Calcium binding proteins distinguish large and small cells of the ventral posterior and lateral geniculate nuclei of the prosimian galago and the tree shrew (Tupaia belangeri).

Two different cell types were identified in the thalamus of galago and Tupaia by using antibodies to two calcium binding proteins, calbindin and parvalbumin. In each species studied, the lateral geniculate nucleus consists of six layers, two of which have smaller relay cells. Previous studies have shown that the small cell layers receive fibers from the superior colliculus and project to the superficial layers of the striate cortex. These are the only geniculate layers that react to a calbindin antibody but not parvalbumin. The ventral posterior nucleus was included in the study and the results for both nuclei show that calbindin is a marker for thalamic cells that receive small fibers and project to superficial layers of koniocortex.

A projection from the parabigeminal nucleus to the pulvinar nucleus in Galago.

A projection from the parabigeminal nucleus (Pbg) to the striate-recipient zone of the pulvinar nucleus in the prosimian Galago was identified by anterograde and retrograde transport methods. In addition to the pulvinar nucleus, Pbg projections were found to terminate in layers 4 and 5 of the dorsal lateral geniculate nucleus and the central lateral nucleus. All three of these structures project to the superficial layers of the striate cortex. Similarities between the Pbg in mammals and the nucleus isthmi in nonmammals in connections and neurochemistry reinforce the idea that these two nuclei are homologous.

Evidence for separate pathways within the tecto-geniculate projection in the tree shrew.

Two layers (3 and 6) in the dorsal lateral geniculate nucleus (GLd) of the tree shrew (Tupaia belangeri) receive projections from the superficial layers of the superior colliculus. The goal of this study was to determine whether the same or different cells in the superior colliculus give rise to the projections to layers 3 and 6 by following individual axons labeled with biocytin from the superior colliculus to the GLd. The results show that the terminal fields differ in the two layers--those in layer 3 are restricted to a line of projection, whereas those in layer 6 are elongated along the dimension orthogonal to a line of projection. Another important difference between axons that project to GLd layers 3 and 6 is that those that project to layer 6 give off collaterals to the posterior pretectal nucleus, whereas at least some axons that project to layer 3 send a collateral to the ventral lateral geniculate nucleus (GLv). These results suggest that the superior colliculus exerts separate influences on these two GLd layers, both of which project to separate targets above layer IV in the striate cortex. The biocytin method has proved useful by showing the dendritic trees of the superior colliculus cells of origin, the pathways taken by the axons (including the presence of collaterals), and the terminal fields both within and outside the GLd.

Cholinergic and monoaminergic innervation of the cat's thalamus: comparison of the lateral geniculate nucleus with other principal sensory nuclei.

The cholinergic and monoaminergic innervation of the lateral geniculate nucleus (GL) and other thalamic nuclei in the cat was examined by using immunocytochemical and tract-tracing techniques. Cholinergic fibers, identified with an antibody to choline acetyltransferase (ChAT), are present in all layers of the GL. They are fine in caliber and exhibit numerous swellings along their lengths. The A layers, the magnocellular C layer, and the medial interlaminar nucleus are rich in cholinergic fibers that give rise to prominent clusters of boutons, while the parvicellular C layers contain fewer fibers that are more uniformly distributed. The interlaminar zones are largely devoid of ChAT-immunoreactive fibers. Double-label experiments show that cholinergic projections to the GL originate from two sources, the pedunculopontine reticular formation (PPT) and the parabigeminal nucleus (Pbg). The PPT contributes cholinergic fibers to all layers, while Pbg projections are limited to the parvicellular C layers. The lateral geniculate nucleus has a much greater density of cholinergic fibers than the other principal sensory nuclei: the density of fibers in the A layers is more than three times greater than that in the ventral posterior nucleus (VP) or the ventral division of the medial geniculate nucleus (GMv). In contrast, serotonin (5-HT)-immunoreactive fibers are distributed with equal density across the principal thalamic nuclei, while tyrosine hydroxylase (TH)-immunoreactive fibers (presumed to contain norepinephrine) are noticeably less dense in the GL than in the others. Monoaminergic fibers also differ from cholinergic fibers in their laminar distribution within the GL: both TH- and 5HT-immunoreactive fibers are distributed evenly across the layers and interlaminar zones and are slightly more abundant in the parvicellular C layers than in the other layers. Other thalamic nuclei rich in cholinergic fibers include the pulvinar nucleus, the ventral lateral geniculate nucleus, the intermediate nucleus of the lateral group, the lateral medial and suprageniculate nuclei (Graybiel and Berson: Neuroscience 5:1175-1238, '80), and the paracentral and central-lateral components of the intralaminar nuclei. This pattern matches the distribution of projections from the PPT and is similar, but not identical, to the pattern of acetylcholinesterase staining. The fact that most of the nuclei rich in cholinergic fibers have been implicated in visual sensory or visual motor functions suggests that cholinergic projections from the reticular formation play an especially important role in visually guided behavior.

