Cytoarchitectonic definition of prefrontal areas in the normal human cortex: II. Variability in locations of areas 9 and 46 and relationship to the Talairach Coordinate System.

The human prefrontal cortex can be divided into structurally and functionally distinct cytoarchitectonic areas, but the extent of individual variation in the position, size, and shape of these areas is unknown. Using criteria described in the preceding companion article (Rajkowska and Goldman-Rakic, 1995), as well as visual inspection, we have mapped areas 9 and 46 in the frontal lobe of seven postmortem human brains, and completely reconstructed these dorsolateral regions in five of the seven cases. The lateral reconstructions in these five cases were analyzed and superimposed on the lateral view of the Talairach and Tournoux (1988) coordinate system in such a way as to render both the variability and the regions of overlap for the two prefrontal areas in the five different brains. Based on this exercise, we developed a set of conservative Talairach coordinates to define area 9 and 46. Area 9 is located on the dorsal, lateral, and dorsomedial surfaces of the frontal lobe extending along the middle third of the superior frontal gyrus and adjacent portions of the middle frontal gyrus in all cases examined. Area 46 lies on the dorsolateral convexity and is either partially or completely surrounded by area 9. It is consistently found on one or more convolutions of the middle frontal gyrus. The superior border of area 46 with adjacent cortex is also variable within the middle and superior frontal sulci, as is the inferior border within the upper wall of the inferior frontal sulcus. The genuine variability in the morphology of the human frontal lobe indicated by our findings suggests that the differences among the classical maps of Brodmann, von Economo and Koskinas, and Sarkissov and others may have been due to normal variation among the brains they analyzed. Such variation may underlie individual differences in the visuospatial and cognitive capacities subserved by these areas.