Sex differences in brain gray and white matter in healthy young adults: correlations with cognitive performance

Abstract

Sex-related differences in behavior are extensive, but their neuroanatomic substrate is unclear. Indirect perfusion data have suggested a higher percentage of gray matter (GM) in left hemisphere cortex and in women, but differences in volumes of the major cranial compartments have not been examined for the entire brain in association with cognitive performance. We used volumetric segmentation of dual echo (proton density and T2-weighted) magnetic resonance imaging (MRI) scans in healthy volunteers (40 men, 40 women) age 18-45. Supertentorial volume was segmented into GM, white matter (WM), and CSF. We confirmed that women have a higher percentage of GM, whereas men have a higher percentage of WM and of CSF. These differences sustained a correction for total intracranial volume. In men the slope of the relation between cranial volume and GM paralleled that for WM, whereas in women the increase in WM as a function of cranial volume was at a lower rate. In men the percentage of GM was higher in the left hemisphere, the percentage of WM was symmetric, and the percentage of CSF was higher in the right. Women showed no asymmetries. Both GM and WM volumes correlated moderately with global, verbal, and spatial performance across groups. However, the regression of cognitive performance and WM volume was significantly steeper in women. Because GM consists of the somatodendritic tissue of neurons whereas WM comprises myelinated connecting axons, the higher percentage of GM makes more tissue available for computation relative to transfer across distant regions. This could compensate for smaller intracranial space in women. Sex difference in the percentage and asymmetry of the principal cranial tissue volumes may contribute to differences in cognitive functioning.

Fig. 1.

Illustration of the MRI segmentation process showing an acquired T2-weighted image (left), a proton density image (middle), and the segmented image (right) in which GM is depicted in white, WM in light gray, and CSF in black.

Fig. 2.

Scatterplots and regression lines for gray matter (GM), white matter (WM), and CSF against cranial volumes in men (left,squares) and women (right,circles).

Fig. 3.

Means ± SEM percentage of tissue and CSF averaged bilaterally (top) and examined as a laterality index (left minus right, bottom) in men (dark bars) and women (light bars).

Fig. 4.

Scatterplots and regression lines for gray matter (left column) and white matter (right column) against average cognitive performance (top row) and verbal and spatial performance (middleand bottom rows, respectively) in men (filled squares, solid regression line) and women (open circles, dashed regression line).