Stepping out of a building and onto the street... for the brain, it's a whole new chapter! This change of scenery causes a radical shift in brain activity, measurable by scientists. But the segmentation of memories is far from passive; it doesn't depend solely on the places you wander. A new American study shows that past experiences and individual priorities influence the division of these "chapters."
"Our results show that we have control over how we organize memories of lived experiences," analyzes Christopher Baldassano, co-author of the study, during an interview for Science and FutureThe study, whose lead author is Alexandra De Soares, was published in the journal Current Biology.
The length of the chapters established by the brain varies
Since 2017, scientists have been able to observe these chapter changes in real time in an individual's brain. A previous study by Christopher Baldassano revealed that this fragmentation was detected throughout the brain, at multiple levels. The length of these chapters varies considerably depending on the brain area studied. It lasts from a few milliseconds in parts of the brain closely linked to sensory information to hundreds of seconds in parts of the brain where higher-order thinking takes place.
Christopher Baldassano's team then made a new discovery: these interconnections are the basis of long-term memory. Indeed, with each new boundary, the hippocampus, a brain area closely linked to memory, shows a spike in activity.
But one mystery remained: what causes the brain to form a boundary around an event it encounters? Researchers are shedding light on the triggers of these chapters in a new study. Changes of location, such as entering a cafe, for example, are not the only elements that initiate a new episode in our brain. The social aspect also comes into play. reveals Pierre Gagnepain, an external researcher at the study, for Science and Future.
Read also“Faced with trauma, being more resilient can (also) be learned!”
The seahorse is a library of memories
Study participants listened to various audio narratives lasting a few minutes each featuring two people in a given setting: a restaurant, an airport, a grocery store, or a conference room. Each audio story dealt with a different social situation: a breakup, a marriage proposal, a business deal, and a romantic encounter. While listening, the researchers examined MRI data of the participants' brains to detect changes in brain activity marking the beginning of new chapters.
In particular, they observed a brain area that plays a complex role in memory retrieval: the medial prefrontal cortex. It is involved in the strategic creation and retrieval of memories that are most relevant and useful in a particular context,” Christopher Baldassano. But how does memory work?
In short, memories of particular past events (episodic memory) are mainly stored in another region of the brain, the hippocampus, which is like a library. In the cartoon Inside Out, it is symbolized by large shelves on which memories are carefully arranged. The medial prefrontal cortex, on the other hand, is more like a librarian, working to ensure that memories are organized in a relevant way and can help you find a specific memory you are looking for,” adds the researcher.
Visualize memory segmentation
Using a mathematical tool developed in 2017 by Christopher Baldassano, scientists can identify sudden changes in brain activity in different regions, indicating when one segment ends and another begins. An event causes a brain pattern in different regions of the brain. It corresponds to the fact that in this brain area, there are more or less active zones," says Pierre Gagnepain.
These nuances are visible thanks to 3D pixels, called voxels. These patterns are quite stable during a given event and when the experience takes a different turn, the distribution of activity is modified, which results in a new pattern,” he adds. As soon as the brain begins a new chapter, the distribution of voxels is modified and therefore visible on MRI.
For example, in their first study, they were able to observe the angular gyrus, another region notably linked to language. In this video, the participants' brain activity was recorded while they watched a series. The recording speed is 35 times faster than the actual speed. On the right, the moving dot, representing brain activity, moves rapidly among nine previously defined "referenced" states of the angular gyrus. Dots of the same color belong to the same "chapter."
In their new study, they analyzed the activity of the medial prefrontal cortex in the same way. While listening to the audio stories, its modulations indicated to us the opening of a new chapter in the brain,” simplifies Christopher Baldassano.
“The brain actually actively organizes our life experiences into chunks that make sense to us.”
Their results prove that how the brain divides a memory depends on what the person is paying attention to and the perspective. When we listen to a story about a marriage proposal in a restaurant, for example, our prefrontal cortex typically organizes the story into events related to the proposal, leading (hopefully) to the final "yes." trace the authors.
But that's not their only discovery. It's possible to influence this division... By asking participants to focus on the couple's ordering of dishes at the restaurant, for example, completely different new chapters were formed, following the steps of the order. And this worked for the other scenarios as well: " A person who heard a story about a breakup at an airport might, if asked to pay attention to the details of the airport experience, record new chapters as they went through security and arrived at their gate." indicate the researchers. The brain actually actively organizes our life experiences into chunks that make sense to us.” But what is the purpose of this segmentation?
Read alsoWe know if our memories are trustworthy, study reveals
Why does the brain segment memories?
These chapters serve as access points to memory, much like the button that skips a chapter on a DVD. Information can be found in memory by first locating the correct chapter to access detailed memories. There is a kind of hierarchy of elements of memory, like a scaffolding, which allows access to increasingly precise details," explains Pierre Gagnepain. Organizing memories into meaningful chapters also helps avoid interference because it ensures that information from unrelated events is not mixed together.
“ One of the main findings of this study is that the brain does not just passively record world events like a VCR. How experiences are organized in memory depends on our attention, goals, and knowledge.Christopher Baldassano concluded. With his team, he is now working on the effects of a gap between expectations and actual experience, " for example, if the waiter throws a glass of water in a customer's face. What is the impact of this shift on the organization of these experiences in memory? he opens.
The brain is constantly making predictions about what's going to happen. It relies on classic patterns, so to speak. For example, when you walk into a restaurant, you expect the waiter to approach the table and hand you a menu. These predictions have a very important function: they help reduce surprise. The risk of error is lower, and the operation is efficient." Pierre Gagnepain emphasizes. They play an important role not only in understanding what happens to us but also in forming and retrieving memories. The influence of this division of memories on the effectiveness of memory is another aspect of the researchers' upcoming work.
English 
French