Figure 1. Cytoarchitectonic divisions of the human and monkey frontal lobe.

Rostral and caudal axes are labelled and the numbers represent the update by Petrides and Pandya¹²⁰ of the Brodmann¹⁵¹ and Walker cytoarchitecture maps. Several investigators have created maps of the frontal cortex based on morphological criteria such as the gross characteristics of cells, the arrangement of these cells in cortical layers, and gross characteristics of myelin in the cortex (for examples see REFS 152,153). However, area boundaries differ significantly between maps. For example, in the lateral prefrontal cortex, Brodmann’s area 9 is extensive and area 46 does not exist¹⁵², whereas Walker includes area 46 and a much more restricted area 9 (REF. 153). Another issue concerns the correspondence between maps of monkey and human cortex. Indeed, a comparison between Brodmann’s monkey and human cortex maps reveals seemingly significant differences; for example, the human map includes area 46, but the monkey map does not^151,152. Likewise, in Walker’s map of the monkey cortex, area 46 is far more extensive than that depicted in Brodmann’s map of the human cortex. To resolve these discrepancies, Petrides and Pandya^10,120,121 performed an extensive comparison of the architecture of the frontal cortex between monkeys and humans. This revealed that both in monkeys and humans, areas 9 and 46 are in the mid-dorsolateral sector of the lateral prefrontal cortex. Also, in both species, area 9 lies dorsal to area 46. However, in humans, area 9 encircles area 46 caudally, which is not the case in monkeys. This caudal portion of area 9 in humans is similar to the caudal portion of area 46 in monkeys. This led Petrides and Pandya to create a new label, area 9/46, which they divided into a dorsal portion (area 9/46d) and a ventral portion (area 9/46v). The nomenclature they put forth in their maps is used in this Review. Figure is reproduced, with permission, from REF. 10 © (2002) Wiley-Blackwell.