Branching and laminar origin of projections between visual cortical areas in the cat

The laminar distribution and branching pattern of corticocortical neurons were studied in areas 17, 18, 19, 20, 21, and the lateral suprasylvian areas of the adult cat neocortex. This was done by examining the laminar position of single-labelled neurons and the proportions of double-labelled cells in these areas after paired injections of the fluorescent retrograde labels fast blue and diamidino yellow in areas 17, 18, and 19 of the ipsilateral hemisphere. After injections in areas 18 and 19, the labelled neurons in area 17 were mostly confined to the supragranular layers, with a small proportion of labelled cells in lamina 5 and upper lamina 6. Double-labelled neurons were rare and were found in the region of overlap between the two populations of labelled cells. They were mostly found in the upper laminae but a few were observed in laminae 5 and 6. The cells projecting to either area were often grouped in patches which were seen to overlap or interdigitate depending on the region examined. As a population, the neurons projecting to area 18 occupied a deeper position in laminae 2 and 3 than those projecting to area 19. Labelled cells in area 18 after injections in areas 17 and 19 were mostly found in the upper laminae with a few double-labelled cells which were restricted to the region of overlap between the two populations of labelled cells. The pattern of labelling in area 19 after injections in areas 17 and 18 was different from the one seen in areas 17 and 18. Neurons were almost equally distributed between the supra- and infragranular layers and there was a substantial proportion of double-labelled neurons (10%) which tended to belong mostly to lamina 5 and upper lamina 6. In area PMLS, the laminar position of corticocortical cells was somewhat similar to the one observed in area 19, in that a substantial number of labelled neurons were found in the deep laminae, especially after injections in 17 or 18. After injections in area 19, labelled cells were mostly found in the upper layers. Double-labelled cells were numerous (20%) when the injections were placed in areas 17 and 18 but quite rare in the other cases (17-19 and 18-19). Most of the double-labelled neurons were found in the deep layers. After injections in areas 17, 18, and 19, labelled cells were found in area 20, thus demonstrating a hitherto unknown projection from area 20 to areas 17 and 18. Labelled cells in area 20 were almost exclusively confined to the infragranular layers.(ABSTRACT TRUNCATED AT 400 WORDS)     

Bihemispheric Collateralization of the Cortical and Subcortical Afferents to the Rat's Visual Cortex

A fluorescent dye (usually fast blue or rhodamine tagged latex microspheres) was injected into cortical area 17 (or area 17 and the lateral part of area 18b) of adult and juvenile (15 - 22 day old) Sprague-Dawley albino rats. Another fluorescent dye (usually diamidino yellow) was injected into cortical areas 17, 18a and 18b of the opposite hemisphere. The injections involved only the cortical grey matter. After postinjection survival of 2 - 14 days the distribution of retrogradely labelled mesencephalic and prosencephalic cells was analysed. Both small and large injections labelled retrogradely a substantial number of cells in specific and nonspecific dorsal thalamic nuclei (lateral geniculate, lateral posterior, ventromedial, several intralaminar nuclei and nucleus Reuniens) as well as a small number of cells in the preoptic area of the hypothalamus and the mesencephalic ventral tagmental area (VTA). While labelled thalamic cells contained only the dye injected into the ipsilateral cortex, a small proportion of hypothalamic and VTA cells was labelled with the dye injected into the contralateral cortex. Virtually none of the cells in these areas were double labelled with both dyes. Both small and large injections labelled cells in the ipsilateral telencephalic magnocellular nuclei of the basal forebrain and the caudal claustrum. A substantial minority of labelled cells in these structures was labelled by the dye injected into the contralateral cortex. Furthermore, a small proportion (about 1%) of claustral cells projecting to the ipsilateral cortex were double labelled with both dyes. In several cortical areas ipsilateral to the injected area 17, associational neurons were intermingled with commissural neurons projecting to the contralateral visual cortex. A substantial proportion of associational neurons projecting to ipsilateral area 17 also projected to the contralateral visual cortex (associational-commissural neurons). Thus, in visual area 18a, the associational-commissural neurons were located in all laminae, with the exception of lamina 1 and the bottom of lamina 6, and constituted about 30% of the neurons projecting to ipsilateral area 17. In paralimbic association area 35/13, associational-commissural neurons were located in lamina 5 and constituted about 20% of neurons projecting to ipsilateral area 17. In the limbic area 29d, the associational-commissural neurons were located in laminae 4, 5 and the upper part of lamina 6 and constituted about 10% of the associational-commissural neurons projecting to ipsilateral area 17. In oculomotor area 8, double-labelled neurons were located in lamina 5 and constituted about 10% of the neurons projecting to ipsilateral area 17. Thus, it appears that the axons of mesencephalic and diencephalic neurons projecting to the visual cortex do not send collaterals into both hemispheres. The bihemispheric projection to the rat's visual cortex originates almost exclusively in the retinotopically organized cortical area 18a and in integrative cortical areas 35/13, 29d and 8.     

