Similar to squirrels, black-capped chickadees conceal their nourishment, monitoring numerous small treasures nestled in crevices or gaps in tree bark.
Upon revisiting one of their many food caches, a distinct group of nerve cells in the memory center of their brains emits a brief surge of activity.
Upon visiting another stash, a disparate combination of neurons illuminates.
These neural configurations function akin to bar codes, and deciphering them might provide fundamental insights into how episodic memories — recollections of specific past occurrences, such as activities on your birthday last year or the location of your wallet — are encoded and retrieved in the brain, as per researchers' findings published on March 29 in Cell.
This type of memory presents challenges in animal research, notes Selmaan Chettih, a neuroscientist at Columbia University. "You can't simply inquire about the memories formed by a mouse each day." However, the precise behavior of chickadees offers an excellent opportunity for researchers. With each cache a chickadee creates, it marks a distinct moment precisely recorded in the hippocampus, a critical brain structure for memory in vertebrates.
To investigate the birds' episodic memory, Chettih and his team constructed a specialized arena comprising 128 small, artificial storage locations. The researchers implanted miniature probes into the brains of five chickadees to monitor the electrical activity of individual neurons. They then compared this activity with meticulous recordings of the birds' bodily positions and actions.
During the caching and retrieval of seeds by the birds, a distinct subset of neurons, constituting 7 percent or less of the entire hippocampus, would momentarily activate, according to Chettih. Each stash seemed to possess its own distinctive combination of neurons, akin to a neural bar code, and these bar codes varied even for caches situated at the same spot.
It's plausible that these bar codes serve as a form of engram, the theorized physical representations of memories. Such bar codes likely have widespread use across various species, given the similarity in hippocampal physiology among animals separated by hundreds of millions of years of evolution, notes Chettih. Nonetheless, further investigation is necessary to validate this hypothesis.
These neural bar codes appear to function alongside another cluster of neurons in the hippocampus known as place cells, which encode details about an animal's whereabouts. Place cells have long been proposed as the cornerstone of episodic memory.
This linkage is partially due to the intertwining of our memory perceptions with spatial context, remarks Kazumasa Tanaka, a neuroscientist at the Okinawa Institute of Science and Technology in Japan, who was not involved in the research. "When you recollect a specific past event, that episodic memory is inseparable from its spatial and temporal context."
The activity of place cells remained constant during caching, which left the researchers surprised. Nonetheless, these results imply a deeper layer of complexity to our comprehension of memory, remarks Chettih. He suggests that the hippocampus may generate a distinct "index" that consolidates various inputs comprising an experience into a cohesive memory.
Tanaka highlights that there are now several contenders for indexing mechanisms within the hippocampus. He posits the possibility that episodic memory emerges from numerous simultaneous coding strategies.
Chettih's group additionally unearthed a "seed code," whereby neurons register the presence or absence of a seed in a cache. The potential correlations among the three distinct patterns of neuron activity — the bar code, the place code, and the seed code — captivate Thomas McHugh, a neuroscientist at the RIKEN Center for Brain Science in Wakō, Japan, who was not part of the study.
"Comprehending their interactions will likely unveil significant insights into the workings of memory," McHugh remarks.
