Understanding the brain’s process of dividing experiences into events

Columbia UniversityOct 3 2024

The moment a person steps off the street and into a restaurant-;to take just one example-;the brain mentally starts a new "chapter" of the day, a change that causes a big shift in brain activity. Shifts like this happen all day long, as people encounter new environments, like going out for lunch, attending their kid's soccer game, or settling in for a night of watching TV.

But what determines how the brain divides the day into individual events that we can understand and remember separately? That's what a new paper in the journal Current Biology aimed to find out. The research team, led by Christopher Baldassano, an associate professor of Psychology, and Alexandra De Soares, then a member of his lab, turned up interesting results.

The researchers wanted to better understand what prompts the brain to form a boundary around the events we encounter, effectively registering it as a new "chapter" in the day. One possibility is that new chapters are entirely caused by big changes in a person's surroundings, like how walking into a restaurant takes them from outdoors to indoors. Another possibility, however, is that the new chapters are prompted by internal scripts that our brain writes based on past experience, and that even big environmental changes might be ignored by our brain if they are not related to our current priorities and goals.

To test their hypothesis, researchers developed a set of 16 audio narratives, each about three to four minutes long. Each narrative took place in one of four locations (a restaurant, an airport, a grocery store, and a lecture hall) and dealt with one of four social situations (a breakup, a proposal, a business deal, and a meet cute).

The researchers found that the way the brain divides up an experience into individual events depends on what a person currently cares about and is paying attention to. When listening to a story about a marriage proposal at a restaurant, for example, subjects' prefrontal cortex would usually be organizing the story into events related to the proposal, leading up (hopefully) to the final "yes."

But the researchers found that they could force the prefrontal cortex to organize the story in a different way if they instead asked study participants to focus on the events related to the dinner orders of the couple. For study participants who were told to focus on these details, moments like ordering dishes became critical new chapters in the story.

"We wanted to challenge the theory that the sudden shifts in brain activity when we start a new chapter of our day are only being caused by sudden shifts in the world-;that the brain isn't really 'doing' anything interesting when it creates new chapters, it's just responding passively to a change in sensory inputs," Baldassano said.

Our research found that isn't the case: The brain is, in fact, actively organizing our life experiences into chunks that are meaningful to us."

Christopher Baldassano, Associate Professor, Psychology, Columbia University

The researchers measured where the brain created new chapters both by looking at MRI scans of the brain to identify fresh brain activity, and, in a separate group of participants, by asking them to press a button to indicate when they thought a new part of the story had begun.

They found that the brain divided stories into separate chapters depending on the perspective they were told to be attuned to-;and it didn't just apply to the proposal-in-a-restaurant scenario: A person hearing a story about a breakup in an airport could, if prompted to pay attention to details of the airport experience, register new chapters as they went through security and arrived at their gate. Meanwhile, a person who heard a story about a person closing a business deal while grocery shopping could be prompted to register either the new steps of the business deal as new chapters, or to be attuned primarily to the phases of grocery shopping instead. The details that the study participants were prompted to pay attention to influenced what their brain perceived as a new chapter in the story.

Moving forward, the researchers hope to investigate the impact that expectations have on long-term memory. As part of this study, the researchers also asked each participant to tell them everything they remembered about each story. They are still in the process of analyzing the data to understand how the perspective they were asked to adopt while listening to the story changes the way they remember it. More broadly, this study is part of an ongoing effort in the field to build a comprehensive theory about how real-life experiences are divided up into event memories. The results indicate that prior knowledge and expectations are a key ingredient in how this cognitive system works.

Baldassano described the work as a passion project. "Tracking activity patterns in the brain over time is a big challenge that requires using complex analysis tools," he said: "Using meaningful stories and mathematical models to discover something new about cognition is exactly the kind of unconventional research in my lab that I am most proud of and excited about."