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.
