A series of experiments has revealed that study participants who employ a form of mental time travel, where they internally recreate the overall experience surrounding the targeted memory, can improve the recall of details of limited or suppressed memories and potentially restore previously lost memories.
The Regensburg University scientists who performed the experiments believe their work also revealed a lasting benefit for participants, which improved long-term and short-term recall of newly revisualized memories, indicating that the phenomenon was not transient.
In the introduction of the team’s study published in the Proceedings of the National Academy of Sciences (PNAS), Regensburg scientists write, “A key characteristic of human forgetting is that memories’ retrievability declines rapidly soon after encoding, but due to ongoing memory consolidation, the rate of forgetting decreases with the passage of time.”
The team stated that this means older memories “not only show reduced immediate retrievability,” but also display “slowed forgetting” over time compared to younger memories. Combined, the two effects make memories increasingly difficult to retrieve over time.
One often proposed method to improve memory retrieval is called the rejuvenation hypothesis. According to the study authors, this hypothesis involves mental time travel, where one tries to internally revisit or even recreate the conditions under which the memory was first encoded. If successful, they say it reverses the two memory diminishing effects and “enhances the memories’ retrievability immediately after the mental time travel as well as their future rate of forgetting.” This process ends up effectively “creating” a new copy of how the memories were at an earlier point in time, which improves the memory’s recall.
In a statement announcing the team’s study, project leader Karl-Heinz Bäuml and his colleagues devised two experiments to test the rejuvenation hypothesis on short- and long-term recall. In the first test, 1,216 study participants were asked to read a passage of text or a series of unrelated words.
After studying the selected text, the participants were instructed to try recalling it at later time points. However, some participants were instructed to reinforce their original memory of the list by practicing mental time travel to recreate the original conditions at four hours, 24 hours, and seven days after studying the material. The proposed methods included either recalling half of the words from the list or writing down other related components of the encoding experience, such as thoughts, feelings, or emotions, just before, during, and immediately after the study.
An examination of the study participants’ text recall efforts at four hours, 24 hours, and seven days showed two distinct patterns. According to the team, participants who did not practice mental time travel “displayed typical forgetting over time.” This result showed that as the memories aged, their “immediate retrievability” decreased correspondingly. The subject’s “rate of forgetting” also weakened.
When the team studied the text or list recall efforts of participants who practiced mental time travel at the assigned times to reencode their memories, the two memory-diminishing effects were reversed. The team stated that the time-traveling participants were “effectively creating a copy of how the memories were at an earlier point in time,” which improved their future recall.
“Mental time travel increased both the memories’ immediate retrievability and their future rate of forgetting when individuals attempted to reinstate context deliberately and actively and when they retrieved other memories sharing a similar temporal context,” they write.
The authors said such a “Sisyphus-like resurrection” of diminished or lost memories suggests that the form of “context reinstatement” mental time travel can induce “is not a transient” phenomenon.
“Mentally reinstating older memories’ temporal context at encoding is a powerful way to rejuvenate memories,” they explain. “Here, we have shown that, while memories’ immediate retrievability and future forgetting decrease as the memories age, mentally traveling back in time to when the memories were encoded reverses these effects and enhances both the memories’ immediate retrievability and future forgetting.”
The team said their findings also suggest that memories considered lost or minimally accessible were likely still encoded, just difficult for the brain to locate.
“Research shows that apparently forgotten memories may not be erased from memory, but only access to the traces [may] be impaired,” they conclude. “Attempts to mentally reinstate older memories’ temporal context at encoding can resolve such inaccessibility by rejuvenating the memories.”
The article “Reinstating memories’ temporal context at encoding causes Sisyphus-like memory rejuvenation” was published in PNAS.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.