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Can you live a normal life with half a brain?

2014-12-19 06:48:27

Tom Stafford

A few extreme cases show that people can be missing large chunks of their

brains with no significant ill-effect why? Tom Stafford explains what it

tells us about the true nature of our grey matter.

How much of our brain do we actually need? A number of stories have appeared in

the news in recent months about people with chunks of their brains missing or

damaged. These cases tell a story about the mind that goes deeper than their

initial shock factor. It isn't just that we don't understand how the brain

works, but that we may be thinking about it in the entirely wrong way.

Earlier this year, a case was reported of a woman who is missing her

cerebellum, a distinct structure found at the back of the brain. By some

estimates the human cerebellum contains half the brain cells you have. This

isn't just brain damage the whole structure is absent. Yet this woman lives a

normal life; she graduated from school, got married and had a kid following an

uneventful pregnancy and birth. A pretty standard biography for a 24-year-old.

The woman wasn't completely unaffected she had suffered from uncertain,

clumsy, movements her whole life. But the surprise is how she moves at all,

missing a part of the brain that is so fundamental it evolved with the first

vertebrates. The sharks that swam when dinosaurs walked the Earth had

cerebellums.

(SPL)

(SPL)

This case points to a sad fact about brain science. We don't often shout about

it, but there are large gaps in even our basic understanding of the brain. We

can't agree on the function of even some of the most important brain regions,

such as the cerebellum. Rare cases such as this show up that ignorance. Every

so often someone walks into a hospital and their brain scan reveals the

startling differences we can have inside our heads. Startling differences which

may have only small observable effects on our behaviour.

Part of the problem may be our way of thinking. It is natural to see the brain

as a piece of naturally selected technology, and in human technology there is

often a one-to-one mapping between structure and function. If I have a toaster,

the heat is provided by the heating element, the time is controlled by the

timer and the popping up is driven by a spring. The case of the missing

cerebellum reveals there is no such simple scheme for the brain. Although we

love to talk about the brain region for vision, for hunger or for love, there

are no such brain regions, because the brain isn't technology where any

function is governed by just one part.

(Thinkstock)

(Thinkstock)

Take another recent case, that of a man who was found to have a tapeworm in his

brain. Over four years it burrowed "from one side to the other", causing a

variety of problems such as seizures, memory problems and weird smell

sensations. Sounds to me like he got off lightly for having a living thing move

through his brain. If the brain worked like most designed technology this

wouldn't be possible. If a worm burrowed from one side of your phone to the

other, the gadget would die. Indeed, when an early electromechanical computer

malfunctioned in the 1940s, an investigation revealed the problem: a moth

trapped in a relay the first actual case of a computer bug being found.

Part of the explanation for the brain's apparent resilience is its 'plasticity'

an ability to adapt its structure based on experience. But another clue comes

from a concept advocated by Nobel Prize-winning neuroscientist Gerald Edelman.

He noticed that biological functions are often supported by multiple structures

single physical features are coded for by multiple genes, for example, so

that knocking out any single gene can't prevent that feature from developing

apparently normally. He called the ability of multiple different structures to

support a single function 'degeneracy'.

And so it is with the brain. The important functions our brain carries out are

not farmed out to single distinct brain regions, but instead supported by

multiple regions, often in similar but slightly different ways. If one

structure breaks down, the others can pick up the slack.

(Thinkstock)

(Thinkstock)

This helps explain why cognitive neuroscientists have such problems working out

what different brain regions do. If you try and understand brain areas using a

simple one-function-per-region and one-region-per-function rule you'll never be

able to design the experiments needed to unpick the degenerate tangle of

structure and function.

The cerebellum is most famous for controlling precise movements, but other

areas of the brain such as the basal ganglia and the motor cortex are also

intimately involved in moving our bodies. Asking what unique thing each area

does may be the wrong question, when they are all contributing to the same

thing. Memory is another example of an essential biological function which

seems to be supported by multiple brain systems. If you bump into someone

you've met once before, you might remember that they have a reputation for

being nice, remember a specific incident of them being nice, or just retrieve a

vague positive feeling about them all forms of memory which tell you to trust

this person, and all supported by different brain areas doing the same job in a

slightly different way.

Edelman and his colleague, Joseph Gally, called degeneracy a "ubiquitous

biological property ... a feature of complexity", claiming it was an inevitable

outcome of natural selection. It explains both why unusual brain conditions are

not as catastrophic as they might be, and also why scientists find the brain so

confounding to try and understand.