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Histological sections of the mouse hippocampus showing labeled neurons
Forming a memory is not a simple matter of “recording” an event. The brain selects, sorts and assembles different elements of an experience. In this study, researchers [1] show that during the formation of a fear memory in mice, not all activated neurons play the same role. Only certain groups appear to form the core of the memory trace, known as the engram.
The team focused on the hippocampus, a key brain region for memory. Using a highly precise optogenetic tool able to label active neurons within very short time windows, they distinguished several groups of cells recruited at different moments of learning: before the shock, during the shock, during episodes of fear-related immobility (“freezing”), and outside these episodes. This temporal precision changes the scale of observation. Instead of treating all neurons activated during the experience as a single population, it becomes possible to break memory down into finer sequences.
The main result is clear. Artificially reactivating the neurons active during the shock or during freezing was enough to trigger a fear response in a different context. By contrast, reactivating neurons recruited before the shock, or outside freezing periods, did not produce this effect. Even more strikingly, inhibiting these same “shock” or “freezing” neurons later disrupted natural memory recall. In other words, not every cell engaged during learning becomes a memory cell. The brain appears to make a selection, as if it retained above all the neurons linked to the most salient moments of the experience.
The researchers also show that these subgroups are largely distinct from one another. Among them, the neurons active during freezing display a particularly notable property: when the memory is recalled, they reactivate in a more coordinated manner, as a true ensemble. This suggests that an engram is not simply a list of neurons, but a collective dynamic in which synchrony matters as much as the identity of the cells.
These findings refine our understanding of memory. A memory may not be stored as a single block, but assembled from neural components recruited at precise moments. This approach opens new avenues for understanding how an experience becomes a memory and why, in some cases, certain memories become disproportionately strong. The results were obtained in mice, within a tightly controlled experimental paradigm, and they shed light on fundamental mechanisms without implying immediate clinical applications.
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Reference
Pouget, C., Morier, F., Autore, L. et al. Deconstruction of a memory engram reveals distinct ensembles recruited at learning. Nat Neurosci (2026).
https://doi.org/10.1038/s41593-026-02230-2
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Contact
Press and Scientific Communication : Paul Turpault, paul.turpault@espci.fr