10 Feb Cutting the Brain in Half
By Alli Glover, UNE Discovery
Cutting the brain in half: What it tells us about brain organisation, language, and human consciousness
Snipping the brain straight down the middle to treat a neurological condition sounds more like the plot of a science fiction film than a legitimate medical intervention. Yet in the mid-20th century, this drastic-sounding procedure became a revolutionary treatment for people with severe, medically intractable epilepsy. What began as a clinical attempt to reduce seizures went on to fundamentally reshape our understanding of brain organisation, language, and even consciousness.
What is epilepsy?
Epilepsy is a neurological disorder characterised by recurrent, unprovoked seizures. According to the World Health Organization, it affects around 50 million people globally, making it one of the most common neurological conditions worldwide (WHO, 2023).
A seizure occurs when groups of neurons in the brain begin firing in an abnormal, excessive, and synchronised way. Under normal circumstances, neurons communicate through carefully regulated electrical and chemical signals. In epilepsy, this balance between excitation and inhibition breaks down, allowing waves of uncontrolled electrical activity to spread through neural networks (Fisher et al., 2014).
Seizures are broadly classified into two main types:
- Focal seizures, which originate in a specific region. Symptoms depend on the affected area and can range from unusual sensations or involuntary movements to changes in emotion or awareness.
- Generalised seizures, which involve both hemispheres of the brain from the onset. These seizures often lead to loss of consciousness, muscle stiffening or convulsions, and can be physically dangerous.
Generalised seizures are particularly disruptive. Beyond the immediate risk of injury, they can impair memory and cognition over time. Repeated generalised seizures are also associated with reduced quality of life, increased risk of sudden unexpected death in epilepsy (SUDEP), and significant social and psychological consequences (Devinsky et al., 2018).
For many patients, seizures can be controlled with medication. However, a substantial minority continue to experience frequent seizures despite optimal drug treatment. It was for these individuals that surgeons began considering more radical solutions.
The solution
In the 1940s, scientists and clinicians were searching for ways to limit the devastating effects of generalised seizures (Markosian et al., 2022). As stopping seizures from occurring altogether was not an option, they asked a different question: what if the spread of seizure activity could be contained?
To understand their reasoning, it helps to know a little about the brain’s wiring. The brain is divided into two hemispheres, left and right, which are connected by a thick bundle of nerve fibres known as the corpus callosum. This structure consists of more than 200 million white matter fibres and serves as the brain’s main communication highway, allowing information to pass rapidly between hemispheres (Fitsiori et al., 2011)
Seizure activity often spreads through these interhemispheric connections. The idea, then, was deceptively simple: if the corpus callosum were severed, seizure activity originating in one hemisphere might be prevented from propagating to the other. In theory, this would transform a generalised seizure into something more focal, reducing its severity and the associated risks.
The goal was not to cure epilepsy, but to improve quality of life by limiting one of the most dangerous aspects of the condition.
The experiments
Surgeons began performing a procedure known as a corpus callosotomy, in which the corpus callosum is partially or completely cut. This effectively creates two cerebral hemispheres that can no longer directly talk to each other.
From a clinical perspective, the results were encouraging. Many patients experienced a marked reduction in the frequency and severity of generalised seizures, with relatively few obvious long-term side effects (van Wagenen & Herren, 1940; Spencer et al., 1988; Sass et al., 1988). Patients could still walk, talk, recognise people, and live relatively normal lives.
But beneath the surface, something extraordinary was happening.
Subtle but striking behavioural anomalies began to emerge, especially under carefully controlled testing conditions. Patients sometimes struggled to verbally identify objects placed in their left hand, even though they could select the correct object using that same hand. At other times, one hand appeared to act independently of the other. Imagine trying to open a drawer with your right hand while your left hand tries to close it!
These oddities caught the attention of neuroscientists Roger Sperry and later Michael Gazzaniga, who recognised that these patients offered a unique natural experiment. By studying individuals whose hemispheres were functionally isolated, they could probe what each half of the brain could do on its own.
This work became known as the split-brain experiments.
In the experiments, visual information was briefly presented to one side of a patient’s visual field. Because visual input from the right visual field is processed by the left hemisphere (and vice versa), researchers could selectively send information to just one half of the brain. When an image was presented to the right visual field (left side of the brain), patients could easily name it. When the same image was presented to the left visual field (right side of the brain), patients often claimed to see nothing at all, yet could correctly draw or point to the object using their left hand (Gazzaniga, 2000; Sperry, 1968).
The explanation lay in hemispheric specialisation. For most people, the left hemisphere is dominant for language production. The right hemisphere, while capable of perception and understanding, lacks the neural machinery required for speech. Without the corpus callosum, the right hemisphere quite literally knew what it had seen but had no way to tell the left hemisphere, and therefore no way to produce a verbal response (Sperry, 1968). It was, in essence, ‘silenced’.
Even more intriguingly, these experiments suggested that each hemisphere could possess its own perceptions, intentions, and goals. In some cases, patients appeared to respond differently depending on which hemisphere was engaged, leading Sperry to argue that split-brain patients might host two parallel streams of consciousness within a single skull!
