Individual variability in cortical organization: its relationship to brain laterality and implications to function.

The human brain and the brains of most mammals studied for this purpose demonstrate hemispheric asymmetry of gross anatomical landmarks and/or architectonic cortical subdivisions. The magnitude as well as the direction of these cortical asymmetries vary among individuals, and in some species there exist significant population directional biases. The magnitude, if not the direction, of cortical asymmetry is found to predict for relative numbers of neurons comprising a given pair of hemispheric architectonic homologues such that the more asymmetric the region is, the smaller the number of neurons. Similarly, the more asymmetric a region is, the smaller the density of interhemispheric connections and (probably) the greater the density of intrahemispheric connections. Developmentally, the decrease in the number of neurons characterizing the more asymmetrical regions appears to reflect mainly increased unilateral ontogenetic cell loss, and diminished callosal connectivity might signify increased developmental axonal pruning. These relationships between cell numbers, callosal connections, and presumed intrahemispheric relationships can be entertained to explain variability in anatomo-clinical correlations for language function and aphasia between left- and right-handers and men and women.