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Who are you? For most of us, our sense of self relies on a personal history of memories that can be dipped into just as readily as turning the pages of a photo album: the child who broke an arm falling out of a tree, the gawky teenager on a first date, the proud parent. But can your memory really be trusted with something as fundamental as your sense of identity?

Psychologists have long known that our memories are easily embellished. We add imaginary details through wishful thinking or to make a more logical story. More controversially, memory may be falsified through suggestion and manipulative questioning, bringing some eyewitness testimony and "recovered" memories into doubt. And we all forget things too. But despite these flaws it was always presumed that the core experiences themselves - the memory traces stamped into the fabric of our brain - were permanent. Look in the right place and we could always dig back to what really happened.

But that's simply not so, according to some surprising new research. A memory is anything but static. Resurrecting a memory trace appears to render it completely fluid, as pliable and unstable as the moment it was first formed, and in need of fixing once again into the brain's circuitry. Any meddling with this fixing process could alter the trace - or even erase it completely. Simply retelling a tale may be enough to change that memory for good. Long-term memory is effectively a myth. What does it mean? Who are we if our personal memories are so volatile that the very act of remembering might allow past experiences to vanish into thin air? How can we trust our minds at all? Well, common sense tells us that our brains aren't that bad at keeping a record of our lives. So maybe what needs changing here is how we think about memory. A memory trace that goes all floppy every time it gets used only seems a disaster if you believe the brain to be something like a computer where data needs to be preserved in fixed form.

Fluidity, on the other hand, may be precisely what is required for memory to work as something much more organic - a living network of understanding rather than a dormant warehouse of facts.

The standard story on memory is that the brain first captures a snapshot of each moment. The firing of a particular arrangement of neurons leaves them electrochemically aroused and ready to fire again in the same pattern, primed to recreate the just-happened experience. But this short-term memory trace lasts barely a few seconds and needs to be turned into something more permanent by a complex cascade of brain events. One of the great goals of neuroscience is to unravel the fine detail of this process of memory consolidation.

In the past few years it has become clear that the transient sensitisation of nerve junctions - the synapses connecting neurons together - leads to an almost immediate swelling. The synapses bulk up with more receptors and more neurotransmitters, and become inflamed to make a stronger connection. Then after a few hours, the neurons begin to physically grow, sprouting new and thicker connections to wire in a permanent memory trace. A mass of protein, produced by a range of genes will be employed to build a remodelled brain circuit.

What makes the process of consolidation so complex is that as well as the neural-level rewiring, the memory trace also migrates. When a memory pattern is fresh, it is stored in specialist memory organs such as the hippocampus, deep within the brain. But over a number of days, weeks or even years, it settles back across the brain and becomes lodged in more general areas. Rather like computer files being transferred from hard disc to back-up tape, old memories eventually get consigned to the vast, wrinkled vaults of the cortex.

Neuroscientists felt this hierarchical filing system was a little long-winded. But it sounded reliable. Once fossilised in some dusty forgotten corner of the brain, a memory trace might become a little harder to retrieve, yet it ought to remain absolutely stable. This was the accepted story until a very simple experiment blew it away.

To study memory consolidation, researchers interfere with steps in the fixing process in order to test their influence on long-term recall. While doing this kind of work, researchers including Karim Nader of McGill University, Montreal, and Joseph LeDoux of New York University noticed something odd. They trained rats to associate an electric shock to their paws with a darkened box. The rats learn that the box is "nasty" and freeze the next time they are put back. If, a few days after training, the animals were given a drug to stop protein synthesis before being reminded of the conditioning stimulus - the sight of the training box - it made no difference to their ability to remember it. The memory seemed fixed and safely stored. But if the rats had a brief reminder of the stimulus just before the drug was given, then a memory that should have been fixed and stable seemed to be erased.

What did it mean? Nader and LeDoux coined the term reconsolidation, suggesting that the act of recalling something renders it flexible, giving the chance to expand or generalise the original memory trace - a form of reaffirmation. The drug given to the rats prevented this reconsolidation step, somehow leading to the decay of the original memory. The publication of their results in Nature (vol 406, p 722) caused quite a stir, and no small amount of scepticism.

But in a detailed follow-up published in the journal Neuron late last year, they made a more convincing case. Again the rats were put in a box and given an electric shock to their paws, and would freeze the next time they were put in the box. According to traditional consolidation theory, such memories are fixed locally by protein changes in a matter of hours and then safely filed to long-term storage in the cortex after about a month. The team waited a full 45 days to test the rats, by which time the memory trace should have been quite immune to interference.

As expected, the rats that were given no reminders of the original experience showed no memory loss when injected with the protein-blocking drugs - they froze when tested. Likewise the complete destruction of the hippocampus left the memory intact, as it was now resident in the vaults of the cortex. But if the rats were reminded of the sight of the box just before the drug was injected, the result was precisely the opposite. Now the protein-blocking drug created amnesia. And destroying the hippocampus also erased the fear association. The rats nosed about the box quite unconcerned. At both the synaptic level and the anatomical level, it was as if the consolidated memory had been released and needed to undergo the whole fixing process again if it were to be remembered. Recall had made an established trace shaky.