A cytoarchitectonic atlas of the medial geniculate body of the opossum, Didelphys virginiana, with a comment on the posterior intralaminar nuclei of the thalamus.

The organization of the medial geniculate body and adjacent posterior thalamus of the Virginia opossum was studied in Nissl-, Golgi-, reduced silver, and myelin-stained preparations. Our chief goals were to define the cytoarchitectonic subdivisions and boundaries in Nissl preparations and to reconcile these with those observed with the Golgi method and in experimental material, to present these results in an atlas of Nissl-stained sections, and to compare the chief nuclear groups in the opossum and the cat medial geniculate body. In the opossum, the ventral division consists chiefly of the ventral nucleus. The ventral nucleus is divided into two main parts: the pars lateralis and the pars ovoidea, the former being relatively smaller in the opossum. The ventral nucleus of both species contains large principal neurons with bushy, tufted dendrites and smaller Golgi type II cells. However, the opossum has far fewer Golgi type II cells, and the texture of the neuropil is correspondingly different, although the primary ascending input from the midbrain arises from the central nucleus of the inferior colliculus in both species. The dorsal division consists of the dorsal nuclei, including the suprageniculate nucleus and the caudal part of the lateral posterior nucleus, the marginal zone, and the posterior limitans nucleus. These nuclei are identified in both species, although they are much smaller in the opossum. The neurons consist of medium-size and small somata with a predominantly radiate mode of dendritic branching and a lower cell concentration than in the ventral division. In both species the afferent brain stem input comes from the inferior colliculus, the lateral tegmental area, the intercollicular tegmentum, and the superior colliculus. The medial division contains several types of cells, which are heterogeneous in form and size, most having radiating dendrites and a low cellular concentration. This division is especially smaller in the opossum, although comparable inputs arise from various auditory and non-auditory sources in the midbrain and spinal cord in both species. A large intralaminar complex of nuclei occurs in the opossum, which have a more extensive distribution than previously appreciated. They not only occupy the intramedullary laminae but form a shell around the medial geniculate nuclei and adjoining main sensory nuclei. The intralaminar complex includes the posterior limitans, posterior intralaminar, posterior, parafascicular, posterior parafascicular, central intralaminar, limitans, and central medial nuclei, and the marginal zone of the medial geniculate body.(ABSTRACT TRUNCATED AT 400 WORDS)

New view of the organization of the pulvinar nucleus in Tupaia as revealed by tectopulvinar and pulvinar-cortical projections.

The projections of the superficial layers of the superior colliculus to the pulvinar nucleus in Tupaia were reexamined by injecting WGA-HRP into the tectum. The main result was finding two different patterns of terminations in the pulvinar nucleus: a zone remote from the lateral geniculate nucleus, which occupies the dorsomedial and caudal poles of the pulvinar nucleus, was almost entirely filled with terminals in every case irrespective of the location of the injection site; and a second division of the pulvinar nucleus, adjacent to the lateral geniculate nucleus, contained irregular patches--much more densely populated--and the distribution of patches varied from case to case. We call the first projection "diffuse" and the patchy projection "specific." Next we injected several divisions of the extrastriate visual cortex to find the cortical target of each pathway. The diffuse path terminates in the ventral temporal area (Tv). The specific path terminates in the dorsal temporal area (Td) and area 18. We speculated about the significance of the two pathways: the specific path may be responsible for the preservation of vision after removal of the striate cortex; the diffuse path may have an important place in the evolution of the visual areas of the temporal and occipital lobe. We argued that the target of the diffuse path is in a position to relate limbic and visual impulses and relay the product of such integration to the other visual areas, striate as well as extrastriate cortex.

Cholinergic projections from the midbrain reticular formation and the parabigeminal nucleus to the lateral geniculate nucleus in the tree shrew.

