Efferent connections of the rostral portion of medial agranular cortex in rats

This study of the rostral part of medial agranular cortex (AGm) was undertaken with two principal aims in mind. First, to delineate the efferent connections of AGm and compare these with the pattern of afferents defined by us in a previous study. Second, to provide a firmer basis for anatomical and functional comparisons with cortical regions in monkeys. Autoradiographic, horseradish peroxidase, and fiber degeneration techniques were used. Rostral AGm has a variety of corticocortical connections--with lateral agranular motor cortex (AGl); visual, auditory, and somatic sensory regions; and limbic/paralimbic areas including orbital, insular, perirhinal, entorhinal, retrosplenial and presubicular fields. The projections to orbital, perirhinal and entorhinal cortices are bilateral. Thalamic projections of rostral AGm are concentrated in the ventral lateral, central lateral, paracentral, mediodorsal and ventromedial nuclei. Moderate terminal fields are consistently seen in the reticular, anteromedial, central medial, gelatinosus, parafascicular, and posterior nuclei. More caudal projections reach the central gray, superior colliculus and pontine gray. The efferents of the adjacent AGl were also examined. Although many of these overlapped those of rostral AGm, there were no efferents to visual or auditory cortex and limbic/paralimbic projections were reduced. Thalamic projections were more focused in the ventral lateral and posterior nuclei and there were no terminal fields in the central gray or superior colliculus. Based on its afferent and efferent connections, role in contralateral neglect, and the results of microstimulation studies, rostral AGm can be viewed as a multimodal association area with strong ties to the motor system. On these structural and functional grounds, rostral AGm bears certain striking resemblances to the frontal eye field, supplementary motor, and arcuate premotor areas of monkey cortex.     

Neuronal populations in the posterior group of thalamic nuclei projecting into the amygdaloid complex and auditory cortex in the cat

Location of neurons in posterior thalamic nuclei and neighbouring structures of the midbrain regions projecting to the amygdaloid complex and auditory cortex of cat was studied by the method of horseradish peroxidase. The main sources of these brain region projections to amygdaloid complex are peripeduncular , subparafascicular and suprageniculate nuclei and caudal division of the medial geniculate body. The cells of origin of projections to the auditory cortex are located in all medial geniculate nuclei and wide regions of the posterior thalamic group. Neuron pools projecting to the auditory cortex and amygdala exist in medial parts of the posterior thalamic nuclei. The role of posterior thalamic nuclei in transmission of auditory signals to amygdala is discussed.     

The associative striatum: organization of cortical projections to the dorsocentral striatum in rats

The dorsocentral striatum (DCS) is the major site of input from medial agranular cortex (AGm) and has been implicated as an associative striatal area that is part of a cortical-subcortical circuit involved in multimodal spatial functions involving directed attention. Anterograde axonal tracing was used to investigate the spatial organization of corticostriatal projections to DCS. Injections of biotinylated dextran amine were made into several cortical areas known to project to DCS based on retrograde tracing data. These included areas AGm, lateral agranular cortex (AGl), orbital cortex, posterior parietal cortex (PPC), and visual association cortex. We discovered a previously undescribed geometry whereby the projection from AGm is prominent within DCS and the main corticostriatal projections from areas other than AGm are situated around the periphery of DCS: visual association cortex dorsomedially, PPC dorsally, AGl laterally, and orbital cortex ventrally. Each of these cortical projections is also represented by less dense aggregates of terminal labeling within DCS, organized as focal patches and more diffuse labeling. Because these cortical areas are linked by corticocortical connections, the present findings indicate that interconnected cortical areas have convergent terminal fields in the region of DCS. These findings suggest that DCS is a central associative region of the dorsal striatum characterized by a high degree of corticostriatal convergence.     

