Memory is a tricky thing. Remembering details of past experiences — details like what you had for lunch last Friday, or what you learned in history class — is often an exercise in futility. But then sometimes, for what appears to be no reason at all, you remember every detail of an exceedingly mundane event. Now, researchers think they might have figured out why that is. As it turns out, emotional events, like those that trigger a fear response, can enhance a person's memory of related pieces of information acquired prior to the emotional moment.

The finding, published today in Nature, is important because it means that memories can be selectively enhanced retroactively. And being able to do that, the researchers say, is pretty huge because it could lead to the development of training regimens that help people learn more efficiently, or that help people in the early stages of Alzheimer’s disease enhance past memories of their loved ones.

To study the effect that an emotional event can have on a previously acquired memory, researchers asked 30 adults to look at 60 images of various animals and tools, at random. The participants didn’t know anything about the experiment other than that they needed to look at these images, says Lila Davachi, a memory researcher at New York University and a co-author of the study. Approximately 10 minutes later, the participants were shown different images of animals and tools. During this phase of the experiment, they also received mild electrical shocks that were tied to either the tools or the animals. A person who received shocks when looking at animals, for instance, didn't receive a shock when looking at tools, and vice versa.

The study was designed this way to ensure that the participants would experience emotional learning. In this case, the shock image category triggered an anticipatory fear response, as measured by the amount of sweat on the participants' skin. This other image category didn't. Once the shock phase was over, the participants were asked to spend time looking at a final set of 60 images of tools and animals. Like the first phase of the experiment, no shock were involved for either category. A day later, the researchers tested the participants to see how well they remembered the animals and the tools in all three phases of the experiment.

To the researchers surprise, the participants who were shocked when they saw animals in phase 2 remembered the animals from the first phase — a phase that didn’t include shocks — far better than they remembered the tools in the first phase (the same can be said for people who were shocked when looking at tools).

"At the time that you were encountering those animals," Davachi says, "you didn’t know that they were going to be important later on." So, nothing could have occurred in phase 1 to change the memory, she says. The change therefore happened retroactively.

This finding, Davachi says, is completely new. Scientists were aware that emotional events could enhance past memories, but they didn’t know that the enhancement could be so selective. This enhancement also occurred with the images in the third phase, but this was less surprising given that the emotional event had already taken place.

"This might be a way to build a really smart person," Davachi says. If scientists can find out more about this works, it might one day be possible to use this phenomenon to create a training regimen that could take advantage of the selective nature of memory. The finding might also be applicable to people with early stage Alzheimer’s disease, she says, because doctors might be able to strengthen past memories relating to loved ones by using images of their faces, and a stimulus of some kind.

Of course, using electrical shocks, even mild ones, isn't ideal. Davachi therefore hopes to find out if other stimuli could have the same effect. "We only did shock here, so we don’t know the what extent [the effect] will generalize to positive learning systems, like rewards," she says. Still, Davachi thinks other forms of learning are likely to work as well.

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"It’s a great study. Wonderful!" says Richard Morris, a neuroscientist at the University of Edinburgh who didn’t participate in the experiment. This work, he says, reveals that information that’s encoded incidentally during the day — information that might ordinarily be forgotten — can be remembered for much longer if related emotional learning takes place later in the day. "This work adds a neat conceptual twist to the idea that [emotion] enhances incidental memory: there is an element of selectivity about what gets enhanced."

Daniel Schacter, a memory researcher at Harvard University who also didn't participate in the study, agrees that the experiment is solid. "This study is important because it shows that an emotionally arousing experience can have a very specific enhancing effect on a previous related experience," he says.

The scientists don’t know what's going on in the brain to cause the memory enhancement. Davachi hypothesizes that there might be only moderate activity in the brain during phase 1 — the kind of activity that wouldn’t support long-term memory formation. But when phase 2 comes around, participants might experience a strong activation in the same areas of the brain that were activated in phase 1. That activation might then retroactively enhance the connections that were made in the brain before the shocks took place. But this is just an idea, she says. It’s also important to note that the enhancement didn’t happen right away. When the researchers tested the participants immediately after the three phases, the memory enhancement wasn’t apparent. "It takes six hours" for the enhancement to occur, she says.

The researchers didn’t test the participants to see if the effect persisted beyond the 24-hour period after the experiment, but they hope to do so soon. "We kind of rushed to get this out because it was really novel and exciting," she says. Davachi and her team also want to find out if it’s possible to enhance memories that were acquired 24 hours prior to phase 2, or even a week prior to phase 2. "If phase 1 had occurred 24 hours before, or a week before, and you had already had a chance to consolidate these memories," she says, "could you still enhance them?"

There’s also the issue of similarity. In this experiment, scientists used images of tools and animals, but what if the emotional event is one relating to an assault perpetrated by a light-haired man? Would past memories relating specifically to light-haired men be selectively enhanced? Or would memories relating to all men be enhanced? "You’re attaching meaning to this individual encounter, and it’s really important that you do so," Davachi says. But under those traumatic circumstances, it not might be beneficial to "change your memories of past encounters with other related people."

Before they perform any of these follow-up experiments, the researchers want to use brain imaging to find out what’s really going on.

Davachi thinks the study is important because it might help researchers understand why humans remember mundane details about their everyday lives. "This is a little clue that [those memories are there] to help us bring back representations that you just don’t know in the moment will be important," she says. "I have been studying memory for much longer than I would like to admit… and to me, this is super exciting."