Normal Sexual Dimorphism of the Adult Human Brain Assessed by In Vivo Magnetic Resonance Imaging

TLDR

The etiology and consistency of normal sexual dimorphisms in the adult human brain remain unresolved. This study evaluates cortical and subcortical sexual dimorphisms in a community sample of 48 adults using in vivo MRI. Participants were matched for age, education, ethnicity, socioeconomic status, intelligence, and handedness, and 45 brain regions were quantified from T1‑weighted 3‑D images acquired on a 1.5‑T scanner. Women had larger relative cortical volumes in frontal and medial paralimbic regions, while men had larger relative volumes in frontomedial cortex, amygdala, and hypothalamus; permutation testing revealed that dimorphisms were strongest in regions homologous to animal areas rich in sex‑steroid receptors, underscoring hormone‑related developmental mechanisms.

Abstract

The etiology and consistency of findings on normal sexual dimorphisms of the adult human brain are unresolved. In this study, we present a comprehensive evaluation of normal sexual dimorphisms of cortical and subcortical brain regions, using in vivo magnetic resonance imaging, in a community sample of 48 normal adults. The men and women were similar in age, education, ethnicity, socioeconomic status, general intelligence and handedness. Forty-five brain regions were assessed based on T1-weighted three-dimensional images acquired from a 1.5 T magnet. Sexual dimorphisms of adult brain volumes were more evident in the cortex, with women having larger volumes, relative to cerebrum size, particularly in frontal and medial paralimbic cortices. Men had larger volumes, relative to cerebrum size, in frontomedial cortex, the amygdala and hypothalamus. A permutation test showed that, compared to other brain areas assessed in this study, there was greater sexual dimorphism among brain areas that are homologous with those identified in animal studies showing greater levels of sex steroid receptors during critical periods of brain development. These findings have implications for developmental studies that would directly test hypotheses about mechanisms relating sex steroid hormones to sexual dimorphisms in humans.