PNAS: Half the brain can fully support text and facial recognition

An unprecedented study of brain plasticity and visual perception found that people who underwent surgery to remove half of their brains in childhood were able to correctly recognize differences between
pairs of words or faces more than 80 percent of the time.
Given the volume of brain tissue removed, the surprising accuracy highlights the brain's ability to reconnect itself and adapt to dramatic surgery or traumatic injury
.

Researchers at the University of Pittsburgh published today in the Proceedings of the National Academy of Sciences (PNAS) are the first attempt to describe human neuroplasticity and understand whether a single brain hemisphere can perform functions
normally separated by the left and right brains.

Senior author Dr.
Marlene Behrmann, professor of ophthalmology and psychology at the University of Pittsburgh and Carnegie Mellon University, said: "The question of whether the brain is an innate ability to function or dynamically organizes its functions as it matures and changes in its environment has driven the development of
vision science and neurobiology.
" "The collaboration with hemispherectomy patients allows us to study the upper limit
of the functional capacity of a single cerebral hemisphere.
With the results of this study, we have now taken the first step towards human neuroplasticity and can finally begin to study the brain's
ability to reorganize.
”

Neuroplasticity is a process that allows the brain to change its activity and rewire itself structurally or functionally in response to changes
in the environment.
Although brain plasticity peaks early in development, our brains continue to change
into adulthood.

As humans age, the two hemispheres of our brain become more and more specialized
.
Although this division of labor is not absolute, the two cerebral hemispheres have different main responsibilities: the left hemisphere matures and is mainly responsible for reading printed words, while the right hemisphere matures and is mainly responsible for recognizing faces
.

But neuroplasticity has limits, and this hemispheric preference becomes stiffer over time
.
In some cases, adults with brain damage due to stroke or tumor may develop dyslexia or face blindness, depending on whether the left or right hemisphere of the brain is affected
.

But what happens when the brain is forced to change and adapt, while it remains highly malleable? To answer this question, the researchers looked at a particular group of patients who underwent complete hemispherectomy (or surgically removed one brain hemisphere to control seizures)
during their childhood.

Because hemispherectomy is relatively rare, scientists rarely reach several patients
at once.
But the Pitt research team found an unexpected glimmer of hope in the COVID-19 pandemic: the normalization of telehealth services, making it possible to recruit 40 hemispherectomy patients, an unprecedented number
in such studies.

To assess word recognition, the researchers showed participants a pair of words, each one differing by only one letter, such as "soap" and "soup," or "tank" and "tack.
"
To test the children's ability to recognize different faces, the scientists showed them a photo of a pair of
people.
Both stimuli appeared on screen for less than a second, and participants had to decide whether the pair of words or the pair of faces were the same or different
.

Surprisingly, the remaining hemisphere supports both functions
.
The control group and those who had their hemispheres resected had different text and face recognition abilities, but the difference was less than 10%, and the average accuracy was more than 80%.

In a direct comparison of matched cerebral hemispheres between patients and controls, the patient's
accuracy in face and word recognition was comparable regardless of which cerebral hemisphere was removed.

"It's reassuring that losing half of your brain doesn't equal losing half of your function," said first author Michael Granovet, Ph.
D.
, a student in
the Medical Scientist Training Program at Pitt Medical School.
"While we cannot definitively predict how cerebral hemispherectomy will affect any child, the results we have seen in these patients are encouraging
.
" The more we know about post-surgery plasticity, the more information we can provide, and perhaps more comfort, to help
parents who are making difficult decisions about their child's treatment plan.
”

Other authors of this article are Sophia Robert, B.
S.
, of Carnegie Mellon University, and Leah Ettensohn, B.
S.

The study was supported
by the National Eye Institute (R01EY027018), the National Institute of General Medical Sciences (T32GM081760), and the American Epilepsy Society (847556).
The research was also supported by the National Eye Institute's P30 CORE Award EY08098, as well as unrestricted support funding
from the Blindness Prevention Study and the Pittsburgh Eye and Ear Foundation.