Heterotopic Cortical Afferents to the Medial Prefrontal Cortex in the Rat. A Combined Retrograde and Anterograde Tracer Study

Cortical afferent projections towards the medial prefrontal cortex (mPFC) were investigated with retrograde and anterograde tracer techniques. Heterotopical afferent projections to the medial prefrontal cortex arise in secondary, or higher order, sensory areas, motor areas and paralimbic cortices. On the basis of these projections three subfields can be discriminated within the mPFC. (1) The ventromedial part of mPFC, comprising the pre- and infralimbic areas, receives mainly projections from the perirhinal cortex. (2) The caudal two-thirds of the dorsomedial PFC, comprising frontal area 2 and the dorsal anterior cingulate area, receives projections from the secondary visual areas, the posterior agranular insular area and the retrosplenial areas. (3) The rostral one-third of the dorsomedial PFC is the main recipient of projections from the somatosensory and motor areas and the posterior agranular insular area. The laminar distribution of cells projecting to the mPFC varies considerably in the different cortical areas, just as the laminar distribution of termination of their fibres within the mPFC does. It is concluded that the corticocortical connections corroborate with subcortical connectivity in attributing to the mediodorsal projection cortex of the rat functions which are comparable to those of certain prefrontal, premotor and anterior cingulate areas in the monkey.     

Topographic organization of the striatal and thalamic connections of rat medial agranular cortex.

The rostral and caudal portions of rat medial agranular cortex (AGm) play different functional roles. To refine the anatomical framework for understanding these differences, axonal tracers were used to map the topography of the connections of AGm with the striatum and thalamus. The striatal projections follow mediolateral and rostrocaudal gradients that correspond to the locations of the neurons of origin within AGm. Projections from rostral AGm are widespread and dense rostrally, then coalesce into a circumscribed dorsocentral region at the level of the pre-commissural septal nuclei. Projections from mid and caudal AGm are less widespread and less dense, and are focused more caudally. Striatal projections from the adjacent anterior cingulate and lateral agranular areas overlap those of AGm but are concentrated more medially and laterally, respectively. Thalamic connections of AGm are organized so that more caudal portions of AGm have connections with progressively more lateral and caudal regions of the thalamus, and the full extent of AGm is connected with the ventrolateral (VL) nucleus. Rostral AGm is interconnected with the lateral portion of the mediodorsal nucleus (MD1), VL, and the central lateral (CL), paracentral (PC), central medial, rhomboid and ventromedial nuclei. Caudal AGm has robust connections with VL, the posterior, lateral posterior and lateral dorsal nuclei, but little or none with MD1, CL/PC and VM. These differences in the subcortical connections of rostral and caudal AGm parallel their known differences in corticocortical connections, and represent another basis for experimental explorations of the functional roles of these cortical territories.     

Organization of corticospinal neurons in the monkey

The retrograde axonal transport method has been employed to identify the cell bodies of cortical neurons projecting directly to the spinal cord in the monkey. The investigation has focused on aspects of the laminar, columnar, and somatotopic organization of corticospinal neurons within each of the cytoarchitectural and functional subdivisions of the sensorimotor cortex. The principle findings of these experiments are that: (i) cortical regions containing cell bodies of corticospinal neurons are the first motor cortex (area 4), the first somatic sensory cortex (areas 3a, 3b, 1, and 2), and part of the immediately adjacent posterior parietal cortex (area 5), the second somatic sensory cortex, the supplementary motor cortex (the medial aspect of area 6), and the medial part of the posterior parietal cortex in a region termed the supplementary sensory area; (ii) corticospinal neurons display a somatotopic organization within each of these functional subdivisions of the sensorimotor cortex; (iii) all corticospinal neurons arise from layer V of the cortex; and (iv) corticospinal neurons within the first motor and first somatic sensory cortex oftern occur in clusters, perhaps reflecting a columnar organization in the sensorimotor cortex. These findings demonstrate the origins of the corticospinal system to be more extensive than previously recognized and show that a number of common features characterize the organization of corticospinal neurons in all cortical areas. Across cortical subdivisions, however, major differences exist in the extent of spinal segmental representations, in the manner in which corticospinal neurons occur in groups, and in the numerical density and sizes of corticospinal neurons. These aspects of the organization of the corticospinal system presumably reflect specialization of the different cortical areas in spinal cord sensory and motor control.     

Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

We studied the connections of eleven auditory cortical areas with the claustrum and the endopiriform nucleus in the cat, by means of cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase, or biotinylated dextran amines. Unlike previously accepted reports, all auditory areas have reciprocal connections with the ipsi- and contralateral claustrum, though they differ in strength and/or topography. The areas that send the strongest projections are the intermediate region of the posterior ectosylvian gyrus and the insular cortex, followed by the primary auditory cortex and the dorsal portion of the posterior ectosylvian gyrus. The high degree of convergence of cortical axons in the intermediate region of the claustrum, arising from tonotopic and nontonotopic areas, suggests that claustral neurons are unlikely to be well tuned to the frequency of the acoustic stimulus. Corticoclaustral axons from any given area cover territories largely overlapping with those occupied by the claustrocortical neurons projecting back to the same area. The location of cortically projecting neurons in the claustrum matches the position of the target cortical area in the cerebral hemisphere, both rostrocaudally and dorsoventrally. These findings suggest that the intermediate region of the claustrum integrates inputs from all auditory cortical areas, and then sends the result of such processing back to every auditory cortical field. On the other hand, the endopiriform nucleus, a limbic-related structure thought to play a role in the acquisition of conditioned fear, would process mostly polymodal information, since it only receives projections from the insular and temporal cortices.     

Connections of cat auditory cortex: I. Thalamocortical system

Despite the functional importance of the medial geniculate body (MGB) in normal hearing, many aspects of its projections to auditory cortex are unknown. We analyzed the MGB projections to 13 auditory areas in the cat using two retrograde tracers to investigate thalamocortical nuclear origins, topography, convergence, and divergence. MGB divisions and auditory cortex areas were defined independently of the connectional results using architectonic, histochemical, and immunocytochemical criteria. Each auditory cortex area received a unique pattern of input from several MGB nuclei, and these patterns of input identify four groups of cortical areas distinguished by their putative functional affiliations: tonotopic, nontonotopic, multisensory, and limbic. Each family of areas received projections from a functionally related set of MGB nuclei; some nuclei project to only a few areas (e.g., the MGB ventral division to tonotopic areas), and others project to all areas (e.g., the medial division input to every auditory cortical area and to other regions). Projections to tonotopic areas had fewer nuclear origins than those to multisensory or limbic-affiliated fields. All projections were organized topographically, even those from nontonotopic nuclei. The few divergent neurons (mean: 2%) are consistent with a model of multiple segregated streams ascending to auditory cortex. The expanded cortical representation of MGB auditory, multisensory, and limbic affiliated streams appears to be a primary facet of forebrain auditory function. The emergence of several auditory cortex representations of characteristic frequency may be a functional multiplication of the more limited maps in the MGB. This expansion suggests emergent cortical roles consistent with the divergence of thalamocortical connections.     

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.     

Posterior parietal cortex in rhesus monkey: I. Parcellation of areas based on distinctive limbic and sensory corticocortical connections

