If you are able to recall a vivid memory and bring it back to your consciousness, it is because it is engraved in your brain in the form of an engram. But can’t everything you do be kept? How do engrams work?

From the German Engram, is a term derived from the Greek in (“in”) and gramma (“writing”).

In neurophysiology, the engram is the trace of memory left in the brain as a result of a past event or a lived experience and which could be reactivated by an appropriate stimulation.

Engrams are the brain imprints that each of our experiences gives us.

Memory is a mechanism that allows humans to store “data” in order to better adjust to their environment. The idea of ​​a physical perception of memory dates back over 2 years. Resumed in 000 BC, Aristotle formalized that “the process of sensory stimulation marks a kind of impression of the percept, just as a seal leaves its imprint in hot wax”.

This idea gradually led to the hypothesis that sets of cells, organized and selectively activated, form the basic building blocks of the memory trace, the engram.

The stronger the bonds between the neurons in the engram, the longer the memory will last. This theory has not been confirmed yet but has been taken up. Since 2013, it has once again been at the center of research on the traces of memory.

Several studies indirectly suggest the presence of engrams. These have been analyzed by two neuroscientists who explain that all of this work would make the engram the basic unit of our memory.

With regard to all intellectual activities and processes that relate to knowledge and the function that carried it out (cognition), investigations into the role of engrams have been carried out in mice.

• A first study reports the loss of memory in mice deprived of a neuron network of the lateral amygdala suggesting the presence of a hypothetical engram;
• In a second, the scientists artificially stimulated a set of active hippocampal dentate gyrus neurons during a fear experiment. As a result, the mice experienced fear, which evokes a memory without an outside signal reactivating it and therefore a potential engram.

The clues are also to be sought on the side of the neurons that make up an engram because they are more sensitive to stimuli and have greater plasticity.

Today we know that a large part of our mental life is based on engrams.

Engrams create our consciousness and are like roots that define everything we are: what makes us react, tremble with fear or happiness, unlike neural networks that are formed as we learn.

Take the example of a flower that we distinguish, we feel a certain pleasure because we anticipate its smell, even if it has not yet reached us. Drinking a cup of chocolate evokes childhood memories, listening to music that is familiar to us, gives us a whole series of feelings of well-being, entertainment and happiness.

In a new study, the findings of which are published in the journal Neuron (Hasan et al., 2019), the international team coordinated by Drs. Alexandre Charlet (France) and Valery Grinevich (Germany) prove that engrams can form in places like the hypothalamus.

To conduct this study, they focused on the neurons that create oxytocin. Oxytocin is a neuropeptide strongly involved in the regulation of emotions such as pain and fear.

The authors discovered the formation and integration of hypothalamic engrams, the manipulation of which categorically modifies the expression and memory of a fear (expression of fear entirely erased).

On the other hand, these neurons show an astonishing plasticity, going from a slow transmission by means of oxytocin to a much faster communication, via the secretion of glutamate.

Understanding the underlying physical and functional circuits of an emotion such as fear could allow the emergence of new therapeutic techniques, when fear becomes pathological for example, as in the case of post-traumatic stress disorders.