Memories are constantly in flux, decaying as soon as they have begun to form. Although you can't count memories, if you could, you'd soon discover that more than half of what we experience is inaccessible to memory within a single hour. For this reason, when learning, it is best to continuously and cyclically review information as you go.

Optimal revision

During the 19th century, Hermann Ebbinghaus, a German psychologist, spent more than 15 years learning random strings of nonsense syllables, and testing himself on their recall. What he found has become one of the few certainties of neuroscience: namely, that all memories grow continuously weaker, but that the rate of "decay" lessens each time you review the information.

Ebbinghaus found that the ideal time to review a memory is just before you are about to forget it. Furthermore, because your memory gets stronger with each review, the times at which you should review the information increase exponentially. In other words, you should first review after a few seconds, then after a few minutes, then an hour, a few hours, a day, a few days, a week, a month, three months, a year, three years, and so on. This is known as spaced repetition, and is a very effective way of learning.

The "forgetting curve" helps to explain why we so often remember nothing shortly after cramming intensely for an exam. Because all the learning takes place in a week or so and is not subsequently reviewed, it begins to be forgotten after only a month.

Continuous testing

Another important way to keep your memories healthy is to practise retrieving them. Actively testing yourself is a significantly better way to strengthen your memory than just passively reviewing the information "contained" within it.

This is actually quite counter-intuitive – you'd think that being reminded that Buzz Aldrin was the second man on the moon would be a more effective way of strengthening that memory than by being asked the question: "Who was the second man on the moon?". But science has repeatedly shown that "active retrieval" is a more effective way to boost your memory power than pure revision.

One way of understanding this is to consider how memories are, in a sense, movements that your mind makes. If recalling a memory is like climbing a hill, then being reminded of it is like being dropped by helicopter on top of the hill. You enjoy the view, and it feels like you've accomplished something, but if you'd climbed to the top yourself, you'd have a better idea of how to get there next time. In other words, when you are tested for a memory, you actively re-create or rediscover it in a way that positively reinforces that memory.

There's an interesting correlate of this that takes place during sleep. Recent studies in neuroscience suggest the brain takes advantage of this "offline" period to repeat, and so select, what should be remembered in the long term. Principally, the brain seeks patterns that exist across different memories that have formed in the recent past. As a result, it is often very helpful to review information you wish to remember just before you fall asleep. In the morning, what seemed complex and cloudy can appear surprisingly lucid.

The danger of getting information wrong

An interesting side effect of how practice reinforces memory is that, when you get a test wrong, you are in some ways strengthening the wrong answer – you are rehearsing failure, which can be dangerous …

I recall once getting into the bizarre habit of not being able to remember Bob Dylan's name. Maybe six or seven times I'd try but fail to recall it. Each time, I'd think it would never happen again. But, sure enough, the next time I had to remember his name, I'd again fail to find the memory.

In the end, I had to perform emergency surgery on the association and imagine him bobbing up and down in a dill salad, croaking away in his inimitable style. This absurd image acted as a temporary crutch or scaffold, and in time my broken memory gradually healed. Now the name Bob Dylan comes to mind without a problem whenever I need it.

How to find a missing memory

This brings us to the question: how do you find a memory that is resisting being recalled? We've all experienced the frustration of setting out confidently to find a familiar memory – the name of an actor or title of a book, for example – only to wind up empty-headed and confused. Intriguingly, this often happens with information that we know we know .

The first thing to realise is that there's nothing remotely shameful or surprising about "failures" of recall. The sum of our memory is an almost infinitely complex and chaotic web of connections: superimposing, jostling, crisscrossing, intermixing, competing; like a jungle, or compost heap, or mad, overcrowded house party. It's a miracle that we can recall anything at all.

When we do find ourselves floundering, however, there are a number of ways we can go about increasing our chances of locating a lost memory. As we've seen, a memory is never an isolated unit of information. There will always be plenty of implicit context or "components" to the memory: the time of day, weather, persons present and so on. Think of these as routes into memory; they are ways of causing the full memory to become active and coercing your brain into reproducing the whole story.

By searching the fringes of the memory, you will increase the likelihood of recalling the nugget of information in the centre that you seek.

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