the intermediate or deep collicular layers. That is, different frontal cortical areas of the cat project separately but not differentially upon different collicular laminae. The greatest numbers of labeled neurons within frontal cortical oculomotor areas are observed following injections placed laterally within the caudal portion of the superior colliculus, regardless of whether the injection sites are located within the intermediate or deep collicular layers. Injections restricted to the superficial gray layer do not result in retrograde labeling in any of the frontal cortical oculomotor areas of the cat. These results are discussed in terms of the patterns of visuomotor integration specific to the cat, as well as in relation to previous evidence showing similarities between frontal cortical areas in cats and monkeys.

The pretectal complex of the cat: cells of origin of projections to the pulvinar nucleus

The pretectal projection to the pulvinar nucleus in the cat was examined using the retrograde transport of wheat germ agglutinin-horseradish peroxidase. These data show that both visual and non-visual areas of the pretectal complex contribute to the projection. Specifically, large numbers of labeled neurons are located within the pretectal olivary nucleus with a substantial number of labeled neurons observed within the nucleus of the optic tract. Labeled neurons are also located within the medial, anterior and posterior nuclei, but not to the degree observed in the other pretectal nuclei. Morphometric analysis of labeled and Nissl-stained neurons indicate that the pretectopulvinar pathway is not correlated to any single cell size.

The medial terminal nucleus of the monkey: evidence for a 'complete' accessory optic system

The retinal projection to the medial terminal nucleus of the accessory optic system of the monkey was examined in several primate species which had received intraocular injections of [3H]proline or [3H]fucose. These data show that the medial terminal nuclei of the slow loris, marmoset monkey, and squirrel monkey all receive a sparse input from the contralateral retina.

Hypothalamic and ventral thalamic projections to the superior colliculus in the cat

The present report describes the organization of collicular afferents that arise within either the hypothalamus or the ventral thalamus. Following the placement of large injections of WGA-HRP into the superior colliculus of the cat, retrogradely labeled neurons are located within the reticular nucleus of the thalamus, the zona incerta, the fields of Forel, and throughout the hypothalamus. Although the dorsal hypothalamic area contains the largest number of labeled hypothalamic neurons, labeled cells are also found within the periventricular, paraventricular, dorsomedial, ventromedial, posterior, lateral, and anterior hypothalamic nuclei. A strikingly similar pattern of distribution of labeled neurons is also observed following placement of small injections of WGA-HRP that are restricted within the stratum griseum intermedium (SGI). In contrast, hypothalamic and ventral thalamic labeling is not seen after placement of injections within the stratum griseum superficiale. Following the placement of injections of tritiated anterograde tracers within the dorsal hypothalamic area, transported label is organized in two bands of clusters over the SGI. When injections of tritiated tracers are placed within the zona incerta, terminal label is also located over the SGI; however, the distribution of silver grains does not appear as clusters or distinct puffs. On the basis of the comparison of the cellular types that give rise to these projections and the differences in terminal distribution, we suggest that the hypothalamic and ventral thalamic projections to the superior colliculus are totally separate and unrelated pathways. The functional implications of the hypothalamotectal pathway are also discussed.

Subcortical connections of area 17 in the tree shrew: an autoradiographic analysis

The present anterograde autoradiographic study reveals several targets of the striate cortex (area 17) of the tree shrew which were not previously observed in studies which used anterograde degeneration methods; our data also confirm several previous findings. The results are discussed in the context of these projections modulating ascending visual information (claustrum, lateral intermediate nucleus, pulvinar, dorsal lateral geniculate, cells of the external medullary lamina, reticular nucleus of the thalamus, superficial collicular layers, and the anterior and posterior pretectal nuclei) or visuomotor information (putamen, caudate, ventral lateral geniculate, pontine gray, and the anterior and posterior pretectal nuclei).

The pretectal olivary nucleus of the cat: evidence for a two-tailed structure

The pretectal olivary nucleus of the cat was reconstructed from Nissl and myelin stained tissue. The olivary nucleus is found to have a medially placed head region and two tail regions which extend obliquely from the head region. The retino-olivary projection was also analyzed using the anterograde autoradiographic tracing method. Retinal input is found bilaterally over both the head of the nucleus and the tail regions. The distribution of silver grains, when compared bilaterally, appears slightly more dense over the contralateral nucleus.