The distribution and sources of putative cholinergic fibers within the lateral geniculate nucleus (GL) of the tree shrew have been examined by using the immunocytochemical localization of choline acetyltransferase (ChAT). ChAT-immunoreactive fibers are found throughout the thalamus but are particularly abundant in the GL as compared to other principal sensory thalamic nuclei (medial geniculate nucleus, ventral posterior nucleus). Individual ChAT-immunoreactive fibers are extremely fine in caliber and display numerous small swellings along their lengths. Within the GL, ChAT-immunoreactive fibers are more numerous in the layers than in the interlaminar zones and, in most cases, the greatest density is found in layers 4 and 5. Two sources for the ChAT-immunoreactive fibers in the GL have been identified--the parabigeminal nucleus (Pbg) and the pedunculopontine tegmental nucleus (PPT)--and the contribution that each makes to the distribution of ChAT-immunoreactive fibers in GL was determined by combining immunocytochemical, axonal transport, and lesion methods. The projection from the Pbg is strictly contralateral, travels via the optic tract, and terminates in layers 1, 3, 5, and 6 as well as the interlaminar zones on either side of layer 5. The projection from PPT is bilateral (ipsilateral dominant) and terminates throughout the GL as well as in other thalamic nuclei. Lesions of the Pbg eliminate the ChAT-immunoreactive fibers normally found in the optic tract but have no obvious effect on the density of ChAT-immunoreactive fibers in the contralateral GL. In contrast, lesions of PPT produce a conspicuous decrease in the number of ChAT-immunoreactive fibers in the GL and in other thalamic nuclei on the side of the lesion but have no obvious effect on the number of ChAT-immunoreactive fibers in the optic tract. These results suggest that there are two sources of cholinergic projections to the GL in the tree shrew which are likely to play different roles in modulating the transmission of visual activity to the cortex. The Pbg is recognized as a part of the visual system by virtue of its reciprocal connections with the superficial layers of the superior colliculus, while the PPT is a part of the midbrain reticular formation and is thought to play a non-modality-specific role in modulating the activity of neurons throughout the thalamus and in other regions of the brainstem.

Terminations of individual optic tract fibers in the lateral geniculate nuclei of Galago crassicaudatus and Tupaia belangeri.

The morphology and laminar distribution of individual optic fibers projecting to the lateral geniculate nucleus (GL) of Galago and Tupaia were studied following iontophoretic injections of horseradish peroxidase (HRP) into the optic tract. In Galago the GL is composed of three functionally matched pairs of layers, each characterized by cells of a given size, one large, one medium-sized, and one small. The results show that there is a close correspondence between the size of the afferent fibers and the size of the neurons in the target layer: large axons project to the magnocellular layers, medium-sized axons project to the parvicellular layers, and small fibers project to the intercalated layers. In Tupaia the GL is composed of two functionally matched pairs and two unmatched layers. Optic fibers that project to the medial matched pair (1 and 2) are only slightly larger than those that project to the lateral matched pair (4 and 5), but both are larger than those that project to the unmatched layers (3 and 6). In both species terminal arbors and the distribution of terminal boutons within layers corresponded closely with the organization of dendritic processes of cells in the target layer. This correspondence was particularly evident in the parvicellular layers in Galago and in layer 6 in Tupaia: parvicellular terminal arbors, like the dendrites of parvicellular cells, are organized in narrow columns oriented along lines of projection, whereas layer 6 terminal arbors, like the dendrites of layer 6 cells, are oriented in elongated strips perpendicular to lines of projection. In both species there was evidence for sublaminar terminations in some layers. These were restricted to the parvicellular layers in Galago and layers 4 and 5 in Tupaia. With the exception of a small number of fine fibers in the intercalated layers in Galago, optic fibers in both species terminated in one and only one layer in a set. The significance of this result depends on the relation between ganglion cell classes and what is being segregated in different GL layers. Lateral geniculate lamination varies even in closely related species and has evolved independently in such distantly related lines as carnivores and primates. It is not surprising, therefore, that what is being segregated varies from species to species.

Laminar organization of geniculocortical projections in Galago senegalensis and Aotus trivirgatus.

