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YOU forgot where the car was parked at the airport yesterday and missed your
mother-in-law鈥檚 birthday last week. The name of that old flame who kissed you in
Kew Gardens way back when is on the tip of your tongue鈥攂ut who was it?
Your memory is atrocious.

Or so you think. Yet if you remembered every place you鈥檇 ever parked, along
with every other trivial detail of daily life, every day, for ever and ever,
you鈥檇 be overwhelmed. Forgetfulness gets a bad rap, yet common sense tells us
that it鈥檚 also vital to a healthy brain, sweeping our minds free of decaying
memorabilia. And so, increasingly, does science.

To date, university labs and drugs companies have been more interested in how
memories lodge in the brain in the first place, driven in no small part by a
desire to invent pills to boost the failing memories of elderly people and those
with Alzheimer鈥檚 disease.

This search continues. But now, a small band of scientists is turning the
quest on its head. They鈥檙e trying to discover not how memories form in the
brain, but how and why the brain chooses to erase some of them days, months or
years later. Their aim isn鈥檛 to boost our ability to remember, nor are they
after some kind of sinister memory-erasing pill. What does interest them is the
prospect of helping people shed the unwanted memories that researchers are
discovering contribute to mental illnesses such as depression and post-traumatic
stress disorder.

If they succeed, the benefits won鈥檛 just be confined to clinics. Even in
mentally healthy people, being unable to forget can dull the mind and generate
misery. There鈥檚 no stronger evidence of this than the life of Solomon
Shereshevski, the world鈥檚 most celebrated mnemonist. Studied for 30 years by the
Russian psychologist Aleksandr Luria, Shereshevski spent much of his
professional life performing amazing feats of memory for paying audiences. He
could memorise strings of dozens of numbers just by looking at a blackboard,
recalling them with stunning accuracy months or years later and reciting them
backwards or forwards.

But Shereshevski paid heavily for his talent. He remembered things by
picturing them in his mind and this, it seems, hindered his intellect. 鈥淚 can
only understand what I can visualise,鈥� he told Luria. Unable to think in the
abstract, his mental capacity never really got beyond that of an adolescent. He
got muddled when a word had two meanings or an object had two names. Indeed, he
could barely read. 鈥淓ach word he read produced images that distracted him and
blocked the meaning of a sentence,鈥� wrote Luria.

Unable to hold down a job, Shereshevski was forced to earn his living as a
memory freak. And though he married and had a son, even this 鈥渉e perceived as
though through a haze鈥�, according to Luria. Many people considered him dull and
awkward, even absent-minded.

Unfortunately, scientists never discovered what is was about Shereshevski鈥檚
extraordinary brain that allowed him to retain details most of us would simply
discard or suppress. Scientists are pretty much agreed on the basics of how new
memories form. Like code woven into cloth, memorable new experiences are
essentially just strengthened connections between neurons. When the experience
is relived, these brain cells are activated together, the connections between
them are strengthened a bit more, and details and feelings flood back to
consciousness.

So you might reasonably expect forgetting to be the reverse of this process,
a simple matter of the connections gradually weakening so that the coding and
the memory vanish. But there鈥檚 a lot more to it than that, as scientists are
starting to discover.

Out of reach

For a start, you can鈥檛 lose what you never had, and much of what we call
鈥渇orgetting鈥� is not so much caused by the brain losing encoded memories as by it
failing to create them in the first place. If you can鈥檛 remember what you had
for breakfast, it鈥檚 probably because your brain never bothered to encode the
information. Moreover, there鈥檚 a growing body of evidence to suggest that many
of the memories we do form, and then later forget, are not so much erased from
the brain as simply put out of reach. When surveyed recently, 84 per cent of
psychologists said they believed that forgotten memories are all still lurking
somewhere in our brains, ready to be cued into consciousness.

