Corpus callosum

The corpus callosum is the largest single bundle of white matter in the human brain. It is a thick, curved band of roughly 200 million axons that connects the left and right cerebral hemispheres, allowing them to share information rapidly.

You can see it clearly on any midline view of the brain: a pale, arched structure sitting just above the lateral ventricles.

What it does

The corpus callosum lets the two hemispheres collaborate. Vision, hearing, and motor control are largely lateralised, with each hemisphere handling the opposite side of the body, and most cognitive functions involve coordinated activity on both sides. The callosum is the main route for that coordination.

It is organised topographically:

• Genu (front): carries fibres between the prefrontal cortices
• Body (middle): fibres connecting motor and sensory regions
• Splenium (rear): visual and auditory traffic

When it's affected

The corpus callosum is sensitive to a number of conditions:

• It thins with age, faster in some people than others
• It is severely affected in multiple sclerosis, where demyelination shows up clearly on FLAIR sequences
• It is often hypoplastic (underdeveloped) or absent in certain congenital syndromes
• Historic surgical "split-brain" procedures cut it deliberately to treat severe epilepsy, producing the classic split-brain experiments that revealed hemispheric specialisation

In imaging

Callosal volume and thickness are standard outputs of structural pipelines. Diffusion tensor imaging is particularly informative for the callosum because the fibres are highly aligned, producing strong fractional anisotropy values that are sensitive to white matter integrity.