The projections of the lateral geniculate nucleus to striate cortex were traced by anterograde and retrograde transport of WGA-HRP in two primates, Galago and Aotus. The goal was to determine the laminar organization of the terminals of individual layers of the lateral geniculate nucleus. The results show that in both species the magnocellular layers project to cortical layer IV alpha, the parvicellular layers project to IV beta, and the intercalated geniculate layers (which term includes layers 4 and 5 in Galago) project to layers III and I. The distribution of terminals in layer III is periodic, which is to say, there are regularly spaced regions of terminals separated by regions devoid of terminals. When the two species are compared to Saimiri, it is clear that the basic organization of the three pathways relaying in the lateral geniculate nucleus is common to all three primates. At the same time, there are some significant species differences. The total pattern of projections in Galago is compressed in comparison to that in the monkey, and this compression seems to be related to the poorer differentiation of cortical layers in the prosimian. For example, layer IIIC is distinct only in the two monkeys and at the same time it is spared from geniculate terminations. Both Galago and Aotus differ from Saimiri in that they lack a projection from the parvicellular geniculate layers to the layer IIIB. These species comparisons are relevant to the questions of the functional significance of the three pathways and the evolution of the primate striate cortex.

The distribution of glutamic acid decarboxylase immunoreactivity in the diencephalon of the opossum and rabbit.

We have examined the distribution of neurons and terminals immunoreactive for glutamic acid decarboxylase (GAD) in the thalamus and adjacent structures of the opossum (Didelphis virginiana) and the rabbit and have compared this distribution with the distributions we described previously for the cat and bushbaby (Galago senegalensis). The significance of these experiments depends, first, on the fact that GAD is the synthetic enzyme for GABA, and therefore that GAD immunoreactivity is a marker for GABAergic inhibitory neurons, and second, on previous findings that suggest that GABAergic neurons in the dorsal thalamus are local circuit neurons. In both cat and Galago, GAD-immunoreactive neurons are distributed essentially throughout the entire thalamus. In the opossum, GAD neurons are chiefly confined to the dorsal lateral geniculate nucleus and the lateral extremity of the lateral posterior nucleus. The distribution of GAD neurons in the rabbit is intermediate between that found in the opossum on the one hand and cat and Galago on the other. Like opossum, about 25% of the neurons in the lateral geniculate nucleus of rabbit are GAD immunoreactive. Unlike opossum, however, as many as 18% of the cells in the ventral posterior nucleus of the rabbit are GAD immunoreactive, and scattered cells are also labeled in other thalamic areas, such as the medial geniculate and the lateral group. Aside from the findings in the dorsal thalamus, the chief observation is that GAD-immunoreactive neurons and/or terminals densely fill all principal targets of the optic tract, including the ventral lateral geniculate nucleus; the superficial gray layer of the superior colliculus; the anterior, posterior, and olivary pretectal nuclei; the nucleus of the optic tract; and the medial and lateral terminal nuclei of the accessory optic tract. These results support the idea first put forward by Cajal that local circuit neurons increase in number during the course of the evolution of complex mammalian brains. If we can assume that the conservative opossum retains characteristics reflecting an early stage of mammalian evolution, the results suggest that thalamic local circuit neurons arose first in the visual system and only later in evolution spread throughout the thalamus.

The laminar organization of the lateral geniculate body and the striate cortex in the tree shrew (Tupaia glis).

The organization of geniculostriate projections in Tupaia was studied using three separate methods, anterograde transport from the lateral geniculate, retrograde transport from the striate cortex, and reconstruction of single geniculostriate axons. The results show that each layer of the lateral geniculate body has a unique pattern of projections to the striate cortex, and each pattern consists of a major and a minor target. The two ipsilateral layers project to thin subtiers of layer IV: the major target of geniculate layer 1 is the top of IVa; the major target of geniculate layer 5 is the base of IVb. The minor target of layer 1 is the major target of layer 5. Two of the contralateral layers can be matched to the ipsilateral layers. Layers 1 and 2 are a matched pair and project to IVa; layers 4 and 5 are a matched pair and project to IVb. Thus, projections of a matched pair overlap. The remaining two contralateral layers, 3 and 6, project chiefly to cortical layer III. Layer 3 projects to layers IIIb and I and seems to be the counterpart of the parvocellular C layers in the cat and the intercalated layers in primates. Layer 6 projects to the base of IIIc in a zone contiguous with IVa. This contiguity raises the issue of whether the base of IIIc might actually be a part of layer IV. If this were the case, the two tiers of layer IV which are separated by a conspicuous cleft might be considered two subdivisions of layer IVb.

Glutamic acid decarboxylase-immunoreactive neurons and horseradish peroxidase-labeled projection neurons in the ventral posterior nucleus of the cat and Galago senegalensis.

