Corpus Callosotomy and Split Brain Patients


The neuronal highway known as the corpus callosum allows the interaction of the two different hemispheres of the brain. This structure consists of about 200 million axons and works to integrate the motor, sensory, and cognitive performances of the cerebral cortex of each hemisphere. For many years, the function of the corpus callosum was unknown, and was even thought to merely fasten the two hemispheres together. This all changed in 1950, when a graduate student of the University of Chicago named Ronald Myers performed research demonstrating the role of the corpus callosum in conscious communication between hemispheres. Later experiments expanded knowledge on this structure, showing that the corpus callosum connects cells present in different hemispheres. Because of this, the receptive field of different brain areas are able to span the midline and the two halves of our world are synthesized into a cohesive whole [5].
Many of these early studies were performed on cats or monkeys in the lab. Research on the corpus callosum was able to advance when more complicated experiments were performed on people whose corpus callosums had been cut, a group known as split brain patients. Split brain patients have bisected connections between their hemispheres due to lesions or surgery. In these individuals, input can be received by one hemisphere without the other hemisphere being consciously aware of said stimuli. Because of the lateralization of hemisphere function, the ability to report a stimulus depends on which side it is presented to. For example, the presence of language centers in the left hemisphere means that a split-brain individual can only report a word shown to them if it is
presented to their left visual field [3]. This concept will be explored in depth in later sections.

Anatomy and Pathways

The location of the corpus callosum is integral to its function. It is found at the midline of the brain, between the left and right hemispheres. It lies below the cerebrum and above the lateral ventricles. The commissural (transverse) fibers of the corpus callosum create the largest bundling of nerve fibers in the entire nervous system [5]. The frontal components of the corpus callosum, the rostrum and genu, connect the hemispheres of the frontal lobes. The two rostral components, the body and splenium, connect the hemispheres of the temporal and occipital lobes [6].

One critical function of the corpus callosum is the interhemispheric communication of signals created by various stimuli. These stimuli include visual information, passed on from the retina. Because of decussation of information from the nasal retina and the ipsilateral transfer of information from the temporal retina, information from the left visual field is sent to the right hemisphere and information from the right visual field is sent to the left hemisphere. As discussed earlier, language centers critical to verbal communication, including Broca's area and Wernicke's area, are located in the left hemisphere. For an individual to report stimuli present in the right visual field, the information only has to travel via pathways within the left hemisphere. To report stimuli from the left visual field, however, the visual input must cross into the right hemisphere to be processed and then into the language centers of the left hemisphere via the splenium of the corpus callosum. [3]

If the visual stimuli consists of a word or sentence to be read, the information is passed to specific higher brain structures. Research on these higher brain structures was expanded in a study performed by Laurent Cohen and his researchers that explored functional MRI (fMRI) and high-density Event Related Potentials (ERP) data of split brain patients. It was found that after processing by occipitotemporal areas contralateral to the stimulated hemifield, visual information is transferred to the visual word form (VWF) system located in the left inferior temporal region. This region is specifically devoted to the processing of letter strings, and can trigger the retrieval of the meaning and pronunciation of a word based on which identified letters are perceived. If the words in question were presented to the left visual field, then this transfer of information from the right to the left hemisphere occurs through the posterior portion of the corpus callosum [3].

Another important neuronal phenomenon pertinent to the research performed on split brain patients is the cortex’s contralateral control of the body. When the left hand writes a word, that planned motor action is initiated by the supplementary motor cortex of the right hemisphere, travelling down the spinal cord via the lateral corticospinal tract and controlling the digits of the hand via synapses on alpha motor neurons or interneurons. When the right hand writes a word, that planned motor action is initiated by the supplementary motor cortex of the left hemisphere [7]. The stage is now set to explore the variety of complications present in split brain patients, including issues with vision, motor output, and conscious awareness.

