The more we recall a memory, the more the detail of that memory fades away

Most people intuitively understand that a memory fades over time.

But why do some parts remain, while other parts disappear? A new study published today in Nature Communications by a team of researchers at the Universities of Glasgow and Birmingham provides some answers.

They found that our memories become less vibrant and detailed over time, and that only the gist remains, which is perhaps a bit obvious.

But less obvious is the finding that the more frequently we recall our recent experiences, the greater the effect of this “gistification” process.

In other words, we contribute to the loss of detail in our own memories every time we think about them.

Every memory fades eventually, but some sooner than others

While memories are ever exact carbon of the past, experts have long suggested that the contents of a memory change each time we bring it back to mind.

But exactly how our memories differ from the original experiences, and how they are transformed over time, has until now proven difficult to measure in laboratory settings.

For this study, the researchers developed a simple computerized task that measures how fast people can recover certain characteristics of visual memories when prompted to do so.

The study consisted of 72 participants, mostly recruited from local universities, with an average age of 20.

The participants learned word-image pairs, and were later required to recollect different elements of the image when cued with the word.

For example, participants were asked to indicate, as fast as possible, if the image was in color or black and white (a perceptual detail), or whether it showed an animate or inanimate object (a semantic element).

These tests took place immediately after learning, and also after a two-day delay.

Reaction time patterns showed that participants were faster to recollect meaningful, semantic elements than surface, perceptual ones.

Our natural preference for gist: we focus on the meaningful part of a story

Many memory theories assume that over time, and as people re-tell their stories, they tend to forget the surface details but retain the meaningful, semantic content of an event, said lead author Julia Lifanov, of the University of Birmingham.

Imagine reminiscing about a dinner with a friend, she said. “You realize that you cannot recall the table décor, but know exactly what you ordered; or you remember the conversation with the bartender, but not the color of his shirt.” Memory researchers call this phenomenon “semanticization.”

This tendency to recall the meaningful, semantic elements that the researchers demonstrate in this study indicates that memories are biased towards meaningful content in the first place, said Co-author Maria Wimber, of the University of Glasgow. “We have shown in previous studies that this bias is clearly reflected in brain signals too,” she said.

And there is good reason for that selectivity. “Our memories change with time and use and that is a good and adaptive thing,” Wimber said. “We want our memories to retain the information that is most likely to be useful in the future, when we encounter similar situations.”

A memory fades more, and more quickly, with repeated remembering

Interestingly, the researchers found that this bias towards semantic (i.e. meaning) memory content becomes significantly stronger with the passage of time, and with repeated remembering.

For example, when participants came back to the lab two days later, they were much slower at answering the detailed, “perceptual” questions, but they showed well-preserved memories for the semantic content of the images.

That effect was much weaker in participants who saw the initial image multiple times, rather than having to effortfully recall it multiple times.

This work could have implications in a number of areas, such as post-traumatic stress disorder, where patients often suffer from intrusive, traumatic memories, and tend to over-generalize these experiences to novel situations

The findings are also highly relevant for understanding how eyewitness memories may be biased by frequent interviews and repeatedly recalling the same event.

And last but not least, these results also demonstrate that testing yourself prior to an exam (for example, by using flashcards) will make the meaningful information stick for longer, especially when followed by periods of rest and sleep.

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Study: “Feature-specific reaction times reveal a semanticisation of memories over time and with repeated remembering“
Authors: Julia Lifanov, Juan Linde-Domingo and Maria Wimber
Published in: Nature Communications
Publication date: May 26, 2021
DOI: https://doi.org/10.1038/s41467-021-23288-5
Photo: by Lisa from Pexels