National Geographic

An injection and a nap: two ways of strengthening memories

The business of encoding new memories is more like writing a document on a computer than inscribing words onto paper. Until you save the file, there’s a chance that you could lose the information. This vulnerable window can last for a couple of days. Only after that point does the memory become strong and long-lasting. This is called ‘consolidation’.

It’s not a permanent state. Whenever we remember something, the fragile window reopens. Again, it’s more like opening a computer document than getting notes out of a drawer. You could easily add, edit or delete information at a flick of a key.  Every time we bring back an old memory, we run the risk of changing it. Again, it takes a while for this window of opportunity to close, for the reactivated memory to strengthen once more. This is called ‘reconsolidation’.

In the last week, two groups of scientists have found two very different ways of boosting both processes, to produce stronger memories.

The injection

Dillon Chen from the Mount Sinai School of Medicine, New York found that a naturally occurring protein called IGF-II (or insulin-like growth factor II, in full) will do the trick. Chen placed rats in a box where one half was electrified and kept in shadow. The rats soon learned that wandering into darkness would give them a mild shock.

A day or so after their shocking learning experience, Chen saw that IGF-II swamped their hippocampus – a part of the brain involved in learning and memory. When he injected the rats with chemicals that neutralise IGF-II, he stopped their new memories from consolidating into long-lasting ones. The rats were just as willing to enter the electrified half as they initially were.

A dose of extra IGF-II did the opposite. If Chen injected the rats within a day of their electrifying experience, he boosted the strength of their memories. They became even more cautious about entering the dark half. The extra IGF-II also prevented them from forgetting the lesson. Three weeks later, they were still wary about the shadows, while normal rats had forgotten about the shocks. Chen also found that IGF-II helps to strengthen the connections between nerve cells, or synapses. This process is called “long-term potentiation” and it’s very important for laying down long-lasting memories.

IGF-II isn’t unique. Other scientists have found molecules with similar properties, including a protein called CREB. Inevitably, discoveries like these could trigger a spurt of people looking to take IGF-II supplements in the hope of achieving photographic memories. Indeed, we already know that a fifth of Nature readers have taken some sort of drug to boost their mental abilities.

But doing the same with IGF-II would be premature. So far, Chen has shown that it can strengthen fearful memories in rats, during the brief windows when they’re vulnerable to outside influences. There are far more unanswered questions. Chen doesn’t really know how it works, whether it does the same thing in humans, which parts of the brain it affects besides the hippocampus, how it affects our ability to remember things other than fears, and what the side effects would be. After all, IGF-II isn’t just a memory molecule. It has many roles. It’s involved in pregnancy and the menstrual cycle, and high levels have been linked to certain cancers.

This is not a molecule to toy with; it’s one to investigate further. And there are far easier ways to boost your memory. For a start, you could try having a good nap.

The nap

If we’re awake, bringing up old memories makes them unstable, until they can set again. But Susanne Diekelmann from the University of Lubeck found that when we’re asleep, activating old memories actually strengthens them. Rather than leaving them open and fragile, triggering a memory during sleep actually shields it from interference. Reconsolidation, it seems, works differently in the Land of Nod.

Diekelmann asked human volunteers to play a game of Concentration, where they had to pair up the images on face-down cards. As they learned, a bad smell wafted past their noses. Later, half of the volunteers stayed awake, while the others had a 40-minute nap. For people in both groups, Diekelmann released the earlier smell to trigger memories of playing the game. Later still, everyone played a slightly different version of the game to potentially distort their memories, before testing themselves on the original set-up.

After the training, the recruits remembered around 60 of the card pairs. Those who stayed awake only remembered 41 after getting a whiff of the familiar smell and playing the distracting second game. As the smell brought their memories, it also made them vulnerable, so that they were easily overwritten by playing the second game.

The nappers actually fared better if their memories were reactivated by the smell. They remembered 84 of the original card pairs, even after playing through the distracting second set-up. Their memories had strengthened during their nap. This fits with previous studies. In 2009, John Rudoy found that he could strengthen someone’s memories while they slept, by providing the right triggers, much like the smells that Diekelmann used.

Perhaps the memories become unstable for a much shorter time, rapidly solidifying before they can be altered. Maybe it’s that sleeping people simply don’t encounter new information that could interfere with their reawakened – they are, after all, asleep. Either way, it’s clear that the act of remembering has different consequences depending on whether we’re awake or asleep.