The pretectal complex of the monkey: a reinvestigation of the morphology and retinal terminations

The cytoarchitecture of the pretectal complex of the squirrel monkey was examined in Nissl- and myelin-stained sections in the coronal, horizontal, and sagittal plane. Five different pretectal subdivisions can be identified on the basis of their nuclear morphology. The general location and cytoarchitecture of these pretectal nuclei are similar to those described for non-primate mammals. Thus, the nomenclature used to designate the pretectal nuclei in other species can now be applied to the squirrel monkey. According to this standard terminology, the pretectal complex of the squirrel monkey consists of the nucleus of the optic tract; the pretectal olivary nucleus; and the medial, anterior, and posterior pretectal nuclei. The pattern of retinal innervation to the pretectum was also determined by placing intraocular injections of 3H-proline into one eye and processing the tissue according to standard autoradiographic techniques. The pattern of transported label is more dense over the contralateral nuclei than over the ipsilateral nuclei. In particular, dense transported label is observed bilaterally over the pretectal olivary nucleus and the nucleus of the optic tract with sparse label over the posterior and medial pretectal nuclei.

Pretectal complex and accessory optic system of primates

In the present report, conflicting results regarding the pretectal complex and accessory optic system of primates are discussed. Subsequently data are presented and used in an attempt to clarify some of the issues. The retinal projections to the pretectal complex and accessory optic system of the tree shrew and squirrel monkey were examined using anterograde autoradiographic methods. These data demonstrate that, following intraocular injections of 3H-proline or 3H-fucose in the tree shrew, silver grains are apparent bilaterally over the pretectal olivary nucleus and the anterior and posterior pretectal nuclei and contralaterally over the nucleus of the optic tract. Following intraocular injections of the 3H-tracer in squirrel monkeys, dense transported label is observed bilaterally over the pretectal olivary nucleus and the nucleus of the optic tract with sparse label over the posterior and medial pretectal nuclei. In both the tree shrew and squirrel monkey, a differential retinal projection is observed, chiefly contralaterally, to all accessory optic terminal nuclei (i.e., the dorsal, lateral and medial terminal nuclei).

Interhemispheric and subcortical collaterals of single cortical neurons in the adult cat

The results of this study demonstrate the existence of single neocortical neurons that send axon collaterals into the corpus callosum, to terminate within the contralateral hemisphere, and subcortically, to terminate within the ipsilateral superior colliculus.

A rapid myelin stain for frozen sections: modification of the Heidenhain procedure

A protocol has been established for the staining of myelin in frozen sections. While the new method is relatively fast and simple, it eliminates the problems routinely encountered with myelin stains such as blotchiness and uneven staining. Modifications were introduced into the procedures in order to obtain excellent fiber staining results on a wide variety of tissue.

The projections of the lateral geniculate nucleus of the squirrel monkey: studies of the interlaminar zones and the S layers

Anterograde and retrograde tracing techniques were used to reveal that axons arising from neurons within the interlaminar zones and the S layers of the lateral geniculate nucleus of the squirrel monkey terminate within the supragranular layers of area 17. Specifically, our data indicate that the axons of the neurons housed within the S layers end in a patchlike fashion in cortical layers IIIa and IIIb, while neurons in the interlaminar zones project primarily to layer I. Both pathways may convey W-cell information from the retina and the superior colliculus to the striate cortex.

The demonstration of a retinal projection to the medial pretectal nucleus in the domestic cat and the squirrel monkey: an autoradiographic analysis

The anterograde autoradiographic tracing technique was used to demonstrate a projection from the retina to the medial pretectal nucleus in the domestic cat and the squirrel monkey. In the cat, the pathway terminates within the contralateral nucleus in two discrete loci. In the squirrel monkey, the retino-medial pretectal pathway is bilateral and terminates in a single locus.

Morphologic and autoradiographic evidence for a laminated pretectal olivary nucleus in the squirrel monkey

The pretectal olivary nucleus of the squirrel monkey was examined in both normal and autoradiographic material. In the Nissl- and fiber-stained tissue the nucleus appears as a laminated structure. The distribution of retinal terminals within the nucleus was examined by the autoradiographic tracing method. These data reveal denser projection to the contralateral pretectal olivary nucleus. When comparing the distribution of the silver grains bilaterally, the pattern of transported label appears to be partially nonoverlapping.

