The human parietal operculum. I. Cytoarchitectonic mapping of subdivisions.

The human secondary somatosensory cortex (SII) is located on the parietal operculum, as shown by intraoperative stimulation and functional imaging studies. The position and extent of the anatomical correlates of this functionally defined region, however, are still unknown. We have therefore histologically mapped the putative anatomical correlates of the SII cortex in cell-body-stained histological sections of 10 human postmortem brains using quantitative cytoarchitectonic analysis. The gray level index (GLI), which is an indicator of the volume fraction of nerve cell bodies, was measured in the parietal operculum. GLI profiles as measures of the laminar pattern of the cortex were extracted perpendicular to cortical layers. Cytoarchitectonic borders were detected observer-independently by multivariate statistical analysis of the laminar profiles. Four cytoarchitectonic areas (termed OP 1-4) were identified. This cytoarchitectonic heterogeneity of the parietal operculum corresponds to results of functional imaging studies on the human SII cortex and data from non-human primates where multiple subregions within SII have been demonstrated by electrophysiological and connectivity studies.