Chettih, S.N., Mackevicius, E.L., Hale, S., & Aronov, D. (2024). Barcoding of episodic memories in the hippocampus of a food-caching bird. Cell, 187(7), 1922-1935.e20. https://doi.org/10.1016/j.cell.2024.02.032
High-density silicon probe recordings in behaving chickadees; 3D postural tracking with millimeter precision
Chickadee anterior hippocampus
In this study, Chettih et al. leveraged chickadees’ natural food-caching behavior – where birds hide seeds in various locations and must remember each cache precisely – to investigate episodic memory encoding. They discovered that each caching event triggers a unique, sparse “barcode” pattern of hippocampal activity distinct from the conventional place code. During caching, most excitatory neurons are suppressed while ~7% fire in brief, high-frequency bursts, creating a site-specific pattern. Crucially, these barcodes reactivate when birds retrieve specific caches, even after 45-minute delays. The barcodes are orthogonal to place cell activity – showing no correlation between adjacent sites just 5cm apart – and both coding systems operate simultaneously in the same neural population. This suggests the hippocampus uses high-dimensional, sparse patterns as unique identifiers for episodic memories, analogous to hash codes in computing.
We propose that animals recall episodic memories by reactivating hippocampal barcodes. Similarly to computer hash codes, these patterns assign unique identifiers to different events and could be a mechanism for rapid formation and storage of many non-interfering memories.