How the brain diminishes certain memories during sleep
- Scientists have long known that the brain replays and consolidates memories during sleep, helping us to store information for the long term.
- But memory replay during sleep can also result in the erosion of certain memories.
- In a recent study, scientists used a technique called targeted memory reactivation to explore how this process occurs in the brain.
One of sleep’s many important functions is memory consolidation. While we sleep, newly formed memories are replayed and transferred into long-term storage. But new research shows that memory replay during sleep can also trigger the forgetting of similar memories.
Memory is associative in nature. It relies heavily on learning relationships between otherwise unrelated objects and events. It is also reconstructive: When we recollect an event, we stitch together fragments of memories rather than recalling the event as a whole. Often, therefore, related memories may “overlap,” with the same or similar fragments being used to compose them.
Some researchers have suggested that replay during slow-wave sleep — the deepest phase of non-rapid eye movement sleep — strengthens the associations between overlapping memories. But a 2017 study showed that it can either strengthen or weaken overlapping memories, depending on how strongly those memories were initially encoded.
To investigate further, Bárður H. Joensen of York University and his colleagues used a technique called targeted memory reactivation to induce recall of specific new memories in study participants while they slept.
Targeted memory reactivation
The researchers had 30 participants learn a series of 60 word triplets, each consisting of objects, locations, and celebrities. The triplets were actually presented in pairs that included overlapping items, with each presented alongside a spoken word specific to each word triplet. For example, “David Beckham-bicycle” was paired with “castle-bicycle,” and participants heard the word “bicycle” while being presented with this triplet.
Importantly, pairs from the same triplet were not presented in order. Rather, pairs from other triplets were presented in between. This allowed the researchers to manipulate the strength of the association between each pair. Those presented in relatively close order formed a stronger association than those presented further apart.
Afterward, the researchers tested participants’ memories of some of the word pairs. They induced recall of overlapping pairs while the participants slept by playing back words that had been presented with the triplet. In the morning, they tested the participants again.
They found that overnight targeted memory reactivation both increased and decreased the participants’ retention of overlapping word pairs, depending on the order in which they were presented. They were better able to remember the first of each pair, but their memory of the second decreased.
This was only the case for overlapping word pairs that were not tested before the participants slept, however. The test performed before sleep consolidated the memory of the tested pairs, and so these were retained.
Thus, targeted memory reactivation increased memory for some word pairs, but also induced forgetting of overlapping pairs that had been presented afterwards.
Previous work has shown that memory retrieval during wakefulness induces forgetting of related memories by inhibiting prefrontal cortex activity, but this is the first demonstration of a similar mechanism occurring during sleep.
The study authors suggest this may be a mechanism that reduces interference by weaker related memories, so that irrelevant information does not oversaturate the memory system.
