Cortical connections between rat cingulate cortex and visual,motor, and postsubicular cortices

The connections of rat cingulate cortex with visual, motor, and postsubicular cortices were investigated with retrograde and anterograde tracing techniques. In addition, connections between visual and the postsubicular (area 48) and parasubicular (area 49) cortices were evaluated with the same techniques. The following conclusions were drawn Area 29 connections: Afferents to area 29 originate mainly from cingulate areas 24 and 25, visual cortex (primarily area 18b), motor cortex area 8, area 11 of frontal cortex, areas 48 and 49, and the subiculum. Efferent connections of area 29 within cingulate cortex and to visual areas differ for each cytoarchitectural subdivision of area 29. Thus, area 29c has limited projections both within cingulate cortex and to areas 48 and 49, while area 29d projects to these areas as well as to area 8, area 18b, and medial area 17. These visual cortex afferents originate mainly from layer V neurons of areas 29b and 29d, while areas 29a and 29c have virtually no projections to visual cortex Area 24 connections: Afferents to area 24 originate primarily from cingulate areas 25 and 29 and visual area 18b and medial area 17. Efferent projections of area 24a are distributed within cingulate cortex, while area 24b has more extensive projections to posterior cingulate and visual cortices. Area 24b is the cingulate subdivision which is both the primary recipient of visual cortex afferents as well as the source of most of the projections of anterior cingulate cortex to visual areas Visual cortex has reciprocal connections with parts of the postsubicular and parasubicular cortices. Neurons of the internal pyramidal cell layer of both areas 48 and 49 project to areas 17 and 18b, while layers I and III of these parahippocampal areas receive projections from areas 17 and 18b In conclusion, areas 29d and 24b have particularly extensive interconnections with visual cortex, while area 29d also maintains projections to area 8 of motor cortex. This connection scheme supports the view that cingulate cortex may have a role in feature extraction from the sensory environment, as well as in sensorimotor integration. Finally, the postsubiculum may be classified as alimbic association cortex in which extensive visual and cingulate efferents converge.     

Macaque monkey retrosplenial cortex: III. Cortical efferents

We have investigated the cortical efferent projections of the macaque monkey retrosplenial and posterior cingulate cortices by using (3)H-amino acids as anterograde tracers. All the injections produced extensive local connections to other portions of this region. There were also a number of extrinsic efferent cortical connections, many of which have not hitherto been reported. Major projections from the retrosplenial cortex were directed to the frontal lobe, with heaviest terminations in areas 46, 9, 10, and 11. There were also very substantial projections to the entorhinal cortex, presubiculum, and parasubiculum of the hippocampal formation, as well as to areas TH and TF of the parahippocampal cortex. Some injections led to labeling of area V4, the dorsal bank of the superior temporal sulcus, and area 7a of the parietal cortex. Projections from the posterior cingulate cortex innervated all these same regions, although the density of termination was different from the retrosplenial projections. The posterior cingulate cortex gave rise to additional projections to parietal area DP and to the cortex along the convexity of the superior temporal gyrus. The ventral portion of the posterior cingulate cortex (area 23v) gave rise to much denser efferent projections to the hippocampal formation than the dorsal portions (areas 23e and i). These connections are discussed in relation to the clinical syndromes of retrosplenial amnesia and topographic disorientation in humans commonly caused by lesions in the caudoventral portions of the retrosplenial and posterior cingulate cortices.     

