Some memories get locked into our brains, persisting for decades. Others slip out by the next day. Researchers have found that the process of recalling a memory actually contributes to this process. When a recent event is recalled, it's possible to consolidate its memory, update it, or even eliminate it entirely. All of that happens when we're conscious. Memories can also get replayed while we sleep, which is thought to influence their long-term storage. A study released over the weekend suggests that recall during sleep has a completely different effect from conscious recall.

The paper that describes the new results includes an extensive discussion of some of the long history of memory studies. It appears that consolidation and destabilization of memories are two sides of the same coin. When a memory is recalled, it appears to be have a period in which it's unstable and can potentially be eliminated. If the memory is left alone or reinforced, storing it again will help consolidate it. If, however, something (such as a researcher) interferes with the re-storing of the memory, then it can be lost entirely.

So, the recall that occurs during sleep could do anything from reinforcing the memory to eliminating it entirely. Unfortunately, figuring out what actually happens is challenging. The procedures involved in monitoring neural activity include implanting electrodes directly into the brain, and interfering with memory storage requires potentially toxic chemicals. Plus, there's no obvious way to ensure a memory is replayed during sleep. Trying to translate these experiments to humans seems like a nonstarter.

That's why the experiments in the new paper seem pretty clever. The authors designed a memory test that could be manipulated without the use of chemicals. It involves the use of a standard 2-D grid of images, from which users had to identify duplicates. This a game many of us have probably played on computers; the images remain hidden, and you have to remember where you previously saw its match. If the images are kept in constant locations, re-exposure to the grid should help strengthen memories. If, however, you shift images around, you can interfere with previously established memories, no chemicals required.

Another bit of cleverness came in researchers' method of ensuring that the memories get evoked, even when their test subject was asleep: during the learning section of the task, they associated the process with an olfactory cue, which they describe as an "unfamiliar, slightly negative smell." When exposed to the same smell in the future, subjects were likely to recall the memories of the task with which it was associated, even if they were asleep. That, in turn, should destabilize the memories, leaving them vulnerable to interference using a test with the images moved around.

They explicitly state that they expected that exposure to the smell would be equally effective regardless of whether the subject was awake or asleep: "We hypothesized that memory reactivations during SWS [short wave sleep], similarly to reactivations during wakefulness, lead to an immediate transient destabilization of memory traces." But that hypothesis turned out to be completely wrong. (Who says you can't publish negative results?)

The researchers started by training test subjects with a grid, while exposing them to that slightly negative smell. After a break, one group was allowed to go to sleep for 40 minutes, a period too short for REM sleep to set in. The rest were kept awake. During this period, the subjects were re-exposed to the scent in order to evoke the memory of the image locations, possibly making them unstable; a control group did not get this second exposure. Then, they were given a grid with the images moved about to interfere with the previous memories. Finally, a test with the initial image locations was repeated.

If the researchers were right, this procedure should re-evoke the memories of the original grid and scramble them, decreasing performance on the final test relative to those who didn't get exposed to the scent a second time. And, among those who were awake, that's precisely what was observed. The control group remembered about 60 percent of the image locations, while those re-exposed to the smell could only manage about 40 percent after having had their memories interfered with.

In contrast, sleep had the exact opposite effect. The control group that slept performed about the same as those who hadn't, remembering 61 percent of the image locations. But those who were exposed to the smell during their short period of sleep managed to get locations right a full 85 percent of the time. In short, re-evoking the memory during sleep consolidated it, even if the sleep was only a short, 40-minute nap. The researchers even got some subjects to sleep in an MRI tube, and showed that the brain activity triggered by re-exposure to the smell was very different from that seen in subjects that were awake.

The stabilization of memories did come at a cost, however. The group that was kept awake may not have been as good at remembering the original locations of the images, but they did better at remembering the new ones presented in the interference portion of the procedure.

It's easy to see this as a practical adaptation: update your memories through the day as conditions change and, if anything's stable at the end of the day, try to lock it into place. But the authors suggest it may simply be a consequence of the difference between sleep and wakefulness. "Because external input is considerably reduced during sleep as compared with the waking state," they write, "memories could enter a labile phase with a distinctly reduced risk of encountering interfering information."

They also point out that consolidation isn't a simple, one-step process, but can take place over days and possibly longer periods of time. So, this brief experiment can't be viewed as a definitive description of the full process of consolidating memories. However, the differences between the two types of recall—sleeping and awake—that the authors have identified may help make sense out of the previous results in the field.

Nature Neuroscience, 2011. DOI: 10.1038/nn.2744 (About DOIs).