The efferent projections of the pretectal complex: an autoradiographic and horseradish peroxidase analysis

Anterograde autoradiographic data reveal that neurons within the pretectal complex of the tree shrew possess axons which terminate within three general categories of targets. First, there are targets of a major ipsilateral descending pathway which include: the dorsal cap of Kooy of the inferior olivary complex, the dorsolateral and dorsomedial regions of the griseum pontis, the mesencephalic reticular formation which lies immediately dorsal and lateral to the red nucleus, the medial terminal nucleus and the superficial layers of the superior colliculus. A second category of targets receive their pretectal input from a large ascending bundle which projects ipsilaterally to: the reticular and lateral nuclei of the thalamus, the zona incerta, the central lateral and paracentral intralaminar nuclei of the thalamus and bilaterally to the ventral lateral geniculate nucleus. A third category of targets include cranial nerve and closely associated nuclei which play a role in eye movements. Pretectofugal fibers projecting to nuclei in this third category terminate ipsilaterally within the nucleus of Darkschewitsch, bilaterally within the nucleus of the posterior commissure and the interstitial nucleus of Cajal, and contralaterally within the somatic cell column of the oculomotor and trochlear nuclei. There are also commissural projections to contralateral pretectal cell groups. Injections of horseradish peroxidase were placed within 11 pretectal targets. These data, which confirm and extend our autoradiographic findings, show that the majority of pretectal targets receive input from several pretectal nuclei, and that the size of pretectal neurons, rather than the cell groups in which they are located, dictates the termination site(s) of their axons.

The precise origin of the tectospinal pathway in three common laboratory animals: a study using the horseradish peroxidase method

The horseradish peroxidase tracing method has been used to study the cells of origin of the tectospinal projections in the opossum, the tree shrew, and the cat. The present data show that only those collicular neurons which occupy the deep (ventral to the stratum opticum) tectal laminae send axons to the cervical spinal cord. In particular, layer IV contains the greatest number of spinal projecting neurons. Our results also reveal that while only the large sized collicular neurons project upon the cervical spinal cord in the opossum and the tree shrew, neurons comprising several different size categories do so in the cat. We thus suggest that several different descending channels exist over which the superior colliculus can influence the neck musculature in the cat.

Patterns of retinal terminations and laminar organization of the lateral geniculate nucleus of primates

Autoradiographic tracing procedures have been used to study the organization of retinogeniculate axons in seven primates, i.e., four species of New World monkeys, one species of Old World monkeys and two species of prosimians. These data suggest that the basic primate pattern of geniculate lamination consists of two parvocellular layers, two magnocellular layers, and two poorly developed and highly variable superficial (S) layers which are ventrally located. Ocular input to each member of each of the three pairs differs. In the macaque, the squirrel, and the saki monkey, the parvocellular layers subdivide and interdigitate into four leaflets so as to give the appearance of four parvocellular "layers." These leaflets are much less extensive in the owl and marmoset monkeys. In some individual macaque monkeys, there is further splitting of the parvocellular leaflets into subleaflets, giving the appearance of six parvocellular "layers." The prosimians (galago and slow loris) have two additional layers that are not found in pithecoid primates, and only one superficial layer is apparent. The two additional layers are termed "koniocellular" since they consist of very small cells. Finally, New and Old World monkeys have both ipsilateral and contralateral retinal input to the interlaminar zones. We conclude that the basic pattern of lateral geniculate organization is six layers, but not the traditional six. Prosimians have evolved two additional layers, the koniocellular layers, and have possibly lost one superficial layer. Both New World and Old World monkeys have elaborated the parvocellular layers by forming leaflets to varying extents. With the possible exception of the single S layer in prosimians, layers form pairs that are similar in cell types, but different in ocular input.

An autoradiographic analysis of the tecto-olivary projection in primates

Autoradiographic tracing methods were used to demonstrate a well-defined projection from the superior colliculus to the inferior olivary complex in the monkey. This projection originates within the deep layers of the superior colliculus, descends within the contralateral tecto-spinal tract, and terminates within the caudal 1/3 of the medial accessory nucleus. The terminal field is restricted to a densely packed, darkly stained group of cells located in the most dorsal segment of subnucleus b. In one animal, another group of olivary afferents was identified. These fibers also descend within the contralateral tecto-spinal tract, and terminate within the dorsal cap of Kooy. While it was not possible to determine the origin of this projection, our data suggest that it arises within a region adjacent to the rostral pole of the superior colliculus. The present study further indicates that in the monkey relatively few axons which course within the classical tecto-spinal tract pass caudal to the medulla.