The human inferior parietal lobule in stereotaxic space

Recently, a new cytoarchitectonic map of the human inferior parietal lobule (IPL) has been proposed, with the IPL consisting of seven cytoarchitectonically distinct areas (Caspers et al. in Neuroimage 33( 2): 430-448, 2006). The aim of the present study was to investigate the different aspects of variability of these IPL areas. As one aspect of variability, we analysed the topographical relationship between the localisation of the borders of the areas and macroanatomical landmarks. Although five areas occupy the surface supramarginal gyrus and two the angular gyrus, their borders cannot be reliably detected by means of macroanatomy. To account for variability in size and extent of the areas in stereotaxic space, cytoarchitectonic probabilistic maps have been calculated for each IPL area. Hemisphere- and gender-related differences have been investigated on basis of volumes of cytoarchitectonic areas. For one of them, area PFcm, a significant gender difference in volume was found with males having larger volumes than females; this difference exceeds that of gender differences in total brain volume. The different aspects of variability and volumetric asymmetry may underlie some of the well-known functional asymmetries in the IPL, observed, for example during fMRI experiments analysing spatial attention or motor attention, and planning. The cytoarchitectonic probabilistic maps of the seven IPL areas provide a robust anatomical reference and open new perspectives for further structure-function investigations of the human IPL.