Ghosts of 'forgotten' memories may linger in the brain – and could even be recalled, study shows

Some forgotten memories may look less like a deleted computer file and more like a dusty book buried deep among the shelves of an ancient library.

They are no longer easy to find. But, under the right circumstances, they could be summoned again.

So, what is forgotten may not be lost – at least when it comes to fruit flies (Drosophila melanogaster).

Scientists have discovered that even after a memory can no longer be recalled, it can persist in a latent state in the brain of a fruit fly, ready to be revived by an appropriate reminder, reports Science Alert.

But there's a catch: the process of retrieving a forgotten memory can also lead to the creation of false memories, according to research by a team led by neuroscientists Wenbin Yang, Benedetta Zatera, and Miguel Pavao-Delgado from the Friedrich Miescher Institute for Biomedical Research in Switzerland.

"We found in Drosophila that forgotten memories can persist as silent memory traces in neural circuits that lack the ability to direct behavior," they state in a paper published in the journal Nature Neuroscience.

"We conclude that switching between silent and active memory states prevents information loss and establishes a conditional memory system that allows the brain to flexibly prioritize memory recall based on recent experiences."

Memories are notorious for how unreliable and elusive they can be. Sometimes we can recall moments from decades ago in minute detail, while other times we find ourselves in the middle of a room trying to remember why we even entered it seconds earlier.

Neuroscientists have found that while some memories can be permanently erased, others can remain present in an inaccessible state and become accessible again with the help of certain reminders.

However, these very reminders can also distort memory, especially, for example, during suggestive questioning. At the same time, very little is known about the brain processes behind the reconstruction of forgotten memories, regardless of whether the result is a real or false memory.

To investigate this, scientists turned to one of the favorite model organisms in neuroscience – the fruit fly.

Flies can be trained to associate a particular odor with a mild electric shock, causing them to avoid that odor. The team found that such learned aversion disappeared after about 24 hours, indicating that the memory had been forgotten.

But then something unexpected happened.

When the researchers returned the flies to the chamber where they had previously been exposed to the aversion training, the memory returned and the flies began to avoid that odor again.

However, the smell alone was not enough. The reminder had to replicate key parts of the original experience – including the texture of the chamber and the lighting – for the forgotten memory to resurface.

The results indicate that contextual cues, such as texture and lighting, are important for recall, suggesting that memory is not simply reproduced, but is actually reconstructed.

To determine what the brain is actually doing while all this is happening, the researchers recorded activity in the memory centers in the flies' brains.

They found that as the flies appeared to forget, the memory trace that triggered the avoidance behavior gradually weakened. At the same time, a second, hidden trace became apparent in another group of neurons that normally do not influence behavior.

Researchers call this the "silent memory trail."

When all the signals serving as reminders coincided, activity would shift back to the neurons involved in avoidance behavior, allowing the memory to once again begin to influence the flies' behavior.

Researchers believe that the brain does not move a forgotten memory from one group of neurons to another, but rather creates two versions of the memory from the very beginning – one that immediately guides behavior and another that remains hidden.

As the first version fades, the hidden version solidifies, allowing the memory to be recalled later under appropriate circumstances.

Scientists also hypothesize that brain activity that makes it difficult to access memories may also help create this hidden backup copy, although this hypothesis has yet to be tested.

It has also been shown that the process of remembering can be imperfect.

During training, the flies were also exposed to another odor that had never been associated with electric shocks.

When the researchers later used this harmless odor as a reminder instead of the dangerous one, the flies began to avoid it as well, as if it had been announcing the electric shock from the start. The flies had, in effect, created a false memory.

Interestingly, the brain used different neural pathways to reconstruct real memories and to create false ones.

This suggests the possibility that the brain possesses separate mechanisms for faithfully retrieving memories and for accidentally reworking them.

The researchers note that the formation of false memories only occurred in the case of odors used in the context of training, suggesting that false memories are the result of inaccurate reconstruction of silent memory traces.

The authors of the study argue that false memories, rather than representing a system flaw, may be part of a mechanism that in other circumstances could encourage flexible adaptation based on learned experiences.

The research was conducted on fruit flies, not humans, so we can't conclude that our brains behave in exactly the same way. However, since the existence of false memories in humans is a well-known phenomenon, this research could give us some insight into how our brains harbor the ghosts of past memories.