Temporal evolution of fearful memories for aversive events experienced close in time

The ensemble of neurons responsible for the storage of a specific memory is called engram. For the memory allocation theory, neurons involved in the engram are not randomly assigned, but specific cellular mechanisms intervene to determine which neurons will be chosen. Specifically, for events experienced close in time, it seems that the neurons that have already been activated for a first event, become more eligible to be chosen to store the memory of the second event, so that the two memories related to these events are likely to be co-allocated to the same engram. Here, we aimed to study how co-allocated memories evolve through time, in relation to a time gap of 6 hours between the two experienced events. The results showed that two co-allocated memories are susceptible to the passing of time that shapes and separates them to form two independent memories encoded by different sets of neurons. In addition, we found that even the inhibitory neuronal network is implied in the separation via the activation of parvalbumin (PV)-expressing neurons, which, once pharmacologically inactivated, impede the memory separation. We found the meaning of this separation in a functional advantage for the animal to keep the specificity of the memory for a unique event without ambiguity and to preserve its entirety from external manipulations, so that the modification of one doesn’t influence any other memory and viceversa. From a clinical point of view the separation represents an advantage since a disfunction of one memory, like it happens in post-traumatic stress disorders, doesn’t interfere with the discrimination with other memories keeping them independent.