Corpus Callosum Connecting the Right and Left
Hemispheres of the Brain
The corpus callosum is a bundle of neural fibers connecting the left and right cerebral hemispheres of the brain. Is is the largest white matter structure in the brain. White matter is primarily made of myelinated nerve cell processes, or axons. These axons interconnect gray matter areas of the brain, or neurons. The axons carry nerve impulses between neurons. The corpus callosum consists of approximately 200-250 million axons.
Much of what we know about the function of the corpus callosum comes from studying patients who don't have a corpus callosum. Some infants are born without a corpus callosum, a condition called agenesis of the corpus callosum. This is a birth defect in which the corpus callosum is partially or completely absent. The reasons for this birth defect are not known. The effects of this disorder range from mild to severe, depending on the level of brain abnormality. In mild cases, the child's intelligence may be normal, but the child may have difficulty with tasks such as matching visual patterns. In the most severe brain malformations, the child may have intellectual retardation, seizures, hydrocephalus, and spasticity.
Transection showing the left hemisphere of the brain and the
Corpus Callosum that would connect to the right hemisphere
A better understanding of the function of the corpus callosum has also come from studying patients where the corpus callosum has been severed surgically. This surgical operation called corpus callosotomy is used as a last resort to treat epilepsy. This results in a condition called the "split-brain syndrome."
To understand split-brain syndrome, you must first recognize that the left hemisphere of the brain controls the right side of the body, and the right hemisphere of the brain controls the left side of the body. It is also important to realize that the left side of the brain is dominant in understanding and using language, while the right side of the brain is more involved in creative aspects such as creating and recognizing pictures and images.
When a patient with a split-brain is shown an object to his left eye, he will not be able to name the object. This is because the left eye is connected to the right side of the brain. The language center in the left side of the brain (i.e., associated with the right eye) did not see the object. Since communication between the two sides of the brain is severed, the left side of the brain cannot name what the right side of the brain is seeing. However, the patient can draw the object with their left hand since that hand is controlled by the right side of the brain. The patient can also recognize the object through touch with their left hand, again because the left hand is controlled by the right side of the brain.
See our corpus callosum videos to see a patient doing split-brain experiments.