CYTOARCHITECTONIC DEFINITION OF PREFRONTAL AREAS IN THE NORMAL HUMAN CORTEX .2. VARIABILITY IN LOCATIONS OF AREAS 9 AND 46 AND RELATIONSHIPTO THE TALAIRACH COORDINATE SYSTEM

The human prefrontal cortex can be divided into structurally and funct ionally 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 (Rajkowsk a and Goldman-Rakic, 1995), as well as visual inspection, we have mapp ed areas 9 and 46 in the frontal robe of seven postmortem human brains , and completely reconstructed these dorsolateral regions in five of t he seven cases, The lateral reconstructions in these five cases were a nalyzed and superimposed on the lateral view of the Talairach and Tour noux (1988) coordinate system in such a way as to render both the vari ability 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 fronta l lobe extending along the middle third of the superior frontal gyrus and adjacent portions of the middle frontal gyrus in all cases examine d. Area 46 lies on the dorsolateral convexity and is either partially Dr 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 s uperior frontal sulci, as is the inferior border within the upper wall of the inferior frontal sulcus, The genuine variability in the morpho logy 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 variat ion among the brains they analyzed, Such variation may underlie indivi dual differences in the visuospatial and cognitive capacities subserve d by these areas.