Amazing stuff!
"... the rodent brains called three different sets of neurons into action to record the memory. The first are known as early-born neurons and are the earliest to develop as a fetus is growing. At the other end of the spectrum are the late-born neurons, which show up late in embryonic development. Between these are neurons that form somewhere right in the middle of growth in the womb.
The imaging study revealed that when the new memory is stored in the early-born neurons, it is initially hard to retrieve, but it becomes stronger as time goes on.
The copy of the memory stored in the late-born neurons, on the other hand, was very strong to start, but faded over time to the point that it eventually became inaccessible by the brain. In the middle, the memory copy showed a higher degree of stability than with either of the other neuronal groups. ..."
"... that in the hippocampus, a brain region responsible for learning from experience, a single event is stored in parallel memory copies among at least three different groups of neurons, which emerge at different stages during embryonic development. ..."
From the editor's summary, abstract and structured abstract:
"Editor’s summary
... the mechanisms that govern the reorganization of neuronal ensembles linked to a specific memory and how these dynamic changes affect memory persistence over time. They found that in the hippocampal network, learning resulted in the parallel establishment of two distinct memory traces. These traces were represented in distinct neurogenesis-defined subpopulations of early- and late-born neurons. Even though temporally restricted, the transient recruitment of late-born neurons was necessary for a memory’s long-term permanence, whereas shifts in the recruitment of early- and late-born neurons had a strong impact on the plasticity of a recently acquired memory. ...
Abstract
... By targeting developmentally distinct subpopulations of principal neurons, we discovered that memory encoding resulted in the concurrent establishment of multiple memory traces in the mouse hippocampus. Two of these traces were instantiated in subpopulations of early- and late-born neurons and followed distinct reactivation trajectories after encoding. The divergent recruitment of these subpopulations underpinned gradual reorganization of memory ensembles and modulated memory persistence and plasticity across multiple learning episodes. Thus, our findings reveal profound and intricate relationships between ensemble dynamics and the progression of memories over time.
Structured abstract
...
RATIONALE
Hippocampal neurons born at different times during embryonic development segregate into subpopulations that preferentially connect across subdivisions and are endowed with distinct genetic, anatomical, and functional properties. We hypothesized that the activation of developmentally and functionally distinct neuronal subpopulations at specific stages of a memory’s lifetime might confer dynamic properties to a memory and underpin its long-term permanence. We thus exploited multiple methods to record and manipulate the activity of hippocampal neurons with specific birth dates—in combination with hippocampus-dependent associative learning paradigms—to dissect the contribution of birth-dated subpopulations to the encoding, persistence, and evolution of a memory over time.
... Late-born neurons were preferentially recruited for retrieval at short latency after acquisition, whereas early-born neurons were preferentially recruited at later times. These divergent trajectories recapitulated reactivation dynamics recorded through longitudinal calcium imaging experiments, which further revealed distinct network-wide responses between subpopulations. Whereas in the late-born subnetwork, learning was associated with a plastic reorganization of coactivity dynamics and functional connectivity, activity patterns among early-born neurons were more rigidly structured and unaffected by learning-induced activity changes. Targeted manipulation experiments indicated that each subpopulation’s activation supported memory expression at specific times after encoding, with late-born neurons supporting recall shortly after acquisition and early-born neurons becoming necessary at later times. Even though temporally restricted, activation of late-born neurons was necessary for a memory’s long-term permanence because silencing their activity at acquisition or during a specific window of consolidation impaired remote-memory expression. Moreover, the systematic shift in recruitment from late- to early-born neurons happened concomitantly to a transient plasticity window occurring shortly after memory encoding, during which mice could combine information acquired across multiple learning episodes to reinforce previously learned associations or make inferences. Manipulating the recruitment of early- or late-born neurons around the closure of such windows had the potential to modulate memory plasticity in opposite ways.
CONCLUSION
We discovered that an underlying logic driving the reorganization of hippocampal memory ensembles over time is anchored in the divergent recruitment of populations of neurons defined by neurogenesis, which is necessary for memory retrieval. Ensemble reorganization is thus not disruptive of memory processes; conversely, the timely recruitment of distinct neuronal subpopulations modulates a memory’s properties at different stages of its lifetime. Our study therefore sheds light on the complex interplay between ensemble activity dynamics, memory encoding, consolidation, and retrieval and the processes governing memory evolution over time. Specifically, we reveal that within the hippocampal network, the divergent recruitment of distinct memory traces emerging in parallel at encoding underpins memory persistence and modulates the plasticity of recently acquired memories."
The brain creates three copies for a single memory (original news release) "A new study ... reveals that the memory for a specific experience is stored in multiple parallel “copies”. These are preserved for varying durations, modified to certain degrees, and sometimes deleted over time ..."
Parallel-emerging memory traces encoded in developmentally defined hippocampal subpopulations underpin memory dynamics.
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