Diekelmann suspects that sleep provides a chance for memories to move from the short-term storage of the hippocampus to more permanent sites elsewhere in the brain. Reactivating the memory sets this process in motion, and dreams could be the result. When people awake, those memories now sit in a protected part of the brain, and they’re shielded against interference by new information coming into the hippocampus. Again, this idea fits with earlier work. For example, in 2007, Stephen Gais showed that when people sleep after learning new information, they form strong connections between the hippocampus and the prefrontal cortex, an area involved in retrieving old memories.

It might seem strange that things as important as our memories could be so regularly vulnerable to change. But while it’s important to store old information, it’s equally important to be flexible. We constantly need to update our information as we encounter new experiences – without this ability, we would never learn. When we run into a new situation, we open up the relevant documents in our minds, add in the new data, and then save it again. As Diekelmann writes, “In this way, ‘successful’ memories are optimized, keeping their potential relevance for future situations, whereas other ‘unsuccessful’ memories become overwritten by new and more useful information.”

References: Diekelmann, S., Büchel, C., Born, J., & Rasch, B. (2011). Labile or stable: opposing consequences for memory when reactivated during waking and sleep Nature Neuroscience DOI: 10.1038/nn.2744

Chen, D., Stern, S., Garcia-Osta, A., Saunier-Rebori, B., Pollonini, G., Bambah-Mukku, D., Blitzer, R., & Alberini, C. (2011). A critical role for IGF-II in memory consolidation and enhancement Nature, 469 (7331), 491-497 DOI: 10.1038/nature09667

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There are 6 Comments. Add Yours.

  1. Rich
    January 27, 2011

    Unfortunately, the link “dreams could be the result” produced a 404 error. It struck me as very odd that the “transfer of memories” during sleep should produce experiences completely unlike the memories. But I doubt anyone would be surprised at the suggestion that “things might be a bit more complicated than that”.

    I’m leery, too, of the idea that memories are “overwritten”. The tendency of the elderly to recall more and more of their early life, recalling things they’ve not thought about in years is well known I think.

    Still I shall use this information: “I wasn’t sleeping, dear, I was consolidating”.

  2. Ed Yong
    January 27, 2011

    Fixed the link error. Re: “overwritten”, perhaps “edited” would be a better analogy. I’m not saying that the memories are completely altered, but they can certainly be tweaked with new information.

  3. bbleeker
    January 27, 2011

    “Whenever we remember something, the fragile window reopens. Again, it’s more like opening a computer document than getting notes out of a drawer. You could easily add, edit or delete information at a flick of a key. Every time we bring back an old memory, we run the risk of changing it.”

    That sounds promising. I have some memories that I’d love to change, or better yet, erase altogether. Are there any techniques for that?

  4. Ewan
    January 27, 2011

    Not to – well, ok, at least in part to – blow my own horn; but the IGF-II data may (possibly; it could also be independent) be a reflection of our work on insulin as a central cognitive modulator (see e.g. http://www.albany.edu/mcnaylab/documents/nlm2010.pdf), given that IGF-II cross-binds to insulin receptors.

    I clearly need a better press office :). [Not only did we show that we could enhance with exogenous insulin, but - more important - direct blockade of endogenous insulin specifically within the hippocampus caused marked impairment, so that insulin signalling appears to be a key component of physiological memory processes. We also showed that systemic insulin resistance causes impaired central insulin responsiveness and cognitive deficits, and that insulin can directly increase hippocampal glucose metabolism; given the long history of data on glucose as a cognitive modulator (e.g. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC16024/) this seems a plausible mechanism by which insulin - and perhaps also IGF-II - act.]

  5. Rich
    January 28, 2011

    Thanks. Perhaps my IT background made me interpret the idea of moving data from one store to another in an overly simplistic way. Certainly the idea of integrating new memories into the existing corpus feels more correct; then there’s a chance of any memory at all surfacing in a dream as links are made to it.

    Was it Martin Gardner who reported a childhood memory of driving with his parents to Devil’s Mountain? He recalled watching it approach as they drove towards it only to discover years later that no such view is possible. Memory can indeed be edited. I wonder if there’s a way to stop it? Would we if we could?

  6. Ben
    January 28, 2011

    I would hate not to be able to edit my memories!
    The monster under my bed would still be terrifying.

    Ooh, but I’d love to use that stuff while running political ads for a campaign!

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