How the brain creates and stores different episodes of our lives as memories has always puzzled us, as it's challenging to understand what aspects of a memory we might be recalling. Remembering the time last year when you cycled to work, instead of routinely taking the bus, and recalling the detailed events that happened along the way, is referred to as an episodic memory, unlike more generic memories, such as a favourite beach. For instance, squirrels and hamsters scatter hoards of food across their home territory and have the wits to find them again, especially during the winter, when food becomes scarce. Similarly, black capped chickadees stash grain in thousands of stores around the area just before winter, which they return to throughout the season. Selmaan Chettih and colleagues at Columbia University, USA, wanted to understand how chickadees have this remarkable ability to remember where they stored food over many sites to determine how these special memories are represented in the brain.
First, Chettih and colleagues collected wild chickadees from multiple locations around New York state between October and February before returning with them to the laboratory, where they recreated the shorter days and longer nights associated with winter, to motivate the bird's hoarding behaviour. The team then constructed an arena with 128 potential seed hiding sites, where the floor was divided with short perpendicular ridges that the birds could perch on, each with a small cavity covered with a rubber flap beneath, in which a bird could deposit sunflower seeds. In addition, the team arranged six cameras around the arena, to track the birds’ movements, with a seventh camera positioned under the arena to film the arrival and retrieval of seeds. Finally, the researchers attached small probes to the birds’ heads to monitor their brain activity as the bird navigated the arena, stashing and retrieving the seeds over 2–3 h periods.
The team recorded that when a bird stashed food in one of the 128 cavities, a unique set of cells in the hippocampus, the brain's memory centre, fired off, correlating to the location of the food hiding place. And when the bird revisited that site again to retrieve the food, that unique set of neurons was reactivated. Chettih and colleagues explain that this specific nerve signal is analogous to the way that barcodes in supermarkets encode details about a product, telling the bird when it had returned successfully to one of its precious food stores.
However, the researchers explain that these new seed-location barcode memories are different from the conventional way in which memories of locations are stored in the hippocampus. The pattern of nerve cells that usually encodes a location in the brain is always similar for nearby locations and is always triggered whenever a bird visits a site, even food stores. However, the pattern of nerve cells that fires off in a special barcode memory is only triggered when a bird revisits the site to retrieve food and not when it hops by randomly. In addition, the group of nerve cells that encodes a barcode memory can vary wildly, even for adjacent locations.
All in all, the hippocampus in chickadees creates a distinct barcode every time the bird stockpiles food and comes back to this treasure during later visits, even after long intervals. This study contributes to our understanding of how certain memories, such as ‘where’ an event occurred, are represented. Whether the barcoding strategy observed in chickadees is used by humans could offer valuable insights into how episodic memories become a part of the human experience.
