Cortico‐cortical connections of somatic sensory cortex (areas 3, 1 and 2) in the rhesus monkey

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Abstract
The connections of areas 3, 1 and 2 in the postcentral gyrus of the rhesus monkey are investigated using ablation‐degeneration techniques following both full depth lesions and lesions which involved fewer than six cortical layers. Analysis of the topographic and laminar organization of these connections reveal that each of these areas has a differential connection pattern both within the parietal lobe and with respect to motor cortex. Area 3 projects predominantly to area 1 via a horizontal, intracortical fiber system which courses through layers III and V without entering the white matter while other efferents of area 3 to areas 2, 3a and second somatosensory cortex (SII) are less dense and course through the white matter. There is no indication that area 3 efferents terminate in areas 4 or 5. In comparison to area 3, area 1 has a wider projection field. Its primary outflow reaches area 2 via a white matter course while moderately strong connections are directed to areas 3a, S II, 4 and supplementary motor cortex (M II) and a minor projection to area 5. Lesions involving the supragranular layers of area 1 demonstrate that efferents from these layers (II‐III) travel directly through the cortex to terminate in layer I of area 3 as well as entering the white matter before terminating in area 2 ventral to the tip of the intraparietal sulcus. Finally, area 2 projects primarily to area 5 via both an intracortical fiber system in layers III‐V as well as through the white matter. While area 2 also has connections with areas 1, 3a, S II, 6 and M II, it was not observed to project to area 3. In addition, layers I‐IV of area 2 ventral to the intraparietal sulcus send a number of horizontally oriented fibers mainly through layer IIIb to terminate in rostral area 7 (area PF of Bonin and Bailey, '47).Thus, the projections of these three subdivisions of the postcentral gyrus indicate that there is a strong and sequential outflow of connections from area 3 to areas 1 and 2, then from area 1 to area 2 and finally from area 2 to area 5 and rostral area 7. Each of these connections originates to a large extent from the supragranular layers. In contrast, connections of these areas in the opposite direction toward the central sulcus are less pronounced.

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