Macaque monkey retrosplenial cortex: II. Cortical afferents

We investigated the cortical afferents of the retrosplenial cortex and the adjacent posterior cingulate cortex (area 23) in the macaque monkey by using the retrograde tracers Fast blue and Diamidino yellow. We quantitatively analyzed the distribution of labeled neurons throughout the cortical mantle. Injections involving the retrosplenial cortex resulted in labeled neurons within the retrosplenial cortex and in areas 23 and 31 (approximately 78% of the total labeled cells). In the remainder of the cortex, the heaviest projections originated in the hippocampal formation, including the entorhinal cortex, subiculum, presubiculum, and parasubiculum. The parahippocampal and perirhinal cortices also contained many labeled neurons, as did the prefrontal cortex, mainly in areas 46, 9, 10, and 11, and the occipital cortex, mainly area V2. Injections in area 23 also resulted in numerous labeled cells in the posterior cingulate and retrosplenial regions (approximately 67% of total labeled cells). As in the retrosplenial cortex, injections of area 23 led to many labeled neurons in the frontal cortex, although most of these cells were in areas 9 and 46. Larger numbers of retrogradely labeled cells were also distributed more widely in the posterior parietal cortex, including areas 7a, 7m, LIP, and DP. There were some labeled cells in the parahippocampal cortex. These connections are consistent with the retrosplenial cortex acting as an interface between the working memory functions in the prefrontal areas and the long-term memory encoding in the medial temporal lobe. The posterior cingulate cortex, in contrast, may be more highly associated with visuospatial functions.     

Corticothalamic connections of paralimbic regions in the rhesus monkey

This study addressed the issue of whether paralimbic regions of the cerebral cortex share common thalamic projections. The corticothalamic connections of the paralimbic regions of the orbital frontal, medial prefrontal, cingulate, parahippocampal, and temporal polar cortices were studied with the autoradiographic method in the rhesus monkey. The results revealed that the orbital frontal, medial prefrontal, and temporal polar proisocortices have substantial projections to both the dorsomedial and medial pulvinar nuclei, whereas the anterior cingulate proisocortex (area 24) projects exclusively to the dorsomedial nucleus. These proisocortical areas also have thalamic connections with the intralaminar and midline nuclei. The cortical areas between the proisocortical regions on the one hand and the isocortical areas on the other, that is, the posterior cingulate region (area 23) and the posterior parahippocampal gyrus (areas TF and TH), project predominantly to the dorsal portion of the medial pulvinar nucleus, the anterior nuclear group (AV, AM), and the lateral dorsal (LD) nucleus. Additionally, the posterior cingulate and medial parahippocampal gyri (area TH) have projections to the lateral posterior (LP) nucleus. Thus, it appears that the proisocortical areas, which are characterized by a predominance of infragranular layers and an absence of layer IV, have common thalamic relationships. Likewise, the intermediate paralimbic areas between the proisocortex and isocortical regions, which also have a predominance of infragranular layers but in addition have evidence of a fourth layer, project to the medial pulvinar and to the so-called limbic nuclei, AV, AM, LD, as well as a modality-specific nucleus, LP.     

Fiber pathways and cortical connections of preoccipital areas in rhesus monkeys

An understanding of visual function at the cerebral cortical level requires detailed knowledge of anatomical connectivity. Cortical association pathways and terminations of preoccipital visual areas were investigated in rhesus monkeys by using the autoradiographic tracing technique. Medial and adjacent dorsomedial preoccipital regions project via the occipitofrontal fascicle to the frontal lobe (dorsal area 6, and areas 8Ad, 8B, and 46); via the dorsal portion of the superior longitudinal fascicle (SLF) to dorsal area 6, area 9, and the supplementary motor area; and via the cingulate fascicle to area 24. In addition, medial and dorsomedial preoccipital areas send projections to parietal (areas PGm, PEa, PG‐Opt, and POa) and superior temporal (areas MST and MT) regions. In contrast, connections from the dorsolateral, annectant, and ventral preoccipital regions are conveyed via the inferior longitudinal fascicle (ILF) to the parietal lobe (areas POa and IPd), superior temporal sulcus (areas MT, MST, FST, V4t, and IPa), inferotemporal region (areas TEO and TE1–TE3), and parahippocampal gyrus (areas TF, TH, and TL). The central‐lateral preoccipital region projects via an ILF‐SLF pathway to frontal area 8Av. The preoccipital areas also have caudal connections to occipital areas V1, V2, and V3. Finally, preoccipital regions are interconnected via different intrinsic pathways. These findings provide further insight into the nature of preoccipital fiber pathways and the connectional organization of the visual system. J. Comp. Neurol. 518:3725–3751, 2010. © 2010 Wiley‐Liss, Inc.     

