While we sleep, the hippocampus reactivates itself unexpectedly by producing activity like that while we are awake. It sends data to the cortex, which response in turn. This exchange is frequently trailed by a time of quiet called a ‘delta wave’ then by rhythmic action called a ‘sleep spindle.’
However, the role of delta waves design to frame stable recollections. Notwithstanding, the job of delta waves in the formation of new memories is as yet a riddle: 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?
Now scientists at the Center for Interdisciplinary Research in Biology (CNRS/Collège de France/INSERM) have shown that delta waves are the reason.
Delta waves are moments of widespread cortical silence that alternate with active states during slow-wave sleep.
Scientists have shown that delta waves emitted while we sleep are not generalized periods of silence during which the cortex rests, as has been described for decades in the scientific literature. Instead, they isolate assemblies of neurons that play an essential role in long-term memory formation.
Scientists carefully observed what happens during delta waves themselves. They found that the cortex is not entirely silent but that a few neurons remain active and form assemblies.
This careful observation suggests that the small number of neurons that activate when all the others stay quiet can carry out essential calculations while protected from possible disturbances.
According to scientists, this work goes even further.
Spontaneous reactivations of the hippocampus determine which cortical neurons remain active during the delta waves and reveal transmission of information between the two cerebral structures. Also, the assemblies activated during the delta waves are formed of neurons that have participated in learning a spatial memory task during the day. Together these elements suggest that these processes are involved in memory consolidation.
In rat models, scientists caused artificial delta waves to isolate neurons associated with reactivations in the hippocampus. They found- when the right neurons were isolated, the rats managed to stabilize their memories and succeed at the spatial test the next day.
Scientists noted, “The study substantially change how we understand the cortex.”
The study is published in Science.