Anatomical organization of the visual system of the mink, Mustela vison

The organization of the retinogeniculocortical visual system of the mink was studied by anterograde and retrograde tracer techniques, by physiological mapping, and by direct recordings from axonal terminals after injection of kainic acid. In the lateral geniculate nucleus, retinogeniculate afferents are segregated according to eye of origin between the two principal layers, A and A1. Within each of these layers there is a further parcellation according to functional type: on-center afferents terminate in the anterior leaflets of A and A1, and off-center afferents in the posterior leaflets. This separation is preserved in area 17: geniculocortical afferents terminate in ocular dominance patches in layer IV, and these patches coexist with an alternating, partially overlapping set of patches for on-center and off-center inputs that we have demonstrated previously (McConnell and LeVay: Proc. Natl. Acad. Sci. USA 81:1590-1593, '84). In both the lateral geniculate nucleus and in area 17, the contralateral eye predominates to a much greater extent than in the cat. Visual cortical areas corresponding to the cat's areas 17, 18, and 19 can be identified in the mink, but they are shifted posterolaterally in the hemisphere, and they show less emphasis on the representation of central retina. Mapping studies also revealed the existence of a fourth visual area in the splenial sulcus (area SV) adjacent to the representation of the far periphery in area 17. This area differs from the corresponding region in the cat in that it receives direct projections from the lateral geniculate nucleus and from areas 17 and 18. The lateral geniculate nucleus projects to each of the four cortical areas that were mapped. The bulk of the projection to area 17 is derived from the principal layers, A and A1, while most cells projecting to areas 18 and SV are found in the C-layer complex. The recurrent projection from area 17 to the lateral geniculate nucleus arises from pyramidal neurons in layer VI, and terminates through all layers of the lateral geniculate nucleus, but most densely in the interlaminar zones. Areas 18 and SV project predominantly to the C layers. Areas 17, 18, and SV are reciprocally connected with the claustrum and the LP-pulvinar complex, and project to the superior colliculus. All four visual cortical areas are mutually interconnected; these associational projections arise from both the supragranular and infragranular layers.     

Bifurcation of subcortical afferents to visual areas 17, 18, and 19 in the cat cortex

We have examined the pattern of axon bifurcation in the thalamic and claustral afferents to visual areas 17, 18, and 19 in the adult cat neocortex. This was achieved by injecting two fluorescent retrograde tracers, fast blue and diamidino yellow, in retinotopically corresponding regions of two of these three cortical areas. The pattern of single- and double-labelled cells was then examined in subcortical structures and the presence of double-labelled cells was interpreted as indicating that these neurons send bifurcating axons to the two injected areas. The size of the cortical region surrounding the injection site where each fluorescent dye is taken up was studied by making side-by-side injections of the two tracers in area 17 and examining the size and the separation of the two groups of labelled cells in the lateral geniculate nucleus (LGN). From these experiments we conclude that the uptake region is smaller than 1 mm and is included in the region of dense coloring surrounding the track of the injection needle. Injections were made in cortical regions which were in retinotopic correspondence as determined by electrophysiological recording. The double-labelled neurons were always found in the zone of overlap of the two populations of colored cells and no double-labelled neurons were found when there was no overlap between these populations. This indicates that the bifurcating axons send branches to strictly retinotopically corresponding regions in the two cortical areas. After injections in areas 18 and 19, numerous double-labelled cells were observed in laminae C of the LGN, in the medial interlaminar nucleus (MIN), the posterior nucleus (PN), and the lateral part of the lateral posterior nucleus (LP), in the retinorecipient zone of the pulvinar (RRZ-Pul), the intralaminar nuclei (ILN), and the claustrum. The proportions of double-labelled cells with respect to the total number of labelled neurons were computed in the region of overlap of the two populations of labelled cells. These percentages ranged between 5 and 20% and were highest in the C laminae of the LGN, the intralaminar nuclei, and the claustrum. After injection of areas 17 and 18, similar proportions of double-labelled cells were observed in the same structures, as well as in the A laminae of the LGN. Here again, the intralaminar nuclei and the claustrum tended to have slightly higher (20-30%) proportions of double-labelled cells. When the nonadjacent areas 17 and 19 were injected, doubled-labelled neurons were also observed in all these structures, except the A laminae of the LGN.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of monocular deprivation on the cat's geniculate neurons projecting to both areas 17 and 18

