Areas 3a, 3b, and 1 of human primary somatosensory cortex.

This study defines cytoarchitectonic areas 3a, 3b, and 1 of the human primary somatosensory cortex by objective delineation of cytoarchitectonic borders and ensuing cytoarchitectonic classification. This avoids subjective evaluation of microstructural differences which has so far been the only way to structurally define cortical areas. Ten brains were fixed in formalin or Bodian's fixative, embedded in paraffin, sectioned as a whole in the coronal plane at 20 microm, and cell stained. Cell bodies were segmented from the background by adaptive thresholding. Equidistant density profiles (125 microm wide, spacing 300 or 150 microm) were extracted perpendicularly to the pial surface across cortical layers II-VI and processed with multivariate statistical procedures. Positions of significant differences in shape between adjacent groups of profiles were correlated with the cytoarchitectonic pattern. Statistically significant borders can be reproduced at corresponding positions across a series of nearby sections. They match visible changes in cytoarchitecture in the cell-stained sections. Area 3a lies in the fundus of the central sulcus, and area 3b in the rostral bank of the postcentral gyrus. Area 1 lies on its crown and reaches down into the postcentral sulcus. Interareal borders, however, do not match macrostructural landmarks of the postcentral gyrus, and they considerably vary in their positions relative to these landmarks across different brains. Hence, only genuine microstructural analysis can define the borders between these cortical areas. Additional significant borders which do not correlate with visible changes in cytoarchitecture can be found within areas 3b and 1. They may represent somatotopy and/or cortical representations of different somatosensory receptors.