A team of researchers at Cedars-Sinai Medical Center in Los Angeles has identified two types of neurons that seem to be involved in the formation of memories. Those neurons are responsible for building boundaries that mark the end of one memory chunk and the beginning of the next memory.
By an unusual method
Most memory studies rely on the standard technique of showing research participants an object to remember, like a photo, for example, and then asking them to recall it later. That experimental setup imposes limits on when a memory starts and stops. “These events are predefined by the experimenter,” says Ueli Rutishauser, co-author of the paper published in the journal Nature Neuroscience.
Instead, they decided to try an unusual method starting with the question, how does our brain form and organize memories? What marks the beginning and end of a memory? It’s a theory known as “event segmentation” and the truth is that we know relatively little about how this process works in the human brain.
In their experiment, they involved 20 patients who were undergoing intracranial recording of brain activity to guide surgery to treat their epilepsy. These epilepsy patients had electrodes surgically implanted in their brains to locate where the seizures were occurring.
Hard and soft limits
They looked at how patients’ brain activity was affected when they were shown movie clips containing different kinds of “cognitive boundaries,” transitions that are believed to trigger changes in the way a memory is stored and mark the beginning and end of a memory. end of the memory “files” in the brain. They focused on what they called “hard” and “soft” boundaries.
“An example of a soft boundary would be a scene with two people walking down a hallway and talking, and in the next scene, a third person joins them, but it’s still part of the same overall narrative,” Rutishauser said. A hard limit evokes a major deviation in the ongoing narrative.
What they found was incredible. Approximately 300 milliseconds after a participant viewed a break in a video clip, whether it was a hard boundary or a soft one, neurons the researchers called “boundary cells” briefly fired. Something similar happened in a different type of cell after hard limits as a dramatic scene change. Thus, they were able to identify two types of neurons that responded to these cognitive limits: “boundary cells,” which responded to both soft and hard limits, and “event cells,” which responded only to hard limits.
This led to the theory that the creation of a new memory occurs when there is a spike in activity from both the boundary and event cells, which is something that only occurs after a hard boundary.
“A limit response is like creating a new folder on the desktop of our desktop computer. Then you can already deposit files there. And when another limit appears, you close the first folder and create another ”, clarifies the expert.
“Theta rhythms are thought to be the ‘temporary glue’ for episodic memory,” said study co-author Jie Zheng of Harvard Medical School. “We think that firing event cells in sync with the theta rhythm creates time-based links across different memory folders.”
The study was supported by the Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) initiative of the National Institutes of Health and published in Nature Neuroscience.
Referencia: Jie Zheng, Andrea G. P. Schjetnan, Mar Yebra, Bernard A. Gomes, Clayton P. Mosher, Suneil K. Kalia, Taufik A. Valiante, Adam N. Mamelak, Gabriel Kreiman, Ueli Rutishauser. Neurons detect cognitive boundaries to structure episodic memories in humans. Nature Neuroscience, Mar. 7, 2022; DOI: 10.1038/s41593-022-01020-w
