The TV sci-fi series Severance (2022–) is based on a radical premise: Biotechnology employees have undergone a medical procedure — severance — that prevents any memories of the outside world while at work and any memories of their work life when they are not:

In a recent article in The Scientist, science journalist Hannah Thomasy uses the sci-fi premise to introduce an article on claims about split-brain syndrome:

If the world of Severance was real, your “innie” would be reading this article at work, oblivious to the fact that your “outie” intends to spend the evening scouring the internet for these very answers. While the Severance procedure—surgical implantation of a chip into the brain to create separate conscious agents with access to separate streams of memory and experience—is purely fictional at present, another brain-splitting surgical procedure, called a corpus callosotomy, is entirely real and has been in use since the 1940s. Instead of separating work life and personal life, this procedure separates the right and left hemispheres of the brain by severing the major line of communication between them, a thick bundle of nerves called the corpus callosum. This surgery was used to treat severe and refractory epilepsy; in many patients, it helped control seizures by preventing aberrant neural activity from spreading between the hemispheres.

“Severance versus Science: The Neuroscience of Split-Brain Syndrome,” March 20, 2025

Two separate minds?

As neurosurgeon Michael Egnor and I discuss in our forthcoming book The Immortal Mind (Worthy June 3, 2025), one treatment for severe epilepsy — one that Dr. Egnor has performed — is corpus callosotomy:

Corpus callosotomy is a type of epilepsy surgery to treat seizures when antiseizure medications don’t help. The procedure involves a careful surgical cut of a band of fibers (the corpus callosum) connecting the two halves of your brain. Afterward, seizure activity can’t pass from one half of your brain to the other, limiting its spread. – Cleveland Clinic

Some neuroscientists have toyed with the idea that two separate consciousnesses result. As one researcher, Gonzalo Munevar of Lawrence Technical University, explains,

This [split-brain surgery] cemented the belief that two consciousness may exist side by side, so to speak. Such a belief is not uncommon, even amongst distinguished researchers. It arose in great part because some split-brain patients have been afflicted by the “wild-hand syndrome,” in which the patient, say, would reach for an object with his right hand, only to have the left hand block or undue the action. This phenomenon made a strong impression on many observers, who then concluded that the two hemispheres, each with its own consciousness, were in conflict with each other. However, many considerations from psychology and neuroscience lead to a simpler and more nuanced explanation without recourse to extraordinary claims: The different hemispheres are conscious at different times, depending on the task.

Munevar, Gonzalo. (2012). The Myth of Dual Consciousness in the Split Brain: Contrary Evidence from Psychology and Neuroscience. International Conference on Brain-Mind Proceedings.

Alien hand (“wild-hand”) syndrome — it’s usually the left hand that acts oddly — is quite rare and is usually temporary :

That is a slender basis on which to hang the idea that the split-brain patient has two separate consciousnesses.

Remarkable resilience

Similarly, Roger Sperry (1913–1994) won a Nobel Prize for carefully constructed experiments that showed limitations in what people with split brains could perceive. For example, a subject might see an apple but — without moving his head — be unable to say what it was. Apart from the experiment, that disability might have gone unnoticed because the subject, left to himself, would simply move his head without thinking about it.

More recent research, Thomasy notes, has tended to illustrate the remarkable resilience of damaged brains:

While the basic ideas about the localization and lateralization hold true—in that certain regions of the cortex seem more important that others for functions like vision, motor control, or language—the human brain is riddled with complexities and exceptions to these “rules.” Functional neuroimaging studies have revealed that the right hemisphere may also play an important role in certain aspects of language processing and production. More recent studies of Broca’s area—once thought to be critical for the production of language—have indicated that damage to this region, either from a stroke or surgical resection is not consistently linked to long-term speech difficulties. Even the motor cortex is more complex than it first appeared: Motor control regions are interwoven with areas that seem to govern action planning rather than the action itself. Split-Brain Syndrome”

So maybe the division between the two halves of the brain was never as rigid as was once thought. It’s still unclear how the halves stay in touch after they have been severed but, thousands of operations later, it seems clear that, for the most part, they do.

Two separate consciousnesses oblivious to each other, one in each half of the brain, is the sort of idea that just writes itself into science fiction. It’s surprising that more use hasn’t been made of it. But in reality, consciousness is still the subject of hot controversies — even if we are only talking about the conventional garden variety we all experience.

You may also wish to read: Consciousness wars still simmer, despite peacekeeping efforts. An article by Mariana Lenharo at Nature discusses the aftermath of the famous Letter against Christof Koch’s IIT theory. If neuroscientists are looking fruitlessly for a material basis for the human mind, progress may always be measured in conflicts, not insights.

Meanwhile, in the fascinating, creative world of science fiction: