Audio Edition: Concept Cells Help Your Brain Abstract Information and Build Memories
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Welcome to the Quanta Audio Edition. In each of these bi-weekly episodes, we bring you a story direct from the Quanta website about developments in basic science and mathematics. I'm Susan Vallett. Individual cells in the brain light up when we think about specific ideas. These concept neurons, once known as Jennifer Aniston cells, help us think, imagine, and remember episodes from our lives. That's next.

Check out this feed every Tuesday for the Quanta podcast. That's where editor-in-chief Samir Patel talks to our writers and editors about more of Quanta's most popular, interesting, and thought-provoking stories. Imagine you're on a first date, sipping a martini at a bar. You eat an olive and patiently listen to your date tell you about his job at a bank.

Your brain is processing the scene, in part by breaking it down into concepts: bar, date, martini, olive, bank. Deep in your brain, neurons known as concept cells are firing. You might have concept cells that fire for martinis but not for olives, or ones that fire for bars, perhaps even that specific bar if you've ever been there before.

The idea of a bank also has its own set of concept cells, maybe millions of them. And there, in that dimly lit bar, you're starting to form concept cells for your date, whether you like him or not. Those cells will fire when something reminds you of him. Concept neurons fire for their concept, no matter how it's presented. In real life or a photo, in text or speech, on television or in a podcast.

Elizabeth Buffalo, a neuroscientist at the University of Washington, says it's more abstract and really different from what you're seeing. For decades, neuroscientists mocked the idea that the brain could have such intense selectivity, down to the level of an individual neuron. How could there be one or more neurons for each of the seemingly countless concepts we engage with over a lifetime?

Neurobiologist Florian Mormann of the University of Bonn says that's inefficient and not economic. But when researchers identified concept cells in the early 2000s, the laughter started to fade. Over the past 20 years, they've established that concept cells not only exist, but are critical to the way the brain abstracts and stores information.

New studies, including one recently published in Nature Communications, have suggested that they may be central to how we form and retrieve memory.

Mathematician Valery Makarov-Slivsneva of the Complutense University of Madrid has done theoretical calculations to prove that concept cells exist. He says we know that the brain processes information about the outside world through the complex dynamics of circuits of neurons, but it's also possible that individual cells have essential roles in the brain's reconstruction of reality.

He says over time, nature has used simple but efficient concepts instead of dealing with complex distributed calculations. He says we're simpler than we thought.

The concept of a concept cell was a joke to neuroscientists, until it wasn't. In 1969, neuroscientist Jerome Letvin gave what became a famous lecture at MIT. He told his students a story in a mocking tone about a fictional neurosurgeon. That doctor was seeing a fictional patient who had a difficult relationship with his mother. To help, the neurosurgeon deleted a cell in his patient's brain that coded for his mother.

All memories of her, gone. The neurosurgeon was so satisfied with his accomplishment that he continued his research by looking for grandmother cells. Theoretically, a grandmother cell is a single neuron, hidden somewhere among the 86 billion in your brain, that codes for one of your grandmothers. You delete it, and poof, everything you know about that person disappears from your brain.

It wasn't an idea that anyone took seriously. A cell for every single person you've ever encountered? Yeah, right.

But not everyone poo-pooed the idea. In the 1990s, a research group at UCLA, led by neurosurgeon Itzhak Fried, developed a new kind of electrode that could look at the activity of individual neurons. It was an unprecedented level of resolution at the time. A scientist, as much as a surgeon, Fried had always been curious about memory and our mental lives. He

He says somehow there's a transformation made of the entire external world into some representation in the brain. This representation could be reflected in vague and abstract concepts, absent of details from the real world. What could that look like?

Fried and Rodrigo Kian Quiroga, a neuroscientist at the Hospital del Mar Research Institute in Barcelona, collaborated with neuroscientist Christoph Koch at the Allen Institute for Brain Science in Seattle to investigate. Epilepsy patients already had electrodes implanted into their brains as part of their medical treatment, so the researchers received consent from those patients to record and analyze their neural activity.

The electrodes accessed each patient's medial temporal lobe, the part of the brain that includes the amygdala, interrenal cortex, and hippocampus, which is the hub for emotion and memory.

and then they showed the patients pictures of objects. In 2000, the researchers reported that individual neurons seemed to represent broad categories such as faces, scenes, houses, or animals by firing for multiple images within each category. The results suggested that something like grandmother cells might exist, but only if those cells were responding to more than the images alone.