Medial temporal lobe projections to the retrosplenial cortex of the macaque monkey

The projections to the retrosplenial cortex (areas 29 and 30) from the hippocampal formation, the entorhinal cortex, perirhinal cortex, and amygdala were examined in two species of macaque monkey by tracking the anterograde transport of amino acids. Hippocampal projections arose from the subiculum and presubiculum to terminate principally in area 29. Label was found in layer I and layer III(IV), the former seemingly reflecting both fibers of passage and termination. While the rostral subiculum mainly projects to the ventral retrosplenial cortex, mid and caudal levels of the subiculum have denser projections to both the caudal and dorsal retrosplenial cortex. Appreciable projections to dorsal area 30 [layer III(IV)] were only seen following an extensive injection involving both the caudal subiculum and presubiculum. This same case provided the only example of a light projection from the hippocampal formation to posterior cingulate area 23 (layer III). Anterograde label from the entorhinal cortex injections was typically concentrated in layer I of 29a–c, though the very caudal entorhinal cortex appeared to provide more widespread retrosplenial projections. In this study, neither the amygdala nor the perirhinal cortex were found to have appreciable projections to the retrosplenial cortex, although injections in either medial temporal region revealed efferent fibers that pass very close or even within this cortical area. Finally, light projections to area 30V, which is adjacent to the calcarine sulcus, were seen in those cases with rostral subiculum and entorhinal injections. The results reveal a particular affinity between the hippocampal formation and the retrosplenial cortex, and so distinguish areas 29 and 30 from area 23 within the posterior cingulate region. The findings also suggest further functional differences within retrosplenial subregions as area 29 received the large majority of efferents from the subiculum. © 2012 Wiley Periodicals, Inc.     

Parietal,temporal, and occipita projections to cortex of the superior temporal sulcus in the rhesus monkey: A retrograde tracer study

The afferent cortical connections of individual cytoarchitectonic areas within the superior temporal sucus (STS) of the rhesus monkey were studied by retrograde tracer techniques, including double tracer experiments. Rostral superior temporal polysensory (STP) cortex (area TPO-1) receives input from the rostral superior temporal gyrus (STG), cortex of the circular sulcus, and parahippocampal gyrus (PHG) (areas 35, TF, and TL). Mid-STP cortex (areas TPO-2 and -3) has input from the mid-STG, cortex of the mid-circular sulcus, caudal inferior parietal lodule (IPL), cingulate gyrus (areas, 23, 24, retrosplenial cortex), and mid-PHG (areas 28, TF, TH, and TL). Caudal STP cortex (area TPO-4) has afferent connections with the caudal STG, cortex of the cauda insula and caudal circular sulcus, caudal IPL, lower bank of the intraparietal sulcus (IPS), medial parietal lobe, cingulate gyrus, and mid- and caudal PHG (areas TF, TH, TL; prostriate area). The most rostral cortex of the lower bank of the STS (areasTEa and TEm), a presumed visual association area, receives input from the rostal inferotemporal (IT) region; more cauda portions of areas TEa and TEm have afferent connections with the caudal IT region, PHG, preoccipital gyrus, and cortex of the lower bank of the IPS. © 1994 Wiley-Liss, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Association fiber pathways to the frontal cortex from the superior temporal region in the rhesus monkey

The projections to the frontal cortex that originate from the various areas of the superior temporal region of the rhesus monkey were investigated with the autoradiographic technique. The results demonstrated that the rostral part of the superior temporal gyrus (areas Pro, Ts1, and Ts2) projects to the proisocortical areas of the orbital and medial frontal cortex, as well as to the nearby orbital areas 13, 12, and 11, and to medial areas 9, 10, and 14. These fibers travel to the frontal lobe as part of the uncinate fascicle. The middle part of the superior temporal gyrus (areas Ts3 and paAlt) projects predominantly to the lateral frontal cortex (areas 12, upper 46, and 9) and to the dorsal aspect of the medial frontal lobe (areas 9 and 10). Only a small number of these fibers terminated within the orbitofrontal cortex. The temporofrontal fibers originating from the middle part of the superior temporal gyrus occupy the lower portion of the extreme capsule and lie just dorsal to the fibers of the uncinate fascicle. The posterior part of the superior temporal gyrus projects to the lateral frontal cortex (area 46, dorsal area 8, and the rostralmost part of dorsal area 6). Some of the fibers from the posterior superior temporal gyrus run initially through the extreme capsule and then cross the claustrum as they ascend to enter the external capsule before continuing their course to the frontal lobe. A larger group of fibers curves round the caudalmost Sylvian fissure and travels to the frontal cortex occupying a position just above and medial to the upper branch of the circular sulcus. This latter pathway constitutes a part of the classically described arcuate fasciculus.     

