Severance versus Science: The Neuroscience of 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.¹ 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.

But researchers were concerned about whether this procedure might have other, less desirable effects. What happens when the right and left sides of the cerebral cortex—responsible for an enormous array of functions including sensation, movement, learning, memory, language, personality, decision-making, and consciousness—can no longer communicate with each other? Can each half carry out its tasks independently? And if so, what are the implications of having two independent centers of sensory processing, or reasoning, or consciousness?

A top-down view of the brain, drawn in light blue, with the corpus callosum highlighted in orange. — The corpus callosum is a thick band of nerve fibers that enables rapid communication between the right and left hemispheres.
iStock, libre de droit

Initial neuropsychological testing in the 1940s revealed surprisingly little: Patients with epilepsy who underwent this surgery performed similarly on tests of vision, handedness, and language before and after the procedure, and seemed to have no difficulties carrying out the activities of daily life.² “The discrepancy between the large size, strategic position, and apparent importance of the corpus callosum on the one hand, and the lack of functional disturbance after its section on the other, posed for many years one of the more intriguing and challenging enigmas of brain function,” wrote California Institute of Technology neurobiologist and Nobel laureate Roger Sperry in a 1961 paper.³

Sperry performed years of experiments with split-brain cats and monkeys, which largely seemed to behave normally, but he observed certain peculiarities under carefully controlled conditions. In these animals, he found, “that each of the divided hemispheres now has its independent mental sphere or cognitive system—that is, its own independent perceptual, learning, memory, and other mental processes.”³ Informed by these experiments, Sperry hypothesized that there might be more going on with callosotomy patients than had been revealed by the initial tests.

In 1960s, Sperry and his graduate student Michael Gazzaniga began their landmark studies of these individuals, carefully presenting visual or tactile stimuli to one hemisphere and evaluating responses generated by the same (or opposite) hemisphere.^4,5 In this way, they could measure the individual processing capabilities of each hemisphere and study which processes patients appeared to be conscious of and which operated outside of their awareness. This research opened a new area of exploration into localization of function within the brain and raised questions about the nature of consciousness that to this day have not been conclusively answered.

“[Prior to] the modern split-brain work, there was a lot of classic neuropsychological work that suggested that certain functions were localized in various parts of the brain,” said Gazzaniga, now an emeritus professor at the University of California, Santa Barbara. For example, in 1865, French anatomist Pierre Paul Broca published his work on language lateralization, identifying a region in the left frontal cortex that appeared to be crucial for speech production, based on the lesions he identified postmortem in the brains of twelve patients with speech difficulties.⁶

In the 1930s and 1940s, pioneering neurosurgeon Wilder Penfield, of the Montreal Neurological Institute, began to construct more detailed maps of the human cortex in the course of his work developing surgical treatments for severe focal epilepsy.⁷ Using local anesthesia, Penfield was able to keep his patients conscious, even as he removed parts of the skull to expose the brain. His patients were then able to report on their experiences as Penfield stimulated different parts of the cortex. This not only helped surgeons identify (and then remove) the exact region where the seizures originated, but it also helped them avoid removing parts of the so-called eloquent cortex—any region involved in crucial functions like vision, hearing, somatosensation, motor control, and language.

Penfield kept meticulously detailed notes, enabling him to localize certain brain functions in individual patients but also to pool these observations and create more widely applicable maps of the human brain. This included the sensory and motor homunculi diagrams that students of neuroscience will likely recognize—these caricature-like humans with outsize faces and hands represent the relative sizes of the brain regions responsible for feeling and moving various regions of the body.⁸ Each hemisphere is responsible for the contralateral side of the body.

Penfield’s work also raised questions about the localization of specific memories. During brain stimulation, most patients reported experiencing things such as flashes of light, a specific smell or taste, or tingling in some region in the body. But others reported vivid and complex experiences—hearing a song played by an orchestra, watching a woman tell her son his coat was on backwards—that might have been memories.⁹ “Where is the place of understanding?” He pondered in a 1952 paper. “Where is the locus of the mind?”⁹

Patients who had undergone the split-brain surgery generously volunteered their time to help researchers search for answers to questions about the functional specializations of each hemisphere: Did one side control language processing, as hypothesized? What about emotion processing, or facial recognition, or memory? Did consciousness reside in one hemisphere or both, and if the latter, what would that mean for patients whose hemispheres were no longer connected?

In a 1967 study of three patients with split-brain syndrome, Gazzaniga and Sperry measured how participants responded to stimuli presented to one hemisphere or the other: Images shown to the right visual field or touch on the right side of the body were received by the left hemisphere and vice versa.⁵ When objects were presented to the right hand or right visual field, participants easily reported the objects’ identity; conversely, they were completely unable to verbally identify any objects presented on the left side of the body (and thus the right brain). It was not simply that the right brain was unable to receive or process information, however: If a picture was presented to the left visual field, participants could use the left hand to select a matching three-dimensional object.

Based on the findings from this study, and others conducted earlier in the 1960s, they concluded that, “conscious awareness is commonly present in the minor [right] as well as in the major [left] hemisphere and that the two separate spheres of conscious experience may proceed concurrently as well as in alternation.”⁵

Further studies by Gazzaniga and others extended these findings. In one study, a patient’s right and left hemispheres appeared to assign different values—indicated by the left and right hands choosing different numbers—when asked to rate people, objects, or concepts on a liking scale from one to seven.¹⁰ In another, researchers reported that patient’s right and left sides could use hand signals indicating higher or lower to guess a number that only one side could see.¹¹

However, more recent studies from researchers at the University of Amsterdam have challenged the idea that severing the corpus callosum results in two separate conscious entities.¹² This study found that when two images were presented to the right and left visual fields, say a circle and a square, the participants could not tell if the shapes were the same or different, which seems to indicate separate visual processing, in agreement with earlier studies. Unlike previous studies, however, the Amsterdam researchers found that when a single shape was presented in either the right or left visual fields, participants could indicate that they had seen the shape, both verbally and by pointing with the right or left hand. According to the authors, this seems to indicate a unified consciousness and at least some exchange of information between the hemispheres. In support of this hypothesis, brain imaging studies in 2008 and 2024 revealed that some functional connectivity remained in patients with split hemispheres.^13,14

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.¹⁵ 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.^16,17 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.¹⁸

Questions about consciousness—where it is located in the brain, whether more than one consciousness can exist in a single brain—are, of course, even trickier to answer. In a 2020 paper, New York University neuroscientist Joseph LeDoux noted that studies of patients with different neurological syndromes seem to indicate that there are important processes happening in the brain that seem to lie outside of conscious awareness.¹⁹ Patients with amnesia, for example, can learn new motor skills and develop conditioned responses to specific stimuli, even though they may not explicitly remember learning the skill or seeing that stimulus before.

Indeed, as Gazzaniga points out, while we can describe the subjective experience of consciousness, “What [consciousness] is, is very hard to put your fingers on. And further on that point, will we recognize the answer when we see it? …There's no settled view on what an answer will look like.”

When asked about what questions remain in this field of research, Gazzaniga jokingly replied, “Other than everything?”

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