Photo by Dean Hochman, CC BY 2.0.

Since the 1940s, neurosurgeons have been performing corpus callosotomy—a surgery that serves as a last resort for treating epilepsy. It included cutting through the corpus callosum, which functions as the main connection between the two hemispheres of the brain. The procedure was considered dangerous by some and less preferred by many, yet it relieved most patients from unbearable epileptic seizures.

Corpus callosotomy “keeps the electrical signals that cause a seizure from crossing over and wreaking havoc,” says Emily Temple-Wood, Wikipedia editor and medical student who was named the Wikipedian of the Year in 2016. “It’s amazing how well these patients adapt and recover, and this is all due to how plastic the brain is.”

Studying those patients helped neuroscientists make sense of how the two halves of the brain work together, what are the functions of each of them and what would happen if they worked separately. In the last mentioned case, the brain behaved as if there were two separate minds, or what they called later split-brain. Wikipedia tells us about it that:

After the right and left brain are separated, each hemisphere will have its own separate perception, concepts, and impulses to act. Having two “brains” in one body can create some interesting dilemmas. When one split-brain patient dressed himself, he sometimes pulled his pants up with one hand (that side of his brain wanted to get dressed) and down with the other (this side didn’t). Also, once he grabbed his wife with his left hand and shook her violently, so his right hand came to her aid and grabbed the aggressive left hand. However, such conflicts are actually rare. If a conflict arises, one hemisphere usually overrides the other.

At the Beyond Belief conference in 2006, neuroscientist Vilayanur S. Ramachandran shocked the audience with a special case where the patient was half atheist, half religious. But how had Ramachandran been able to interrogate the two halves of his patient?

Since the right hemisphere manages the left side of the body and vice versa, Ramachandran found his way to separately communicate with the two sides by whispering into the patient’s right ear to ask a question to the left hemisphere. He did the same with the left ear to communicate with the right hemisphere.

The major concern with that plan was how to get answers from the right hemisphere. Having the communications center (that controls speaking) on the left side means that it is only possible for the left hemisphere to verbally communicate. So, to get answers from the right hemisphere, the patient was shown a piece of paper with yes and no options to choose from using their left hand.

Studying split-brain patients helped tremendously with distinguishing the differences between the two hemispheres’ functions. While the left side is usually responsible for the language computation, the right side is the face-recognizing expert.

Painting by Vertumnus, public domain.

When a normal person is shown a painting by the Italian artist Giuseppe Arcimboldo, known for drawing portraits composed of objects (like the one above), they will usually recognize it as a face made of vegetables, fish or flowers. This is not the case for split-brain patients, however—their left side will recognize a face in the painting, while the right side will only see the objects.

“What I find particularly interesting is that consciousness is maintained as a unified state even when the two hemispheres of the brain are disconnected,” Temple-Wood explains. “We don’t understand consciousness very well at all, and it used to be thought that disconnecting the hemispheres would lead to being ‘of two minds’, quite literally. But there’s no connection between something like dissociative identity disorder and callosotomy!”

Michael Gazzaniga, one of the leading researchers in cognitive neuroscience, has dedicated a large amount of his life to studying the split-brained patients. He was concerned about how they act emotionally and physically in comparison to those who do not have a split brain—and if every one of us has two different minds, “why [do] people, including split-brain patients, have a unified sense of self and mental life”?

Nature magazine featured one of Gazzaniga’s favorite examples when he recalled “flashing the word ‘smile’ to a patient’s right hemisphere and the word ‘face’ to the left hemisphere, and asked the patient to draw what he’d seen. “His right hand drew a smiling face,” Gazzaniga recalled.

– “’Why did you do that?’ I asked.
– The patient said, ‘What do you want, a sad face? Who wants a sad face around?’.”

The patient’s left hemisphere had made up a story to verbally justify his drawing, and it had no idea why he made the face smiling because he hadn’t seen the word ‘smile’. “The left-brain interpreter,” Gazzaniga says, “is what everyone uses to seek explanations for events, triage the barrage of incoming information and construct narratives that help to make sense of the world.”

You can read more about split-brain in Wikipedia’s article about it.

Samir Elsharbaty, Digital Content Intern
Wikimedia Foundation