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, 18 (12), 2920-31

Size Matters: Cerebral Volume Influences Sex Differences in Neuroanatomy

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Size Matters: Cerebral Volume Influences Sex Differences in Neuroanatomy

Christiana M Leonard et al. Cereb Cortex.

Abstract

Biological and behavioral differences between the sexes range from obvious to subtle or nonexistent. Neuroanatomical differences are particularly controversial, perhaps due to the implication that they might account for behavioral differences. In this sample of 200 men and women, large effect sizes (Cohen's d > 0.8) were found for sex differences in total cerebral gray and white matter, cerebellum, and gray matter proportion (women had a higher proportion of gray matter). The only one of these sex differences that survived adjustment for the effect of cerebral volume was gray matter proportion. Individual differences in cerebral volume accounted for 21% of the difference in gray matter proportion, while sex accounted for an additional 4%. The relative size of the corpus callosum was 5% larger in women, but this difference was completely explained by a negative relationship between relative callosal size and cerebral volume. In agreement with Jancke et al., individuals with higher cerebral volume tended to have smaller corpora callosa. There were few sex differences in the size of structures in Broca's and Wernicke's area. We conclude that individual differences in brain volume, in both men and women, account for apparent sex differences in relative size.

Figures

Figure 1.
Figure 1.
Sagittal images. (A) Partial volume estimate (PVE) image of gray matter in individual with low cerebral volume. The percentage of gray matter in each pixel is estimated on a scale ranging from 0 (black) to 1 (white). (B) PVE image of white matter in individual A. (C) PVE image of gray matter in individual with high cerebral volume. (D) PVE image of white matter in individual C. (E) T1 weighted image of left hemisphere with small pars triangularis (PTR), Heschl's gyrus (HG), planum temporale (PT), and relatively large planum parietale (PP). Boxes depict regions magnified in (F and G). (H) T1-weighted image of left hemisphere with large PTR, HG, PT, and absent PP. Boxes depict regions magnified in (I and J). White lines in (F), (G), (I), and (J) are tracings of surface areas.
Figure 2.
Figure 2.
Histograms of the distribution of cerebral volume (top) and left hemisphere planum temporale surface area (bottom) in 100 men and 100 women. Note the large range of individual differences in each measure. Although there is a relatively small degree of overlap in cerebral volume, the distributions of left planum temporale surface area overlap completely.
Figure 3.
Figure 3.
Top: Cerebellar volume and corpus callosum area have similar relationships with cerebral volume in men and women. Bottom: The slopes of gray matter, white matter and CSF against cerebral volume are similar in women (left) and men (right). Correlation coefficients are given in Table 3.
Figure 4.
Figure 4.
Gray matter proportion and the relative size of the corpus callosum and Heschl's gyrus plotted as a function of cerebral volume. All measures show a negative relation with cerebral volume (see Tables 3, 5, and 6).
Figure 5.
Figure 5.
Effect of age and image processing method on the ratio between gray and white matter. Data and references are given in Table 8. Although the gray/white ratio declines with age and is generally higher in women, gray white ratios appear to be more consistent in images that have not been normalized (rescaled) to a template. Open symbols: women; solid symbols: men; Squares: images normalized and modulated (values adjusted for cerebral volume); Triangles: images segmented and measured in native (actual) space, although aligned to standard axes.

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