MEMORIES fade quickly, as we all know too well. “All things being equal, it’s harder to remember things from a long time ago compared to more recent events,” says neuroscientist Marc Howard of Boston University.
But forgetting doesn’t just happen by accident. Evidence suggests that it is largely down to active processes in the brain.
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In the hippocampus, for instance, which plays an important role in memory, new cells are formed throughout life. It takes energy to do this, yet these cells seem to overwrite established memories and induce forgetting.
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Why should the brain invest energy in dismantling its own memories? The issue isn’t storage space: given the number of cells and connections in the brain, there is reason to think we could remember much more than we do.
According to Blake Richards at the University of Toronto, Canada, the goal of memory isn’t to store information indefinitely, but to optimise decision-making in the future (see “Do we even know what memory is for?”). And it seems that forgetting most of our experiences actually helps us learn important lessons.
Each memory is thought to be stored in an interconnected network of brain cells. To retrieve a memory, you need some part of its content: for example, to recall who came to your last birthday party, you might start by picturing where the party took place.
Artificial intelligence researchers have built computer programs that work on the same principles, known as neural networks. They have found that when memories are distributed across interconnected units like this, there is a lot of potential for what’s called interference, in which one memory effectively impedes the recall of another. This is especially true if they share some of the same content, so you might end up confusing memories of two birthday parties that happened at the same venue. In addition, if you store memories that are no longer useful, there’s a high risk that they will hamper the storage of new memories.
Having fewer memories can also make it easier to spot important patterns that help us plan for the future. By remembering instances when traffic was bad on your commute, for example, you would learn which times to avoid. Remembering every single journey would make it impossible to identify such patterns.
Even so, there are less practical reasons for forgetting, especially as we age. Our bodies simply aren’t adapted to living quite as long as we do, says Aoife Kiely from UK charity the Alzheimer’s Society.
Aside from some new cells in the hippocampus, “the neurons you are born with are pretty much the ones you are going to live your whole life with”, she says. “It is certainly an issue of wear and tear of the brain.” As we age, we lose many of the connections between these neurons, and immune cells in the brain can also begin to run amok.
Still, factors like health and education play a large role in how our memory fares as we age, Kiely points out. And even when memories seem to have disappeared, they are often still lurking somewhere, it is just that we can’t or don’t retrieve them – until the right moment comes along.
This article appeared in print under the headline “What happens to memories over time?”
