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This could explain why large brains are more susceptible to mental illness

Explains a lot.

DAVID NIELD
24 JUL 2016
 

A new study by an international team of researchers suggests that bigger brained animals - such as humans - are more prone to mental illness because signals have to travel further inside larger brains than in smaller ones.

Just like a neurological game of Chinese whispers, insider large brains, messages have to travel further along a limited number of connections, which means things have more chance to go awry, the team has shown.

 

"One could speculate that the low weight of human long-range connections may contribute to an increased susceptibility to disconnection syndromes, such as have been proposed for Alzheimer's disease and schizophrenia," write the authors, led by Henry Kennedy from the Université Claude Bernard Lyon 1 in France.

Not only does the research provide insight into why mental illness is more common in species such as humans rather than mice or rats, it also confirms the idea that common principles govern the layout of the brain, no matter what its size. 

The new hypothesis is based on a few previous findings, primarily the exponential distance rule or EDR, which was modelled in 2013 by some of the same academics behind the new research.

Basically, it combined tracing studies on macaques with some advanced network theory to come up with the prediction that the brain contains far less long-range nerve fibres than short-range ones.

That means the closer two cortical areas are in the brain, the more connections there are between them, and the further apart two areas are, the less connections link them.

Fast forward to the new study, and the EDR was put to the test on a mouse brain, which is much smaller than that of a macaque.

 

Analysis showed the fundamental rules were the same as in the 2013 work – so even as mammal brains evolve to different sizes, it seems there are underlying principles that apply to them all.

The findings "suggest that the EDR plays a key role across the mammalian order", and could explain how larger brains stay efficient by having fewer long-range connections.

It also suggests that in our big brains – about five times the size of a macaque brain – the (relatively) long-distance connections could be quite weak, with only a few connections between regions.

That, in turn, the researchers conclude, could trigger disconnection syndromes, where these connections become damaged, and some scientists think diseases like Alzheimer's and schizophrenia are the end result.

If you're thinking that's a lot of 'coulds' in there, you're right – the brain is notoriously difficult to map and monitor, and researchers are still feeling their way into a lot of these ideas about how it actually works.

For now, this is just another hypothesis that needs to be tested, but the researchers have had it peer-reviewed and published in PLOS Biology.

As the authors put it themselves: "Understanding brain networks is arguably one of the major challenges of the 21st century. The mammalian cortex is an extraordinary computational device."

However, as research like this shows, we're making progress, and that will eventually lead to better and more informed ways to treat problems within the brain as they happen – as well as hopefully being able to prevent them in future.

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