Columnar organization of corticocortical projections in squirrel and rhesus monkeys: similarity of column width in species differing in cortical volume.

To compare the size and pattern of the terminal distribution of corticocortical projections in two primate species with brains of different size, tritiated amino acids were injected into the prefrontal cortex of New World squirrel monkeys (Saimiri sciureus) and Old World rhesus monkeys (Macaca mulatta), and their brains were processed for light microscopic autoradiography. In both species, prefrontal efferents are directed to a number of cortical targets in the same and opposite hemispheres, where in coronal sections, they generally terminate as radially oriented columns. In the rhesus monkey, the median width of the columns in transverse sections is 685 micrometers. In squirrel monkey, corresponding columns have a median with of 555 micrometers. Considering that the volume of the neocortex in rhesus monkey is approximately 4.5 x larger than that of squirrel monkey, the dimensions of cortical columns in the two species are surprisingly similar. This finding suggests that phylogenetic expansion in cortical surface area is accompanied by an increase in the number, rather than the width of afferent fiber columns. The increase in number of modular units may be relevant to the increasing computational and information processing capacity of the cerebral cortex in the course of evolution.