When a kitten is reared with one eyelid sutured closed, there are profound changes in the developing visual system. In the lateral geniculate nucleus, the neurons in the laminae innervated by the deprived eye are smaller than normal, and some of these neurons may lose connections with the visual cortex. In the present study a variety of double label retrograde transport methods were used to define the effects of monocular deprivation on cortical projections of geniculate neurons. One marker was injected into area 17 and the other was injected into area 18. Neurons projecting to area 17 are on average 16.4% smaller than those in the nondeprived laminae. The neurons that normally would project to both areas 17 and 18 by an axon that branches are the most severely affected by monocular deprivation. These cells are nearly 40% smaller than their counterparts in the nondeprived laminae, and many of the neurons appear to lose their projection to one of the cortical areas. These neurons may be at a distinct disadvantage, since they must compete with neurons from the nondeprived laminae for a considerable amount of cortical territory in two different cortical areas. This competition may be so severe that some of the neurons are no longer capable of maintaining connections with both cortical areas.     

Anatomical organization of primary visual cortex (area 17) in the ferret

The present report describes the intrinsic and extrinsic cortical connectivity of striate cortex (area 17) in the ferret. Injections of horseradish peroxidase demonstrate periodic intrinsic connections over an extent of 2.5-3.0 mm, mainly in the supragranular layers but also occurring secondarily in layer 5. These connections have a stripelike configuration, with a center-to-center spacing of 0.5-0.7 mm. Their laminar distribution and stripelike configuration resemble the pattern in the cat (Gilbert and Wiesel, '83), another member of the carnivore family, but not that in monkeys. In both macaque and squirrel monkeys, these connections have a bilaminar distribution in layers 2-3 and 4B, and a more complicated latticelike geometry (Rockland and Lund, '83). Their interperiod spacing, of about 0.5 mm, however, is relatively constant across species. Extrinsic connections in the ferret link striate cortex with territories probably homologous to feline areas 18 and 19, and to the suprasylvian region. Callosal connections extend on the lateral surface about 1.5 mm into area 17 and 4.0 mm into area 18 beyond their common border. There are homotopical connections between striate cortices and heterotopical connections from at least areas 18 and 19 to contralateral area 17. In addition to gray matter connections, intracortical injections also result in labeled interstitial neurons in the subgriseal white matter. These occur both subjacent to an injection site in area 17, and below labeled foci in area 18 projecting back to area 17, as if interstitial neurons shared the connectivity of overlying layer 6.     

Laminar origins of visual corticocortical connections in the cat

The interconnections among visual areas in cat cortex were studied with respect to the specific laminae in which the cortically projecting neurons are located. Single injections of HRP were made through recording micropipettes into nine different visual areas. In 15 cortical areas the laminar distribution of neurons which were retrogradely filled with HRP was plotted. In this way we determined the laminar origins of the cortical projections to the nine separate cortical visual areas which were injected. There are three major observations. First, areas 17 and 18 are the only two visual areas in which layers II and III are the primary site of cortically projecting cells; in the other 13 areas the deeper layers of cortex provide a large percentage of such neurons. Second, within any one cortical area, cortically projecting neurons may be distributed among different layers; the specific layer depends upon the cortical target of those neurons. Third, any one cortical area receives projections from several different cortical layers, the specific layers being dependent upon the area from which the projection originates. An individual cortical area, therefore, contributes to several different cortical visual circuits, with each of these circuits defined by the laminar connections of its neurons.     

