Corpus callosum

Also known as: corpus callosum function, corpus callosum location, callosum, callosal fibers, splenium, genu of the corpus callosum, agenesis of the corpus callosum

In one sentenceThe corpus callosum is the thick bundle of nerve fibers joining the brain's left and right hemispheres, letting sensory, motor and cognitive information pass between them.

What does the corpus callosum do?

The corpus callosum is a thick bundle of nerve fibers that connects the left and right hemispheres of the brain, so that the two sides can communicate and send signals to each other. A combination of sensory, motor and cognitive information is constantly passing between the hemispheres along it. It is the brain’s main commissural pathway, made of white matter tracts that cross from one hemisphere to the other, and it is the largest white matter structure in the brain. It sits in the white matter of the cerebrum and is about 10 cm long at the midline.

Counts of its fibers differ between reference sources. The Queensland Brain Institute gives about 300 million axons in an average corpus callosum, while a StatPearls reference chapter gives about 200 million heavily myelinated fibers. According to the Queensland Brain Institute, the structure evolved only in placental mammals.

Where it is and its parts

The corpus callosum lies in the middle of the brain, between the two hemispheres. It is closely related to the fornix and the lateral ventricles, and together with the fornix it forms a physical barrier that separates the two lateral ventricles.

Anatomists divide it into four parts, listed from front to back, each connecting distinct areas of the cortex:

• Rostrum. Its fibers connect the orbital regions of the frontal lobes.
• Genu. Its fibers cross over and form the forceps minor, a connection between regions of the frontal cortex on each side.
• Body. Its fibers run sideways and form the corona radiata and other large white matter pathways.
• Splenium. Its fibers run backward and form the forceps major, which connects the occipital lobes.

The isthmus is the narrow region between the body and the splenium. Callosal fibers can link an area of cortex either to the matching area on the other side (homotopic projections) or to a different area (heterotopic projections), and they end on neurons in the same cortical layer.

Which fibers go where

Earlier tracing studies had already shown that fibers for different parts of the cortex cross at different points along the corpus callosum. The best-known summary, Witelson’s scheme, is based mainly on data from primates and divides its midline cross-section into five vertical zones.

In 2006 Sabine Hofer and Jens Frahm retested this in individual healthy people with diffusion tensor imaging, an MRI method based on the movement of water in tissue, combined with a fiber-tracing algorithm. They also distinguished five vertical segments, carrying fibers to:

1. the prefrontal cortex,
2. the premotor and supplementary motor areas,
3. the primary motor cortex,
4. the primary sensory cortex, and
5. the parietal, temporal and occipital areas.

The third and fourth segments correspond to the motor and touch strips pictured in the cortical homunculus. The largest differences from Witelson’s scheme were in the middle and front of the corpus callosum. In particular, motor fibers crossed much further back than previously indicated, and the authors proposed a revised classification.

The fibers also differ in thickness along the structure. In 1992 Francisco Aboitiz and colleagues measured fiber densities in ten regions of the corpus callosum in 20 human brains, ten from women and ten from men. Thin fibers were densest at the front, in the genu, became sparser toward the back of the body and grew denser again in the splenium. Large-diameter fibers showed the opposite pattern, peaking toward the back of the body. Brains with a larger callosal area had more fibers crossing in total, but this held only for the thin fibers, most of which are believed to connect association cortex.

How it grows

The corpus callosum expands rapidly during infancy as the number of axons, their diameter and their myelin all increase. Its development is complete by age 4, but it continues to grow at a much slower rate until the third decade of life.

Split-brain research

When Roger Sperry began experiments on animals in the early 1950s, the function of the connections between the hemispheres was entirely unknown. He found that when these connections were cut in monkeys, each hemisphere could still learn, but what one hemisphere had learned was not available to the other.

A similar operation, called commissurotomy, had by then also been carried out in a number of patients with severe epilepsy that did not respond to other treatment. The Queensland Brain Institute describes surgically cutting the corpus callosum as a last-resort method for untreatable epilepsy, used to stop seizures from spreading across the brain. Most of these early patients had fewer seizures afterward. Their general behavior showed no obvious changes, and psychological tests found no impairment in their ability to perceive and learn.

In the early 1960s Sperry, working with Michael Gazzaniga and Joseph Bogen, began testing these patients. Their 1962 report described functional effects of cutting the commissures in a person, and Gazzaniga later wrote that these studies gave rise to much of what is known about how the hemispheres specialize and work together. With carefully designed tests, Sperry showed that in these patients each hemisphere learned and remembered independently of the other. The isolated left hemisphere could speak, write and calculate. The right hemisphere was mute, yet it was better than the left at some tasks, including spatial awareness and recognizing melodies. Sperry received half of the 1981 Nobel Prize in Physiology or Medicine “for his discoveries concerning the functional specialization of the cerebral hemispheres”. The other half went to David Hubel and Torsten Wiesel for work on the visual system.

According to Gazzaniga’s 2005 review, split-brain methods, alone and combined with brain imaging, have since shown how specific different regions of the corpus callosum are and how the structure helps integrate the perception of causality and a unified sense of self.

The textbook picture has also been questioned. In the standard account, a split-brain patient can respond to an image in the left half of the visual field only with the left hand, and to an image in the right half only with the right hand or in speech. In 2017 Yair Pinto and colleagues studied two patients whose corpus callosum had been completely cut, confirmed by brain imaging. They replicated the classic finding that the patients could not compare images across the two halves of the visual field. But across many tasks, both patients were fully aware that an image was present anywhere in the visual field and identified its location, orientation and identity well above chance, whether they answered with the left hand, the right hand or in words. The authors suggested that cutting the corpus callosum splits visual perception but does not create two independent conscious perceivers in one brain.

When it does not form

Agenesis of the corpus callosum is a failure to develop this bundle of fibers. It occurs in about 1 in 4,000 people. The Queensland Brain Institute describes the developmental disorder, called corpus callosum agenesis or dysgenesis, as one in which the corpus callosum is malformed or missing altogether. It usually arises during a critical period between 12 and 16 weeks of pregnancy, and it can be associated with a range of physical, intellectual and social disabilities, while some people with it live full and productive lives.

Research has looked at how the structure normally forms. Queensland Brain Institute researchers found that a particular type of glial cell in the fetal brain plays a role in building the corpus callosum, and that disrupting these cells is one source of the condition. They also found that mutations in a gene called DCC led to malformation of the corpus callosum. The same gene is linked to mirror movement disorder, in which one limb moves involuntarily along with the other.

A 2007 review by Lynn Paul and colleagues described how genetics, animal models and detailed brain imaging were shedding light on the developmental and molecular basis of agenesis. Studies using neuropsychological tests, EEG and fMRI were examining the impairments in emotional and social functioning that can result. Researchers are also interested in how the brain rewires itself to make up for the missing connection, a question about neuroplasticity.

Sex differences

It has been claimed, and widely believed, that the splenium, the back of the corpus callosum, is larger in women than in men. In 1997 Katherine Bishop and Douglas Wahlsten carried out a meta-analysis of 49 studies published since 1980 and found no significant sex difference in the size or shape of the splenium, whether or not brain size was taken into account. They argued that simply dividing callosal size by whole-brain size can create a false impression of a difference. The studies did confirm that men have, on average, a larger brain and a larger corpus callosum overall. The authors concluded that the belief that women have a larger splenium and therefore think differently is untenable. Aboitiz’s 1992 fiber count likewise found no sex differences in the fiber composition of the corpus callosum.