"The dogma was that once a memory trace has been consolidated, it is permanent," says Nader. "But here it was labile - subject to interference in exactly the same way as a brand-new experience." The old static picture of memory could not be right. "We were showing memory to be something incredibly dynamic."

Some researchers, such as James McGaugh and Larry Cahill of the University of California at Irvine, were sceptical. They didn't think this "reconsolidation" effect would be replicated. Cahill describes the work as "iffy" and feels a simpler interpretation of the results - such as some kind of suppression mechanism - might still be forthcoming.

But confirmation has been pouring in. "Reconsolidation has now been demonstrated in all sorts of situations and all sorts of animals - crabs, slugs, chickens," says Nader. "This tells me it's a basic feature of the memory system." He now believes that there's no longer any question about whether the effect exists. Instead he's keen to find out what it means.

For example, does it imply that our memory is always unreliable? Clearly not. But it isn't completely stable either. But even if reconsolidation doesn't yet convince all memory researchers of that fact, there's another reason to suggest instability is inevitable - molecular turnover. The proteins, fats and other complex molecules making up a cell generally last an astonishingly short time, anything from a matter of days down to just a few minutes, and so need constant replacing. Cells are not static creations but fragile things that are continually renewing themselves.

For brain cells - where their shape and synaptic structures determine their function - the issue is all the more acute. The protein filaments that give the cells their internal shape have a half-life of just a few minutes. And the receptor proteins that stud the synapses need replacing every few days. As Joe Tsien, a neurobiologist at Princeton University in New Jersey, says, the brain you have this week is not the one you had last week. Even the DNA needs to be repaired. So if "you" are essentially a pattern of synaptic connections, a tangled web of memories, then there is a big problem of how this pattern endures. "I don't know how people ever got this static picture of the brain," says Tsien. "A memory trace would have to be a dynamic thing just because of molecular turnover." The idea that we enjoy a photographic record of the past is a myth that has also been exploded by experiments such as the eyewitness research of Elizabeth Loftus of the University of California at Irvine. Here, subjects incorporated overheard details about a staged bank robbery or car crash in their own memories of the event (New Scientist, 23 July 1994, p 32). Loftus's work seems clear proof that our memories are fluid creations that can be edited or embroidered.

And the more you think about it, the more such dynamism makes sense. Susan Sara of the Pierre and Marie Curie University in Paris, who found indications of a reconsolidation effect in her own experiments in 1997, says the real problem for the brain is not how well it can preserve the past but how successful it is at integrating new learning with old learning. Memories exist to make sense of the present - to recognise and understand the world - and the brain needs to be able to optimise all its circuits, strengthening or generalising some connections while weakening or erasing others. Reconsolidation may seem a radical and unnecessary step for a brain that just wants to be a dormant warehouse. But, Sara says, if a memory becomes completely plastic every time it is roused, then it can be refiled in a carefully updated way. Active choices can be made about whether to merge the old and the new - or by contrast, to reinforce their separateness.

"It's just an accident that reconsolidation has been demonstrated by erasing memories," says Sara. This is what is creating the misconception of the shaky trace. It's easier to interfere with the reconsolidation in this way than it is to show the opposite - a memory being strengthened. But Sara and her team hope to show that drugs which arouse the brain will reinforce an activated memory.

The bottom line is that there is no reason to believe that rousing your personal memories is a risky affair, she says. Thinking about the past probably does require a surprisingly drastic change in the state of a memory trace. They may not be simple snapshots of events that are passively read out but constructive and ever changing. However, there is no cause to think the brain does a poor job of reintegrating a memory that has been roused. Our memories may not be an infallible recording device, but frequently recalling a childhood memory seems more likely to reinforce it than erase it.

By the same token, the reconsolidation finding goes a long way to support the claims of psychologists like Loftus, who say that society needs to take better account of the essentially reconstructive nature of human memory. Loftus first demonstrated more than 30 years ago the ease with which the memories of eyewitnesses could be biased. But, she says, it is only recently that advances in DNA testing have brought a spate of wrongful convictions to light, forcing US legal authorities to take the issue more seriously. Loftus has also been at the centre of the false memory syndrome controversy, where the suggestive questioning style of psychotherapists has been blamed for creating imagined incidents of childhood abuse.

"In most avenues of life, it doesn't really matter if you make a few mistakes in your memory," says Loftus. "But if somebody's liberty is at stake, or they are going to be involved in a horrible lawsuit of some sort, then very precise memory does matter." She has been calling for changes in legal and therapeutic interview practices so as to minimise the chances of contaminating the memory of witnesses.

Yet even Loftus confesses to be a little taken aback that neurology might prove the brain to be quite so labile. She often quotes the Uruguayan novelist, Eduardo Galeano, who said: "Memory is born every day, springing from the past, and set against it." Now, says Loftus, this may be even truer than anyone ever suspected.

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