The projection of the lateral geniculate nucleus to area 18

The present study is concerned with the projection of the lateral geniculate nucleus onto cortical area 18. Horseradish peroxidase (HRP) was injected into area 18 of 15 cats. Drawings were made to determine the location of the injection site and the distribution of labeled neurons in the lateral geniculate nuclei of each cat. The local retinotopic maps constructed prior to the injections and the reconstructions of the lateral geniculate nucleus were used to determine the location and the extent of each of the HRP injections. In 15 of the 25 hemispheres studied, the ratio of the number of HRP-labeled neurons in lamina A relative to the number of labeled neurons in lamina A1 was calculated. This ratio varied from 1.06 to 0.28, indicating that at least some regions of area 18 are dominated by inputs from lamina A1. However, if the HRP-labeled neurons in lamina C are included in the counts for lamina A, then the ratio A + C/A1 has a mean of 1.11, suggesting that area 18 receives a balanced input, with inputs from the contralateral eye being relayed through laminae A and C, and inputs from the ipsilateral eye being relayed through lamina A1. When the distribution of HRP-labeled neurons in lamina A was plotted onto a dorsal view of the lateral geniculate nucleus, the labeled neurons formed an ellipse with the long axis of the ellipse oriented parallel to the isoelevation lines. The representation of azimuth is compressed in area 18 relative to the lateral geniculate nucleus. In six hemispheres the injections were restricted to a few layers of the area 18. Following small injections into layer IV of area 18, the HRP-labeled neurons occupied an extensive region of the lateral geniculate nucleus, indicating a considerable amount of convergence of the inputs to area 18. In hemispheres where the injections were restricted to layers I and II, labeled neurons were only seen in the medial interlaminar nucleus and the C laminae.     

Effects of early monocular eyelid suture upon development of relay cell classes in the cat's lateral geniculate nucleus

Horseradish peroxidase (HRP) was injected into visual cortex of four normal cats and five cats raised with monocular lid suture, and retrograde labelling was assessed in cells of the lateral geniculate nucleus. In all but one of the sutured cats (noted below) focal injections were carefully limited to area 17 or 18 and analysis of labelling focused on laminae A and A1. The effects of deprivation were indistinguishable whether lamina A or A1 was deprived, and in all cases, the nondeprived laminae had labelling essentially identical to that seen in normal cats. After area 17 injections (bilateral in one normal cat and unilateral in 3 deprived cats), roughly 77% of the cells in nondeprived laminae were labelled and they were mostly small to medium in size. Deprived laminae, when compared to nondeprived laminae, had two abnormalities: (1) cells, both labelled and unlabelled, were smaller; and (2) roughly 11% fewer cells (i.e., 66%) were labelled, and this represents a small but statistically significant difference for each cat. After area 18 injections (bilateral in one normal cat plus unilateral in 3 other normal and 3 deprived cats), roughly 15% of the cells in nondeprived laminae were labelled, and they tended to be large in size. Deprived laminae, when compared to nondeprived laminae, had three abnormalities: (1) only 5–6% of the cells were labelled, and these tended to be quite faintly labelled; (2) the volume occupied by labelled cells was small; and (3) both labelled and unlabelled cells were reduced in size. Finally, large bilateral injections were made throughout occipitotemporal cortex in one lid sutured cat in an effort to label completely the terminal zones of cells in the medial interlaminar nucleus (MIN), a division of the lateral geniculate nucleus; this cat also had a prior intraocular injection of tritiated proline to provide through subsequent autoradiography a delineation of deprived and nondeprived portions of MIN. Roughly 78% of the cells in nondeprived portions of MIN were labelled in this cat. In the deprived portions, only about 51% of the cells were labelled, and these tended to be faintly labelled. Also, labelled cells were smaller, and unlabelled cells were larger in deprived than they were in nondeprived portions. Since prior studies have shown that, within the A laminae, X-cells project exclusively to area 17 whereas the Y-cell population projects to areas 17 and 18, these data are taken as further support of the conclusion that geniculate Y-cells are more seriously affected by the early deprivation than are geniculate X-cell. That is, these data are consistent with the suggestion that a similar population of Y-cells in deprived laminae (roughly 10% of the overall cell total) fail to transport HRP from area 17 or area 18 injections. This can be extended to the MIN, which seems to be comprised nearly exclusively of Y-cells. However, these conclusions must be considered tentative, since interpretation of HRP data can be difficult as evidenced by discrepancies in the literature.     