Convergence of Limbic Input to the Cingulate Motor Cortex in the Rhesus Monkey

Limbic system influences on motor behavior seem widespread, and could range from the initiation of action to the motivational pace of motor output. Motor abnormalities are also a common feature of psychiatric illness. Several subcortical limbic-motor entry points have been defined in recent years, but cortical entry points are understood poorly, despite the fact that a part of the limbic lobe, the cingulate motor cortex (area 24c or M3, and area 23c or M4), contributes axons to the corticospinal pathway. Using retrograde and anterograde tracers in rhesus monkeys, we investigated the ipsilateral limbic input to area 24c and adjacent area 23c. Limbic cortical input to areas 24c and 23c arise from cingulate areas 24a, 24b, 23a, 23b, and 32, retrosplenial areas 30 and 29, and temporal areas 35, TF and TH. Areas 24c and 23c were also interconnected strongly. The dysgranular part of the orbitofrontal cortex and insula projects primarily to area 24c while the granular part of the orbitofrontal cortex and insula projects primarily to area 23c. Afferents from cingulate area 25, the retrocalcarine cortex, temporal pole, entorhinal cortex, parasubiculum, and the medial part of area TH target primarily or only area 24c. Our findings indicate that a variety of telencephalic limbic afferents converge on cortex lining the lower bank and fundus of the anterior part of the cingulate sulcus. Because it is known that this cortex gives rise to axons ending in the spinal cord, facial nucleus, pontine gray, red nucleus, putamen, and primary and supplementary motor cortices, we suggest that the cingulate motor cortex forms a strategic cortical entry point for limbic influence on the voluntary motor system.     

Superior area 6 afferents from the superior parietal lobule in the macaque monkey

Superior area 6 of the macaque monkey frontal cortex is formed by two cytoarchitectonic areas: F2 and F7. In the present experiment, we studied the input from the superior parietal lobule (SPL) to these areas by injecting retrograde neural tracers into restricted parts of F2 and F7. Additional injections of retrograde tracers were made into the spinal cord to define the origin of corticospinal projections from the SPL. The results are as follows: 1) The part of F2 located around the superior precentral dimple (F2 dimple region) receives its main input from areas PEc and PEip (PE intraparietal, the rostral part of area PEa of Pandya and Seltzer, [1982] J. Comp. Neurol. 204:196–210). Area PEip was defined as that part of area PEa that is the source of corticospinal projections. 2) The ventrorostral part of F2 is the target of strong projections from the medial intraparietal area (area MIP) and from the dorsal part of the anterior wall of the parietooccipital sulcus (area V6A). 3) The ventral and caudal parts of F7 receive their main parietal input from the cytoarchitectonic area PGm of the SPL and from the posterior cingulate cortex. 4) The dorsorostral part of F7, which is also known as the supplementary eye field, is not a target of the SPL, but it receives mostly afferents from the inferior parietal lobule and from the temporal cortex. It is concluded that at least three separate parietofrontal circuits link the superior parietal lobule with the superior area 6. Considering the functional properties of the areas that form these circuits, it is proposed that the PEc/PEip-F2 dimple region circuit is involved in controlling movements on the basis of somatosensory information, which is the traditional role proposed for the whole dorsal premotor cortex. The two remaining circuits appear to be involved in different aspects of visuomotor transformations. J. Comp. Neurol. 402:327–352, 1998. © 1998 Wiley-Liss, Inc.     

