How memory updates over time

How do neurons manage to encode the memory of an event experienced, and integrate it into a life course? A study, published on November 6 in the journal Naturebrings new pieces to the brain palace of memory, this “sentinel of the spirit”selon William Shakespeare (1564-1616).

A team from the Mount Sinai School of Medicine in New York wanted to understand how a memory could be integrated into recent life experiences. In adult mice, the authors followed the evolution of the memory of a stressful event: a small electric shock on a paw, which the rodent learns to associate with a given spatial environment. For example, the animal receives no shock when placed in a blue triangular room, but only when it is in a red rectangular room.

The researchers mapped the neurons activated at two times: first, while the mice were learning about these negative experiences, and second, while they were resting, motionless, a few minutes later (duration periods). “calm awakening”). To do this, they used calcium imaging, which consists, by injecting a virus into the brain, of making neurons produce a particular protein: a probe. which changes fluorescence depending on the calcium ion concentrations in these cells. However, these concentrations increase when a neuron is activated. Measuring changes in fluorescence therefore makes it possible to quantify the activity of each of them.

Consolidate a memory

The authors focused on the neurons of the hippocampus, this pillar of memory encoding. This brain structure is, in particular, a champion of comparisons on life experiences: it creates links between the sensory and the “where”, the “what” and the “when” of the events experienced..

First observation: a few minutes after each experience, while the mouse is in a period of calm wakefulness, its brain “replays” this experience. Clearly, the neuron circuit activated during this experience, in the hippocampus, is then reactivated. A phenomenon, in reality, already known.

“In 1989 and then in 1994, we discovered that the neurons which encode a recent memory can be reactivated during animal sleep”indicates Raphaël Brito, postdoctoral researcher in neuroscience at the Collège de France (CNRS, Inserm). Then, « in 2006 and in 2007, we realized that these neurons could also be reactivated during periods of wakefulness, at the time of acquisition of the memory and shortly after.adds Céline Drieu, postdoctoral researcher at Johns Hopkins University in Baltimore (Maryland).

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