Brain activity ‘barcodes’ may be linked to specific memories in birds

Neural recordings from chickadees have found unique patterns of activity that occur when they hide food and then retrieve it later, and they may represent how memories are indexed in the brain.

Black-capped chickadees remember the locations of stashed food
Peter Llewellyn RF/Alamy

Unique patterns of brain activity that are linked to specific memories have been discovered in songbirds. These “barcode” patterns may be related to memory recall in the birds and could also be present in other animals such as humans.

The black-capped chickadee (Poecile atricapillus) stores foods such as seeds in hiding places and retrieves them later when sustenance is scarce. This makes it a useful organism to study memory, says Selmaan Chettih at Columbia University in New York.

A pair of structures in the birds’ brains called the hippocampi are vital for this ability. Previous studies have found that impairing the hippocampi of chickadees massively affects their ability to find stashed food.

To find out more, Chettih and his colleagues created an indoor set-up with 128 places where the chickadees could store and retrieve food in a similar way to how they would in the wild. “We built this floor that has rubber flaps that mimic peeling tree bark,” says Chettih. “The birds can peel back these flaps and insert seeds inside.”

The team then implanted electrodes into the brains of five chickadees to measure the activity in their left hippocampus and monitored the birds as they explored the site for several hours at a time. One bird would store and retrieve seeds around 100 times in a session, says Chettih.

Each time the chickadees stored seeds, the researchers saw a distinct pattern of brain activity in their hippocampus. They saw the same pattern when the birds went back to the site to retrieve the seeds.

Chettih and his colleagues call these patterns of brain activity barcodes, as they seem to be unique for each seed-storing event. “Birds may make multiple caches and retrievals at the same location over time, but these events are all represented differently from each other,” he says.

Moreover, these barcodes don’t appear to be related to the nature of the hiding locations. For example, when a bird stores seeds in two sites right next to each other, they result in completely different barcodes in the brain.

Chettih suspects that the barcodes occur when the birds recall a memory. If that is true, the researchers should be able to observe the brain barcode firing up when the bird remembers where they stored the seeds, before they go back to the hiding place.

But they haven’t been able to do this yet, as there is too much noise in the brain activity data while the bird roams around, says Chettih. The birds could be thinking about several things during these periods, and it is hard to isolate when they actually remember a storage site, he says.

We don’t know if this same mechanism exists in humans, but it is plausible, says Chettih. “There have been studies done where people have done neural recordings in humans while they watch a movie and then find that these neurons are reactivated when that person thinks about the same scene in the movie,” he says.

The latest findings support an idea called the indexing theory, says Pedro Jacob at the University of Oxford. This suggests that the role of the hippocampus is to index the brain activity that occurs when we form a memory, and the reactivation of this index results in memory recall. Brain recording and imaging studies in humans have suggested that the human hippocampus might also work in this way, says Jacob.


bioRxivDOI: 10.1101/2023.05.27.542597

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