Broca's region revisited: cytoarchitecture and intersubject variability

Abstract

The sizes of Brodmann's areas 44 and 45 (Broca's speech region) and their extent in relation to macroscopic landmarks and surrounding areas differ considerably among the available cytoarchitectonic maps. Such variability may be due to intersubject differences in anatomy, observer-dependent discrepancies in cytoarchitectonic mapping, or both. Because a reliable definition of cytoarchitectonic borders is important for interpreting functional imaging data, we mapped areas 44 and 45 by means of an observer-independent technique. In 10 human brains, the laminar distributions of cell densities were measured vertical to the cortical surface in serial coronal sections stained for perikarya. Thousands of density profiles were obtained. Cytoarchitectonic borders were defined as statistically significant changes in laminar patterns. The analysis of the three-dimensional reconstructed brains and the two areas showed that cytoarchitectonic borders did not consistently coincide with sulcal contours. Therefore, macroscopic features are not reliable landmarks of cytoarchitectonic borders. Intersubject variability in the cytoarchitecture of areas 44 and 45 was significantly greater than cytoarchitectonic differences between these areas in individual brains. Although the volumes of area 44 differed across subjects by up to a factor of 10, area 44 but not area 45 was left-over-right asymmetrical in all brains. All five male but only three of five female brains had significantly higher cell densities on the left than on the right side. Such hemispheric and gender differences were not detected in area 45. These morphologic asymmetries of area 44 provide a putative correlate of the functional lateralization of speech production.