Although physically speaking the brain looks perfectly symmetrical, separated down the middle by a deep groove, cerebral functions are actually asymmetrical or literalized. The left half of your brain is more heavily involved in controlling speech, emotions, reading, writing, and learning, while the right hemisphere is the primary place for recognizing faces, spatial orientation, controlling memory and reasoning, as well as problem-solving.

This lateralization is perfectly illustrated by the effects of stroke or injury to the left side of the brain, which often disrupts the ability to speak. On the flip side, people with right brain injuries are prone to get lost even in familiar surroundings and may become unable to draw.

But if brain lateralization is so sensitive, one might imagine that a person who loses half their brain cannot possibly have a normal life, doomed to live the role of a human vegetable — yet reality couldn’t be further from the truth, a new study out of the University of Pittsburgh.

One of the rarest and most radical forms of brain surgery is a procedure called hemispherectomy. Some children have such severe epileptic seizures that the only viable treatment is the surgical removal of literally half of their brain (an entire hemisphere).

Rehabilitation after such a drastic surgery is tough, but thanks to the marvels of neuroplasticity, most of these patients grow up into highly functional, normal adults.

“The question of whether the brain is prewired with its functional capabilities from birth or if it dynamically organizes its function as it matures and experiences the environment drives much of vision science and neurobiology,” said senior author Marlene Behrmann professor of ophthalmology and psychology at the University of Pittsburgh and Carnegie Mellon University. “Working with hemispherectomy patients allowed us to study the upper bounds of functional capacity of a single brain hemisphere. With the results from this study, we now have a foot in the door of human neuroplasticity and can finally begin examining the capabilities of brain reorganization.” 

Neuroplasticity refers to the brain’s ability to reorganize itself by forming new neural connections throughout life in response to changes in the environment. Examples of situations where your brain demonstrates neuroplasticity include learning a new language, practicing music, and memorizing how to navigate a new unfamiliar city. Although neuroplasticity peaks in early development as a child, the human brain is constantly undergoing structural and functional changes throughout our entire adult lives.

Over time, the human brain becomes increasingly compartmentalized and the left/right hemispheric preference becomes more rigid. This is why adults who develop brain lesions in one of their hemispheres end up experiencing major disruption in the functions associated with the damaged brain hemisphere.

But that’s not necessarily the case at all for children, whose brains are still highly plastic. Pitt and colleagues were able to find and enroll 40 hemispherectomy patients — an unprecedentedly large number of patients who’ve undergone such a rare procedure. The participants’ word recognition ability was tested using pairs of words that only differed by one letter, such as “soap” and “soup” or “tank” and “tack.” Facial recognition was tested using pairs of photos that looked similar. Either a pair of words or human faces showed up on a computer screen for only a fraction of a second and the participants had to judge whether the words or faces that flashed on the screen were the same or different.

Despite the fact that they only had a single surviving hemisphere, the word and face recognition abilities of patients with hemispherectomies were only 10% less good than control subjects. Their average accuracy exceeded 80%. And in direct comparisons between matching hemispheres in patients and controls, patients’ accuracy on both face and word recognition was comparable regardless of the hemisphere removed.  

“Reassuringly, losing half of the brain does not equate to losing half of its functionality,” said first author Michael Granovetter, Ph.D., a student in the Medical Scientist Training Program at Pitt’s School of Medicine. “While we can’t definitively predict how any given child might be affected by a hemispherectomy, the performance that we see in these patients is encouraging. The more we can understand plasticity after surgery, the more information, and perhaps added comfort, we can provide to parents who are making difficult decisions about their child’s treatment plan.”  

The findings appeared in the journal Proceedings of the National Academy of Sciences (PNAS).