How do we store new memories while we sleep?

Despite being a universal and essential aspect of life for all animals, sleep remains one of the most enigmatic aspects of human biology and behavior.

Some animals can spend up to 20 hours sleeping every day, during which they are vulnerable to predators and lose time for feeding or reproduction.

So why do we need to sleep?

Besides allowing the brain to clean itself, sleep has also been proven to give time for the brain to organize memories.

However, the role of sleep in memory formation and consolidation is a very complex process, and there is still much that we do not understand.

Despite this complexity, the Pixar movie 'Inside Out' from 2015 managed to accurately describe complex brain mechanisms, such as how our emotions work and how we form and store new memories.

The filmmakers did this with the help of two professors in psychology, Paul Ekman and Dacher Keltner from the University of California. So, what is it that the movie does so well?

When fictions mimics reality

‘Inside Out’ tells the story of an 11-year-old girl named Riley who is struggling to adjust to a new life in San Francisco after moving with her family from Minnesota.

A large part of the movie takes place inside Riley's head, where her emotions - Joy, Sadness, Anger, Fear, and Disgust (each their own character) - control her actions and reactions. The emotions live in Headquarters, the control center of Riley's mind known to us in reality as the hippocampus.

The hippocampus is the area of the brain responsible for learning and memory. We know this because people missing a hippocampus cannot form new memories.

During the day, as Riley learns new things, new memories come to the headquarters represented as memory spheres.

At night, when Riley goes to sleep, all of her memories are transported from headquarters to long-term storage that looks sort of like a library. Here, they get consolidated into long-term memories just as it happens in reality.

The role of dreams in memory

In ‘Inside Out,’ Riley’s brain has its very own dream production studio.

It is a production studio in the classical Hollywood sense, with a director, screenwriters, and makeup artists.

The studio produces Riley’s dreams with the events that Riley experienced during the day.

And this is actually a quite accurate representation of what happens in real life.

Several studies have managed to demonstrate that during sleep, memories are replayed in the hippocampus much faster than we experienced them in reality. By doing this, the hippocampus interacts with the long-term memory and thus consolidates our new memories.

Why do we forget some things and randomly remember other?

The new memories arriving at headquarters during the day shine with a bright glow. But some of the memories in Riley’s long-term memory have lost their glow. These are the memories that are no longer relevant to her.

Our current knowledge suggests that we keep memories that have emotional value or contain important information that we need to remember.

Conversely, we have a tendency to forget less important memories like a telephone number.

But this does not mean that memories that have lost their glow are gone forever.

When we walk down the street, we can sometimes experience a stimulus such as a smell or hearing an old song from our childhood. This can bring an old, otherwise forgotten memory clearly back in our minds. This is what we know as memory recall.

In ‘Inside Out’, this is elegantly represented by a train that carries old memories from the long-term storage (the cortex) to the headquarters (hippocampus), where memories are replayed when Riley needs to remember them.

While this is a simplified version of the process, the basic idea is quite accurately represented by ‘Inside Out’.

Where is reality different from fiction?

Even though ‘Inside Out’ does a good job at explaining the complexity of our minds, there are some areas where it could have been more precise given our current knowledge.

For example, the use of memory spheres is great as a simplistic view of memories, but it can make memories look like nice and clean spheres.

In reality, memories are not that simple, and they can simultaneously activate different neurons in several brain areas, making these connections quite messy.

Another example is that in the movie, memory spheres were replayed over and over without consequences.

While it was, for a long time, the view that memories in long-term storage were permanent, studies from before the movie's release have shown that this is not the case.

When a memory is recalled, it actually returns to an unstable state, where it can be altered or even deleted entirely. This is a process known as memory re-consolidation.

Finally, the dream production studio only uses events that Riley experienced during the day. But in our dreams, we can also recall events that happened weeks, months, or even years ago.

The movie could have shown this by having the dream production studio replaying some of the memories from the shelves of the library.

After all, 'Inside Out’ has done a pretty good job of explaining how memories are stored in our brains.

But the reality of memory consolidation is much more complex than what a movie can show.

There are many different brain regions involved in the process, and the mechanisms by which sleep facilitates memory consolidation are still something we are trying to understand fully.

Further research is needed to better understand the role of sleep in the process of generating and storing new memories in our brains. And it is precisely some of these questions that I try to answer in my own research.

This article was originally published on our Danish sister site Forskerzonen.

References:

• Macarena Gomez de Salazar's profile (ResearchGate)
• 'The Legacy of Patient H.M. for Neuroscience', Neuron (2009), DOI: 10.1016/j.neuron.2008.12.023
• 'Communication between neocortex and hippocampus during sleep in rodents', Proc Natl Acad Sci U S A (2003), DOI: 10.1073/pnas.0437938100

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