Cortical projections of the lateral geniculate nucleus in the cat

A new application of the retrograde transport method designed to demonstrate neurons that project to two cortical areas has been developed. This method depends on the retrograde axonal transport of two markers, each of which is uniquely detectable by histological methods. In this study, horseradish peroxidase (detectable by the enzymatic reation product only) and tritiated proteins (either enzymatically inactive tritiated horseradish peroxidase or tritiated bovine serium albumin, both of which are detectable by the tritium label only) were used. One of these markers was injected into cortical area 17 and the other was injected into area 18. In layers A and A₁ of the lateral geniculate nucleus, 10% of the cells project to both area 17 and area 18 by axons that branch, 70% of the neurons project to area 17 only, less than 1% of the neurons project to area 18 only, and approximately 20% of the cells are probably interneurons. In the C laminae 50% of the cells project to both areas 17 and 18 by axons that branch, approximately 20% of the neurons project to area 17 only, 10% of the cells project to area 18 only, and about 20% of the neurons are unlabeled. The cells in the medial interlaminar nucleus project to area 17 only, to area 18 only, or to both of these areas by axons that branch. In addition the retrograde markers were injected into single cortical areas, either area 17, area 18, or area 19. The injections of area 17 and those of area 18 confirmed the results of the double-label experiments, with 80% of the cells in the A laminae projecting to area 17 and approximately 10% projecting to area 18. Following injections of area 19, labeled neurons were seen in the medial interlaminar nucleus and the C laminae. Therefore, the medial interlaminar nucleus contains cells that project to areas 17, 18, and 19, with some cells projecting to both area 17 and area 18 by axons that branch. In the C laminae the analysis of the projection pattern could be carried further, for it was possible to determine the percentage of labeled neurons in this region. Since 80% of the cells project to areas 17 and 18, and since 60% of the cells of the C laminae were labeled following injections of area 19, many of the cells which project to area 19 must also project to either area 17 or area 18, and some cells must project to all three (areas 17, 18, and 19) by branching axons.     

Auditory Neurons with Transitory Axons to Visual Areas Form Short Permanent Projections

The kitten's auditory cortex (including the first and second auditory fields AI and AII) is known to send transient axons to either ipsi- or contralateral visual areas 17 and 18. By the end of the first postnatal month the transitory axons, but not their neurons of origin, are eliminated. Here we investigated where these neurons project after the elimination of the transitory axon. Eighteen kittens received early (postnatal day (pd) 2 - 5) injections of long lasting retrograde fluorescent traces in visual areas 17 and 18 and late (pd 35 - 64) injections of other retrograde fluorescent tracers in either hemisphere, mostly in areas known to receive projections from AI and AII in the adult cat. The middle ectosylvian gyrus was analysed for double-labelled neurons in the region corresponding approximately to AI and AII. Late injections in the contralateral (to the analysed AI, AII) hemisphere including all of the known auditory areas, as well as some visual and 'association' areas, did not relabel neurons which had had transient projections to either ipsi- or contralateral visual areas 17 - 18. Thus, AI and AII neurons after eliminating their transient juvenile projections to visual areas 17 and 18 do not project to the other hemisphere. In contrast, relabelling was obtained with late injections in several locations in the ipsilateral hemisphere; it was expressed as per cent of the population labelled by the early injections. Few neurons (0 - 2.5%) were relabelled by large injections in the caudal part of the posterior ectosylvian gyrus and the adjacent posterior suprasylvian sulcus (areas DP, P, VP). Multiple injections in the middle ectosylvian gyrus relabelled a considerably larger percentage of neurons (13%). Single small injections in the middle ectosylvian gyrus (areas AI, AII), the caudal part of the anterior ectosylvian gyrus and the rostral part of the posterior ectosylvian gyrus relabelled 3.1 - 7.0% of neurons. These neurons were generally near (<2.0 mm) the outer border of the late injection sites. Neurons with transient projections to ipsi- or contralateral visual areas 17 and 18 were relabelled in similar proportions by late injections at any given location. Thus, AI or AII neurons which send a transitory axon to ipsi- or contralateral visual areas 17 and 18 are most likely to form short permanent cortical connections. In that respect, they are similar to medial area 17 neurons that form transitory callosal axons and short permanent axons to ipsilateral visual areas 17 and 18.     

