CYTOARCHITECTURE AND INTRAFRONTAL CONNECTIONS OF THE FRONTAL-CORTEX OF THE BRAIN OF THE HAMADRYAS BABOON (PAPIO-HAMADRYAS)

A study was made of the cytoarchitecture of the lateral and medial frontal cortex in the hamadryas baboon (Papio hamadryas). The frontal cortico-cortical connections of areas 46, 8, 6, and 4 were investigated by injection of wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into different regions of areas 46, 8, and 6. The lateral region of the frontal lobe of the baboon consists of broad areas of motor (area 4), premotor (area 6), and the dorsolateral prefrontal cortex, each of which is further divided into subdivisions with distinct cytoarchitectural features: areas 4a, 4b, 4c; 6a-alpha, 6a-beta, 6a-gamma, 6b-alpha, and 6b-beta; 8A and 8B; 45; 46 and 46ps; 9; 10; and 12. Although the frontal cortex of the baboon brain exhibits the same basic cytoarchitectural features as the frontal corticies of the cercopithecus (campbelli?) (Vogt and Vogt, '19) or the macaque (Walker, '40; Barbas and Pandya, '87, '89), the baboon frontal cortex is very different from that of the macaque and cercopithecus in terms of cytoarchitecture: (1) the baboon frontal cortex has an additional area, termed here "6a-gamma", within area 6, which has cytoarchitectural characteristics that are intermediate between those of areas 6 and 8; (2) the aggregation of giant pyramidal cells (> 50-mu-m in diameter) is found only in area 4a in the baboon, whereas such aggregates are found in areas 4a and 4b and, occasionally, in area 4c in the macaque; and (3) area 46 of the prefrontal cortex of the baboon can be subdivided into the cortex that surrounds the principal sulcus (area 46) and the upper and lower banks of the principal sulcus (area 46ps). Retrogradely WGA-HRP labeled cells and anterogradely WGA-HRP labeled terminals coexisted in the frontal cortex in a columnar fashion, indicative of a reciprocity among the connections. The frontal cortico-cortical connections of areas 46, 8, 6, and 4 in the hamadryas baboon were organized as follows: (1) areas 46, 8, and 6 were connected to one another, (2) area 4 was connected only to area 6, and (3) these connections showed a gross ventrodorsal topography: the ventral regions of each of areas 46, 8, and 6 were connected more strongly to the ventral than the dorsal regions of the other areas; the dorsal regions of each of areas 46, 8, and 6 were connected more strongly to the dorsal than the ventral regions of the other areas. Moreover, the ventral region of area 6 was more strongly connected to the ventral than to the dorsal region of area 4, whereas the dorsal region of area 6 was more strongly connected to the dorsal than to the ventral region of area 4.