Brain storm

David Bodanis pokes around in our skulls to get an idea of how we work

I like to think of the brain as a little spaceship. It has video cameras sticking out of the front, so it can see what's going on, and it has two microphones stuck on its sides - our ears! - so it can hear what's happening outside.

There are also cables coming in from the nose and the mouth, but that's not all. The brain also has millions of nerve cell cables stretching in from the body that carries this wondrous little thinking machine around.

Those cables from the body are so important that they are wrapped inside the thick bony tubing of the spinal column. Wriggle your toes and the news that this is happening zooms upwards, through long narrow nerve cells inside those cables, travelling at about a motorcycle's speed, until it reaches the brain.

There are detection units all over the body at the end of the nerve cells to let us know what's going on. In our important parts, such as the fingertips or tongue, those detection units - our nerve reception cells - are bunched closely together.

Even a tiny paper cut on your tongue feels huge. On your back, however, the nerve receptors are much further apart. If you take two finely sharpened pencils, and lightly press someone on the back with them an eighth or even a quarter of an inch apart, they usually won't be able to tell that you're using two pencils, and will think you only touched them in one place. The receptors are so far apart that only one gets struck. That's because the brain doesn't need so much information from there.

It would be no good if all the signals that come racing into your brain - from eyes and ears; from fingers and toes - just fell into an empty space once they arrived up there. Instead, there are dozens of different regions for analyzing what information arrives. At one time scientists thought it was very simple. Sound would go to a hearing unit, vision would go to a vision unit, and memories would go to some sort of miniature library, a bit like the storage disk on a computer.

There's a problem with that simple view, though. If the nerve cables from our eyes just went into a vision unit - something like a miniature cinema - then how would we know what we were seeing? You couldn't just say there was a little copy of yourself sitting watching everything, for what would be going on in "his" or "her" brain? Another, even smaller, little film watcher?

No one has ever been able to work out exactly what it is that defines our consciousness. What exactly is the "self" that perceives the world? And where is it to be found in the brain? Scientists don't really know.

The best we can say is that our brain can do a lot of information sorting without any little watching man inside having to do the work.

If something dangerous is moving quickly towards us, our brain will start sending signals down the nerve cables to our muscles to tell us to run away. To help us escape, some of the nerve cables stretch inside down to clusters of cells in our lower back that are on top of the kidneys. Since the Latin for "on top of" is "ad", and the Latin for kidneys is "renal", those clusters of cells are called "adrenal" glands, and what pours out is adrenalin. This sloshes through our body, and for a few minutes makes our muscles stronger and our vision sharper.

But not everything we see makes our brains hurry to get us to escape or stand still. Other aspects of what we see get brought into our memory and these are then compared with what we already know. Maybe the "thing" rushing towards us is actually a friend! We'll recognise the face, and instead of starting an adrenalin rush and running away, our brain will begin to prepare sentences to be spoken; it will give instructions to our mouth and tongue and chest to pour out air, causing it to vibrate so that it will produce the words we want to speak.

You might say thousands of words this way in a typical school day, but you won't remember most of them a few weeks later (how many sentences that you said exactly one week ago can you remember?). This is because most of the things we do or see or hear only get switched into short-term memory storage inside our brain. It's like remembering a phone number for just long enough to dial it. After a few seconds the nerve cells in our brains stop firing in the code that carries the phone number, and we "forget" it.

Sometimes though, for something that's really important, the incoming signals that we see or hear get transferred out of short-term memory. They get pushed through a switching centre called the hippocampus. This looks a bit like a tiny horse, and as the Greek for horse is "hippo" that's how it got its name. Once the signal is pushed through, it gets locked into long-term memory - something like a computer hard disk. Once it's there it has a chance of stay for many years.

When people get older, their long-term memory often remains good, but their hippocampus stops working as well as it used to, and they can't push so many of the signals they receive through it. But it is not just old people who can have this problem. When we are bored, we are less likely to transfer what we are seeing or thinking into permanent storage. When there is something that interests us, though - a sports match or gossip about a friend - we are suddenly ultra-alert. Our hippocampus and other parts of the brain are working full out, and the news gets through and locked into place.

The whole brain is divided pretty neatly into two sides. Most of the things the two sides do are similar, but there are some differences. The left side, for example, is usually better at understanding words and solving problems than the right, which is more emotional, and responds more to art and music. But this doesn't mean that if you're especially good at language it's because the left side of your brain is bigger than the other. All of us have a thick cable between the two sides, which constantly carries messages back and forth. What's thought or worked out on one side, gets quickly linked with what's going on in the other side.

A century ago, a lot of researchers were convinced that they'd found deep differences in the brains of people from different countries. They always seemed to find that people from their own countries had the biggest brains! When more reputable scientists went back to the research later, they found that there were no differences at all.

It's true, for example, that women tend to have smaller brains than men. But this is only because they're shorter on average. When you take two people of the same size, whether male or female, whether from Lancashire or Latin America, their brains work just the same: the nerve cells inside transmit information at just the same speed; the areas where their reasoning takes place are in the same place.

Nor is there any advantage for the taller folks. The overall weight of their brains will be slightly more, but that's because their bodies are bigger. The brain needs to use up more space just to supervise all that extra mass. The amount left over for reasoning and remembering and emotions is just the same.

One curious difference which does hold up though, is that for some reason women are often better at reading upside down than men. It's easy for most girls to read the following phrase: But most boys are probably already turning the page around to see what it says.

All the vision and memory and reasoning usually works fine, even though the cells crammed inside our brain are very small. There are about one thousand million of them in there - so there are almost the same number of cells in each of our brains as there are stars in the Milky Way.

About 20 per cent of all the food we eat - a fifth of every chocolate bar, hamburger, apple or chip - is transformed into the energy that feeds our brain. In young children it's higher, and up to 50 per cent of what a baby eats is turned into brain battery power. That's why your head gets so hot: the best way to stay warm on a cold day is to keep a hat on, so the heat that your brain produces can't escape.

Every few hours, bits of each brain cell that are damaged or used up get discarded, and fresh bits get built to take their place. By the end of a school day your brain is a bit different from what it was in the morning, and by the end of a week or two almost all the solid material inside your brain has been replaced by fresh sugars or fats taken from your food.

We make our brain out of our food, which means that our most personal thoughts - the way in which we think about ourselves whenever we look in a mirror - are actually made up of leftover bits of hamburgers and chocolate and other food. Think about that!

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