Scaling Down: Proportionally Smaller Corpora Callosa in Larger Brains

Background: Larger brains are believed to rely more heavily on intra-hemispheric than inter-hemispheric processing, which may lead to a proportionally reduced callosal size. Methods: To test this hypothesis, we used T1-weighted magnetic resonance images from a large population sample (n = 38,034). The sample was drawn from the UK Biobank and included 19,947 females and 18,087 males, aged between 44 and 83 years (mean ± SD: 64 ± 7.72 years). Linear modelling was used to assess the relationship between proportional callosal volume and total intracranial volume, with sex, age, and handedness included as covariates and interaction terms. Results: We observed a significant negative relationship between proportional callosal volume and total brain volume, such that larger brains had proportionally smaller corpora callosa. Conclusion: These findings support the hypothesis that increasing brain size is associated with reduced inter-hemispheric connectivity, potentially due to conduction constraints that promote greater intra-hemispheric processing in larger brains.