Immunocytochemical methods were used to identify neurons in the ventral posterior nucleus of the cat and Galago senegalensis that contain glutamic acid decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter, GABA. In both species GAD-immunoreactive neurons make up about 30% of the total neurons in the ventral posterior nucleus and form a distinct class of small cells. After cortical injections of horseradish peroxidase (HRP), GAD-immunoreactive cells are not labeled with HRP and may, therefore, be GABAergic local circuit neurons. Comparison of the dendritic morphology of GAD-immunoreactive neurons with that of HRP-filled projection neurons reveals that the morphology of the GAD-containing neurons is distinct and, in particular, that the GAD-immunoreactive neurons display fewer primary dendrites. The relay neurons, in turn, can be divided into classes based on dendritic morphology and cell body size.

The laminar organization of the lateral geniculate body and the striate cortex in the squirrel monkey (Saimiri sciureus).

The organization of the projection from the lateral geniculate body to the striate cortex in the squirrel monkey has been re-examined using the anterograde and retrograde transport of horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP. The results confirm earlier findings that the projections of the magnocellular and parvocellular layers of the lateral geniculate body terminate in separate sublaminae of layer IV of striate cortex; a more superficial projection of the parvocellular layers to a narrow strip at the base of layer III (IVA in Brodmann's terminology) has also been confirmed. In addition to these well characterized pathways, our results show that the projections of the lateral geniculate body terminate in more superficial levels of layer III and sparsely in layer I of striate cortex. The projections to the upper portion of layer III terminate in distinct patches which coincide precisely with patches of cytochrome oxidase activity previously identified in this zone. The projections to the patches originate primarily from small, pale-staining cells of the "intercalated layers" which surround the magnocellular layers of the lateral geniculate body. A comparison of the organization of the geniculo-cortical projections in the squirrel monkey with that of the cat, Galago, and Tupaia suggests that, despite marked species differences in the laminar organization of the lateral geniculate body and striate cortex, there are striking similarities in the pathway which terminates in the most superficial layers of striate cortex.

Retinal ganglion cell projections to individual layers of the lateral geniculate body in Galago crassicaudatus.

The genus Galago provides an unique opportunity to study the relation between layers of the lateral geniculate body and classes of retinal ganglion cells. In the present experiments HRP was restricted to individual layers of the lateral geniculate body with the following results: After injections of the magnocellular layers, layers 1 and 2, labeled retinal ganglion cells ranged in size from 8 to 20 micrometers. After injections of the parvocellular layers, layers 3 and 6, labeled retinal ganglion cells ranged in size from 6 to 12 micrometers. After injections involving layers 4 and 5, which layers contain only very small, pale cells, labeled retinal ganglion cells ranged in size from 5 to 14 micrometers. Thus, the very largest ganglion cells were labeled only after injections of magnocellular layers 1 and 2, while small and medium retinal ganglion cells were labeled after HRP injections in every layers of the lateral geniculate body. Because the magnocellular layers actually contain a mixture of large, medium, and small-sized cells, we suggest that retinal ganglion cells of different size-classes project to geniculate relay cells of the corresponding size-class.

Projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago.

We have studied the projections from the superior colliculus and the neocortex to the pulvinar nucleus in Galago senegalensis by using the retrograde transport of horseradish peroxidase (HRP). Injections of various parts of the pulvinar complex, both the inferior and superior divisions, both the tectorecipient zone and the nontectorecipient zone as defined by Glendenning et. al. ('75), produce labeled cells in the lower tier of stratum griseum superficiale. The distribution of labeled cells in the superior colliculus varies with the locus of the injection, indicating a retinotopic projection system from the entire superior colliculus to all sectors of the pulvinar complex. These experiments also provide an opportunity to study the distribution and laminar origin of neurons giving rise to cortical descending projections. The entire visual cortex projects onto the pulvinar complex. The cells or origin can be divided into two populations--one located in layer V and the other in layer VI. In seven of the nine cases reported, the layer V population is restricted entirely or mainly to the striate area. In the two exceptional cases, the layer V population is located in the adjacent extrastriate cortex, areas 18 and 19. The difference in the layer of origin of the cortical descending fibers reflects a difference in the layer of termination of the reciprocal ascending projection. These findings identify the entire visual field as primary visual cortex. The importance of this conclusion is underscored by the fact that the visual field comprises as much as one-half of the whole neocortex.

