Chickadees have unique neural “barcodes” for memories of stashing away food
BIRD BRAIN NO LONGER A PERJORATIVE
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CHICKADEES CACHING SEEDS IN THE TESTING ARENA
view moreCREDIT: CELL/CHETTIH ET AL.
Your ability to remember and recall moments in time is important for recording life-defining moments and everyday information like where you parked the car. Now researchers reporting in the journal Cell on March 29 have new insight into how those episodic memories are encoded in the brain based on studies of how chickadees store food.
Their study finds that chickadees activate unique neural patterns, which they liken to barcodes, each time they cache food in a certain spot. When they go back to retrieve that stored food, their brains light back up with that precise pattern.
“We find that each memory is tagged with a unique pattern of activity in the hippocampus, the part of the brain that stores memories,” said Dmitriy Aronov of Columbia University’s Zuckerman Institute. “We called these patterns ‘barcodes’ because they are extremely specific labels of individual memories—for example, barcodes of two different caches are uncorrelated even if those two caches are right next to each other.”
Scientists have known for decades that the hippocampus of the brain is required for episodic memory, but it had been much harder to understand exactly how those memories were encoded. That’s in part because it’s hard to know in most cases what an animal might be remembering at a particular time.
To get around this problem in the new study, Aronov and colleagues looked to chickadees. They realized chickadees offered a unique opportunity to study episodic memories because the birds cache food items and then must remember to go back for them later.
“Each cache is a well-defined, overt, and easily observable moment in time during which a new memory is formed,” Aronov says. “By focusing on these special moments in time, we were able to identify patterns of memory-related activity that had not been noticed before.”
It still wasn’t easy to do. The researchers had to engineer arenas that allow detailed and automated tracking of behavior as chickadees cache and retrieve food. They also had to develop technologies for large-scale, dense neural recordings in their brains as the birds moved about freely.
Their brain recordings during caching revealed very sparse, transient barcode-like patterns of firing across hippocampal neurons. Each barcode involves only about 7% of the cells in the hippocampus.
“When a bird makes a cache, about 7% of the neurons respond to that cache,” Aronov says. “When a bird makes a different cache, a different group of 7% of neurons respond.”
Those neural barcodes happened together with conventional activity of neurons in the brain that are triggered in response to particular places, appropriately called place cells. But, interestingly, the episodic memory barcodes for caching locations that were close to each other had no resemblance.
“It was widely assumed that when an animal forms a new memory, place cells change,” Aronov said. “For example, place cells might increase or decrease their firing near the location of a cache. Although this was the prevailing hypothesis, our data did not support it. It seems that place cells do not represent information about caches and rather remain relatively stable as a chickadee caches and retrieves food in the environment. Instead, episodic memories are represented by an additional pattern of activity—the ‘barcode’—which coexists with place cells.”
The researchers liken the newly discovered hippocampal barcodes to computer hash codes, which are patterns assigned as unique identifiers to different events. They suggest that the barcode-like patterns could be a mechanism for rapid formation and storage of many non-interfering memories.
Aronov says that perhaps the biggest outstanding question is whether and how barcodes are used by the brain to drive behavior. It’s not clear whether chickadees activate the barcodes and use those memories of food-caching events as they make decisions about where to go next, for example. The researchers say these are questions they plan to address in future studies through more complex environments in the lab in which they’ll record brain activity while the birds make choices about which food caches to visit.
CREDIT
Cell/Chettih et al.
Chickadee
Chickadee
CREDIT
Dmitriy Aronov
This work was supported by the Beckman Foundation, the New York Stem Cell Foundation, the Simons Society of Fellows, and the NIH.
Cell, Chettih et al. “Barcoding of episodic memories in the hippocampus of a food-caching bird” https://cell.com/cell/fulltext/S0092-8674(24)00235-6
Cell (@CellCellPress), the flagship journal of Cell Press, is a bimonthly journal that publishes findings of unusual significance in any area of experimental biology, including but not limited to cell biology, molecular biology, neuroscience, immunology, virology and microbiology, cancer, human genetics, systems biology, signaling, and disease mechanisms and therapeutics. Visit http://www.cell.com/cell. To receive Cell Press media alerts, contact press@cell.com.
JOURNAL
Cell
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Barcoding of episodic memories in the hippocampus of a food-caching bird
ARTICLE PUBLICATION DATE
29-Mar-2024
Chickadees are memory geniuses. Their barcode-like neural activity may be to thank
Scientists find new patterns of electrical activity that may help the brain record and recall memories, not just in birds, but humans too
NEW YORK, NY — Black-capped chickadees have extraordinary memories that can recall the locations of thousands of morsels of food to help them survive the winter. Now scientists at Columbia's Zuckerman Institute have discovered how the chickadees can remember so many details: they memorize each food location using brain cell activity akin to a barcode. These new findings may shed light on how the brain creates memories for the events that make up our lives.
