Why you can live a normal life with half a brain

Posted on December 31, 2014

Photo: flickr

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.

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.

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.

Category(s):Health / Illness / Medical Issues

Source material from Mind Hacks

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