This, of course, has been the basis of a whole string of claims that 鈥渓ost鈥�
childhood memories can be recovered intact, paving the way for cases of
long-forgotten child abuse. It鈥檚 an issue that continues to divide experts. Some
believe that the memories are genuine, others that they鈥檙e inadvertently planted
by therapists. The debate looks set to run and run, with neither side able to
clinch its case. But what has become clear is that we have more control than we
might think over what we forget.

Michael Anderson, a memory researcher at the University of Oregon in Eugene,
has found evidence that the brain has an inbuilt ability to suppress unwanted
memories. In at least some cases, forgetting is not a passive process of decay,
but rather an active process of inhibiting (Nature, vol 410, p 366).

Anderson and his colleague Collin Green, also at the University of Oregon,
asked volunteers to memorise a list of 50 or so simple word pairs, such as
鈥渙rdeal, roach鈥�. They then showed the volunteers the first word and asked them
either to recall the second or to avoid thinking about it altogether by
concentrating on the displayed word.

Anderson and Green found that the more times volunteers had been asked to
repress the memory of the associated word, the worse they were at recalling it
later on when asked to鈥攅ven when they were offered a cash incentive. 鈥淚鈥檓
not making the claim that you鈥檙e forgetting the memory,鈥� says Anderson. 鈥淚t鈥檚
inhibited, not erased.鈥�

Shereshevski himself devised elaborate ways to try to suppress his memories
but found it almost impossible. He described one technique to Luria: 鈥淚n my mind
I erase the blackboard and cover it, as it were, with a film that鈥檚 completely
opaque and impenetrable. . . Even so, when the next performance starts and I
walk over to that blackboard, the numbers I had erased are liable to turn up
补驳补颈苍.鈥�

And even the rest of us find there are certain memories that we can鈥檛 get rid
of with willpower alone. Sometimes it鈥檚 just an annoying tune in your head
(New Scientist, 28 July, p 44).
But more often, it鈥檚 an emotionally charged
event鈥攖he death of a loved one, say, or a violent attack.

According to Daniel Schacter, a psychologist at Harvard University who
specialises in memory, there鈥檚 a good evolutionary reason for remembering
traumatic events: they pose a threat to survival, so animals and people who can
remember and learn from them would have a better chance of staying alive. But
sometimes the system works overtime and becomes counterproductive. 鈥淧ersistence
is the flipside of an adaptive feature of memory,鈥� he says.

And again, not being able to forget can have serious consequences. Susan
Nolen-Hoeksema at the University of Michigan has found that people who tend to
mull things over excessively experience longer episodes of depression than those
who don鈥檛. She studied people after the 1989 earthquake in California鈥檚 Bay
Area, for instance, and found that those who鈥檇 鈥渞uminated鈥� a lot in the
immediate aftermath were more likely to be seriously down about it weeks after
others had come to terms with the experience.

Traumatic memories

Chris Brewin at University College London has confirmed that people suffering
from depression or post-traumatic stress disorder (PTSD) are especially prone to
this sort of persistent memory. It鈥檚 been observed that people with lower IQs
are generally more likely to suffer from PTSD. Brewin speculates it may have
less to do with intelligence per se, and more to do with something related to
intelligence鈥斺�漺orking memory capacity鈥�. This is basically the
right-here-right-now attention a person can call upon. 鈥淚鈥檓 starting to think
that how much working memory capacity a person鈥檚 got can affect the likelihood
of whether they鈥檒l get these illnesses,鈥� Brewin says.

In a recent study, Brewin asked volunteers to check through a series of short
strings of arithmetic and then remember single words that appeared after each
equation. The volunteers then tried to suppress thoughts of a white bear, and
rang a bell whenever the unwanted thought came to mind. Interestingly, he found
that the people who were best at remembering the words, indicating that they had
more working memory capacity, were also best at not thinking of the bear.