Direct connections of rat visual cortex with sensory,motor, and association cortices

Each division of rat visual cortex, areas 17, 18a, and 18b, has connections with sensory, motor, and association cortices. These corticocortical connections were sampled using anterograde autoradiographic and retrograde horseradish peroxidase labeling techniques. Area 17 is connected via reciprocal pathways with each division of visual cortex, the posterior one-third of motor area 8, association area 7, and posteroventral area 36 of temporal cortex. It also receives projections from perirhinal areas 13 and 35. Area 18a has reciprocal connections with areas 17 and 18b, a patch in posterior somatosensory area 3, and dorsal auditory area 41. Like area 17, area 18a receives afferents from and projects to the posterior one-third of motor area 8. The connections of area 18a with association cortices are extensive; these regions include parietal areas 7, 39, 40, and 14, posteroventral and dorsal area 36, and perirhinal cortex. Area 18b is connected with areas 17 and 18a, a patch in medial area 3, and dorsal area 41. There are reciprocal projections between area 18b and posterior area 8. As for association cortex, area 18b projects to frontal area 11, area 7, posteroventral and dorsal area 36, and perirhinal cortex. In addition, area 18b receives input from and projects efferents to the dorsal claustrum. Most of the interconnections among areas 17, 18a, and 18b originate from neurons in layers II, III, and V and end in terminal fields in layers I–III and V. In contrast, projections of other sensory, motor, and association cortices to visual cortex originate mainly from neurons in layer V and to a lesser extent from layer II. The reciprocal pathways from visual cortex terminate predominantly in the supragranular layers. In conclusion, these corticocortical pathways provide the basis for cortical visuosensory and visuomotor integration that may aid the rat in the coordination of visually guided behaviors.     

Post-rolandic cortical projections of the superior temporal sulcus in the rhesus monkey.

The efferent connections of different cytoarchitectonic areas of the superior temporal sulcus (STS) in the rhesus monkey with parieto-temporo-occipital cortex were investigated using autoradiographic methods. Four rostral-to-caudal subdivisions of cortex (area TPO) in the upper bank of the STS have distinct projection patterns. Rostral sectors (areas TPO-1 and -2) project to the rostral superior temporal gyrus (areas Ts1, Ts2, and Ts3), insula of the Sylvian fissure, and parahippocampal gyrus (perirhinal and prorhinal cortexes, areas TF, TH, and TL); caudal sectors (TPO-3 and -4) project to the caudal superior temporal gyrus (areas paAlt and Tpt), supratemporal plane (area paAc), circular sulcus of the Sylvian fissure (area reIt), as well as medial paralimbic (areas 23, 24, and retrosplenial cortex) and extrastriate (areas 18 and 19) cortexes. Area TPO-1 does not project to the parietal lobe; area TPO-2 projects to the inferior parietal lobule; area TPO-3 to the lower bank of the intraparietal sulcus (IPS) (area POa); and area TPO-4 to medial parietal cortex (area PGm). Vision-related cortex (area TEa) in the rostral lower bank of the STS sends fibers to the rostral inferotemporal region (areas TE1, -2, and -3) and parahippocampal gyrus (perirhinal cortex, areas TF and TL). Visual zones in the caudal lower bank and depth of the sulcus (area OAa, or MT and FST) project to the caudal inferotemporal region (areas TE3 and TEO), lateral preoccipital region (area V4), and lower bank of the IPS (area POa). A zone in the rostral depth of the STS (area IPa) projects to the rostral inferotemporal region, parahippocampal gyrus, insula of the Sylvian fissure, parietal operculum, and lower rim of the IPS (area PG). STS projections to parieto-temporo-occipital cortex have "feedforward," "feedbackward," and "side-to-side" laminar patterns of termination similar to those of other cortical sensory systems. The differential connectivity supports the cytoarchitectonic parcellation of the STS and suggests functional heterogeneity.     

