Amazing stuff! Too bad, the study is limited to fear memory acquisition/formation!
"... Engrams are essentially permanent physical and/or chemical changes in the brain associated with the assimilation of specific information or with the formation of new memory associations. A brain region that is known to play a key role in the learning of links between stimuli and outcomes is the CA1 area in the hippocampus.
Researchers ... carried out a study involving mice that was aimed at better understanding how groups of CA1 neurons contribute to the formation of memories. Their findings ... suggest that different groups of cells are active at different phases of learning and encode distinct aspects of experiences. ..."
"... In this study, researchers 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.
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. ..."
From the abstract:
"The mechanisms of associative memory formation, including which cells encode a memory and the timing of their engagement, remain poorly understood. By visualizing and tagging cells based on their calcium influx with unparalleled temporal precision, we identified nonoverlapping dorsal CA1 neuronal ensembles that are differentially active during associative fear memory acquisition. We dissected the acquisition experience into periods during which salient stimuli were presented, or certain mouse behaviors occurred, and found that cells associated with specific acquisition periods are sufficient alone to drive memory expression and contribute to fear engram formation. This study delineated the distinct identities of the cell ensembles active during learning and revealed which ones form the core engram and are essential for memory formation and recall."
How the hippocampus builds memory piece by piece (original news release)
Histological sections of the mouse hippocampus showing labeled neurons
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