Individual variation of cortical surface area asymmetries.

Asymmetries in the size of cortical regions are regularly associated with functional lateralization. Assessment of cortical asymmetry is often confounded by measurement artifact and a lack of information about the normal variance of asymmetry in regions that are functionally symmetric. In order to measure hemispheric asymmetries in the surface area of cortical gyri, magnetic resonance (MR) images were acquired from 10 normal, right-handed males. Computer representations of the cortical surface in all 20 hemispheres were reconstructed from the images by first creating a white matter model and then 'inflating' it to approximate the cortical surface. The advantage of this approach is that it accurately models the deep sulci as well as the cortical surface. Surface area measurements of the whole hemisphere, the postcentral and the cingulate gyrus were collected from each subject. For each region an asymmetry score was computed based on the difference in the surface area of the left and right regions. Many subjects showed asymmetries in these two gyri; however, the mean asymmetry scores were never significantly asymmetric. The large variability of individual asymmetry scores indicates that cortical asymmetries may be present even in the absence of clear functional asymmetry. An understanding of the degree of asymmetry in structures that do not show clear functional lateralization is critical for interpreting data gathered from cortical regions that are functionally asymmetric.