Anatomical evidence for glutamate and/or aspartate as neurotransmitters in the geniculo-, claustro-, and cortico-cortical pathways to the cat striate cortex

Data obtained by using various experimental approaches suggest that in the mammalian brain, most neurons within the visual system projecting to the striate cortex employ excitatory amino acids as transmitters. In order to investigate further the neurotransmitter phenotype of the ipsilateral afferents to area 17 of the cat, we have injected D-[³H]-aspartate, a retrograde tracer which selectively reveals putative glutamatergic and/or aspartatergic pathways, into this area. Retrogradely labelled neurons were observed in the dorsal lateral geniculate nucleus, visual claustrum, cortical areas 18, 19, 21a, and in both posteromedial and posterolateral parts of the suprasylvian areas but not in other known thalamic afferents such as the lateral posterior-pulvinar complex and the intralaminar nuclei. The distribution and localization of the labelled cells in all these regions were similar to that observed by using the non-selective tracer horseradish peroxidase conjugated to wheat germ agglutinin, though the number of cells was higher with the latter. Our findings provide additional evidence for the presence of excitatory amino acids as neurotransmitters in the major afferents to the cat striate cortex. © 1996 Wiley-Liss, Inc.     

Visual thalamocortical projections in normal and enucleated rats: HRP and fluorescent dye studies

Visual thalamocortical projections of neonatally enucleated and control rats were studied after tracer injections into the striate and peristriate areas of adult pigmented rats. The distribution of retrogradely labeled neurons in the visual thalamic nuclei was mapped after (a) small localized injections of horseradish peroxidase into either area 17, 18, or 18a and (b) simultaneous injections of three different retrograde tracers (fast blue, HRP, and diamidino yellow) into the anterior, medial, and posterior regions of area 17. It was shown in both normal and neonatally enucleated rats, that the dorsal lateral geniculate nucleus projects to the striate cortex (area 17), whereas the laterodorsal thalamic nucleus of the lateral thalamus projects to the medial peristriate area 18, and the lateral posterior thalamic nucleus has a projection to the lateral peristriate area 18a. Additionally, both extrageniculate visual thalamic nuclei project to area 17. Neurons in the dorsoanterior region of the dorsal lateral geniculate nucleus project to the posterior part of area 17, while neurons in the ventroposterior region of the nucleus send their axons to the anterior part of area 17. A similarly inverted projection of anterior and posterior divisions of the lateral posterior thalamic nucleus to visual area 18a was detected. In enucleated rats, the general topography of the projections from the thalamic neurons to the striate and peristriate cortices was indistinguishable from that in the controls. Nonetheless, there was noticeable shrinkage of the dorsal lateral geniculate nucleus and lateral thalamus and a significant decrease in the size of the somata of projecting neurons. Mean somal area of the HRP-labeled neurons in the dorsal lateral geniculate nucleus of enucleated rats was reduced by 19.0% and the mean maximum cell diameter by 14.3% compared with controls.     

Projections to the visual cortex in the golden hamster.

Retrograde transport of horseradish peroxidase (HRP) was used to determine the origins of afferent connexions to the visual cortex (areas 17, 18a and 18b) in the hamster. The distribution of neurons projecting to the visual cortex from other cortical areas, from the thalamus and from the brainstem was studied using a computer technique for three-dimensional reconstruction. There is a topographically organized projection from the dorsal lateral geniculate nucleus to area 17, but probably to no other of the areas studied. The lateral posterior nucleus of the thalamus (LP) projects to area 18a and weakly to area 17. The lateral nucleus (L) projects to area 18b and also, probably, weakly to area 17. The cortical projections from LP and L are also organized topographically but relatively grossly compared with the geniculo-cortical pathway. There are reciprocal association projections between area 17 and areas 18a and 18b. Areas 18a projects weakly to 18b. The main commissural connexions of the posterior neocortex are between the area 17/18a boundary zones in the two hemispheres, with little between the bodies of area 17. Labelled neurons were found bilaterally in the locus coeruleus, more ipsilaterally than contralaterally, after multiple injections into the visual cortex: single, small injections sometimes resulted in the labelling of a single cell body in the locus coeruleus.     

