During sleep, the hippocampus reactivates spontaneously, generating activity similar to that of wakefulness. It then transmits information to the cortex, which reacts accordingly. This exchange is often followed by a period of silence known as the "delta wave", followed by rhythmic activity known as the "sleep spindle". This is when cortical circuits reorganize to form stable memories. However, the role of delta waves in the formation of new memories remains an enigma: why does a period of silence interrupt the sequence between hippocampal-cortical information exchanges and the functional reorganization of the cortex?
The researcher and co-author of this article took a closer look at what happens during the delta waves themselves. Surprisingly, they discovered that the cortex is not entirely silent, but that some neurons remain active and organize themselves into assemblies, i.e. small coactive sets encoding information. This unexpected observation suggests that the few neurons that are active while all the others are silent may thus be able to carry out important calculations, sheltered from possible disturbances. And the findings of this study go even further! Spontaneous hippocampal reactivations determine which cortical neurons remain active during delta waves, revealing the transmission of information between the two brain structures. In addition, the assemblies activated during delta waves are made up of neurons that have been highly solicited when learning a spatial memory task during the course of the day. All this suggests that these processes are involved in memory consolidation. To demonstrate this, the scientists induced artificial delta waves in rats to isolate either neurons associated with hippocampal reactivations, or neurons at random. The result: when the right neurons were isolated, the rats stabilized their memories and passed the spatial test the following day.
These results have led to a profound revision of our understanding of the cortex. Delta waves could be a means of selectively isolating selected assemblies of neurons, which transmit crucial information between periods of hippocampal-cortical dialogue and cortical circuit reorganization, to form long-term memories.
Bibliography
Isolated Cortical Computations during Delta Waves Support Memory Consolidation. Ralitsa Todorova and Michaël Zugaro, Science, October 18, 2019. DOI : 10.1126/science.aay0616