In the early 2000s, Quiroga was fiddling with an algorithm he'd created to analyze electrode data. It let him identify many more neurons than was previously possible, even cells that rarely fired and thus were harder to detect. He says he could see neurons that people couldn't see before because he was using tricks he learned from physics and math. He wanted to see what those neurons do.

At first, Quiroga showed epilepsy patients images of scientists, such as Richard Feynman and Albert Einstein, to see whether neurons responded to individual people. When the patients couldn't identify the scientists, he tried showing them photos of more recognizable places and people, including the actor Jennifer Aniston, a star of the hit sitcom Friends. To his delight, he found a neuron that responded to the actor.

That raised a new question: is the neuron responding to the picture of Jennifer Aniston, or is it responding to the concept of Jennifer Aniston? In a follow-up experiment, he showed patients seven different pictures of Aniston, and found that the same neuron fired for all of the photos, but not for images of other actors or objects. He then started to identify neurons for other famous places and people.

He found one that responded only to Halle Berry, and another that fired only for the Leaning Tower of Pisa. Quiroga wrote out the name, Oprah Winfrey. The same neurons that had fired for her picture also fired for her written name. That meant that the neurons weren't responding to features of the picture, like brightness or color. They were context independent. They were responding to Oprah as a concept.

He knew that his observation of one neuron firing didn't mean there was only one neuron for every concept. If that were true, the chance of finding it would be close to zero. Kiroga says he used to joke that if that were the case, he would quit science and start gambling because he'd be the luckiest person ever. Instead, he believed the brain must have many neurons for every concept, but he didn't know how many.

In 2005, the team published their results in Nature, and the cells became known as Jennifer Aniston cells. At first, Koch says it was the hardest thing to get people to accept the possibility of such cells, largely because of the long-running negative connotations around grandmother cells. But were these grandmother cells?

Kiroga says he's very against that view. Sure, these cells were highly selective, firing only for Aniston, or sometimes also for closely related people who might evoke her, such as other Friends cast members. But the concept of the Grandmother cell assumed a one-to-one concept-to-cell ratio, and that wasn't the case with these cells.

A year after publishing their data, the team crunched some numbers. Using an estimate from psychologists that the brain can distinguish about 20,000 semantic concepts, they calculated that millions of cells would code for every concept, and that each concept cell could code for dozens of different, though often related, concepts.

For example, cells that fire for Harry Potter might also fire for his wizarding school pals Ron Weasley or Hermione Granger. Maybe they'd even fire for Gandalf, the wizard from The Lord of the Rings.

Neurobiologist Florian Mormann points out that it's the same profession, even if it's a different story. He says sometimes you have narrow tuning to a single individual person and no one else. And sometimes you have wider tuning, like to the category of wizards. He says the same concept cell might also fire for wand or old men in gowns with beards.

Concept cells could code for anything and everything, but they're not used for object recognition. They're too slow for that. These cells fire after a delay of about 300 milliseconds.

Yuli Rudishauser, a neuroscientist at Cedars-Sinai Medical Center in Los Angeles, says it's not clear why it takes so long. Rather, these cells seem to dip into a more internal process, forming an abstract representation informed by past experiences and memory.

Everyone has a different set of concepts and cells that encode them. Not everyone has seen friends or follows celebrity culture. Instead, concept cells develop for people or objects that we care about or have some history with. For example, your brain could form an association between your date and the bar where you met him, so that your concept cells for the man might also fire for the bar. However, that's only true if the bar is tightly coupled with the person.

Mormont says if it's a place you go all the time, it's unlikely that the same neuron will fire for both. For years after publishing the work, Quiroga, who wasn't happy to be known as the Jennifer Aniston neuron guy, tried to get the term concept cells to stick. It failed to catch on until 2012, when he published a paper in Nature titled "Concept Cells: The Building Blocks of Declarative Memory Functions."

The paper presented his hypothesis that the brain uses concept cells to convert information from the world into memory. The process requires abstraction. We extract relevant information from experience, strip it of unnecessary detail, and store it.

Concept cells are abstract representations of ideas, such as specific people or objects. So Kiroga proposed that concept cells might link together to form new associations, like words in a sentence, and serve as building blocks for memories, like building a story out of sentences. Kiroga says this is the skeleton of how we store memories.

To many scientists, the idea that concept cells link up and interweave to form memories intuitively makes sense. Memories are important for our survival. So Sina McKay, a graduate student at the University of Bonn who works with Mormont, says it's the best explanation for why our brain can afford the luxury of having such high specialization to independent semantic concepts.