What does this tell us about consciousness?
The split-brain studies profoundly challenged the intuitive idea that consciousness is a single, unified experience. Instead, they suggested that consciousness might emerge from distributed neural processes that are normally tightly integrated, but can function independently when disconnected.
Gazzaniga later proposed the idea of the “interpreter”, a mechanism in the left hemisphere that constructs narratives to explain our thoughts, actions, and feelings, even when it lacks full information (Gazzaniga, 2000). In split-brain patients, this interpreter would often confidently explain actions initiated by the right hemisphere, unaware that it was confabulating.
For example, imagine the word walk is briefly presented to the left visual field, so it is processed primarily by the right hemisphere. You then stand up and begin to walk out of the room. When the experimenter asks why you are leaving, you do not report having seen the word walk, because that information never reached the left hemisphere’s language centres. Instead, you offer a different explanation, such as going to get a drink or use the bathroom. From your own perspective, this explanation feels entirely reasonable. Unaware of the true trigger for the behaviour, the left hemisphere constructs a coherent and plausible narrative after the action has already occurred.
So, rather than revealing a brain neatly divided into logic on the left and creativity on the right, split-brain research painted a subtler picture: one in which different cognitive functions are specialised, yet deeply interdependent under normal circumstances.
A lasting legacy
Today, the corpus callosotomy is still used, though far less commonly and with more refined techniques (Markosian et al., 2022). More importantly, the insights gained from split-brain patients continue to shape neuroscience, cognitive psychology, and philosophy of mind.
What began as a desperate attempt to reduce the burden of severe epilepsy ultimately reshaped how scientists think about the mind itself. By cutting the brain’s primary communication bridge, researchers revealed that our thoughts, perceptions, and sense of self depend less on any single region than on the cooperation between many. Split-brain patients show that the brain can sustain parallel streams of awareness, normally fused so seamlessly that their separation feels almost inconceivable. In the end, split-brain research forces us to confront the uncomfortable idea that the feeling of being a single, unified self may not be as fundamental as it seems. Instead, it may be an emergent product of constant communication between the halves of our brain. It was only when these connections were cut that scientists were able to uncover just how constructed our unified sense of self really is.
Want to see the Split Brain Experiments in action? Check them out in this Youtube video: https://www.youtube.com/watch?v=ZMLzP1VCANo
References
- Devinsky, O., Spruill, T., Thurman, D., & Friedman, D. (2018). Recognizing and preventing epilepsy-related mortality: A call for action. Neurology, 90(17), 779–786. https://doi.org/10.1212/WNL.0000000000002253
- Fisher, R. S., Acevedo, C., Arzimanoglou, A., Bogacz, A., Cross, J. H., Elger, C. E., Engel, J., Jr., Forsgren, L., French, J. A., Glynn, M., Hesdorffer, D. C., Lee, B. I., Mathern, G. W., Moshé, S. L., Perucca, E., Scheffer, I. E., Tomson, T., Watanabe, M., & Wiebe, S. (2014). ILAE official report: a practical clinical definition of epilepsy. Epilepsia, 55(4), 475-482. https://doi.org/10.1111/epi.12550
- Fitsiori, A., Nguyen, D., Karentzos, A., Delavelle, J., & Vargas, M. I. (2011). The corpus callosum: white matter or terra incognita. British Journal of Radiology, 84(997), 5-18. https://doi.org/10.1259/bjr/21946513
- Gazzaniga, M. S. (2000). Cerebral specialization and interhemispheric communication. Brain, 123(7), 1293–1326. https://doi.org/10.1093/brain/123.7.1293
- Markosian, C., Patel, S., Kosach, S., Goodman, R. R., & Tomycz, L. D. (2022). Corpus callosotomy in the modern era: Origins, efficacy, technical variations, complications, and indications. World Neurosurgery, 159, 146-155. https://doi.org/https://doi.org/10.1016/j.wneu.2022.01.037
- Sperry, R. W. (1968). Hemisphere deconnection and unity in conscious awareness. American Psychologist, 23(10), 723–733.
- Spencer, S. S., Spencer, D. D., Williamson, P. D., Sass, K., Novelly, R. A., & Mattson, R. H. (1988). Corpus callosotomy for epilepsy. I. Seizure effects. Neurology, 38(1), 19-24. https://doi.org/10.1212/wnl.38.1.19
- Sass, K. J., Spencer, D. D., Spencer, S. S., Novelly, R. A., Williamson, P. D., & Mattson, R. H. (1988). Corpus callosotomy for epilepsy. II. Neurologic and neuropsychological outcome. Neurology, 38(1), 24-28. https://doi.org/10.1212/wnl.38.1.24
- van Wagenen, W. P., & Herren, R. Y. (1940). Surgical division of commissural pathways in the corpus callosum. Archives of Neurology and Psychiatry, 44(4), 740–759.
- World Health Organization. (2023). Epilepsy fact sheet. https://www.who.int/news-room/fact-sheets/detail/epilepsy