Topographical organization of the cortical afferent connections of the prefrontal cortex in the cat

The topographical distribution of the cortical afferent connections of the prefrontal cortex (PFC) in adult cats was studied by using the retrograde axonal transport of horseradish peroxidase technique. Small single injections of the enzyme were made in different locations of the PFC, and the areal location and density of the subsequent neuronal labeling in neocortex and allocortex were evaluated in each case. The comparison of the results obtained in the various cases revealed that four prefrontal sectors (rostral, dorsolateral, ventral, and dorsomedial) can be distinguished, each exhibiting a particular pattern of cortical afferents. All PFC sectors receive projections from the ipsilateral insular (agranular and granular subdivisions) and limbic (infralimbic, prelimbic, anterior limbic, cingular, and retrosplenial areas) cortices. These cortices provide the most abundant cortical projections to the PFC, and their various subdivisions have different preferential targets within the PFC. The premotor cortex and the following neocortical sensory association areas project differentially upon the various ipsilateral PFC sectors: the portion of the somatosensory area SIV in the upper bank of the anterior ectosylvian sulcus, the visual area in the lower bank of the same sulcus, the auditory area AII, the temporal area, the perirhinal cortex, the posterior suprasylvian area, area 20, the posterior ectosylvian area, the suprasylvian fringe, the lateral suprasylvian area (anterolateral and posterolateral subdivisions), area 5, and area 7. The olfactory peduncle, the prepiriform cortex, the cortico-amygdaloid transition area, the entorhinal cortex, the subiculum (ventral, posteroventral, and posterodorsal sectors), the caudomedial band of the hippocampal formation and the postsubiculum are the allocortical sources of afferents to the PFC. The dorsolateral PFC sector is the target of the largest insular, limbic, and neocortical sensory association projections. The dorsomedial and rostral sectors receive notably less abundant cortical afferents than the dorsolateral sector. Those to the dorsomedial sector arise from the same areas that project to the dorsolateral sector and are more abundant to the dorsal part, where the medial frontal eye field cortex is located. The rostral sector receives projections principally from all other PFC sectors, and from the limbic and insular cortices. The projections from the allocortex reach preferentially the ventral PFC sector. Intraprefrontal connections are most abundant within each PFC sector. Commissural interprefrontal connections are largest from the site homotopic to the HRP injection.(ABSTRACT TRUNCATED AT 400 WORDS)     

Parcellation of cortical afferents to three distinct sectors in the parahippocampal gyrus of the rhesus monkey: an anatomical and neurophysiological study

The posterior parahippocampal gyrus (PHG) of the rhesus monkey (Macaca mulatta) is comprised of three distinct cortical areas based on cytoarchitecture, connectivity, and neurophysiological response properties. Fluorescent retrograde tracers placed in each PHG area demonstrated unique patterns of cortical afferent input to areas TH, TL, and TF. Area TF receives inputs from the multimodal cortices of the superior temporal sulcus including areas PGa, TPO, and MST, from the visuospatial parietal area PG-Opt, and from visual areas V3A and dorsal V4. Area TL receives afferents from the inferotemporal region including visual areas TE1 and TE2 as well as from areas TEa, IPa, and FST in the lower bank and depth of the superior temporal sulcus. In contrast, the input to area TH is from the rostral part of superior temporal gyrus, including the auditory association areas TS1-3, and from the middle sector of area TPO in the superior temporal sulcus. Frontal and cingulate areas also project to the PHG in largely differential patterns. To further investigate this a correlative electrophysiological study of the three PHG areas resulted in a confirmation of these differential cortical inputs such that visually responsive neurons were found in areas TF and TL, auditory responsive neurons or bimodal auditory/visual-responsive neurons in area TH, and somatosensory-responsive neurons at the TF/TL border. Since each PHG area also receives differential hippocampal input, these data suggest that the processing of unimodal or multimodal information may be related to memory processing functions that are largely segregated within areas TH, TL, and TF.     

Afferent connections of the perirhinal cortex in the rat

Connections of the perirhinal cortex in the.rat brain were studied using anterograde (³H-proline/leucine) and retrograde (horseradish peroxidase) tracers. The perirhinal cortex receives major projections from medial precen-tral, anterior cingulate, prelimbic, ventral lateral orbital, ventral and posterior agranular insular, temporal, superior and granular parietal, lateral occipital, agranular retrosplenial, and ectorhinal cortices, and from the pre-subiculum, subiculum, and diagonal band of Broca. Rostral neocortical areas project predominantly to rostral perirhinal regions while more caudal neocortical and subicular areas project predominantly to caudal perirhinal regions. Terminal fields are further segregated within perirhinal cortex to either the dorsal or ventral banks of the rhinal sulcus. All afferents from frontal areas terminate predominantly in the deep layers of its ventral bank; afferents from temporal, parietal, and lateral occipital areas terminate predominantly in the deep and superficial layers along its dorsal bank; and afferents from ectorhinal cortex terminate in a column within its dorsal bank. Cortical cells which project to perirhinal areas are found predominantly in layer II and the superficial part of layer III. However, ventrolateral orbital, parietal, and lateral occipital cortex projections originate predominantly from layer V. Perirhinal areas also receive afferents from the nucleus reuniens of the thalamus, lateral nucleus of the amygdala, claustrum, supramammillary nuclei, and the dorsal raphe nuclei.     

