When surgeons removed one sixth of a child's brain, here's what happened
Posted July 31, 2018 11:51 a.m. EDT
What does a child's brain, which repaired itself after a full third of the right hemisphere was surgically removed, teach us about all brains?
Our developing brains find unique ways to rewire themselves as necessary, suggests a new case study of a nearly 11-year old boy referred to only as "UD" (his privacy is respected by never revealing his true name).
The operation when he was 6 years 10 months old eliminated UD's entire occipital lobe, home of the brain's vision processing center, and most of his temporal lobe, where both visual and auditory signals land and then get sorted. Yet, UD's left hemisphere compensated for any losses on the right side of his brain by assuming the roles of both hemispheres. As a result, both his cognitive and visual function are now intact.
"UD's case [has] essentially shown us that one hemisphere is enough for normal visual function," said Marlene Behrmann, senior author of the study and a neuroscientist at Carnegie Mellon's Department of Psychology and Center for the Neural Basis of Cognition.
This is the "key finding" of the new case study published Tuesday in the journal Cell Reports, said Behrmann, though she added that UD's vision remains partially impaired by the surgery. Seemingly, a small price paid to be cured of a life-threatening disorder.
At age 4, when most children have begun to somersault, to dress themselves and to name the many colors they see, UD suffered his first epileptic seizure. Soon, his seizures became frequent and severe.
Over the next couple of years, his doctors made many attempts to try and control UD's seizures, mostly through medication, explained Behrmann. Though some drugs reduced his seizures, none stopped them.
"As a last resort, the decision was made for him to undergo lobectomy," said Behrmann, who explained that this procedure requires a surgical plan to remove the focal point of epilepsy without affecting other regions of the brain.
"The surgery eliminates, completely, the seizures in roughly 60% to 70% of the children [who undergo the operation], so it's really highly effective," said Behrmann, who took no part in the surgery, which is performed on about 4% to 6% of all patients with uncontrollable epilepsy.
In UD's case, a localized tumor in his right hemisphere was the cause of his epilepsy. The surgery, which took place when he was age 6 years and 9 months old, removed the tumor along with most of two of his four lobes situated within the right hemisphere.
"We saw him almost a year later, when he was fully stable and no longer on medication and ready to participate," said Behrmann. Prior to the surgery, he had undergone extensive behavioral and visual testing, which was necessary "because they were going to remove part of the visual system," she explained.
Over the next three years, Behrmann and her team studied UD's post-surgical progress using high tech scans to measure his brain activity at five separate timepoints.
Essentially, then, she and her colleagues watched how UD's brain rewired itself after the surgery.
What they witnessed was the left hemisphere assuming the functions usually performed by the missing regions of his brain. His left hemisphere, then, did its usual work of word recognition yet also took on the role of recognizing faces, usually the responsibility of the right hemisphere.
"So as word recognition was emerging, we could see a kind of jostling for position between word recognition and face recognition in the same left hemisphere," said Behrmann. "They kind of like pushed each other around a little bit and then settled down."
"And now [UD's] face and word recognition skills are entirely normal," she said, noting that the two skills "sort of settled down in neighboring and abutting regions" of his brain in the left hemisphere.
Both word and face recognition is thought of as "a complex pattern recognition problem," explained Behrmann. The reason is that words are all "visually very similar" to each other just as faces are.
Even in typically-developing children, the brain takes a long time to acquire these separate recognition abilities, said Behrman, probably because of how "fine-grained the mechanism needs to be" to tease apart similar appearing words and similar appearing faces.
Not only did she and her colleagues compare UD's abilities to his peers, they also tested him with "the most challenging tests that we could do," she said. "We really drilled down to understand whether or not there was any loss or alteration of function. And we weren't able to see any."
Regions of UD's brain involved in other complex visual functions -- object recognition and scene recognition -- were present and normal when he was first scanned by the study researchers at age 7 years 10 months. Like his peers, these developed functions became more sophisticated as time passed, proving that part of his brain remained in good shape, said Behrmann.
UD did lose one skill due to surgery and it will not return. Visual information appearing in UD's left visual field now has "nowhere to go," explained Behrmann. Still picked up by his functioning eyes, it is transmitted along the visual processing circuit, yet there is no longer any brain region that can receive it.
To compensate, UD moves his eyes and head. We all (unconsciously) do to this cover for our own blind spots that occur where nerves, which run from the eye to the brain, block our sight. UD likely is unaware of doing this and his appearance is not unusual, according to Behrmann.
Now almost 11 years old, UD receives vision therapy and sits on the left side of classrooms so he can take in more of the school scene. His test scores range from average to advanced.
Mark Johnson, associate director of the Centre for Brain and Cognitive Development at Birkbeck College, University of London, said the case study of UD is "very interesting."
"While some of the functions of the brain are known to be working from birth, it is clear that many other specialisations require extensive experience," Johnson, who was not involved in the research, wrote in an email.
Because processing faces is such a crucial skill, some scientists have argued that the areas of our cerebral cortex (mainly in the right hemisphere) responsible for recognizing faces must be "tuned up" for this purpose from birth, he said.
Yet UD's post-surgical experience -- where words and faces competed to capture space within his brain -- suggests something more along the lines of the model of brain function called "Interactive Specialization," which Johnson originally proposed in 2000.
According to Johnson's model, brain regions develop increasing specialization over time. Like sibling rivals within a family, brain regions interact and compete with each other to acquire their role, which may sharpen or become more restricted as they collectively mature.
Behrmann said that she and her colleagues "are now undertaking a very large scope study that evaluates many kids," who, like UD, have undergone similar brain surgeries.
"The surgeries are becoming much better, there's a lot of computer-aided mapping for getting the geography just right, and there's a lot of work on the drug front as well," said Behrmann, who has received funding from the National Institutes of Health.
"We are very much in the midst of significant progress and advances in medicine and neuroscience," she explained, all due to the development of new analytic and statistical ways of handling large data sets alongside the expansion of new technologies.
"There are so many unanswered questions," said Behrmann.