What Is a Split Brain?

(Last Updated On: February 17, 2020)

What Is a Split Brain?

Have you heard of the term split brain? At first glance, it might sound like a terrifying prospect, like you’ve got two brains inside of your skull or something. But split brain is not science fiction. It’s a real condition. So, what exactly is a split brain?

The Anatomy of a Split Brain

First, a quick crash course on brain anatomy. You’ve probably heard the terms left-brained and right-brained before. They refer to the two hemispheres of your brain, and these two halves are generally considered to be better at different things. We’ll get a little bit more into hemispheres later, but suffice to say that they exist. In order for your brain to function properly, these two hemispheres need to be able to communicate. Which means you can probably see where this is going.

The hemispheres of your brain are connected by this nifty little thing named the corpus callosum. It comes from the Latin for “tough body.” 

So, what makes the corpus callosum so important? Well, cutting it makes for a split brain–bringing us back to the title of this post. Heck, the name of the operation is colloquially called “split-brain surgery.” Though technically it’s called a “corpus callosotomy.”

Why Cut the Corpus Callosum?

As we alluded to earlier, yes the corpus callosum can be cut. It’s also possible for a corpus callosum to never develop properly, as can happen with most parts of the body.

But why would one cut the corpus callosum intentionally? Normally, the surgery is done for those with very severe epilepsy. Though typically, the cases warranting a corpus callosotomy are so extreme anti-seizure medication just doesn’t work. 

Given that the two hemispheres of the brain share information, sometimes not all of the corpus callosum is cut so they can continue to share some information. Typically we’re talking vision, because that’s pretty important. If problems aren’t fixed following that, they’ll go back for another surgery to get the rest of the callosum. 

Typically the success rate ranges between 50% and 75%, with a 2009 study pinning it around 77%. More recent studies have generalized to just “greater than 50%.” Ultimately, it depends on how you define success. The very severe seizures that the procedure is used to treat are normally fixed. Patients may still experience minor seizures though.

Brain Hemispheres

So our brain hemispheres are typically in communication, because they take in information differently. Well at least, separately. They also dole out information and commands separately. In addition, they can push out differing commands. 

Example; the left hemisphere of your brain is typically associated with language and word formation, while the right is associated with more holistic things. So your left hemisphere is concerned with what you say, while the right hemisphere is more concerned with how you say it. However, the left hemisphere is typically the one that gets to control the speaking part.

Half your body is also bound to one hemisphere, while the other half is bound to… The other hemisphere. Your left hemisphere receives visual information from the right eye, and sends motor commands to the right side of the body (ergo, it tells your right hand what to do). The exact opposite applies to the right hemisphere, receiving input and sending commands to the left side of the body.

What Happens When the Brain Is Split?

So given how important it is for the left and right hemispheres to communicate, you’d imagine there might be some issues if they couldn’t communicate anymore. And it turns out, yes, there are.

At a basic level, you can prove how information doesn’t go between hemispheres. Scenario: you’ve shown a split brain patient a dog, but they can only see the dog out of their left eye. Which means the right hemisphere can see the dog, but the left hemisphere cannot. 

Daily Shenanigans

Remember how the left hemisphere controls speech? So if you ask the patient to say what they’re looking at, they will say they can’t see anything. But conversely, they can write the word “dog” with their left hand–as it is controlled by the hemisphere that can see the dog.

But that seems kind of… expected. Here’s where it gets a little nutty. Turns out, the two hemispheres can disagree with each other. So left brain can say “I want to drink coffee,” and your right arm will reach out to grab a cup. A normal operation. 

Except right brain, which cannot say “I want tea” can actually slap your right hand out of the way and reach out for a tea bag with your left arm. 

Split-brain patients have discussed this phenomena in their daily lives, like choosing outfits and stuff. Which leads to the development of… Distinct personalities between left and right hemispheres. Generally speaking, though, split-brain patients can typically operate with their hemispheres in relative concert–you probably wouldn’t be able to tell their motions apart. Despite the fact that the right leg is technically no longer really aware of what the left leg is doing. 

So there’s probably another personality in your head that can’t speak because it’s physically unable to. Literally “I have no mouth but I must scream.”

Think you know your brain anatomy? Quiz yourself here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.