Different distribution of large and small retinal ganglion cells in the cat after HRP injections of single layers of the lateral geniculate body and the superior colliculus.

Retinal ganglion cells were labeled with HRP after injecting single layers of GL or single strata within the stratum griseum superficiale (SGS). Only small cells were labeled after injecting small cell C layers and upper SGS. Only large cells were labeled after injecting lower SGS. Small and large cells were labeled after injecting medial interlaminar nucleus (MIN) and layers A and A1.

Distribution of acetylcholinesterase in the geniculo striate system of Galago senegalensis and Aotus trivirgatus: evidence for the origin of the reaction product in the lateral geniculate body.

This inquiry began with the discovery that just two layers of the lateral geniculate nucleus (GL) of Galago contain large amounts of acetylcholinesterase (AChE). These two layers (layers 3 and 6) are similar in cell size and Nissl-staining characteristics and project to the same layer in the striate cortex. To find out whether the pattern of staining is unique in the Galago, we examined the distribution of AChE in the lateral geniculate nucleus of the owl monkey, Aotus trivirgatus. In this species we found that the parvocellular layers (3 and 4) stained darkly for AChE while the magnocellular layers (1 and 2) were only slightly stained. The interlaminar zones as well as the "S" layers were also distinguished by a high level of AChE staining. In order to determine the source of the cholinesterase staining in layers 3 and 6 of Galago, we studied, in separate experiments, the effects of kainic acid injections into GL, of eye enucleation, and of lesions of the striate cortex. Injections of kainic acid, followed by survival times of 2 and 11 days, produced severe cellular destruction in GL, yet the AChE staining of layers 3 and 6 was undiminished. Eye enucleations had no effect upon the AChE staining of GL even after a survival period of 3 years. In contrast, a small lesion of the striate cortex, followed by a 9-day survival period, produced conspicuous gaps in the AChE staining of layers 3 and 6. These results indicate that the AChE in layers 3 and 6 is not attributable to the cells within the layers, or to retinal fibers, but is dependent upon descending projections from the striate cortex. Because of the dependence of the AChE reaction product in layers 3 and 6 of GL upon an intact striate cortex, we turned our attention to the distribution of AChE in the striate cortex. In Galago, cholinesterase-positive cells were found in layer VI of the striate cortex; and in both Galago and Aotus, the striate cortex was distinguished from other cortical areas by a prominent band of cholinesterase activity within layer IV. This band ended abruptly at the 17-18 border. The precise origin of this cholinesterase staining within layer IV of the striate cortex remains to be determined.

Cortical connections of the pulvinar nucleus in Galago.

In the first series of experiments, small amounts of HRP were injected into areas 17, 18, and 19 and each of the cytoarchitectonic areas of the temporal lobe. The resulting distributions of labeled cells fell into a number of distinctive classes. For example, after injecting the temporal anterior area (Ta), the labeled cells occupied a band on the ventral border of the inferior division of the pulvinar complex; after injecting the temporal ventral area (Tv), the labeled cells were concentrated in the medial extremity of the superior division. In spite of the distinctiveness of these different distributions, there was evidence that in areas that we know to contain a representation of the visual field, the injection of the same part of the field led to labeled cells in the same part of the pulvinar nucleus. Thus, when the representation of the center of the field was injected in areas 17 or 18 or Tm (the temporal middle area), labeled cells were found in the dorsal part of the rostral half of the inferior division. The distinctiveness of the different distributions did not obscure certain features common to all experiments: labeled cells were always found in both subdivisions of the pulvinar complex, and there was always continuity between the population of labeled cells in the inferior division and the population of cells in the superior division. Wherever the site of the injection in the extrastriate region, some labeled cells were found in the causal half of the inferior division. Since the caudal half of the inferior division corresponds approximately to the tecto-recipient zone as defined earlier, the entire temporal lobe, except for the auditory areas, is visual cortex. Evidence was also found for an overlap in the striate cortex between the projections of the lateral geniculate body and the pulvinar nucleus. In conclusion, the pulvinar complex projects to a vast area of cortex, including all of the occipital lobe and most of the lateral surface of the temporal lobe. This entire region, which comprises a good part of the whole of the neocortex, can be regarded as visual cortex. A second series of experiments involved cortical injections of tritiated amino acids. The results showed that the projections from the thalamus to cortex were precisely reciprocated by descending projections from cortex to thalamus. The results also served as a way of confirming the results of the first set of experiments.