The human inferior parietal cortex: Cytoarchitectonic parcellation and interindividual variability

The inferior parietal cortex (IPC) integrates information from different sensory modalities and plays an important role in a variety of higher cognitive functions. Brodmann (Brodmann, K., 1909. Vergleichende Lokalisationslehre der Großhirnrinde. Barth, Leipzig) proposed a cytoarchitectonic subdivision of the IPC into only two cortical areas, a rostral (BA 40) and a caudal (BA 39) area. Although his scheme was repeatedly challenged by other observers, it is still used for the anatomical localization of functional imaging data. The apparent differences between all these cyto- and myeloarchitectonic maps may be caused partly by the observer-dependent procedure of defining cytoarchitectonic borders by pure visual inspection of histological sections and partly by the interindividual variability of cytoarchitecture. The present observations and the resulting cortical map of the IPC are based on quantitative, observer-independent definitions of cytoarchitectonic borders and take into account each area's topographical variability across brains. Ten human postmortem brains were scanned using an MRI 3-D FLASH sequence prior to histological processing. After embedding in paraffin, serial sections through whole brains were prepared, and the sections were stained for cell bodies. Following high-resolution digitization of sections containing the IPC, we defined the cytoarchitecture and borders of each cortical area of this brain region using a multivariate statistical analysis of laminar cell density profiles. In contrast to previous observations, we found seven cytoarchitectonic areas in the IPC: five in the rostral (covering the region of BA 40) and two in the caudal part (covering the region of BA 39). We observed considerable interindividual variability in the topography of each area. A consistent correspondence between macroanatomical landmarks and cytoarchitectonic borders was not found. This new cytoarchitectonic map of the human IPC demonstrates regional differences in the cortical microstructure that is suggestive of functional differentiation. Furthermore, the map is registered in three dimensions and thereby provides a robust anatomical base for interpreting functional imaging studies.