THE MODULAR EFFERENT ORGANIZATION OF THE INFERIOR PARIETAL LOBULE AND CAUDAL PRINCIPAL SULCUS FOR THEIR CALLOSAL AND RECIPROCAL ASSOCIATION PROJECTIONS: COMPUTER-ASSISTED MAPPING OF THE TANGENTIAL AND RADIAL DISTRIBUTIONS OF HORSERADISH PEROXIDASE/TETRAM

The distributions of cortico-cortical projection neurons in the homotypic neocortex of the inferior parietal lobule (IPL) and caudal principal sulcus (CPS) of Rhesus monkeys (M. fascicularis) were studied, employing the retrograde tracer Horseradish Peroxidase (HRP) with its sensitive chromogen Tetramethylbenzidine (TMB), and computerized data acquisition, storage, analysis, and display.; Following injections of HRP into the IPL of two monkeys, detailed maps were made of the distributions of neurons projecting callosally from the IPL and associatively from the CPS; following injections of HRP into the CPS of two monkeys, detailed maps were made of the distributions of neurons projecting callosally from the CPS and associatively from the IPL.; It was found that cortico-cortical projection neurons in the homotypic neocortex of the IPL and CPS are modularly organized for their callosal and reciprocal association projections. The unit module has the solid-geometric form of a slab. Each slab extends approximately 4 mm along a major axis, is restricted to 1.5 mm along its minor axis, and spans the depth of cortex. A slab typically contains 25 to 30 labelled neurons per 100 microns tangential displacement in 50 micron sections; radially, approximately 75% of the projection neurons are aggregated in the deepest 200 microns of layer III; most of the remainder are in the superficial 200 microns of layer V; few projection neurons are found in layers VI and IV, and none in layers I and II. When wide-spread, homogeneous injections of HRP are made, families of slabs are subsequently revealed in the connected areas. Each slab in a family is parallel to the others, and all are roughly parallel to the long dimension of the gyrus in which they reside. These pluricellular slabs of projection neurons are segregated from each other by paucicellular zones of approximately the same dimensions. The difference in the cell counts between the slabs and the flanking paucicellular zones is typically 8 to 15 cells per 100 microns of tangential displacement in 50 micron sections, whether the slab is in the center or at the periphery of a projection zone within the homotypical neocortical area.