We knew already the essential role of sleep in the consolidation of memory. During its various phases, sleep allows the brain to sort between important and unnecessary information. On the other hand, we did not know which method the neurons use to perform this work. The work of a team from INSERM that has just been published in Science Advance makes it possible to see more clearly this process which proves to be much more complex than imagined.
Cells of the brain constantly exchange information and, during the phases of sleep, this activity serves in particular to consolidate memory. The electroencephalogram, which measures the global activity of the brain, shows regular waves, more or less fast, depending on the phase of sleep but does not allow to know how each information is treated at the level of the neuron.
“We imagined that neurons functioned in a very precise and repetitive way to transmit information or store it,” explains Christophe Bernard of the Institute of Neuroscience Systems at INSERM in France. Using electrodes recording the electrical activity of a hundred or so neurons concentrated in a given region of the brain, his team found that the path of information actually changes constantly: groups of neurons are organized among themselves for very short periods of time to store and transmit information by constantly relaying it, with only a few of these neurons playing a leading role within each group.
“There is thus a succession of sub-states with, in the end, about half of the neurons of the three regions, the hippocampus, the prefrontal cortex and the entorhinal cortex, which play a key role in the processing of information at one time or another, in other words, there is no established hierarchy within the neurons but rather a balanced distribution of roles,”says Christophe Bernard.
And it is in the discovery of this complex process that lies the surprise of researchers: “The dominant theory was that the transfer of information followed a fixed path, a little like a well-tuned industrial machine, however this is not the case!,” recognizes Christophe Bernard.
“It happens a little like on the internet, an email that leaves from Paris to Sydney will go through servers in different countries during its routing, and these servers will vary during the day depending on the traffic. In the brain, it is the same, even when the information is the same, the routes it borrows only are not fixed and the partners are constantly changing!”
These works have also made it possible to better understand the “language” of neurons: a sub-state corresponds to a word, a sequence of sub-states constitutes a sentence, that is to say that this language is complex.
This complexity is greater during REM sleep (the one during which dreams are expressed) than during slow sleep. This offers a research track to study in particular the possible link between memory loss in epileptic subjects and the complexity of neuronal language.