In mental life, nothing which has once been formed can perish," said Sigmund Freud. Which means that, in theory, everything you have ever thought, learned or experienced may be stored somewhere in your brain. But however therapeutic it might be to access a memory that has been repressed, if every piece of information your brain has ever processed was immediately accessible to your conscious mind, you would be so bombarded with mental data you would be unable to function.
This was the unfortunate experience of Solomon Sherekevskii, whose amazing photographic memory was studied by the great Russian psychologist, Alexander Luria, for more than 30 years from the 1920s. S, as he was known, could memorise lists of several hundred random nonsense syllables and recall them perfectly months and even years later.
He also had a rare mental ability - synaesthesia, which meant he could "see", "feel" and "taste" sounds, making his memory of them even more vivid. But the vast amounts of information he recalled made it difficult for him to hold down a normal job and he ended up as a professional memory man.
Although S was a unique example of hypermnesia, or super-memory, about one person in a million has extraordinary powers of recall. Five times world memory champion Dominic O'Brien can memorise a single pack of shuffled cards in 38.29 seconds and, as a result, has been banned from playing blackjack in any casino in the United Kingdom. And Mozart apparently once wrote down all nine parts of Allegri's Miserere after hearing it only once.
But the rest of us, according to Professor Steven Rose, director of the Open University's brain and behaviour research group, have fairly equal brains when it comes to memory, though it is possible to train yourself to use yours more effectively.
"I am not sure 'good' or 'bad' memories exist," he says, "because a person's capacity to remember something depends largely on the amount of attention they give it.
"For example, you may not remember the name of someone who has just been introduced to you at a party because you were feeling so self-conscious or distracted that your mind didn't take it in. Similarly, you tend not to store information that bores you. People remember things that are important or interesting to them."
Which is why a teenager who appears to have no recollection of last week's history lesson, can tell you the score of every match ever played by his favourite football team.
"Harold Wilson was famous for never forgetting a name when he was prime minister, but it wasn't because he had an amazing memory. He simply trained himself to remember people's names," Rose says.
One well-known method of doing this is to create a visual image of the name and attach it to the person in your mind. For example, if someone is called Green, you imagine grass growing on their head and forever associate them with that.
One of the proofs that our brain stores many more memories than we can recall lies in the fact that our capacity to "recognise" is far greater than our capacity to remember. A good test is to put 20 small objects on a tray, look at them for a minute and then cover it over and try to recall them. You probably won't remember them all (15 is a good score), but if you are then shown a new tray of, say, 100 objects which includes a few of the items you didn't remember the first time, you will recognise them instantly.
No one knows where in the brain, or in what form, the things we don't consciously remember are stored, but the current received wisdom is that they are probably lurking somewhere. Even the things that we do remember are not stored in any recognisable form. No one knows what a thought or a memory "trace" is or looks like, even though a great deal is now known about the 100 billion neurones which move them around the brain.
It has, however, been established that there are five main categories of memory: * short-term memory, for things that we don't need to remember for long, like the beginnings of sentences or mental arithmetic; * procedural memory, for skills such as swimming or driving a car; * episodic memory, for personal experiences; * semantic memory for information, facts and figures; * prospective memory, for things which have to be done in the future, like keeping an appointment.
But before the countless bits of information our senses receive every second are transferred to memory, they pass through a "filtering" system. Irrelevant material is forgotten instantly, things you need to use immediately (such as a phone number you have just looked up) go to your short-term memory (where they may be forgotten within a few minutes) and everything that survives is transferred to long-term memory, where it may stay forever, even if you can't recall it.
The brain has an almost infinite storage capacity and a very precise filing system. But, according to Steven Rose, the popular theory that the right side of the brain is intuitive and the left side logical is purely metaphorical because we use all of our brain for everything. However, studies of people who have lost particular areas of their brain (through accidents or strokes) reveal that specific memories are stored in specific places. Depending on where it is, a brain lesion can destroy long or short-term memory, or take away something as specialised as the ability to recognise faces, name colours or use verbs - though it may be a part of the brain's communication system that has been destroyed rather than the memory itself.
The reason people with normal, healthy brains keep forgetting things that they want to remember is probably because old memory traces are easily eclipsed by more recent ones. We can remember what we had for lunch today and possibly yesterday, but few people can tell you what they had for lunch every day last week. We do, however, remember special events and "firsts" - the first date, the first day at school - and we tend not to forget procedural skills such as how to ride a bike.
Most of us have very good memories for smells and tastes, as these once helped our ancestors avoid poisonous or rotting foods, and smells and tastes can themselves evoke powerful memories, possibly because they are processed in an area of the brain very close to the ones that lay down long-term memories.
We only find it difficult to memorise foreign vocabulary and pin numbers and to remember names, dates and lists, because we do not use and repeat the information enough times for it to become lodged firmly and accessibly in our long-term memory. Long-term memory will tell you exactly where to find eggs, apples and coffee in your regular supermarket, but it won't tell you whether these items are on your mental shopping list because that data is in your less reliable short-term memory.
Next week: Can you master your memory? We look at the power of association and other tools.