Injections of HRP-WGA in four cytoarchitectonic subdivisions of the posterior parietal cortex in rhesus monkeys allowed us to examine the major limbic and sensory afferent and efferent connections of each area. Area 7a (the caudal part of the posterior parietal lobe) is reciprocally interconnected with multiple visual-related areas: the superior temporal polysensory area (STP) in the upper bank of the superior temporal sulcus (STS), visual motion areas in the upper bank of STS, the dorsal prelunate gyrus, and portions of V2 and the parieto-occipital (PO) area. Area 7a is also heavily interconnected with limbic areas: the ventral posterior cingulate cortex, agranular retrosplenial cortex, caudomedial lobule, the parahippocampal gyrus, and the presubiculum. By contrast, the adjacent subdivision, area 7ip (within the posterior bank of the intraparietal sulcus), has few limbic connections but projects to and receives projections from widespread visual areas different than those that are connected with area 7a: the ventral bank and fundus of the STS including part of the STP cortex and the inferotemporal cortex (IT), areas MT (middle temporal) and possibly MTp (MT peripheral) and FST (fundal superior temporal) and portions of V2, V3v, V3d, V3A, V4, PO, and the inferior temporal (IT) convexity cortex. The connections between posterior parietal areas and visual areas located on the medial surface of the occipital and parieto-occipital cortex, containing peripheral representations of the visual field (V2, V3, PO), represent a major previously unrecognized source of visual inputs to the parietal association cortex. Area 7b (the rostral part of the posterior parietal lobe) was distinctive among parietal areas in its selective association with somatosensory-related areas: S1, S2, 5, the vestibular cortex, the insular cortex, and the supplementary somatosensory area (SSA). Like 7ip, area 7b had few limbic associations. Area 7m (on the medial posterior parietal cortex) has its own topographically distinct connections with the limbic (the posterior ventral bank of the cingulate sulcus, granular retrosplenial cortex, and presubiculum), visual (V2, PO, and the visual motion cortex in the upper bank of the STS), and somatosensory (SSA, and area 5) cortical areas. Each parietal subdivision is extensively interconnected with areas of the contralateral hemisphere, including both the homotopic cortex and widespread heterotopic areas. Indeed, each area is interconnected with as many areas of the contralateral hemisphere as it is within the ipsilateral one, though less intensively. This pattern of distribution allows for a remarkable degree of interhemispheric integration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Separate,parallel sensory and hedonic pathways in the mammalian somatosensory system

We propose that separate sensory and hedonic representations exist in each of the primary structures of the somatosensory system, including brainstem, thalamic and cortical components. In the dorsal horn of the spinal cord, the hedonic representation, which consists primarily of nociceptive-specific, wide dynamic range, and thermoreceptive neurons, is located in laminae I and II, while the sensory representation, composed primarily by low-threshold and wide dynamic range neurons, is found in laminae III through V. A similar arrangement is found in the caudal spinal trigeminal nucleus. Based on the available anatomical and electrophysiological data, we then determine the corresponding hedonic and sensory representations in the area of the dorsal column nuclei, ventrobasal and posterior thalamic complex, and cortex. In rodent primary somatosensory cortex, a hedonic representation can be found in laminae Vb and VI. In carnivore and primate primary and secondary somatosensory cortical areas no hedonic representation exists, and the activities of neurons in both areas represent the sensory aspect exclusively. However, there is a hedonic representation in the posterior part of insular cortex, bordering on retroinsular cortex, that receives projections from two thalamic areas in which hedonics are represented. The functions of the segregated components of the system are discussed, especially in relation to the subjective awareness of pain.     

The associative striatum: cortical and thalamic projections to the dorsocentral striatum in rats

Corticostriatal projections to the dorsocentral striatum (DCS) were investigated using retrograde fluorescent axonal tracing. The DCS is of interest because of its role in directed attention and recovery from multimodal hemispatial neglect following cortical lesions of medial agranular cortex (AGm), an association area that is its major source of cortical input. A key finding was that the multimodal posterior parietal cortex (PPC) also contributes substantial input to DCS. This is significant because PPC and AGm are linked by corticocortical connections and are both critical components of the circuitry involved in spatial processing and directed attention. Other cortical areas providing input to DCS include visual association areas, lateral agranular cortex and orbital cortex. These areas also have reciprocal connections with AGm and PPC. Less consistent labeling was seen in somatic sensorimotor areas FL, HL and Par 1. Thalamic afferents to DCS are prominent from the intralaminar, ventrolateral, mediodorsal, ventromedial, laterodorsal (LD) and lateral posterior (LP) nuclei. Collectively, these nuclei constitute the sources of thalamic input to cortical areas AGm and PPC. Nuclei LD and LP are only labeled with injections in dorsal DCS, the site of major input from PPC, and PPC receives its thalamic input from LD and LP. We conclude that DCS receives inputs from cortical and thalamic areas that are themselves linked by corticocortical and thalamocortical connections. These findings support the hypothesis that DCS is a key component of an associative network of cortical, striatal and thalamic regions involved in multimodal processing and directed attention.     