Split Brain Patients

As discussed in the introduction, many split brain patients have bisected corpus callosums due to surgical intervention. These surgeries, known as corpus callosotomies, are performed on severely epileptic individuals that do not respond to pharmaceutical intervention. The first callosotomy was performed in 1940 by Dr. William P. van Wagenen, a doctor who also indirectly assisted in the start of the study of split-brain patients [10]. In this procedure, the connection between the two hemispheres of the brain is cut. This interrupts the interhemispheric spread of seizure activity. In some cases, the surgery is performed in two stages. In the first procedure, only the front two-thirds of the structure is cut, leaving the back intact. If the seizures persist after this first procedure, the rest of the corpus callosum is cut [4]. This procedure is typically very effective, with seizure cessation or >90% seizure reduction in 85% of epileptic patients that had previously suffered from drop attacks. The family of split brain patients reported feeling satisfied with the results of the procedure in 83% of patients [9].

There are many complicated outcomes of this procedure. In split-brain patients, the VWF system in the left hemisphere is only activated by stimuli from the right visual field because the information given by visual input cannot cross via the corpus callosum. Brain imaging data has revealed that the VWF systems in split brain patients were activated by words presented in the right visual field, but not by words presented in the left visual field. Since the VWF system is not activated, some of these split brain patients were unable to report these words presented in the left visual field [2].,+a+circle.+He+can+also+pick+out+the+circle+by+merely+touching+shapes+with+his+right+hand,+out+of+sight+behind+a+screen.+However,+his+left+hand+can%E2%80%99t+identify+the+circle.+If+a+triangle+is+flashed+to+the+patient%E2%80%99s+right+brain,+he+can%E2%80%99t+say+what+he+saw+(speech+is+controlled+by+the+left+hemisphere).+He+also+can%E2%80%99t+identify+the+triangle+by+touch+with+the+right+hand.+Now,+however,+the+left+hand+has+no+difficulty+picking+out+the+triangle.+In+other+tests,+the+hemispheres+reveal+distinct+skills,+as+listed+above+the+drawing..jpg,+a+circle.+He+can+also+pick+out+the+circle+by+merely+touching+shapes+with+his+right+hand,+out+of+sight+behind+a+screen.+However,+his+left+hand+can%E2%80%99t+identify+the+circle.+If+a+triangle+is+flashed+to+the+patient%E2%80%99s+right+brain,+he+can%E2%80%99t+say+what+he+saw+(speech+is+controlled+by+the+left+hemisphere).+He+also+can%E2%80%99t+identify+the+triangle+by+touch+with+the+right+hand.+Now,+however,+the+left+hand+has+no+difficulty+picking+out+the+triangle.+In+other+tests,+the+hemispheres+reveal+distinct+skills,+as+listed+above+the+drawing..jpg

This phenomenon becomes even more interesting when considering research on the output split brain patients are capable of, as it probes deeper into what these individuals are actually conscious of. In one of Robert Sperry’s classic experiments, patients were shown a stimulus in their left visual field and asked to identify an object by touch that corresponded to that stimulus. When told to retrieve that object using their right hand, they are unable to do so. However, when told to retrieve the object using their left hand, they are able to do so due to the contralateral control the right hemisphere has over the left hand. The object appears in their left visual field, is processed by the right hemisphere, and is able to be discovered by the left hand. Though they report seeing nothing, they are still able to identify what it was they were exposed to. Split brain patients fail to perform visual-tactual association tasks involving interhemispheric communication because the severed corpus callosum is unable to pass information from the right hemisphere to the left, causing a sort of blindness known as alexia. This is further illustrated through the testing of other sensory pathways; if one nostril of a split brain patient is is exposed to a specific odor, that same odor is not recognized when presented to the other nostril [7].

The lack of interhemispheric communication in split brain patients can also lead to complications in motor output. In one study, the rate of simultaneous finger tapping was monitored during a variety of situations for both control subjects and split-brain subjects. Split-brain subjects could tap one finger at a time at a fairly normal rate, but abnormalities became apparent when more complicated tasks were presented concurrently with finger tapping. Alternation or synchronization of finger tapping took significantly longer in split-brain subjects, and some subjects were unable to tap their fingers while performing complicated verbal tasks. This may be due to the complexities involved in the mental processing of split-brain individuals. While normal subjects could tap their fingers in a highly automatic manner, finger tapping in split-brain patients appeared to cause an extra load on the cerebral control of processes such as thinking or speaking. It is also possible that corpus callosotomies disconnect the left hand from a mechanism in the left hemisphere which allows for higher rates of finger tapping [8].

