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September 22, 2008 | by  | in Opinion | [ssba]

Bits of your brain that you can live without

The brain is the part of the body that most of us think of as ‘me’. Imagine if it was possible to ‘swap’ brains with someone else. Consider your body with the other person’s brain, and your brain in the other person’s body: which one is ‘you’?

Neuroscientists have learnt a lot about the function of the brain by studying the abilities and disabilities of people who’ve experienced brain damage resulting from injury, infection, or disease, and people who’ve had parts of their brains removed for medical reasons. You might be surprised to hear how much of your brain you can live without. You might also be surprised how much – or how little – losing parts of your brain can change ‘you’.

The corpus callosum is a large bundle of nerve fibres that make up the main connection between the left and right hemispheres (halves) of the brain, allowing them to ‘communicate’ with each other. Functions of the human brain are either performed in one hemisphere, or in both – either as a whole, or broken up into parts performed in one or the other hemisphere. In most people, language production is largely the domain of the left hemisphere. Visual perception and processing occurs in parallel in both hemispheres: images from the right of a person’s visual field are processed in the left hemisphere, and vice versa (the crossing-over occurs in an area called the optic chiasma, and is not associated with the corpus callosum).

Some people who experience severe cases of a certain type of epilepsy – in which a seizure can spread until it affects the entire brain – have their corpus callosum (and sometimes other minor communication channels) surgically cut to try to limit the severity of the epilepsy, by limiting how far the seizures spread.

In specialised testing it is clear that after such surgery, the two hemispheres of the brain are functionally isolated. In one test, an image is flashed in one side of a post-operative patient’s visual field, and she asked to name the image. If a picture is flashed in her right visual field, she has no difficulty naming it (remember that images in the right visual field are processed in the left hemisphere, the hemisphere responsible for language production). However, if an image is flashed in her left visual field, try as she might, she cannot name the object: her right hemisphere cannot send information to her left, naming, hemisphere.

But outside of such specialised testing, in day-to-day life, the loss of this major connection between the hemispheres doesn’t have a noticeable effect: people who’ve undergone this type of surgery can carry on just as before they had the main communication channel between their brain’s hemispheres cut.

Phineas Gage, a nineteenth century railway worker, is remembered for having survived a railway spike (with a diameter of about three centimetres) blasted through the front of his head – destroying a lot of the frontal portion of his brain (the prefrontal areas of the frontal lobes). Amazingly, Gage survived the injury, and was able to continue to function quite normally – with little or no disruption to his ability to speak and move, for example. However, after the accident, Gage had difficulty planning ahead, lost his patience easily, and began, according to the original medical report: “indulging at times in the grossest profanity (which was not previously his custom).” His personality had changed so dramatically that “his friends and acquaintances said he was ‘no longer Gage’’” Gage had damaged part of his brain responsible for what is now often termed the “executive functions” of the brain: involved in guiding thought and behaviour to keep it aligned with overall goals. This includes forward planning, and response inhibition (hence the “gross profanity”).

Towards the more dramatic end of the scale is a man known in scientific literature as “HM”. HM had epilepsy, with frequent and severe seizures of the type that start in a specific place and then ‘spread’ throughout the brain. In the mid-twentieth century, doctors identified his temporal lobes (in the lower left and right sides of the brain) as the starting point of the seizures, and in experimental surgery, removed most of the lobes. In the surgery, most of HM’s hippocampus – associated with learning and memory formation – was destroyed. The effect of HM’s surgery was severe. He experienced acute anterograde amnesia: HM could not form new memories. Try to imagine what it must be like to be stuck forever in one point in time, and the distress HM felt in looking in the mirror: the young man in his mind seeing the old man in reality reflected before him.

So, look after your brain: although it is not uncommon to survive drastic damage to the brain, the you that survives may be a very different person to who you think of as ‘you’.


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