The corpus callosum is a huge bundle of nerve fibers found in mammalian brains. It connects the left and right hemispheres of the brain, and is responsible for most of the communication between the two. It is composed of white matter, that is, myelinated nerve cells, or axons, whose primary function is to connect grey areas together with neural impulses. The corpus callosum is the largest white matter structure in the brain, found in its interior. Grey matter occupies the periphery.
Although this area is largely composed of uniform material, the back (posterior) portion is named the splenium, while the front (anterior) portion is called the genu. In 1982, an article was published that claimed the corpus callosum is larger in women than in men, permitting greater crosstalk between the two hemispheres, but this was subsequently found to be false.
In severe cases of epilepsy, the corpus callosum is sometimes surgically severed. This is called a corpus callosectomy. Information from experiments involving patients who have undergone this procedure, sometimes called split-brain patients, has provided substantial insight into the functioning of the brain. In some cases, split-brain patients develop bizarre pathologies, such as Alien hand syndrome, in which one's hand seemingly takes on a life of its own.
Split-brain experiments have found that, when a patient is shown an object in his or her left visual field, the patient cannot name the object, despite recognizing it fully. This is because the speech control center is in the left side of the brain, and information from the left visual field only goes to the right side, which is then unable to relay this information to the other hemisphere. Split-brain patients may also develop a dual personality, one loosely associated with each hemisphere, a sort of "Jekyll and Hyde" effect. For this reason, the removal of the corpus callosum is highly controversial, and only performed in cases in which epileptic seizures are extremely resistant to drug-based treatments.