Why Do We Remember Bad Things?

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Henry Ray Abrams / AFP / Getty

People watch the World Trade Center burn after the terrorist attack on September 11, 2001.

You remember where you were on the morning of 9/11, but you have no recollection of what you had for lunch last Thursday. One of life's great mysteries is why certain experiences get lodged immovably in our memory, while others are forgotten. Fortunately, recent advances in neuroscience have helped spur major breakthroughs in scientists' understanding of the nature of memory. To explain, TIME asked Matt Wilson, a professor of neurobiology at MIT's Picower Institute for Learning and Memory.

Q: Why do we remember unpleasant events better than ordinary ones?

A: We think of memory as a record of our experience. But the idea is not just to store information; it's to store relevant information. [The idea is] to use our experience to guide future behavior.

There's been a lot of really interesting research that points to a connection between our memory of the past and our ability to imagine the future. In our studies of animal models of memory, where we're able to go in and actually watch the pattern of [a rat's] brain activity, we can see that the brain activity while the animal is in a behavior-based situation, [such as navigating a maze,] directly corresponds to its future behavior: what it can, may and will do in the future. We can see that the animal does in fact — I hesitate to use the word, but I'll use it anyway — "think." In terms of brain activity, anticipating the future and remembering the past seem to be related.

The speculation is that we process memory in order to solve problems. And things we should learn from, things that are particularly important or that have strong emotions tied to them, may be things that are going to be important in the future. If you present stimuli with a strong negative emotional component, the memories do seem to be more easily retrieved than neutral stimuli or even those that are somewhat positive, for example happy faces versus angry faces.

One main question is whether or not we really "stamp in" important events just by making them emotionally charged, whether we lay down the memories in a form that's somehow stronger. It could be that they're simply more accessible, that there are more "hooks" into the memory that make it easier to retrieve.

There's evidence on both sides. The systems that are involved in adding emotional content include a brain structure called the amygdala, which gets activated when we experience strong emotions, particularly negative emotions, and it does influence memory systems — in particular, a structure known as the hippocampus. So there's opportunity there to influence how strongly memories get laid down. But the hippocampus is involved in both the storage as well as the retrieval of memory. Things that are emotionally charged may simply be memories that are more likely to be accessed or used.

[Emotional content] does not necessarily mean that events are remembered more accurately, and that's an important distinction. In fact, there's a lot of evidence that all memories can be altered. It's a normal process — we're constantly taking our experience and revising it, even twisting it to our own benefit. We might be able to take control of that process in some ways, which would be particularly useful in cases of abnormal, pathological memory processing — for instance, traumatic memory processing. There have been efforts to find ways of undoing that emotional bias. We don't know yet whether that's going to work.