All lobar volumes were larger in men than in women , but no significant sex differences remained after linearly scaling the brains to adjust for individual differences in brain size. Nevertheless, the voxel-based maps detected significant sex differences at a more local level.
Fig. 1 Lobar volumes in men and women before scaling (bottom) and after scaling (top). Means and standard deviations are shown for the two hemispheres. In the unscaled data, all lobes were significantly bigger in men, whereas in the scaled data, there was no (more ...)
shows voxel-wise significance maps of volumetric differences between men and women, after images were scaled to adjust for TBV. Ratio maps indicating the mean male volume divided by the mean female volume are also shown at the corresponding slices.
Fig. 2 Three-dimensional maps of sex differences in cerebral volumes. The top right panel shows P values corresponding to local volume expansion or shrinkage. Red indicates significance at the voxel level. To assess the direction of changes, we also mapped the (more ...)
In agreement with our prior hypothesis (that women have larger temporal lobe volumes, particularly in the left hemisphere language areas – see Introduction), in scaled images, women had proportionally larger left temporal lobe regions (pcorrectedλ=λ0.029), left parietal, and left occipital lobes (pcorrectedλ=λ0.038 and pcorrectedλ=λ0.016, respectively). Applying a strict Bonferroni correction to adjust for the multiple statistical tests performed, the left temporal lobe sex differences should be considered significant, with trends in the left parietal and left occipital lobes (see for results in each lobe). Proportionally larger regions were found in women in the left but not the right hemisphere (pcorrectedλ=λ0.044, left; pcorrectedλ=λ0.12, right).
P values (two-tailed) showing the overall significance of the volumetric differences in each lobe
In the statistical maps (left panel of ), women had proportionally greater volumes for the left (a) and the right (b) superior temporal gyri (which house the primary auditory cortex), in agreement with .
One unexpected finding was that men had proportionally larger volumes in the left occipital lobes (c), in regions housing primary and secondary visual cortices. This was not hypothesized, and is less intuitive than the other findings, as the volume difference is unlikely to be associated with a functional difference (as sex differences in primary visual function are not anticipated). This difference needs to be replicated in future studies, but it remains possible that these occipital volumetric differences may not have any obvious functional correlate.
The anterior cingulate cortex (d), which is involved in affective regulation and autonomic function, was also proportionally larger in women. Men still showed greater volumes, even after adjusting for brain scale, in the left superior lateral fasciculus (e), a white matter tract connecting the caudal inferior parietal cortex to the dorsolateral prefrontal cortex.
One major confound in studies of sex differences is the TBV difference between men and women. Proportional differences in brain volumes may therefore be because of nonlinear scaling effects, in which substructure volumes scale nonlinearly rather than proportionately to TBV [16
]. If direct fitting of this nonlinear power law would predict sex differences in the direction found here, they may be meaningless, in the sense that selected groups of men who are matched for brain size would not be distinguishable from women.AQ7 To test this, after linear scaling of the images, we determined whether the relative volume at each voxel still correlated with the TBV. shows the correlation coefficient between TBV and local volume; maps are similar within each sex (not shown). Pale green (0) indicates no remaining correlation between regional volumes and the TBV, that is, the TBV effect was statistically removed by the linear one-to-one scaling, and there is no evidence for a nonlinear power law. Some regions (blue colors) are proportionally larger in volunteers with small TBV, whereas occipital regions (in red) are proportionally larger in volunteers with larger TBV. Importantly, occipital regions are shown to be proportionally larger when TBV increases, so the effects found in favoring men might be because of how the brain scales, regardless of sex. However, no residual correlations were detected in the temporal and cingulate regions where women were proportionally larger (a, b, d), nor in the SLF II (e) which is larger in men, showing that these effects are likely to be mediated by biological sex operating independently of how the brain scales.
Fig. 3 This map shows any remaining correlations between regional volumes and total brain volume, after images were all scaled to the same overall scale. The lack of any residual correlation in right superior temporal gyrus (b), cingulate (d), and the left superior (more ...)