How The Human Brain Rewires Itself After Half A Hemisphere Is Surgically Removed

An fMRI scan showing one single slice through the brain of an adult who had an entire hemisphere removed during childhood due to epilepsy. Caltech Brain Imaging Center

Since the early 20th century, surgery to remove half a person’s brain has been performed on dozens of people to reduce extreme epileptic seizures. Yet how can anyone exist – much less thrive – with half their brain gone? It seems impossible. Improbable, at the very least. But such patients do exist.

"The people with hemispherectomies that we studied were remarkably high functioning. They have intact language skills; when I put them in the scanner we made small talk, just like the hundreds of other individuals I have scanned," said Dorit Kliemann, a cognitive neuroscientist at the California Institute of Technology, and first author of a new study. "You can almost forget their condition when you meet them for the first time."

The new study, published in Cell Reports, peers into the patients' brains to see how they are functioning so well after surgery. The team found that specific brain networks reorganize themselves, a hallmark of plasticity, where the brain adapts and modifies its own connections from a previous norm.

Six adults with childhood hemispherectomy were scanned with an fMRI machine while the team tracked brain activity. They found that various regions of the brain for vision, sensorimotor information, and social cues strengthened previous connections, communicating more frequently and with higher activity than the controls. It’s as if certain parts of the brain took up some of the slack from the missing regions and took on more roles. 

It is generally believed that hemispherectomy is best done when the patient is young to maximize the strength of plasticity in childhood. Although this is still true, there is a growing body of evidence that suggests there isn’t necessarily a solid cutoff line. The age in which the study participants had the surgery ranged from 3 months old to 11 years old.

Half-brain hemisphere removal is a last resort in patients with extreme seizures and only performed when medication and other treatment options have not worked. The patients’ condition before surgery is often progressive and can inflict damage on the rest of their brain if not stopped.

"As remarkable as it is that there are individuals who can live with half a brain, sometimes a very small brain lesion like a stroke or a traumatic brain injury like a bicycle accident or a tumor can have devastating effects," said Kliemann.

"We're trying to understand the principles of brain reorganization that can lead to compensation. Maybe down the line, that work can inform targeted intervention strategies and different outcome scenarios to help more people with brain injuries.”

 

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