Brain size, sex, and the aging brain

Abstract

This study was conducted to examine the statistical influence of brain size on cortical, subcortical, and cerebellar compartmental volumes. This brain size influence was especially studied to delineate interactions with Sex and Age. Here, we studied 856 healthy subjects of which 533 are classified as young and 323 as old. Using an automated segmentation procedure cortical (gray and white matter [GM and WM] including the corpus callosum), cerebellar (GM and WM), and subcortical (thalamus, putamen, pallidum, caudatus, hippocampus, amygdala, and accumbens) volumes were measured and subjected to statistical analyses. These analyses revealed that brain size and age exert substantial statistical influences on nearly all compartmental volumes. Analyzing the raw compartmental volumes replicated the frequently reported Sex differences in compartmental volumes with men showing larger volumes. However, when statistically controlling for brain size Sex differences and Sex × Age interactions practically disappear. Thus, brain size is more important than Sex in explaining interindividual differences in compartmental volumes. The influence of brain size is discussed in the context of an allometric scaling of the compartmental volumes.

Keywords: brain size; magnetic resonance imaging; morphometry; neuroanatomy; sex differences.

Figure 1

Coronal, sagittal, and horizontal slices demonstrating the segmentations given by FreeSurfer.

Figure 2

Partial correlations (r) between ICV and FBV and the compartmental volumes (left panel). On the right panel partial correlations between the proportional volume measures and ICV as well as FBV are shown. All partial correlations are computed with Age and Sex as covariates. The partial correlations are color‐coded to demonstrate the strength of the effects according to Cohen [1992]. Large effect sizes with a partial correlation r > 0.5 are coded in red; moderate effect sizes with a partial correlation r > 0.3 are coded in yellow, and small effect sizes with a partial correlation r > 0.1 are coded in green.

Figure 3

Mean volumes for the raw and adjusted volumes (in ml) and standard deviations.

Figure 4

Mean proportional volumes (volumes related to ICV in %) and standard deviations broken down for Age and Sex. Proportional volumes for women are shown in black and the proportional volumes for men are shown in gray bars.

Figure 5

Demonstration of Eta² values (as % of explained variance) for the main effects Age (A) and Sex (S) as well as the interaction between Age × Sex (A × S) broken down for the compartmental volumes (raw, adjusted, and proportional). The effect sizes are color coded to demonstrate the strength of the effects according to Cohen [1992]. Large effect sizes with an Eta² (in %) >14% are coded in red; moderate effect sizes with an Eta² (in %) >6% are coded in yellow and small effect sizes with an Eta² (in %) >1% are coded in green. Degrees of freedom (df) for the main and interaction effects for the ANOVA are (1,852); the for main and interaction effects for the ANCOVAs are: (1,851).