"We see the world through our memories of objects, places and people," said Dmitriy Aronov, PhD, a principal investigator at Columbia’s Zuckerman Institute and an assistant professor of neuroscience at Columbia’s Vagelos College of Physicians and Surgeons. "Memories entirely define the way we see and interact with the world. With this bird, we have a way to understand memory in an incredibly simplified way, and in understanding their memory, we will understand something about ourselves."
This barcode-like formatting of memory, revealed for the first time today in the journal Cell, may be a common tactic in animal brains, including those of humans. "There are many findings in humans that are totally consistent with a barcode mechanism," said postdoctoral research fellow Selmaan Chettih, PhD, the study's co-first author along with Emily Mackevicius, PhD.
Chickadees are “memory geniuses,” said Dr. Aronov, the study’s corresponding author. They are masters of episodic memory — the brain's ability to recall specific moments, such as stashing a bit of food away under tree bark or in a knothole. This can prove a matter of life or death for them, since unlike most birds that live in cold places, chickadees don't migrate during the winter. This means their survival hinges on remembering where they hid food during warmer months, with some making up to 5,000 of these stashes per day.
Scientists have long known that these birds rely on the hippocampus–a brain structure critical for memory in all vertebrates, including humans–for storing memories of their caches. However, no one had identified the specific neural activity in the hippocampus that encodes episodic memories such as food-caching events.
"The question we're trying to answer is, 'What physically is a memory?'" Dr. Chettih said.
Chickadees may help scientists unlock this mystery. To explore the prodigious memories of chickadees, Dr. Aronov and his team built indoor arenas inspired by the birds’ natural habitats.
"Scientists have marveled at the memory of these birds for decades, but what has been a mystery is what was going on in their brains to support these memories," said Dr. Aronov. "Now we have neural recording and behavior tracking tools at our disposal to advance our knowledge of how these birds are capable of these feats of memory."
In typical experiments, a black-capped chickadee instinctively hides sunflower seeds in holes in the arenas as the researchers monitor activity in the hippocampus. Meanwhile, six cameras also record the birds as they flit about, with an artificial intelligence system automatically tracking them as they stash and retrieve seeds.
The scientists unexpectedly found that each time a chickadee stashed a seed, hippocampal neurons fired in a unique pattern. These fleeting patterns reactivated when the birds retrieved that specific food cache.
"These are very striking patterns of activity, but they're very brief — only about a second long on average," Dr. Chettih said. "If you didn't know exactly when and why they happened, it would be very easy to miss them." As the researchers mulled over their data, the idea of neural barcodes as unique labels for different events began to make sense, they said.
These barcode patterns exist independently from the activity of hippocampal neurons, called place cells, which encode memories of locations. Each barcode remains distinct, even when it comes to stashes hidden at the same place but different times, or at neighboring stashes made in quick succession.
"Many hippocampal studies have focused on place cells, with the Nobel Prize awarded for their discovery in 2014," Dr. Aronov said. "So the assumption in the field was that episodic memory must have something to do with changes in place cells. We find that place cells don't actually change when birds form new memories. Instead, during food caching, there are additional patterns of activity beyond those seen with place cells."
Going forward, the researchers want to see if the chickadees activate barcodes when looking for caches from remote locations.
"That's what we might expect if they are planning to retrieve a cached item before they actually do it," Dr. Chettih said. "We want to identify those moments when a bird is thinking about a location but it's not there yet, and see if activating a barcode might drive a bird to go to a cache."
The researchers are also eager to know if the barcoding tactic they have uncovered chickadees is in widespread use among other animals, including humans. Such research may help shed light on a core part of the human experience.
"If you think about how people define themselves, who they think they are, their sense of self, then episodic memories of particular events are central to that," Dr. Chettih said. "That's what we're trying to understand."
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The paper, "Barcoding of episodic memories in the hippocampus of a food-caching bird," was published online in Cell on March 29, 2024.
The full list of authors includes Selmaan N. Chettih, Emily L. Mackevicius, Stephanie Hale, and Dmitriy Aronov.
The authors report no conflicts of interest.
JOURNAL
Cell
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Barcoding of episodic memories in the hippocampus of a food-caching bird
ARTICLE PUBLICATION DATE
29-Mar-2024