鈥淧eople with greater working memory capacity may be better able to suppress
unwanted memories of important life experiences, such as traumatic experiences,鈥�
Brewin speculates. He says that inhibiting memories, or forgetting, uses up
mental resources. 鈥淭hey are not overwhelmed by the memories bursting into their
minds.鈥� He agrees with Anderson and Green that forgetting is often an active
process鈥攜ou have to work at it.

But there are other ways to forget, which prove surprisingly important for
useful brain function. Many of our everyday memories aren鈥檛 so much lost or
thrown away, as merged. As time passes, the details drop away and only the broad
outlines remain. You don鈥檛 remember every breakfast you ate as a child, but you
may well remember that you hated toast and tea and that your brother always
argued with you. You remember only the gist of breakfasts past.

Barry Gordon, director of the memory clinic at Johns Hopkins University in
Baltimore, likens the human memory to the 鈥渓ossy鈥� system used in MP3 music
files. Information that鈥檚 not essential is deleted and what鈥檚 left is
compressed.

Gordon even believes this process of forgetting is necessary in order to
think creatively. 鈥淩emembering too much detail prevents you from seeing the
pattern,鈥� he says. Here again, Shereshevski provides a telling example. He could
remember the numbers 2345, 3456, 4567, 5678, for instance, in any direction and
after several years, but he couldn鈥檛 recognise the pattern. It鈥檚 no accident
that children learn language before they start forming explicit memories, Gordon
contends. Toddlers can be sensitive to general patterns鈥攗nderstanding that
a dog is a dawg is a dug鈥攚ithout worrying about the specific
pronunciation.

鈥淧art of the advantage of having a sloppy memory is that the slop, the
fuzziness, allows you to take creative leaps,鈥� he says. Some individuals with
superior mnemonic ability, he says, lament that they never have original
thoughts. They recall where every detail came from, and never learn to
synthesise, he says.

All this is not to suggest that forgetfulness doesn鈥檛 come at a price. Each
year, Anderson gets his undergraduates to calculate that cost by asking them to
keep 鈥渇orgetting diaries鈥�. For six days, the students keep detailed records of
everything they have forgotten鈥攖hings they鈥檝e neglected to do, items
they鈥檝e misplaced, and words, facts and ideas that don鈥檛 come readily to mind
when beckoned. Then they are asked to estimate how much time or money each lapse
of memory cost.

Based on several years of data, Anderson estimates that the average person
probably wastes about 40 days a year compensating for things they鈥檝e
forgotten鈥攁nd people who are more forgetful than average squander about 60
days. 鈥淎nd that may be an underestimate,鈥� admits Anderson. 鈥淭hey may have
forgotten to write things down.鈥�

SOMETIMES when you think you鈥檝e forgotten something, it鈥檚 not that the memory
has gone forever. All you really need to do is create the right conditions for
remembering. Michael Anderson from the University of Oregon recalls borrowing
his neighbour鈥檚 car to go to the supermarket. After he finished his shopping he
couldn鈥檛 locate his car. He had no recollection of parking it. He looked
everywhere and couldn鈥檛 find it. Then it dawned on him: he didn鈥檛 park his car,
he parked his neighbour鈥檚 car. And he promptly remembered where.

This particular memory lapse, says Anderson, was due to an ineffective
retrieval cue鈥攏ot uncommon in our everyday lives. Getting the setting
right is vital too: memories are very context-dependent, he says. You鈥檒l have a
better chance of remembering something if you go back to where the memory was
first encoded.

This was wonderfully illustrated in a study by Alan Baddeley of the
University of Bristol. He taught two groups of volunteers a long list of words.
One group learned the words while lying on a beach, and the other learned them
while underwater, wearing scuba gear. Baddeley then tested their recall, both on
the beach and underwater in full gear. He reported that those who learned on the
beach remembered better on the beach. And remarkably, those who learned
underwater, encased in rubber with breathing apparatus on their backs, performed
better that way too.

So it won鈥檛 surprise you to hear that getting back to the original state of
mind also aids memory. The same background music, sounds and smells help. And
what鈥檚 learned drunk is far better remembered drunk.