In recent years, there has been definite romanticization of the lateralization of brain function. Though the claims of being a “right brained” or a “left brained” individual are not fully grounded in science, it is true that each hemisphere has certain functions not fully present in the other. Much of this information on the functions of the different hemispheres have arisen from studies done on split-brain patients. For example, split-brain patients that were previously right-handed can actually draw better with their left hands due to the right hemisphere’s dominance for spatial relationships [13]. Though there are generally lateralized functions of each hemisphere (such as language in the left and spatial judgement in the right), the ventral stream of visual processing appears to be bilateral [2].

There has been some controversy surrounding what it actually means to live with a split brain. In his paper titled Consciousness, Personal Identity, and the Divided Brain, Roger Sperry explores the implications of living with a severed corpus callosum. Some research has suggested that split brain patients actually live with two different domains of inner conscious awareness, controlled by their separated left and right hemispheres. A subject may claim that they do not register a visual stimulus in their right visual field, but with their left hand they will identify an object corresponding to the exact stimulus they were exposed to. Sperry brings these cases forward, which demonstrate individuals unaware of a function performed by one side of their body, and questions whether this shows that two different consciousnesses reside within the same human being [12]. There have even been reports of split-brain individuals uncontrollably performing two different movements with the two sides of their body. One side receives orders reflecting rational goals, while the other side receives orders reflecting hidden desires [1]. This dual motivation is rare in split-brain patients, but the fact that these individuals exist is fascinating. Research on these patients does not necessarily support the theory that there are two separate consciousnesses within the human mind, but these studies do add an interesting component to the discussion on what it means to be consciously aware and able to plan motor actions.

The structure known as the corpus callosum allows for a communication between the hemispheres of the brain critical for the combination of stimuli from either side of our world, but can also facilitate the spread of seizure activity within the brain of an epileptic individual. It is fascinating that individuals previously haunted by seizures can lead a fairly normal life after a surgery that involves severing this important neural structure. Follow up studies on these individuals have proven the role of the corpus callosum in communication between the hemispheres, and have complicated our idea of a unified consciousness within each individual human being.

Glossary [11]

  • Corpus callosum: a broad band of nerve fibers joining the two hemispheres of the brain
  • Split-brain: (of a person or animal) having the corpus callosum severed or absent, so as to eliminate the main connection between the two hemispheres of the brain
  • Corpus callosotomy: a palliative surgical procedure for the treatment of medically refractory epilepsy. In this procedure the corpus callosum is cut through in an effort to limit the spread of epileptic activity between the two halves of the brain.
  • Cerebral cortex: the outer layer of the cerebrum, composed of folded gray matter and playing an important role in consciousness
  • Lateralization: localization of function or activity (as of verbal processes in the brain) on one side of the body in preference to the other
  • Commissural fibers: white-matter structures that connect the two hemispheres of the brain
  • Broca’s area: a region of the brain concerned with the production of speech, located in the cortex of the dominant frontal lobe
  • Wernicke’s area: a region of the brain concerned with the comprehension of language, located in the cortex of the dominant temporal lobe
  • Retina: a layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.
  • Right visual field: area right of the vertical meridian in the binocular field
  • Left visual field: area left of the vertical meridian in the binocular field
  • Functional MRI (fMRI): measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled.
  • Event Related Potential (ERP): measured brain response that is the direct result of a specific sensory, cognitive, or motor event. More formally, it is any stereotyped electrophysiological response to a stimulus.
  • Visual word form system: functional region of the left fusiform gyrus and surrounding cortex (right-hand side being part of the fusiform face area) that is hypothesized to be involved in identifying words and letters from lower-level shape images, prior to association with phonology or semantics.
  • Alexia: aphasia marked by loss of ability to read, caused by a defect in the brain
  • Supplementary motor cortex: part of the primate cerebral cortex that contributes to the control of movement.
  • Lateral corticospinal tract: descending motor pathway that begins in the cerebral cortex, decussates in the pyramids of the lower medullaand proceeds down the contralateral side of the spinal cord
  • Alpha motor neuron: large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction.
  • Interneuron: a neuron that transmits impulses between other neurons
  • Epilepsy: a neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain

Relevant Links -- a video of a split brain patient being tested by Michael Gazzaniga -- Mental imaging of motor activity in humans. Includes research on handedness recognition in split brain patients. -- Function of corpus callosum in contralateral transfer of somesthetic discrimination in cats. An experiment performed by John S. Stamm and Robert Sperry, further exploring the role of the corpus callosum in information transfer.

