The granular frontal association cortex of the strepsirhine primate Galago: Comparative anatomy and evolutionary implications

Do primates possess evolutionary specializations of brain regions involved in cognition? We addressed this issue with a comparative anatomical study of granular frontal cortex (GFC), an associational region implicated in higher-order cognitive processes. Published studies of GFC using modern anatomical techniques have been virtually restricted to a single group of anthropoid primates, Macaca. Here, we have extended the study of GFC to Galago, a group of relatively small-brained, anatomically conservative strepsirhine ("prosimian") primates.;Connectional evidence supports our architectonic evaluation. Cortical injections of tracer substances (HRP and tritiated amino acids) revealed that Galago GFC is connected with a vast array of cortical areas, as reported for Macaca; in both genera, connections with parietal and temporal association areas are especially prominent. However, there are fewer discrete foci of parieto-frontal connectivity in Galago than in Macaca, consistent with the conclusion that Galago possesses fewer GFC areas. Moreover, Galago GFC possesses connections resembling those of Macaca arcuate cortex, but lacks connections distinctive of principalis cortex.;Our evidence indicates that new areas arose in the GFC during the evolution of anthropoid primates. Moreover, the GFC areas shared by strepsirhines and anthropoid primates possess characteristics not found in the frontal cortex of other mammals. These results are difficult to reconcile with classical accounts of comparative neuroanatomy, which assert the fundamental sameness of frontal lobe organization across mammals.;Using myelo- and cytoarchitectonic techniques to make an initial parcellation of GFC, we were able to distinguish only about half as many GFC areas in Galago as in Macaca. Differences are much less marked in other regions of frontal, parietal, and temporal cortex. Most of the GFC areas of Galago resemble areas located near the arcuate sulcus in Macaca. However, Galago possesses no obvious counterparts of the distinctive, myelin-poor areas of the Macaca principal sulcus.