Scientists link two crucial types of brain cells used for memory creation

Neuroscientists watched individual neurons draw boundaries between memories
Grant Currin
A photograph showing how this location appeared long ago.Unsplash / Anita Jankovic 

A team of researchers at Cedars-Sinai hospital in Los Angeles have discovered two types of brain cells that seem to be involved in forming memories.

Those neurons are responsible for constructing boundaries that mark the end of one chunk of memory and the beginning of the next.

The results of the study are published today in the academic journal Nature Neuroscience.

Senior author Ueli Rutishauser tells IE that his team found stunning results by using an unusual method.

Most studies of memory rely on the standard technique of showing research participants one thing to remember — a photo, for example — and then asking them to recall it later.

That experimental setup imposes bounds on when a memory starts and stops.

"Those events are predefined by the experimenter," Rutishauser says.

But that's extremely artificial. 

"We move through life, and there is no clear beginning and end of when a memory should be," he says.

His team is interested in how brains divide up everything we experience into the discrete chunks that make up our memories.

"The question we attacked in this paper is, 'What defines when an episode begins and ends?'"

Researchers listened to individual neurons while participants watched videos

The sample size for the study was small — just 19 participants — but those people offered the researchers a rare chance to listen to single neurons inside living human brains.
"We had the opportunity to work with patients [at Cedars-Sinai Hospital] who have focal epilepsy," he says.

As part of their treatment, those patients have probes implanted deep into their brains. That special case enables researchers to use a method called single-neuron recording — common in animal studies but typically unethical in humans — to eavesdrop on specific neurons.

Rutishauser calls it a "very precious and rare opportunity to measure electrical activity directly inside the brain."

They’re divided into three groups based on interruptions to their narrative structure, which the researchers call “boundaries.”

In the clips with no boundaries, nothing really changes. No new people enter or leave the scene. For the researchers, that means they shouldn't trigger a new chunk of memory.

Other clips have a soft boundary, meaning something changes but not everything. For instance, you might see a group of people leave the office and then cut to the same group walking into a coffee shop. It's the same narrative.

Finally, clips with a hard boundary start with one narrative and then cut to something completely different. Maybe an old man is playing chess in a park on a sunny morning and then, all of the sudden, a group of cyclists are crossing a bridge. These clips clearly showed scenes from two different stories.

Epilepsy patients make this insight possible

The research took place in the participants’ hospital beds while they waited, usually for two or three weeks, to have a seizure.

For the participants, the research was very simple. Rutishauser compared it to playing a video game.

The participants watched all 90 video clips on a computer and answered questions about what they saw.

At the same time, the researchers were using the electrodes in their brains to record the electrical activity of specific neurons.

"We can see individual pulses of a brain cell," he says. Those signals — action potentials, in the language of science — are the binary messages that individual neurons use to communicate with one another. By themselves, those signals carry very little information. But together, across all 86 billion neurons in the brain, they are what make it possible to perceive, think, remember, breathe, and many other things.

What they found was incredible. Roughly 300 milliseconds after a participant saw a cut in a video clip — either a hard boundary or a soft one — cells the researchers named “boundary cells” would briefly activate. Something similar happened in a different type of cell after hard boundaries. The researchers called those “event cells."

The researchers call what they observed a “boundary response,” and they think it’s part of the system the brain uses to form a new memory.

“A boundary response is kind of like creating a new folder on your computer,” Rutishauser says. “You can then deposit files in there. And when another boundary comes around, you close the first folder and create another one.”

New treatments require a better understanding of how the brain works

Rutishauser says he was "very much" surprised that these cells "react highly reliably to cognitive boundaries," meaning the separation between memories.

He says the research in this study won't be put to use in the near future. But that doesn't mean it's useless. Researchers who are working on treatments and therapies for neurological and psychological problems are currently working in the dark, without a basic understanding of how the organ they're trying to fix works in the first place.

"One of the reasons we really don't have any good treatment for memory disorders is that we don't understand well enough how it works. If we don't know how it works, we can't really engineer a new treatment," he says