Science Gazette

Researchers have figured out how the human brain divides, stores, and recalls memories

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The brain cells that define the borders between distinct events have been identified in a study.

Two kinds of cells in human brains have been discovered as being important in arranging separate memories depending on when they happened, according to researchers. This discovery advances our knowledge of how the human brain develops memories and may have consequences for memory diseases like Alzheimer’s disease. The research was published in Nature Neuroscience with funding from the National Institutes of Health’s Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative.

“This discovery has changed the way researchers look at how the human brain works,” said Jim Gnadt, Ph.D., program director at the National Institute of Neurological Disorders and Stroke and the NIH BRAIN Initiative. “By recording directly from neurons that generate ideas, it brings to human neuroscience a method that has previously been applied in nonhuman primates and rodents.”

This research began with a deceptively basic question: how does our brain develop and organize memories? It was directed by Ueli Rutishauser, Ph.D., professor of neurosurgery, neurology, and biomedical sciences at Cedars-Sinai Medical Center in Los Angeles. We live our waking lives as a one continuous experience, yet human behavior studies suggest that we remember life experiences as separate, different moments. What distinguishes a memory’s beginning and end? “Event segmentation” is the name given to this hypothesis, yet we know very little about how it works in the human brain.

Rutishauser and his colleagues investigated this by working with 20 patients who were having intracranial monitoring of brain activity to guide surgery for drug-resistant epilepsy therapy. They studied how different types of “cognitive boundaries” — transitions thought to trigger changes in how a memory is stored and that mark the beginning and end of memory “files” in the brain — affected the patients’ brain activity when they were shown film clips containing different types of “cognitive boundaries.”

The first, known as a “soft border,” is a video that contains a scene before cutting to another scene that continues the same tale. Consider a baseball game in which a pitch is delivered and the camera pans to a fielder making a play after the batter smacks the ball. A “hard boundary,” on the other hand, is a transition to a whole other scenario — picture the hit ball being followed by a commercial.

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The crucial difference between the two borders was revealed by Jie Zheng, Ph.D., a postdoctoral fellow at Children’s Hospital Boston and the study’s lead author.

“Is this a fresh scene in the same narrative, or are we viewing something entirely different? The sort of cognitive limit is determined by how much the narrative varies from one clip to the next “Zheng said.

The researchers monitored the participants’ brain activity while they viewed the films and discovered two unique groups of cells that increased their activity in response to various sorts of boundaries. In reaction to a soft or hard border, a set of cells known as “boundary cells” became more active. A second type of cells, dubbed “event cells,” only reacted to strict borders. This led to the hypothesis that the formation of a new memory occurs when the activity of both border and event cells peaks, which can only happen after a hard boundary.

The way images are saved and accessible on your phone or computer is one analogue for how memories could be kept and retrieved in the brain. Photos are often automatically categorized into events based on when and where they were shot, and then presented to you as a significant picture from that event. You may dive deeper into that exact event by tapping or clicking on that picture.

“A boundary reaction is similar to establishing a new picture event,” Dr. Rutishauser said. “It’s as though additional photographs are being added to the event as you develop the recollection. When a hard boundary is reached, the previous event ends and a new one starts. Soft borders may be compared to the new pictures that emerge from a single occurrence.”

The researchers next looked into memory retrieval and how it connects to border and event cell firing. They hypothesized that the brain used border peaks as markers for “skimming” through old memories, similar to how important photographs are used to identify events. When the brain recognizes a familiar firing pattern, it “opens” the event.

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Two separate memory tests were utilized to investigate this notion. The participants in the first experiment were given a series of still photographs and asked whether they were from a scene in the film clips they had just viewed. Images that happened just after a hard or soft border, when a new “picture” or “event” would have been formed, were more likely to be remembered by study participants.

In the second test, individuals were shown pairings of pictures taken from film segments they had just seen. After then, the participants were asked to choose which of the two pictures appeared first. It showed discovered that picking the proper picture was significantly more difficult when the two images appeared on opposite sides of a hard border, probably because they were put in distinct “events.”

These discoveries provide light on how the human brain constructs, stores, and retrieves memories. These findings might be used to create novel medicines since event segmentation is a process that can be disrupted in persons with memory impairments.

Dr. Rutishauser and his colleagues aim to investigate two potential pathways for developing medicines based on their results in the future. First, neurons that employ the chemical dopamine, which are well recognized for their function in reward systems, may be stimulated by border and event cells, indicating a potential target for memory reinforcement.

Second, the theta rhythm, one of the brain’s typical intrinsic rhythms, has been linked to learning and memory. Participants had a better difficulty recalling the sequence of the pictures they were presented if event cells fired in sync with that rhythm. Because theta rhythms are affected by deep brain stimulation, it might be another option for treating people with memory problems.

Through the NIH BRAIN Initiative’s Research on Humans initiative, a multi-institutional cooperation made this experiment feasible. Cedars-Sinai Medical Center, Children’s Hospital Boston (site PI Gabriel Kreiman, Ph.D.), and Toronto Western Hospital were all participating in this research (site PI Taufik Valiante, M.D., Ph.D.). The NIH BRAIN Initiative (NS103792, NS117839), the National Science Foundation, and Brain Canada supported the research.

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