Mind & Body

Studying 'Split-Brain' Patients Taught Scientists Strange Things About the Brain

Some of the most amazing discoveries in neuroscience have come out of unusual brain conditions. For instance, Phineas Gage's devastating railway accident showed 19th-century scientists that the brain controls behavior, and a man who could only utter a single word helped 20th-century scientists learn about the brain area devoted to speech. Some of the most fascinating and unsettling studies happened in the 1960s when a neurobiologist ran a series of experiments on "split-brain" patients — that is, people with two brain hemispheres that aren't connected. Those experiments taught us how the brain allocates tasks and led to modern research that's demonstrating just how adaptable our brains can be.

Cerebral Schism

In 1961, Michael Gazzaniga was a Ph.D. student studying neurobiology at Caltech. He wanted to know how the brain's two halves communicate. Scientists knew that certain areas of the brain took over particular tasks — one region might be responsible for speech, another for motor control, yet another for memory. What was less known was the way parts of the brain connected with each other. To learn more, Gazzaniga examined twelve people who had undergone a surgery called a corpus callosotomy.

To illustrate this surgery, imagine there's a brain on the table in front of you. It looks like a grey, moist walnut and weighs close to 3 pounds (1,400 grams). Sitting on top of the brainstem and the small, round structure known as the cerebellum is the brain's largest, wrinkliest portion: the cerebrum. The cerebrum is where you perform your high-level thinking, and it's divided into left and right hemispheres. If you gently pull those hemispheres apart, you'll see that the main connection between them is a small bundle of white tissue. This the corpus callosum. It connects roughly 80 percent of the cerebral cortex — the outermost layer of the brain — and is the central line of communication between the two hemispheres. A corpus callosotomy involves cutting the corpus callosum and separating the two halves of the cerebrum.

Gazzaniga didn't actually cut people's brains in half for his experiments — that would be unethical. Instead, his participants had already had the surgery as a last-resort treatment for epilepsy. Epileptic seizures are caused by bursts of electrical activity throughout the brain, and this surgery ensured that this activity stayed on one side of the brain where they could limit their damage.

The people within this group of subjects became known as "split-brain" patients.

On the Other Hand

Gazzaniga's split-brain studies yielded fascinating and eerie insight into the wiring of the brain. He found out that when the two hemispheres were cut off from one another, they could each act on their own. "It is as if the brain contained two separate spheres of consciousness," he wrote in a 1967 Scientific American article.

For example, one of his experiments involved flashing lights in each eye independently and asking the patient whether they saw anything. Each hemisphere of the brain controls the opposite side of the body, so when a patient's right eye saw the light, it transmitted the image to the left hemisphere. The left hemisphere controls speech, so the patient was able to say yes, they saw the light.

When the left eye saw the light, the image went to the right hemisphere. Because the right hemisphere doesn't contain a region devoted to speech, the patient couldn't say they saw the light. However, the right hemisphere does have control over motor movements. In a similar experiment where the patient was asked to raise a hand if they saw the lights, a light flashed in the left eye led the patient to raise their left hand.

Weirder still is what happened when Gazzaniga showed an image to the left eye. The patient couldn't read the word out loud, but they could use their left hand to draw a picture of what the left eye saw.

While the split-brain condition wasn't debilitating for any of Gazzaniga's patients, it's worth mentioning that there were some side effects. One patient even experienced antagonism between his hands. One hand would pick up a newspaper and the other would abruptly set it down, forcing the first hand to pick it up again. Others who have had this surgery report similar issues getting dressed or selecting items at the grocery store.

"I'd reach with my right for the thing I wanted, but the left would come in and they'd kind of fight," Vicki, a woman who had a corpus callosotomy, told Nature. "Almost like repelling magnets."

Gazzaniga's experiments showed that while certain regions might do the heavy lifting on certain tasks, the brain distributes cognitive tasks across the hemispheres in a precise way. They also showed that the hemispheres work in tandem on tasks, not independently.

Brains Find a Way

Those early experiments weren't the last we've learned about how the brain is organized. New research recently published in the journal Cell Reports shows that the brain also has a remarkable ability to rewire itself. Researchers at Caltech are finding out that even if one hemisphere is missing, the brain will adapt to perform all of its necessary functions as long as the damage happens early enough.

Hemispherectomy Brain Anatomy - Six adult participants with left (n = 2, HS2 and HS3) or right (n = 4, HS1, HS4, HS5, and HS6) hemispherectomy. Axial slices were taken minimally above the anterior/posterior commissure line. L, left; R, right.

This study looked at six adults who had a whole or partial hemisphere removed during childhood (this kind of "hemispherectomy" is another extreme treatment for intractable epilepsy). fMRI scans of these patients' brains showed interconnectivity that's different from a typical brain, yet surprisingly functional. These patients feel and act normally even though, in most cases, half of their cerebrum is missing. For example, patients whose left hemisphere has been removed are often capable of speech. In these cases, the right hemisphere seems to be taking on those functions.

That might further change how we think about the way the brain works. Gazzaniga's experiments showed how the developed adult brain delegates tasks, but this new information shows that there's more than one way to configure a brain. Maybe the "typical brain" doesn't exist.

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Hear Gazzaniga's entire fascinating life story in his autobiography, "Tales from Both Sides of the Brain: A Life in Neuroscience." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Cameron Duke December 6, 2019

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