Half-forgotten memories can be resurrected using “mental time travel,” a new study suggests.
The research, published Monday (July 28) in the journal PNAS, showed that a person can rejuvenate their fading memories by recalling the emotions and thoughts they had when they first stored that memory. In fact, the researchers found that the refurbished memories were then almost as retrievable as newly formed memories.
The study specifically focused on memories of learned information, as opposed to memories of events, for instance. When we learn something, that new memory teeters on a forgetting curve, like a boulder perched atop a tall mountain. As that boulder rolls downhill, we lose some details of the memory. But as it approaches the base of the memory mountain, where the incline is less steep, the rate of forgetting slows down.
There are processes that “make the memories more and more stable and less sensitive to any type of forgetting processes,” said study co-author Karl-Heinz Bäuml, a psychologist at the University of Regensburg in Germany. Some details remain etched into your memory, while others fade with time. But this forgetting might not be inevitable, Bäuml argued.
“You can reduce this type of forgetting if you mentally travel back in time to the context of encoding,” meaning when you made the memory. In the new study, Bäuml and colleagues explored how this mental time travel affected memory retrieval.
The team recruited over 1,200 volunteers. Half were tasked with studying a short passage, while the other half studied lists of unrelated nouns. Each group was then split into four subgroups, which were asked to remember the material in different ways.
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One group, which served as a point of comparison, was asked to recall the information they had just learned several times over the next hour, without performing any extra steps. The three other groups had a gap of four hours, 24 hours or seven days between learning the material and having their memories tested.
Upon being tested, these three groups were asked to mentally time travel, either by recalling the thoughts and feelings they had during their first session in the lab or by looking at a subset of the information they’d learned, as a kind of primer to remember the rest. The comparison group was also retested at these later time points, and their recall, sans time travel, was used as a benchmark.
Both types of mental time travel helped restore the participants’ recollections, rolling their memories up the mountain to some degree. At the four-hour and 24-hour marks, these tricks improved recall by “reactivating” the memories. Remembering emotions from the earlier encoding restored about 70% of the targeted memories after four hours and 59% after 24 hours, while selective priming restored about 84% and 68% of the target memories at these time points.
However, after a week, the effect of mental time travel had waned. Remembering emotions didn’t restore any memories, while priming restored only 31% of the target memories.
Deniz Vatansever, a cognitive neuroscientist at Fudan University in China who was not involved in the study, said the new work refines our understanding of memory. “Memory is not just linearly decaying, but actually we’re able to reset it almost into its original form,” he said. However, he said the real test would be to see how these findings generalize to life outside the lab.
“Autobiographical memories or other experiences that we have in daily life — they’re rich in emotional content; they’re rich in sensory modalities,” he noted. By comparison, memories of short passages and word lists lack these features.
Bäuml agreed that the degree of memory rejuvenation will vary with factors not explored in the current study, such as the richness of the experiences being remembered. But for now, he said the evidence suggests that, if you’re aiming to ace an exam, it might be best to schedule revision sessions with only short intervals in between.
“The best way would be to distribute your mental reinstatement a little bit and do it not only after seven days, but do it after three days, six days and so forth,” he said. This would “create recurring rejuvenation cycles, which keep the memories all in all at a higher level,” he suggested.
While this study found that single instances of mental time travel might nudge memories back up the mountain, other research has suggested that repeated practice might make it harder for the memory to roll down in the first place, said Justin Hulbert, a neuroscientist at Bates College who was not involved in the study.
This might mean that memories need to be refreshed regularly at first — after an hour or two — but that later refreshes could wait longer, maybe months or years, Hulbert said. “That might mean that you have to push the boulder up the mountain fewer times to still preserve that memory over long periods,” he said.