Scientists at the Center for Interdisciplinary Research in Biology (CNRS / Collège de France / INSERM) have shown that delta waves emitted during sleep are not generalized periods of silence during which the cortex rests, as has been described for decades in the scientific literature . On the contrary, they isolate groups of neurons that play an essential role in the formation of long-term memory. These results were published on October 18, 2019 in Science.
While we sleep, the hippocampus spontaneously reactivates generating activities similar to those while we are awake. Send information to the cortex, which reacts in turn. This exchange is often followed by a period of silence called "delta wave", hence by rhythmic activity called "melt of sleep". This is when cortical circuits reorganize to form stable memories. However, the role of delta waves in the formation of new memories is still an enigma: why does a period of silence interrupt the sequence of information exchanges between the hippocampus and the cortex and the functional reorganization of the cortex?
The authors here have examined more closely what happens during the same delta waves. They found, surprisingly, that the cortex is not completely silent but that some neurons remain active and form assemblies, or small coactive sets that encode information. This unexpected observation suggests that the small number of neurons that are activated when all others remain silent can perform important calculations while they are protected from possible disturbances. And the discoveries of this work go even further! The spontaneous reactivations of the hippocampus determine which cortical neurons remain active during delta waves and reveal the transmission of information between the two brain structures. Furthermore, the assemblies activated during delta waves are formed by neurons that have participated in the learning of a spatial memory task during the day. Together, these elements suggest that these processes are involved in memory consolidation. To prove this, scientists in rats caused artificial delta waves to isolate the neurons associated with reactivations in the hippocampus or random neurons. Result: when the right neurons were isolated, the rats were able to stabilize their memories and succeed in the spatial test the next day.
These results substantially change the way we understand the cortex. Delta waves are therefore a means to selectively isolate assemblies of selected neurons, which send crucial information between the periods of hippocampus-cortical dialogue and the reorganization of cortical circuits, to form long-term memories.
Source of the story:
. (tagsToTranslate) Nervous system; Research in psychology; Research on insomnia; Parkinson