Connections between the thalamus and the somatosensory areas of the anterior ectosylvian gyrus in the cat

The thalamocortical and corticothalamic connections of the second somatic sensory area (SII) and adjacent cortical areas in the cat were studied with anterograde and retrograde tracers. Injections consisted of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or a mixture of equal parts of tritiated leucine and proline. The cortical regions to be injected were electrophysiologically studied with microelectrodes to determine the localization of the selected components of the body representation in SII. The distribution of recording points was correlated in each case with the extent of the injection mass in the cortex. Distributions of retrograde and anterograde labeling in the thalamus were reconstructed from serial coronal sections. The results from cases with injections of tracers exclusively confined to separate parts of the body map in SII indicated a fairly precise topographical organization of projections from the ventrobasal complex (VB) to SII. The labeled cells and fibers were located within a series of lamella-like rods that curved throughout the dorsoventral and rostrocaudal axis of VB. The position and extent of these lamellae shifted from medial and ventral, in the medial subdivision of ventral posterior lateral nucleus (VPLm) for radial forelimb digit zones of SII, to dorsal, Posterior, and lateral, in the lateral subdivision of ventral posterior lateral nucleus (VPLl) for proximal leg and trunk regions in SII. For every injected area in SII the densest clustering of labeled cells and fibers was usually more posteriorly represented in VB. The distribution in these dense zones of labeling often extended through the central core of VB. SII projecting neurons were also consistently noted in the extreme rostral portion of the medial subdivision of the posterior nuclei (Pom) that lies dorsal to VB. Corticothalamic and thalamocortical connections for SII Were entirely reciprocal. Injections of tracers into cortical areas surrounding SII labeled other parts of the posterior complex but failed to label any part of VB except when the injection mass also diffused into SII. Injections into the somatic sensory cortex located lateral to SII, within the lips and depth of the upper bank of the anterior ectosylvian sulcus (AES), heavily labeled the central and posterior portions of Pom. Substantial labeling was noted in the lateral (Pol) and intermediate (Poi) divisions of Po only when the injections involved some part of the auditory area that occupies the most posterior part of the AEG and both banks of the immediately adjoining AES. The magnocellular nucleus of the medial geniculate (MGmc) was labeled only when some part of the auditory cortex was injected. The suprageniculate nucleus (SG) was labeled from the insula and lower bank of the AES. These results indicated that medial (rostral and caudal Pom) and lateral components (Poi, Pol, MGmc) of the Posterior complex have separate cortical projection zones to somatic sensory and auditory cortical regions, respectively. SIV and the lateral extent of area 5a located in the medial bank of the anterior suprasylvian sulcus sent projections to the deep layers of the supe- rior colliculus and the ventrolateral periaqueductal gray. No cortico-tectal projections were seen from SII.     

Multiple cortical targets of one thalamic nucleus: the projections of the ventral medial nucleus in the cat studied with retrograde tracers

The organization of the cortical projections of the ventral medial thalamic nucleus (VM) was studied in the cat with retrograde tracers. The extent of the VM-cortical projections was first investigated with horseradish peroxidase injected in different cortical fields. The results obtained in the experiments indicated that the main target of VM efferents is represented by a large territory anterior to the cruciate sulcus involving area 6 and the gyrus proreus and extending into the anterior part of the medial cortical surface. The afferents to these precruciate fields arise from throughout the VM. In addition, the lateral third of VM projects upon the lateral precruciate cortex that is coextensive with the precruciate part of area 4, whereas VM efferents do not extend into the posterior sigmoid gyrus. A second major target of VM efferents is represented by the insular cortex in the anterior sylvian gyrus. VM projections also reach the prepyriform cortex and the cingulate gyrus. An anteroposterior decrease of density was found in the VM-cingulate projections. Sparse VM projections reach the temporal cortex, the adjacent posterior sylvian and ectosylvian fields, and the anterior ectosylvian gyrus. No VM projections were found either upon the visual areas 17 and 18 or upon the primary auditory cortex. The interrelations between some VM-cortical cell populations and their divergent collateralization were studied by using double retrograde labeling with fluorescent tracers. The results of these experiments demonstrated that a relatively high number (at least 20%) of VM cells projecting to the insula are also connected to the precruciate fields by means of axon collaterals. This finding indicates that VM is a highly collateralized structure of the cat's thalamus. Very few branched cells were found in the other combinations of cortical fields here examined (precruciate vs. posterior sylvian fields, lateral precruciate vs. proreal cortex, anterior vs. posterior cingulate fields). Altogether these data indicate that VM branched cells preferentially interconnect the two main cortical targets of the nucleus, i.e., precruciate and insular fields. The results of the present study are discussed in regard to the literature on the VM projections in the rat and the previously available data in the cat, to the afferent VM organization in the cat, to the relationships between VM and the nucleus submedius, and to the anatomical and functional role of VM in relation to the so-called "nonspecific" thalamocortical system.     

