Much like squirrels, black-capped chickadees hide their food, keeping track of many thousands of little treasures wedged into cracks or holes in tree bark. When a bird returns to one of their many food caches, a particular set of nerve cells in the memory center of their brains gives a brief flash of activity. When the chickadee goes to another stash, a different combination of neurons lights up.
These neural combinations act like bar codes, and identifying them may give key insights into how episodic memories — accounts of specific past events, like what you did on your birthday last year or where you’ve left your wallet — are encoded and recalled in the brain, researchers report March 29 in Cell.
This kind of memory is challenging to study in animals, says Selmaan Chettih, a neuroscientist at Columbia University. “You can’t just ask a mouse what memories it formed today.” But chickadees’ very precise behavior provides a golden opportunity for researchers. Every time a chickadee makes a cache, it represents a single, well-defined moment logged in the hippocampus, a structure in the vertebrate brain vital for memory.
To study the birds’ episodic memory, Chettih and his colleagues built a special arena made of 128 small, artificial storage sites. The team inserted small probes into five chickadees’ brains to track the electrical activity of individual neurons, comparing that activity with detailed recordings of the birds’ body positions and behaviors.
When the birds were caching and retrieving their seeds, a specific subset of neurons representing 7 percent or less of the entire hippocampus would briefly light up with activity, Chettih says. Each cache appeared to have its own unique combination of neurons, or neural bar code, and those bar codes differed even for individual caches at the same location.
It’s possible bar codes are a type of engram, the proposed physical manifestations of a memory (SN: 1/24/18). Such bar codes are probably used across many species, considering how similar hippocampus physiology is between animals separated by hundreds of millions of years of evolution, Chettih says. However, more research is needed to confirm this.
Those bar codes seem to work in parallel with another group of neurons in the hippocampus called place cells, which encode information on an animal’s location. Place cells have been widely theorized as the foundation of episodic memory.
This is partly because our perceptions of memory are enmeshed with location, says Kazumasa Tanaka, a neuroscientist at the Okinawa Institute of Science and Technology in Japan who was not involved with the study. “When you recall some specific event that happened in the past, that episodic memory cannot be dissociated from where that event happened, or when that event happened.”
Place cells didn’t change their activity during caching, surprising the researchers. But the findings suggest an added nuance to this understanding of memory, Chettih says, where the hippocampus creates a separate “index” that binds together all the different inputs making up an experience into a distinct memory.
Tanaka notes that there are now multiple candidates for indexing systems in the hippocampus, and that it’s possible that episodic memory arises from multiple, concurrent coding schemes.
Chettih’s team also discovered a “seed code,” where neurons encode the presence or absence of a seed in a cache. The potential interconnections between the three different neuron activity patterns — the bar code, the place code and the seed code — intrigues Thomas McHugh, a neuroscientist at the RIKEN Center for Brain Science in Wakō, Japan, who was not involved with the research.
“Understanding how they interact is probably going to tell us a lot more about how memory works,” McHugh says.
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