To read more on the research performed by neuroscientist Robert Sperry, follow this link:


1) How many axons make up the structure known as the corpus callosum?
  1. 500-1000
  2. Half of a million
  3. 200 million
  4. 3 billion

2) Where is the corpus callosum located?
  1. At the midline of the brain, below the cerebrum and above the lateral ventricles
  2. At the midline of the brain, within the lateral ventricles
  3. At the midline of the brain, on the surface above the cerebrum
  4. At lateral fissure, connecting the frontal lobe to the rest of the cerebrum

3) The hemispheres of the frontal lobe are connected by
  1. The rostrum and the genu
  2. Projection fibers
  3. Association fibers
  4. The body and splenium

4) Information instructing the left hand to draw a picture is sent down the spinal cord via the
  1. The rubrospinal tract
  2. The vestibulospinal tract
  3. The anterior corticospinal tract
  4. The lateral corticospinal tract

5) Wernicke’s area and Broca’s area are both located in the left hemisphere, and assist in speech. True/False

6) Reporting a stimulus present in the right visual field involves many pathways that run back and forth between the hemispheres. True/False

7) The spatial regulatory functions of the right hemisphere can be attributed to the unilateral presence of the ventral stream, or the “what” pathway. True/False

Short answer:
8) Why is it that split brain patients are unable to report visual stimuli present in their left visual field? What happens when they are told to retrieve an object that corresponds with the stimulus with their right and left hands, respectively?

9) How does the lateralization of brain function pertain to split brain patients? Why is it that previously right-handed split-brained patients now prefer to draw with their left hand?

10) Discuss the corpus callosotomy as a treatment for epilepsy. When was the first procedure performed? Is it effective?

Answers to Multiple Choice & True and False

  1. C
  2. A
  3. A
  4. D
  5. True
  6. False
  7. False


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[2] Corballis, P. (2002). Hemispheric asymmetries for simple visual judgments in the split brain.
Neuropsychologia,40(4), 401-410.

[3] Cohen, L., Dehaene, S., Naccache, L., Lehéricy, S., Dehaene-Lambertz, G., Hénaff, M. A.,
Michel, F. (2000). The visual word form area: Spatial and temporal characterization of an
initial stage of reading in normal subjects and posterior split-brain patients. Brain, 123(2), 291-307.

[4] Corpus Callosotomy. (2017). Retrieved from

[5] Dafny, N. (1997). Overview of the Nervous System. Retrieved December 05, 2017, from

[6] Hubel, D. Eye, Brain, and Vision.

[7] Knierim, J. (1997). Motor Cortex. Retrieved from

[8] Kreuter, C., Kinsbourne, M., & Trevarthen, C. (1972). Are deconnected cerebral hemispheres
independent channels? A preliminary study of the effect of unilateral loading on bilateral
finger tapping. Neuropsychologia,10(4), 453-461.

[9] Maehara, T., & Shimizu, H. (2008). Surgical Outcome of Corpus Callosotomy in Patients
with Drop Attacks. Epilepsia,42(1), 67-71. doi:10.1046/j.1528-1157.2001.081422.x

[10] Mathews, M. S., Linskey, M. E., & Binder, D. K. (2008). William P. van Wagenen and the
first corpus callosotomies for epilepsy. Journal of Neurosurgery,108(3), 608-613.

[11] Medical Terms and Abbreviations: Merriam-Webster Medical Dictionary. Retrieved
December 07, 2017, from

[12] Sperry, R. (1984). Consciousness, personal identity and the divided brain.
Neuropsychologia,22(6), 661-673.

[13] Wright, A. (1997). Higher Cortical Functions: Associative and Executive Processing.
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