Afferent connections of the medial frontal cortex of the rat. II. Cortical and subcortical afferents

In order to compare the frontal cortex of rat and macaque monkey, cortical and subcortical afferents to subdivisions of the medial frontal cortex (MFC) in the rat were analyzed with fluorescent retrograde tracers. In addition to afferent inputs common to the whole MFC, each subdivision of the MFC has a specific pattern of afferent connections. The dorsally situated precentral medial area (PrCm) was the only area to receive inputs from the somatosensory cortex. The specific pattern of afferents common to the ventrally situated prelimbic (PL) and infralimbic (IL) areas included projections from the agranular insular cortex, the entorhinal and piriform cortices, the CA1–CA2 fields of the hippocampus, the subiculum, the endopiriform nucleus, the amygdalopiriform transition, the amygdalohippocampal area, the lateral tegmentum, and the parabrachial nucleus. In all these structures, the number of retrogradely labeled cells was larger when the injection site was located in area IL. The dorsal part of the anterior cingulate area (ACd) seemed to be connectionally intermediate between the adjacent areas PrCm and PL; it receives neither the somatosensory inputs characteristic of area PrCm nor the afferents characteristic of areas PL and IL, with the exception of the afferents from the caudal part of the retrosplenial cortex. A comparison of the pattern of afferent and efferent connections of the rat MFC with the pattern of macaque prefrontal cortex suggests that PrCm and ACd areas share some properties with the macaque premotor cortex, whereas PL and IL areas may have characteristics in common with the cingulate or with medial areas 24, 25, and 32 and with orbital areas 12, 13, and 14 of macaques. © 1995 Wiley-Liss, Inc.     

Architecture and connections of retrosplenial area 30 in the rhesus monkey (Macaca mulatta).

Because of the sharp curvature of the retrosplenial region around the splenium of the corpus callosum, standard coronal sections are not appropriate for architectonic analysis of its posteroventral part. In the present study, examination of the posteroventral retrosplenial region of the rhesus monkey in sections that were orthogonal to its axis of curvature (and therefore appropriate for architectonic analysis) has permitted definition of its architecture and precise extent. This analysis demonstrated that areas 29 and 30 of the retrosplenial cortex, as well as adjacent area 23 of the posterior cingulate cortex, extend together as an arch around the splenium of the corpus callosum and maintain their topographical relationship with one another throughout their entire course. Injections of anterograde and retrograde tracers confined to retrosplenial area 30 revealed that this area has reciprocal connections with adjacent areas 23, 19 and PGm, with the mid-dorsolateral part of the prefrontal cortex (areas 9, 9/46 and 46), with multimodal area TPO in the superior temporal sulcus, as well as the posterior parahippocampal cortex, the presubiculum and the entorhinal cortex. There are also bidirectional connections with the lateroposterior thalamic nucleus, as well as the laterodorsal and the anteroventral limbic thalamic nuclei. The connectivity of area 30 suggests that it may play a role in working memory processes subserved by the mid-dorsolateral frontal cortex in interaction with the hippocampal system.     

