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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 (1) four prefrontal sectors (rostral, dorsolateral, ventral, and dorsomedial) can be distinguished, each exhibiting a particular pattern of cortical afferents. (2) 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. (3) 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. (4) The olfactory peduncle, the prepiriform cortex, the cortico-amygdaloid transition area, the entorhinal cortex, the subiculum (ventral, posteroventral, and posterodorsal sectors), the caudomedial band off the hippocampal formation and the postsubiculum are the allocortical sources of afferents to the PFC. (5) 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. (6) Intraprefrontal connections are most abundant within each PFC sector. Commissural interprefrontal connections are largest from the site homotopic to the HRP injection. Contralateral heterotopic areas also project to the PFC. The present findings are compared to the patterns of distribution of cortical afferents in the PFC reported in macaque monkeys and similarities and differences are considered.

We have examined the circuitry connecting the posterior parietal cortex with the frontal lobe of rhesus monkeys. HRP-WGA and tritiated amino acids were injected into subdivisions 7m, 7a, 7b, and 7ip of the posterior parietal cortex, and anterograde and retrograde label was recorded within the frontal motor and association cortices. Our main finding is that each subdivision of parietal cortex is connected with a unique set of frontal areas. Thus, area 7m, on the medial parietal surface, is interconnected with the dorsal premotor cortex and the supplementary motor area, including the supplementary eye field. Within the prefrontal cortex, area 7m's connections are with the rostral sector of the frontal eye field (FEF), the dorsal bank of the principal sulcus, and the anterior bank of the inferior arcuate sulcus (Walker's area 45). In contrast, area 7a, on the posterior parietal convexity, is not linked with premotor regions but is heavily interconnected with the rostral FEF in the anterior bank of the superior arcuate sulcus, the dorsolateral prefrontal convexity, the rostral orbitofrontal cortex, area 45, and the fundus and adjacent cortex of the dorsal and ventral banks of the principal sulcus. Area 7b, in the anterior part of the posterior parietal lobule, is interconnected with still a different set of frontal areas, which include the ventral premotor cortex and supplementary motor area, area 45, and the external part of the ventral bank of the principal sulcus. The prominent connections of area 7ip, in the posterior bank of the intraparietal sulcus, are with the supplementary eye field and restricted portions of the ventral premotor cortex, with a wide area of the FEF that includes both its rostral and caudal sectors, and with area 45. All frontoparietal connections are reciprocal, and although they are most prominent within a hemisphere, notable interhemispheric connections are also present. These findings provide a basis for a parcellation of the classically considered association cortex of the frontal lobe, particularly the cortex of the principal sulcus, into sectors defined by their specific connections with the posterior parietal subdivisions. Moreover, the present findings, together with those of a companion study (Cavada and Goldman-Rakic: J. Comp. Neurol. this issue) have allowed us to establish multiple linkages between frontal areas and specific limbic and sensory cortices through the posterior parietal cortex. The networks thus defined may form part of the neural substrate of parallel distributed processing in the cerebral cortex.

The existence of interhemispheric cortical afferent connections to the prefrontal cortex of the cat is investigated by means of the retrograde axonal transport of horseradish peroxidase technique. Labeled neurons are found in contralateral hemisphere in sites homotopical and heterotopical to the injection. The heterotopical contralateral projection arises principally from prefrontal, insular, prelimbic, premotor, cingular and retrosplenial cortices.

Afferent connections of the prefrontal cortex of the cat arising in allocortical regions have been investigated using the horseradish peroxidase retrograde transport technique. Our results demonstrate the existence of projections from the olfactory peduncle, anterior and posterior prepiriform cortices, cortico-amygdaloid transition area, entorhinal cortex, ventral, caudal and dorsal subiculum and postsubiculum to the prefrontal cortex.

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. These findings provide evidence that each major subdivision of posteriorparietal cortex has a unique set of reciprocal connections with limbic and sen-sory areas in both hemispheres. For the most part, each parietalsubdivision, rather than being a site of multimodal convergence, receives input from only one sensory modality, though often from different channels of information within that modality. For example, the two streams of visual information tra-ditionally linked to pattern and motion seem to converge in both areas 7a and 7ip. The areal parcellation of parietal cortex byits afferent and efferent con-nections provides an anatomical foundation for a parallel processing model of higher cortical functions.

Ipsilateral cortical projections to the prefrontal cortex of the cat were investigated by means of the retrograde axonal transport of horseradish peroxidase technique. Labeled neurons were identified in frontal, insular, parietal and temporal regions, as well as in cingular, retrosplenial and perirhinal areas. These intrahemispheric cortical connections can be compared to those described in the rhesus monkey.