Between the 2 halves of our brains could be a gap stuffed with fluid and spanned only by the corpus callosum: a bridge of neural fibres that ferries information to either side.
Some people, however, are born without a pathway, meaning they lack the same old neural highway of roughly 190 million axons that may typically carry information between the left and right hemispheres. How their brains adapt is actually extraordinary.
Studies have shown that in individuals who never develop this major connection, the brain rewires itself and creates entirely new connective pathways that re-route signals through other brain regions, keeping communication flowing across the divide via the mid-and fore-brain.
The new fibres built to reconnect the brain's two hemispheres, called Probst bundles in some cases, aren't an ideal replacement for the nerve tract, and yet "[r]emarkably, communication between the 2 hemispheres is maintained," explains neuroscientist Vanessa Siffredi from the University of Geneva in Switzerland.
Much about the method remains a mystery, but new research from Siffredi and colleagues has now revealed that the brain plasticity in children lacking a pathway runs even deeper still.
Imaging results from a study of Australian children show that brains lacking a nerve pathway create a noteworthy number of recent connections inside each hemisphere in addition, to assist maintain overall brain function.
About one in every 4,000 people are born without a nerve pathway, which usually measures 10 centimetres (almost 4 inches). It first develops within the womb by around 20 weeks into gestation.
Around half people born without a nerve tract (called agenesis of the corpus callosum) have learning and memory difficulties. Some may also have poor attention and fewer cognitive flexibility than their peers.
Others will be even more severely affected, and yet 1 / 4 of individuals with the diagnosis show no visible signs of any impairment whatsoever.
The researchers thought that viewing the connectivity of neural fibres within each hemisphere – except for being another marker of the brain's malleability and brilliance – may also go away to explaining this variability in effects.
In the study, 20 children (aged 8 to 17) who were born either without a pathway or with an incomplete one had their brains scanned using an MRI machine so the researchers could examine connections between different parts of their brain. These were compared with scans taken from 29 healthy children with an intact pathway.
As well as having their brains scanned, the kids also completed tasks to check their memory, attention, and verbal learning abilities.
By analysing the structural connections within the whole-brain images from the youngsters without a completely formed tract, the team found the brain responds by strengthening neural pathways inside each of its two hemispheres.
The children with nerve tract agenesis showed greater connectivity within each brain hemisphere than the group of healthy children.
In a healthy brain (top), the two hemispheres are connected by the corpus callosum fibres, shown in red. These fibres are absent in a brain with corpus callosum agenesis (bottom). (Vanessa Siffredi/UNIGE)
Incredibly, when analysing the functional links across the brain scans (in only 16 children with tract agenesis this time), there was no significant difference between the 2 groups.
Even without a nerve tract, parts of the children's brains were still actively communicating with other areas, both within hemispheres and between them. This functional connectivity was to love their healthy peers with a normally developed pathway.
This shows, again, how brain plasticity can overcome what appears to be, on first impressions, gaping structural deficiencies in brain development.
"We think that plasticity mechanisms, like the strengthening of structural bonds within each hemisphere, compensated for the shortage of neuronal fibres between hemispheres," says Siffredi.
"New connections are created and therefore the signals may be re-routed so communication is preserved between the 2 hemispheres."
Focusing just on the children with tract agenesis and their test results, the researchers also discovered that greater structural connectivity within hemispheres was related to better learning outcomes, long-term and dealing memory, and a spotlight.
Although the number of youngsters involved during this study may appear small, it's sizeable for research examining this condition. There are few studies thus far that pair brain imaging with behavioural outcomes like this research does.
It's also worth noting that a number of the kids studied had other brain abnormalities additionally to their missing pathway, though these kids showed similar levels of plasticity to their group mates.
All in all, this remains a spectacular example of brain plasticity at work, and also the new findings could help explain why some children with this extraordinary condition fare better than others.
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