Organization of cortical and subcortical projections to anterior cingulate cortex in the cat

The aim of the experiments reported here was to identify cortical and subcortical forebrain structures from which anterior cingulate cortex (CGa) receives input in the cat. Deposits of retrograde tracers were placed at nine sites spanning the anterior cingulate area and patterns of retrograde transport were analyzed. Thalamic projections to CGa, in descending order of strength, originate in the anteromedial nucleus, lateroposterior nucleus, ventroanterior nucleus, rostral intralaminar complex, reuniens nucleus, mediodorsal nucleus, and laterodorsal nucleus. Minor and inconsistent ascending pathways arise in the paraventricular, parataenial, parafascicular, and subparafascicular thalamic nuclei. The basolateral nucleus of the amygdala, the hypothalamus, the nucleus of the diagonal band, and the claustrum are additional sources of ascending input. Cortical projections to CGa, in descending order of strength, derive from posterior cingulate cortex, prefrontal cortex, motor cortex (areas 4 and 6), parahippocampal cortex (entorhinal, perirhinal, postsubicular, parasubicular, and subicular areas), insular cortex, somesthetic cortex (areas 5 and SIV), and visual cortex (areas 7p, 20b, AMLS, PS and EPp). In general, the limbic, sensory, and motor afferents of CGa are weak. The dominant sources of input to CGa are other cortical areas with high-order functions. This finding calls into question the traditional characterization of cingulate cortex as a bridge between neocortical association areas and the limbic system.     

Cortical projections of the non-entorhinal hippocampal formation in the cynomolgus monkey (Macaca fascicularis)

Episodic memory consolidation requires the integrity of the anatomical pathways between the cerebral cortex and the hippocampal formation. Whilst the largest cortical output of the hippocampal formation originates in the entorhinal cortex, direct projections from CA1, subiculum and presubiculum to the cortex have been reported. The aim of this study is the assessment of the extent, topography and relative strength of those projections, as a parallel/alternate route of memory processing. A total of 45 injections in 28 Macaca fascicularis monkeys were used. Cortical deposits of fluorescent tracers (20 cases, 3% Fast Blue, 2% Diamidino Yellow) or 1% WGA-HRP (eight cases) were made in different cortical areas of the frontal, temporal and parietal lobes, as well as cingulate cortex by direct exposure of the cortical surface. After appropriate survival, animals were perfused and the brains serially sectioned at 50 microm and the retrograde labelling charted with an X-Y digitizing system. Retrograde neuronal labelling was observed in CA1, subiculum, presubiculum and parasubiculum; it was absent in the dentate gyrus, CA3 and CA2. Compared to other portions of the hippocampal formation, the CA1-subiculum border had the highest number of labelled neurons (especially after deposits in the rostral perirhinal cortex), followed by medial frontal cortex, temporal pole, orbitofrontal, anterior and posterior cingulate cortices, parietal and inferotemporal cortices, and no labelling after posterior inferotemporal and lateral frontal cortices. Our results indicate that CA1, subiculum, presubiculum and parasubiculum send direct output to cortical areas. This nonentorhinal, hippocampal formation cortical output may be relevant in memory processing.     

Perirhinal and parahippocampal cortices of the macaque monkey: projections to the neocortex

We investigated the topographic and laminar organization of the efferent cortical projections of the perirhinal and parahippocampal cortices. Area 36 of the perirhinal cortex projects preferentially to areas TE and TEO, whereas area TF of the parahippocampal cortex projects preferentially to the posterior parietal cortex and area V4. Area TF projects to many regions of the frontal lobe, whereas area 36 projects mainly to the orbital surface. The insular and cingulate cortices receive projections from areas 36 and TF, whereas only area TF projects to the retrosplenial cortex. Projections to the superior temporal gyrus, including the dorsal bank of the superior temporal sulcus, arise predominantly from area TF. Area 36 projects only to rostral levels of the superior temporal gyrus. Area TF has, in general, reciprocal connections with the neocortex, whereas area 36 has more asymmetric connections. Area 36, for example, projects to more restricted regions of the frontal cortex and superior temporal sulcus than it receives inputs from. In contrast, it projects to larger portions of areas TE and TEO than it receives inputs from. The efferent projections of areas 36 and TF are primarily directed to the superficial layers of the neocortex, a laminar organization consistent with connections of the feedback type. Projections to unimodal visual areas terminate in large expanses of the cortex, but predominantly in layer I. Projections to other sensory and polymodal areas, in contrast, terminate in a columnar manner predominantly in layers II and III. In all areas receiving heavy projections, the projections extend throughout most cortical layers, largely avoiding layer IV. We discuss these findings in relation to current theories of memory consolidation.