Organization of rat vibrissa motor cortex and adjacent areas according to cytoarchitectonics, microstimulation, and intracellular stimulation of identified cells

The relationship between motor maps and cytoarchitectonic subdivisions in rat frontal cortex is not well understood. We use cytoarchitectonic analysis of microstimulation sites and intracellular stimulation of identified cells to develop a cell-based partitioning scheme of rat vibrissa motor cortex and adjacent areas. The results suggest that rat primary motor cortex (M1) is composed of three cytoarchitectonic areas, the agranular medial field (AGm), the agranular lateral field (AG1), and the cingulate area 1 (Cg1), each of which represents movements of different body parts. Vibrissa motor cortex corresponds entirely and for the most part exclusively to AGm. In area AG1 body/head movements can be evoked. In posterior area Cg1 periocular/eye movements and in anterior area Cg1 nose movements can be evoked. In all of these areas stimulation thresholds are very low, and together they form a complete representation of the rat's body surface. A strong myelinization and an expanded layer 5 characterize area AGm. We suggest that both the strong myelinization and the expanded layer 5 of area AGm may represent cytoarchitectonic specializations related to control of high-speed whisking behavior.     

Direct projections from the medulla oblongata to selective cortical areas in the rat

Injections of horseradish peroxidase into different cortical areas reveal that neurons located in the rostroventral medulla oblongata innervate restricted cortical areas: the anterior and posterior cingulate cortex and the insular cortex. Other cortical areas do not receive medullary projections. Neurons projecting to both the anterior and the posterior cingulate cortex lie scattered throughout a wide territory of the ventrorostral medulla, whereas neurons projecting to the insular cortex are restricted to more medial regions.     

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.     

Topographic organization in the corticocortical connections of medial agranular cortex in rats

Medial agranular cortex (AGm) is a narrow, longitudinally oriented region known to have extensive corticortical connections. The rostral and caudal portions of AGm exhibit functional differences that may involve these connections. Therefore we have examined the rostrocaudal organization of the afferent cortical connections of AGm by using fluorescent tracers, to determine whether there are significant differences between rostral and caudal AGm. Mediolateral patterns have also been examined in order to compare the pattern of corticocortical connections of AGm to those of the laterally adjacent lateral agranular cortex (AGl) and medially adjacent anterior cingulate area (AC). In the rostrocaudal domain, there are notable patterns in the connections of AGm with somatic sensorimotor, visual, and retrosplenial cortex. Rostral AGm receives extensive afferents from the caudal part of somatic sensorimotor area Par I, whereas caudal AGm receives input largely from the hindlimb cortex (area HL). Middle portions of AGm show an intermediate condition, indicating a continuously changing pattern rather than the presence of sharp border zones. The whole of the second somatic sensorimotor area Par II projects to rostral AGm, whereas caudal AGm receives input only from the caudal portion of Par II. Visual cortex projections to AGm originate in areas Oc1, Oc2L and Oc2M. Connections of rostral AGm with visual cortex are noticeably less dense than those of mid and caudal AGm, and are focused in area Oc2L. The granular visual area Oc1 projects almost exclusively to mid and caudal AGm. Retrosplenial cortex has more extensive connections with caudal AGm than with rostral AGm, and the agranular and granular retrosplenial subregions are both involved. Other cortical connections of AGm show little or no apparent rostrocaudal topography. These include afferents from orbital, perirhinal, and entorhinal cortex, all of which are bilateral in origin. In the mediolateral dimension, AGm has more extensive corticocortical connections than either AGl or AC. Of these three neighboring areas, only AGm has connections with the somatic sensorimotor, visual, retrosplenial and orbital cortices. In keeping with its role as primary motor cortex, AGl is predominantly connected with area Par I of somatic sensorimotor cortex, specifically rostral Par I. AGl receives no input from visual or retrosplenial cortex. Anterior cingulate cortex has connections with visual area Oc2 and with retrosplenial cortex, but none with somatic sensorimotor cortex. Orbital cortex projections are sparse to AGl and do not appear to involve AC.(ABSTRACT TRUNCATED AT 400 WORDS)     