Indeed, in a recent study in Nature Communications, their team found the strongest experimental hints yet that concept cells may link specific objects to locations in our long-term memory. For decades, researchers have studied cells that store location information in our brains. The study found that the firing patterns of concept cells and location cells correlated with patients' ability to remember an object's location.

The authors wrote that concept cells are the "what" to our memories, while location cells are the "where." Concept cells are also linked to working memory, which is activated temporarily when you're grocery shopping or remembering a phone number. Neuroscientist Uli Rudishauser says this type of memory is low capacity, and it has high demands. He says if you get slightly distracted, it's gone.

In 2017, his team found that concept cells remain active for several seconds as you try to hold items in working memory. And in a study published in Neuron at the end of 2024, his team found that working memories are more likely to migrate into long-term memory when patients' concept cells are active. Working memory also turns on when you imagine a scenario or tell a story.

Peter Rolfsema studies vision, perception, and memory at the Netherlands Institute for Neuroscience. He suggests this story: Shrek and Jennifer Aniston walk into a bar. Maybe Shrek orders a beer. As you listen to this sentence, the concepts of Aniston, Shrek, and a bar stitch together, one by one. It's likely that concept cells play a role in this imagining.

Rolfsema says you're building something in your working memory that's incrementally becoming richer and maybe more realistic. And then the story unfolds. Rolfsema's group recently found that concept cells respond to pronouns as well. In the study, the pronoun he, standing in for Shrek, lit up the same concept cells as Shrek did.

Rolfsema says the pronoun then directs attention to the concept, Shrek, who's going to be the subject of the next sentence. He thinks it's beautiful that you can actually measure that. Researchers debate how concept neurons fit in with other modes of how the brain represents the external world. Yuri Buzhaki, a neuroscientist at New York University who's researched the hippocampus for decades, says concept neurons are a fantastic discovery.

But the representation of concepts happens at different scales in the brain, at the level of a single neuron and also at the level of cellular populations. Buzsaki says the question is, what's more important? One obstacle to finding an answer is that concept cells are hard to locate.

Currently, they can only be studied in a clinical setting, where patients are undergoing surgery to have electrodes implanted for medical reasons. That limits who can study the cells and how. Plus, it's not easy to define them, says Corey Miller, a neuroscientist at the University of California, San Diego. Part of the problem is the vague definition of concept itself. For example, no one can say whether we have concept cells for experiences like emotions.

One intriguing possibility is that the varied hippocampal cells can be remapped to do different jobs in different contexts. But Buzsaki says when you start looking at the history and the overall picture, you start scratching your head. He says there are time cells, place cells, border cells, boundary vector cells, concept cells. He says at one point, you've got to wonder how that works if there's a limited number of neurons in the hippocampus.

Neuroscientist Elizabeth Buffalo says it's possible that these neurons can play different roles and take on different identities based on the task at hand. When it needs to be a concept cell for Jennifer Aniston, that's what it is. When it needs to be a place cell to help you navigate toward the martini at the bar, it's a place cell. Corey Miller says it's kind of like the Swiss Army knife of cells.

The few groups with access to patients and the technology to record the activity of single neurons are excitedly continuing their experiments. Mormont wants to understand just how abstract concept cells can get. In preliminary data, he found some concept cells that respond to broad, amorphous concepts, such as government and taxes, but more that respond to concrete concepts, such as Jennifer Aniston.

Meanwhile, Kiroga is hoping to prove that concept cells are specific to humans. That's a hotly debated idea with potentially profound implications. He says if no other animal can represent concepts in the brain, this may be the basis of our intelligence. Okay, now, poof!

After hearing all of this, you might have already formed concept cells that code for concept cells. And that's a concept that we're somehow able to wrap our brains around. Michael Kanyangolo helped with this episode. I'm Susan Vallott. For more on this story, read Yasmin Sapakoglu's full article, Concept Cells Help Your Brain Abstract Information and Build Memories, on our website, quantummagazine.org.

I'm Steve Strogatz. And I'm Jana Levin. And this season on The Joy of Why from Quantum Magazine, we're sitting down together. We have our own research areas, but we don't always get the chance to speak deeply about science and math beyond our fields. We'll ask researchers about moments big and small that inspire them and lead to incredible discoveries. Lots of science news outlets provide coverage on applied work like health and tech, but

But we cover big questions in the study of life, reality, numbers, and information. We hope these stories spark your curiosity too. Join us for the joy of why from Quantum Magazine.