Barcoding of episodic memories in the hippocampus of a food-caching bird

The hippocampus is critical for episodic memory. Although hippocampal
activity represents place and other behaviorally relevant variables, it is
unclear how it encodes numerous memories of specific events in life. To
study episodic coding, we leveraged the specialized behavior of chickadees
– food-caching birds that form memories at well-defined moments in time
whenever they cache food for subsequent retrieval. Our recordings during
caching revealed very sparse, transient barcode-like patterns of firing
across hippocampal neurons. Each “barcode” uniquely represented a caching
event and transiently reactivated during the retrieval of that specific
cache. Barcodes co-occurred with the conventional activity of place cells,
but were uncorrelated even for nearby cache locations that had similar
place codes. 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.