In the Harry Potter films, Hogwarts headmaster Albus Dumbledore shows the young wizard memories that he keeps in glass vessels. The franchise portrays memories as things that possess a physical structure that can be moved around. Although they appear to look like wispy bits of fluff, they are given form and structure.

Is this in anyway similar to real life? Do memories have structure? How are they formed? To answer these questions, I spoke to Lila Davachi, who runs the Memory Lab at New York University.

I had just been reading a new publication from Davachi’s team that showed how the brain is changed after the experience of some disturbing or stimulating event. This suggests that events such as the Great East Japan Earthquake can go so far as to change the brain.

Davachi calls this an emotional “hangover,” and her latest work suggests it influences how we remember future events. “Remembering the future” sounds like something out of a movie about time travel, but it merely reflects how an emotional event can influence the way in which we remember subsequent events.

“How we remember events is not just a consequence of the external world we experience, it is also strongly influenced by our internal states — and these internal states can persist and color future experiences,” she says.

We all know that we operate differently if we are experiencing emotions such as anger, grief or love, for example, but we assume that once the intensity of the emotion has died down we go back to our base states — our regular selves. Davachi’s work casts doubt on such a notion. “‘Emotion’ is a state of mind,” she says. “These findings make clear that our cognition is highly influenced by preceding experiences and, specifically, that emotional brain states can persist for long periods of time.”

To test the theory, Davachi and her team asked people to watch a series of images that contained emotional content.

Psychologists and neuroscientists need a universal standard way of measuring emotional arousal, which led to the creation of the International Affective Picture System (IAPS). IAPS is a set of images ranging in emotional power from everyday objects to mutilated bodies and naked people in erotic poses.

In Davachi and her team’s study, a group of subjects was first shown some provocative and stimulating pictures, followed a little later by a bunch of ordinary, boring images. A second group were tested the other way around, seeing the boring pictures first, followed by the arousing images. Six hours later, both groups were tested on their memories of the images. The people who were shown the exciting images first had better long-term memories about the images they saw subsequently — in short, they could remember the future better.

By scanning the subjects’ brains, Davachi’s team found that the brain states associated with emotional experiences persisted for 20 to 30 minutes and influenced the way people processed and remembered future experiences. The results are published in the journal Nature Neuroscience.

It’s worth noting that the people in the test just looked at a few images. What about people who see sights such as people dying in real life? Would there be a similar “hangover”?

Davachi says that something more arousing should create stronger memories. “However,” she cautions, “we do know that prolonged stress over days as compared to acute stress, can be detrimental to memory.”

In other words, as we know from the Great East Japan Earthquake and the Fukushima disaster (with the latter case, in particular, featuring ongoing and misleading stories about the threat from radiation), when stressful events are experienced over days and weeks, people can suffer harmful psychological effects. Indeed, Koichi Tanigawa from Fukushima Medical University said that the most damaging outcome of the Fukushima crisis was the psychological problems that followed. We know, therefore, that the emotional stress of an event changes the brain, but do we know more details about the composition of a memory? Is a memory a physical thing or is it a network of neural connections that needs to be traced in order to retrieve it?

Davachi admits that scientists aren’t yet sure. “But it will involve a distributed code, and activation of several neurons across different brain structures to retrieve a single memory,” she says.

A memory, Davachi says, does not “live” in one set of neurons alone in one region. “There is a sort of path that needs to be followed, we think, to unlock a memory,” she says. “A cue — for example, a word or a smell — can unlock a pattern of activation in the hippocampus.”

The hippocampus is a seahorse-shaped structure in the brain that is critical in memory. The activation of the hippocampus, Davachi says, can lead to a cascade of activity in the cortex where we suspect the actual memory representations are activated. Do we know where do memories go and how are they stored? What about those times we think we can just about remember something but it never comes?

“It’s called the tip-of-the-tongue effect of the feeling of knowing,” Davachi says. “It is clear that we can still remember some things but need to be in the appropriate retrieval state to recover those memories.”

A group of people with a condition called Highly Superior Autobiographic Memory (HSAM) can remember every detail of every day of their entire lives. For most of us, it’s strange that we process information in our brains, but it doesn’t all “stick.” Every day, as we walk to work and go about our daily lives, we are bombarded with thousands of images, noises and smells. Almost all this information is processed in the brain at some low level, but most doesn’t rise to the level of conscious contemplation. Most of the information is scanned and apparently discarded. But does it leave a trace? It does for people with HSAM, but what about the rest of us?

Davachi says she doubts we can gain access to everything we ever encountered. “While maybe a comforting idea, I also don’t see the utility in it,” she says. People with HSAM don’t sail through exams because they’ve remembered all the relevant facts — it may be that their brains can’t distinguish between useful and useless memories. What it does show, however, is that the brain has an extraordinary capacity for memory, a capacity that most of us come nowhere near reaching.

Rowan Hooper is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”).

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