Interchange of callosal and association projections in the developing visual cortex

Neurons projecting transitorily into the corpus callosum from area 17 of the cat were retrogradely labeled by the fluorescent tracer Fast Blue (FB) injected into contralateral areas 17 and 18 on postnatal days 1-5. During the second postnatal month these neurons were still labeled by the early injection, although they had eliminated their callosal axon. At this time, 15-20% of these neurons could be retrogradely relabeled by injections of Diamidino Yellow (DY) into ipsilateral areas 17 and 18, but few or none by similar injections in the other areas that receive from area 17 (19, 21a, PMLS, 20a, 20b, DLS). Similarly, area 17 neurons projecting transitorily to contralateral area PMLS during the first postnatal week could be relabeled by DY injections in ipsilateral areas 17 and 18 but not in PMLS. Already around birth, many transitorily callosal neurons in area 17 send bifurcating axons both to contralateral areas 17 and 18 and ipsilateral area 18. It is probable that during postnatal development some of these neurons selectively eliminate their callosal axon collaterals and maintain the projection to ipsilateral area 18. In fact, some transitorily callosal neurons in area 17 can be double-labeled by simultaneous perinatal injections of FB in contralateral areas 17 and 18 and of a new long-lasting retrograde tracer, rhodamine-conjugated latex microspheres, in ipsilateral area 18. The same neurons can then be relabeled by reinjecting ipsilateral area 18 with DY during the second postnatal month. This finding, however, does not exclude the possibility that some transitorily callosal neurons send an axon to ipsilateral area 18 after eliminating their callosal axon. In conclusion, area 17 neurons that project transitorily through the corpus callosum later participate, probably permanently, in ipsilateral corticocortical projections but selectively to areas 17-18. The mechanism responsible for this selectivity is unknown, but it may be related to the differential radial distribution (i.e., to birth date) of area 17 neurons engaged in the various corticocortical projections. The problems raised by the use of long-lasting retrograde fluorescent tracers in neurodevelopmental studies and by the quantification of results of double- and triple-labeling paradigms are also discussed.     

Connections of the multiple visual cortical areas with the lateral posterior-pulvinar complex and adjacent thalamic nuclei in the cat

The present report describes the patterns of cat thalamocortical interconnections for each of the 13 retinotopically ordered visual areas and additional visual areas for which no retinotopy has yet emerged. Small injections (75 nl) of a mixture of horseradish peroxidase and [3H]leucine were made through a recording pipette at cortical injection sites identified by retinotopic mapping. The patterns of thalamic label show that the lateral posterior-pulvinar complex of the cat is divided into three distinct functional zones, each of which contains a representation of the visual hemifield and shows unique afferent and efferent connectivity patterns. The pulvinar nucleus projects to areas 19, 20a, 20b, 21a, 21b, 5, 7, the splenial visual area, and the cingulate gyrus. The lateral division of the lateral posterior nucleus projects to areas 17, 18, 19, 20a, 20b, 21a, 21b, and the anterior medial (AMLS), posterior medial (PMLS), and ventral (VLS) lateral suprasylvian areas. The medial division of the lateral posterior nucleus projects to areas AMLS, PMLS, VLS, and the anterior lateral (ALLS), posterior lateral (PLLS), dorsal (DLS) lateral suprasylvian areas, and the posterior suprasylvian areas. In addition, many of these visual areas are also interconnected with subdivisions of the dorsal lateral geniculate nucleus (LGd). Every retinotopically ordered cortical area (except ALLS and AMLS) is reciprocally interconnected with the parvocellular C layers of the LGd. The medial intralaminar nucleus of the LGd projects to areas 17, 18, 19, AMLS, and PMLS. Finally, each cortical area (except area 17) receives a projection from thalamic intralaminar nuclei. These results help to define the pathways by which visual information gains access to the vast system of extrastriate cortex in the cat.     