Corticocortical projections to area 1 in squirrel monkeys (Saimiri sciureus)

Cortical area 1 is a non‐primary somatosensory area in the primate anterior parietal cortex that is critical to tactile discrimination. The corticocortical projections to area 1 in squirrel monkeys were determined by placing multiple injections of anatomical tracers into separate body part representations defined by multiunit microelectrode mapping in area 1. The pattern of labeled cells in the cortex indicated that area 1 has strong intrinsic connections within each body part representation and has inputs from somatotopically matched regions of areas 3b, 3a, 2 and 5. Somatosensory areas in the lateral sulcus, including the second somatosensory area (S2), the parietal ventral area (PV), and the presumptive parietal rostral (PR) and ventral somatosensory (VS) areas, also project to area 1. Topographically organized projections to area 1 also came from the primary motor cortex (M1), the dorsal and ventral premotor areas (PMd and PMv), and the supplementary motor area (SMA). Labeled cells were also found in cingulate motor and sensory areas on the medial wall of the hemisphere. Previous studies revealed a similar pattern of projections to area 1 in Old World macaque monkeys, suggesting a pattern of cortical inputs to area 1 that is common across anthropoid primates.     

Organization of subcortical pathways for sensory projections to the limbic cortex. II. Afferent projections to the thalamic lateral dorsal nucleus in the rat

Afferent projections to the thalamic lateral dorsal nucleus were examined in the rat by the use of retrograde axonal transport techniques. Small iontophoretic injections of horseradish peroxidase were placed at various locations within the lateral dorsal nucleus, and the location and morphology of cells of origin of afferent projections were identified by retrograde labeling. For all cases examined, subcortical retrogradely labeled neurons were most prominent in the pretectal complex, the intermediate layers of the superior colliculus, and the ventral lateral geniculate nucleus. Labeled cells were also seen in the thalamic reticular nucleus and the zona incerta. Within the cerebral cortex, labeled cells were prominent in the retrosplenial areas (areas 29b, 29c, and 29d) and the presubiculum. Labeled cells were also seen in areas 17 and 18 of occipital cortex. Peroxidase injections in the dorsal lateral part of the lateral dorsal nucleus result in labeled neurons in all of the ipsilateral pretectal nuclei, but especially those that receive direct retinal afferents. Labeled cells were also seen in the ventral lateral geniculate nucleus and the rostral tip of laminae IV-VI of the superior colliculus. In contrast, peroxidase injections in ventral medial portions of the lateral dorsal nucleus result in fewer labeled pretectal cells, and these labeled cells are found exclusively in the pretectal nuclei that do not receive retinal afferents. Other labeled cells following injections in the rostral and medial portions of the lateral dorsal nucleus are seen contralaterally in the medial pretectal region and nucleus of the posterior commissure, and bilaterally in the rostral tips of laminae IV and V of the superior colliculus. Camera lucida drawings of HRP labeled cells reveal that projecting cells in each pretectal nucleus have a characteristic soma size and dendritic branching pattern. These results are discussed with regard to the type of sensory information that may reach the lateral dorsal nucleus and then be relayed on to the medial limbic cortex.