Corpus callosum

The corpus callosum is a bundle of nerves that connects the left and right hemispheres of the brain. Although scientists knew about its existence for a number of years, it was not until the middle of the twentieth century that they were able to determine its function. The corpus callosum is the principal means by which the two halves of the brain communicate with each other. If the corpus callosum is damaged in some way prior to an individual's birth, this intercommunication between the brain halves is permanently impaired and may lead to developmental, behavioral, and social issues. In patients with severe epilepsy, the corpus callosum might be intentionally severed either partially or totally in an effort to control seizures.

Background

The term corpus callosum is Latin for "tough body." The presence of the corpus callosum was known for many years before its function was adequately understood. In 1906, anatomist R.B. Bean put forth a statement that the male human brain had a larger corpus callosum than the female brain. He also suggested that there were differences in its size among different races; his statement carried an implication of superiority in those with a larger corpus callosum. Less than three years later, Franklin Mall, the physician in charge of the same lab where Bean worked, refuted Bean's claims. While there was debate on both sides of this hypothesis over the next century, magnetic resonance imaging (MRI) examinations conducted in 2006 found no consistent difference.

During the 1950s, physicians and scientists were finally able to document the function of the corpus callosum. The experiments that identified its function were conducted at the University of Chicago by graduate student Ronald Meyers. Meyers devised an experiment in which he taught cats to react to two images projected on screens in front of them by touching the image that included a circle.

He tested the cats by covering one eye and teaching them to find the circles, then switching the cover to the other eye and projecting the images again. The cats were able to perform the task even though the uncovered eye had not been trained to look for the images. He then severed the part of the optic nerves that cross from the left eye to the right half of the brain, and vice versa. This meant that the left eye was only connected to the left side of the brain and the right eye to the right side. Once again, he taught the animals to touch the desired image while covering one eye and then tested it by switching the covering. The animals were still able to perform the task. This led him to the conclusion that the information was being shared through the corpus callosum. When he tested this by severing the nerve bundle and again testing the cats, they were no longer able to perform the task when they were taught with one eye and tested with the other. This proved the function of the corpus callosum as the information conduit from one side of the brain to the other.

Overview

The corpus callosum is a bundle of somewhere between 200,000 and 250,000 nerve fibers that connect the two halves of the brain. If the brain were cut in half, the corpus callosum would be found in the center of the cut area. It resembles the rounded top portion of a mushroom.

These halves, the right and left hemispheres of the brain, are made up of what is known as gray matter. This kind of brain tissue stores and interprets information, making it the part of the brain used to think, compute, store memories, and do many of the other functions associated with thought and intelligence. The corpus callosum is considered to be white matter. This type of brain tissue has a higher level of a substance called myelin. Myelin is a fatty material that coats nerve fibers and allows them to transfer information quickly.

The body is designed so that the left side of the brain controls the functions of the right side of the body, and vice versa. In addition, each side of the brain is responsible for certain specific functions. The right side of the brain is responsible for the more artistic, creative functions of the mind while the left side is responsible for the functions that involve logic. However, few functions are purely artistic or purely logic related; as a result, the two sides of the brain need to communicate and share information. This is the function of the corpus callosum.

A human's corpus callosum starts to form during the first trimester of pregnancy and is generally completely developed by the time the child is born. However, it continues to increase in function throughout childhood, reaching its peak at about age twelve. From that point through adulthood, the corpus callosum is as fully functioning as it will ever be.

Sometimes something goes wrong as the fetus is developing before birth. This can result in one of several known disorders of the corpus callosum. These disorders are diagnosed before or after birth through tests such as ultrasounds, MRIs, and computerized axial tomography (CAT) scans. These disorders can be caused by chromosomal abnormalities, toxic metabolic conditions such as drug or alcohol abuse by the mother, certain infectious diseases such as rubella, and cysts or other conditions that affect the physical growth of the corpus callosum. The condition cannot be corrected; children born with these disorders may never have full communication between the two halves of their brain and may suffer developmental, social, and behavioral issues that can present in childhood and become increasingly noticeable as they move into adolescence and adulthood.

In some cases, physicians intentionally sever the corpus callosum. This is usually done in patients with severe epilepsy. Because the communication between the two halves of the brain can increase the intensity of seizures, some patients experience a reduction in the number or intensity of the seizures if the corpus callosum is partially or fully severed. However, this procedure, which is called a corpus callosotomy, leaves the patient with permanent side effects that can include problems with coordination, speech, and lack of awareness of one or the other side of the body.

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