Visual cortex development in the ferret. I. Genesis and migration of visual cortical neurons

The production of ferret visual cortical neurons was studied using 3H-thymidine autoradiography. The genesis of cortical neurons begins on or slightly before embryonic day 20 (E20) of the 41 d gestational period, continues postnatally until 2 weeks after birth (P14), and follows an inside-out radial gradient with neurons for the deeper cortical layers being generated before those for the superficial layers. Layer I neurons are generated both early (E20-E30) and late (P1-P14) in the period of cortical neurogenesis and, thus, provide at least a partial exception to the inside-out gradient of cortical neurogenesis. Tangential gradients of cortical neurogenesis extend across areas 17 and 18 in both the anterior-to-posterior and lateral-to-medial directions. Neither of these gradients bears a meaningful relationship to the cortical representation of the visual field. Most infragranular and granular layer neurons are generated prenatally, while most supragranular layer neurons are produced postnatally. Neurons destined for a given layer are produced over a period of several days, and the neurons generated on any given day contribute to the formation of 2 or more cortical layers. In general, prenatally generated neurons complete their migration in 1 week or less, while most postnatally generated neurons require approximately 2 weeks to complete their migration.     

Spiny stellates as cells of origin of association fibres from area 17 to area 18 in the cat''s neocortex

Neurons in the cat's area 17 were stained in Golgi-like fashion following injection of horseradish peroxidase into area 18. Such staining allows classification of neurons on the basis of dendritic morphology. The types of neurons found in area 17 are: pyramidal cells in layers 2,3 and 4ab; spiny stellate cells in the lower part of layer 3, and in layer 4ab; and a few pyramidal and spindle cells in layer 5. The axons of the spiny stellate cells are finer than those of pyramidal cells; they give off collatera;s in deeper cortical layers and may bifurcate when entering the white matter. Spiny stellates in area 17 do not project to area 19; after injections are made into area 17, these neurons are found neither in area 18 nor in area 19. The spiny stellate cell with a long axonis thus categorized as a projection neuron which takes part in the pathway from area 17 to ipsilateral and contralateral¹⁰ area 18.     

Divergence of single axons in afferent projections to the cat's visual cortical areas 17, 18, and 19: a parametric study

The proportions of neurons projecting via axon collaterals to two areas in the cat's occipital cortex (diverging neurons) were determined quantitatively in subcortical and cortical afferents by making use of the retrograde axonal transport of two different tracers. The proportions of diverging neurons were determined for that part of the afferent sites in which neurons filled with tracers from both injected areas occurred (overlap zone). A number of experimental variables were tested for their role in possibly influencing the results of quantitative double-label experiments, among them the types and the combinations of retrograde tracers, the position of the injections, the survival time, and the histological procedure. The most important variable was the position of the cortical injection, which had to be restricted clearly to the cortical grey matter and to one cortical area in order to avoid false-positive double labeling. Other experimental variables affected the total number of retrogradely labeled neurons and/or the ratio between neurons labeled with the two different tracers rather than the proportions of double-labeled neurons. In particular DL proportions were largely independent of the number and density of labeled neurons. They only deviated significantly from mean values in those sections in which the number of labeled neurons amounted to less than 20% of the maximal number of labeled neurons found in one section throughout the overlap zone. Our results show that divergence is common in afferents to the cat visual cortex. The amount of divergence, however, varies considerably according to the origin of the afferent projection. The proportion of diverging neurons expressed as the percentage of the total number of neurons projecting to areas 17 and 18 was 3% in the A-laminae of the dorsal part of the lateral geniculate nucleus, about 8% in the posteromedial lateral suprasylvian area, and about 15% in the C-laminae of the dorsal part of the lateral geniculate nucleus, in the medial interlaminar nucleus, in the lateral part of the lateral posterior nucleus, and in the claustrum. The proportions of diverging neurons in the afferent projections to areas 17 and 19, and to areas 18 and 19 were about 10%. Diverging neurons were also found in the projections of the intralaminar thalamic nuclei to the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Projections from cortical visual areas 17, 18, and MT onto the dorsal lateral geniculate nucleus in owl monkeys.

Projections from cortical visual Areas 17, 18, and MT to the dorsal lateral geniculate nucleus of owl monkeys were revealed by the use of the autoradiographic tracing method. Restricted injections of 3H-proline into Area 17 resulted in dense columns of labeled tissue extending through all layers of the nucleus and roughly perpendicular to the layers. There was some lateral spread of the label in the interlaminar zones. Projections from Areas 18 and MT were distributed over the ventral layers of the nucleus including the magnocellular layers, layer S, and the adjoining interlaminar zones. The input from Area 18 was relatively sparse. The projections from all three cortical visual areas appear to connect homotopic locations in the geniculate and cortical representations of the visual field.