As we grow older, our brains reorganize themselves, pruning and streamlining nerve fiber linkages to reduce overall network connectivity while selectively preserving long-distance connections that are crucial to information integration.
Now, a team led by Newcastle University in the UK has found this brain streamlining starts earlier in girls, suggesting it may explain why they mature before boys in their teen years.
The researchers also suggest this gradual, selective reduction of connections may be the reason brain function does not deteriorate – it even improves – during network pruning.
One of the study leaders, Dr. Marcus Kaiser, reader in Neuroinformatics at Newcastle, says:
“Long-distance connections are difficult to establish and maintain but are crucial for fast and efficient processing.”
To explain the value of the long-distance connection, Dr. Kaiser uses the idea of social networks:
“If you think about a social network, nearby friends might give you very similar information – you might hear the same news from different people. People from different cities or countries are more likely to give you novel information.”
“In the same way, some information flow within a brain module might be redundant whereas information from other modules, say integrating the optical information about a face with the acoustic information of a voice, is vital in making sense of the outside world.”
For their study, researchers at Newcastle, Glasgow and Seoul Universities evaluated brain scans of 121 healthy individuals aged between 4 and 40. They picked this age range because these are the years when the major pruning of connections occurs.
The scans were done using a highly sensitive form of magnetic resonance imaging (MRI) known as diffusion tensor imaging (DTI), which tracks water traveling along nerve fibers.
The DTI scans showed that the age range 4 to 40 was indeed a period when the brain was busy pruning fibers.
However, the researchers found that in the case of long-range connections – called projections – selective pruning was going on, and some types were pruned more heavily than others.
The projections that tended to be preferentially preserved were short-cuts that link different processing modules and allow fast information transfer and synchronous processing. Examples include connections between vision and sound modules.
The researchers say changes in these connections have been linked to developmental brain disorders, such as epilepsy, autism and schizophrenia.
The social network analogy also serves to explain why reducing some projections in the brain helps to focus on essential information. For instance, if you are lost and need to find your way, instead of talking to lots of people at random, it is best to speak to a select few who have been in the neighborhood a while.
With this study, the researchers have shown for the first time that the loss of white matter fibers between brain regions is a highly selective process – one that they call preferential detachment.
They found reduction in brain connections between distant regions, between hemispheres, and between processing modules were fewer during brain maturation than expected, and they conclude this may explain how the brain maintains a stable network as it matures.
The study, which forms part of the Human Green Brain project that is examining human brain development, was funded by the British Engineering and Physical Sciences Research Council (EPSRC).
Meanwhile, another team in the US published a study recently in the Proceedings of the National Academy of Sciences that showed striking differences in brain wiring between men and women.
In that study, researchers at the University of Pennsylvania also used DTI to reveal that in one brain region, women have more connections between left and right hemispheres, and men within hemispheres, while in another brain region, the pattern is the other way around.