Split-brain

Disease/Disorder

Anatomy or system affected: Brain

Definition: A condition that results from cutting the corpus callosum, which is the major connection between the two cerebral hemispheres of the brain.

Key terms:

cerebral hemispheres: refers to the two halves of the cerebrum, which includes the cerebral cortex, containing both white and gray matter of the brain

commissure: a connection or link between two structures, such as the two cerebral hemispheres

contralateral: referring to the opposite side

epilepsy: a neurological condition that results in seizure activity

epileptic focus: the point in the brain where a seizure originates

glia: the most numerous cells within the brain; in contrast to neurons, they do not conduct impulses but provide various supportive functions such as cleaning up waste products

laterality: refers to unique functions that occur within only one cerebral hemisphere

myelin: glia cells that wrap themselves around nerve axons

Causes and Symptoms

Split-brain is a condition that develops when the two cerebral hemispheres in the brain cannot easily and efficiently communicate with each other. In early usage, it was referred to as the bisected brain. The corpus callosum (CC) is the largest of several connections between the two hemispheres. The CC is made up of mostly nerve axons (approximately 350 million in the human brain), which are covered with glia cells that form a myelin sheath. The density of the myelinated axons is such that it alters the color of the CC, giving it a whitish appearance, hence, the name “white matter” as opposed to “gray matter” found in the brain. The central purpose of the CC is to allow for the transference of massive amounts of information to flow across the two hemispheres. When this connection is severed, the brain receives information but becomes limited in how it can process it.

Immediately after the CC is surgically cut, a patient will typically experience trouble coordinating their behavior. Investigators have described the split-brain condition as though two people are in control of a single body. For example, a split-brain person who removes an item from a closet with one hand could find that the opposite hand attempts to put it back. Fortunately, the internal competition that results from two independent hemispheres dissipates after a few weeks. The brain, through the process of neuroplasticity, begins to alter itself in ways that allow the two hemispheres to cooperate better. One way it does this is by allowing additional commissures (e.g., the anterior commissure) to transfer information that had been previously sent across by the CC. It is also believed that the left hemisphere suppresses the right hemisphere’s ability to interfere with coordination by taking control.

Although severing the CC is an intrusive surgical procedure, it usually does not lead to a permanent, debilitating outcome. Most patients lead a normal life post-surgery. For physicians and neuroscientists, one unexpected windfall from the procedure was that it created an opportunity to learn more about the independent nature and functionality of the two hemispheres. Although it was previously known that each hemisphere controlled the sensory and motor information on the contralateral side of the body, the split-brain condition revealed the laterality for different language functions, memory processes, and different types of attentional processes. Experimental testing performed by Michael Gazzaniga and his colleagues found that visual information could be presented in such a way that only one hemisphere could learn it. If a word such as “spoon” was presented to the right hemisphere, the split-brain person showed signs of knowing the word yet could not verbalize it. The inability to say “spoon” was due to the lack of language centers in the left hemisphere for the vast majority of people. However, when the person is asked to point to the object from a group of objects, the person has no difficulty identifying the spoon. This apparent lack of “knowing” how to verbalize the object is due to the fact that information was placed into the nonspeaking hemisphere, which could not transfer the information into the left hemisphere via the CC.

Treatment and Therapy

The split-brain procedure is most often performed on patients who suffer from epilepsy. Epilepsy occurs in one to two percent of the population. In the majority of cases, epileptic seizures can be controlled using antiseizure medication. When medicines do not adequately control seizure activity, surgical procedures are considered to remove the epileptic focus with the goal of eliminating the seizures. In rare instances, a patient might have multiple focus points within the same hemisphere, whereby surgically removing all of them would be too dangerous. An alternative surgical procedure is a corpus callosotomy. This is a procedure that severs the CC, resulting in the split-brain condition. Surgeons will consider cutting about two-thirds of the CC and then evaluating the results as a means to avoid some of the split-brain symptoms. This procedure does not prevent seizures from occurring; instead, it isolates the seizures in one hemisphere by preventing the malevolent electrical signals from propagating through the CC to the opposite hemisphere, thus minimizing their disruptive influence on the brain. A corpus callosotomy is a last resort treatment to stop the hemispheres from sharing seizure activity, but it can come at a cost.

Different degrees of split-brain conditions can occur as a result of abnormal neural development. A rare genetic disorder, callosal agenesis, occurs when the CC forms improperly or not at all. This condition can lead to developmental delays and difficulties in language as well as spatial reasoning. Whether the split-brain condition arises from corpus callosotomy or from callosal agenesis, cognitive rehabilitation can be prescribed as a means to improve overall cognitive functioning.

Perspective and Prospects

While William P. Van Wagenen and R. Yorke Herren, in 1940, were the first to create the split-brain condition by surgically cutting the CC as a means to control epileptic seizures, it was Michael Gazzaniga, Joseph Bogen, and Roger Sperry who adapted more modern techniques, which were initially performed on animals, to human patients in the early 1960s. Sperry was one of three winners of the Nobel Prize in Physiology or Medicine in 1981 for his work on the functional specialization of the cerebral hemispheres. While corpus callosotomy surgeries are performed to help patients suffering from epileptic seizures, specifically those that are resistant to medication, most neuroscientists have turned to functional brain imaging technologies such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) scanning as a means to investigate the unique processing capabilities of the two cerebral hemispheres.

Bibliography

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“Epilepsy Foundation.” Epilepsy Foundation, www.epilepsy.com. Accessed 28 Mar. 2026.

“Epilepsy.” World Health Organization, 7 Feb. 2024, www.who.int/news-room/fact-sheets/detail/epilepsy. Accessed 28 Mar. 2026.

Gazzaniga, Michael. “Forty-Five Years of Split-Brain Research and Still Going Strong.” Nature Reviews Neuroscience, vol. 6, no. 8, 1 Aug. 2005, pp. 653–9, doi:10.1038/nrn1723. Accessed 28 Mar. 2026.

Gazzaniga, Michael. “The Split Brain Revisited.” Scientific American, 1 July. 1998, www.scientificamerican.com/article/the-split-brain-revisited/. Accessed 28 Mar. 2026.

“The Nobel Prize in Physiology or Medicine 1981.” Nobel Prize, www.nobelprize.org/prizes/medicine/1981/summary. Accessed 28 Mar. 2026.

Reuter-Lorenz, Patricia Ann, and Michael S. Gazzaniga. The Cognitive Neuroscience of Mind: A Tribute to Michael S. Gazzaniga. MIT Press, 2010.

Springer, Sally, and Georg Deutsch. Left Brain Right Brain Perspectives from Cognitive Neuroscience. W.H. Freeman and Company, 2003.

Stirling, John D. Cortical Functions. Routledge, 2000.