Corpus callosum · Grey Matter

The corpus callosum is the thick bridge of white matter that joins the two cerebral hemispheres, letting each side share what it perceives and plans with the other.

Corpus callosum. The corpus callosum is the thick bridge of white matter that joins the two cerebral hemispheres, letting each side share what it perceives and plans with the other.

It holds about 200 million axons, most of them linking matching areas on the two sides, and it lies under the longitudinal fissure, roofing the lateral ventricles. Its front part (the genu) carries prefrontal fibres, its body motor and somatosensory fibres, its back part (the splenium) visual and parietal fibres. Because each hemisphere sees the opposite half of the visual field and moves the opposite half of the body, this bridge is what lets the two halves act as one.

Cutting it shows what each hemisphere knows on its own. In split-brain patients, whose callosum was severed to stop seizures from spreading, an object shown only to the right hemisphere can be picked out by the left hand but not named, because naming lives in the left hemisphere in most people.

In everyday life the cut is hard to notice. Both hemispheres still see most of the world by moving the eyes, and subcortical routes carry some information across.

What each side specialises in is on the lateralisation card.

Questions: What happens when the corpus callosum is cut? The operation was done to keep seizures from spreading between the hemispheres, and in daily life its effects are hard to see. In the laboratory, Sperry and Gazzaniga showed each hemisphere words or pictures separately by flashing them in one half of the visual field: an object shown to the left hemisphere could be named, the same object shown to the right hemisphere could not be named but could be picked out by touch with the left hand. Each hemisphere was perceiving and acting on its own, and the speaking left hemisphere would sometimes invent a reason for what the left hand had done.