How our memories stabilise during sleep decoded

Researchers have shown that delta waves - a pattern of high amplitude electrical activity in the brain emitted during sleep - are not silent resting periods for the brain's cortex, but play an essential role in long-term memory formation.

The study, published in the journal Science, noted that when humans sleep, the seahorse shaped brain region called hippocampus reactivated itself spontaneously to a state similar to its activity during wakefulness. According to the researchers from Centre national de la recherche scientifique (CNRS) in France, during sleep, the hippocampus sends information to the cortex, which reacts in turn.

The study noted that this exchange is often followed by a period of silence called a 'delta wave,' then by rhythmic activity called a 'sleep spindle'. This is when the cortical circuits reorganize to form stable memories, the researchers said. However, they added that the role of delta waves in the formation of new memories is still a puzzle.

"Why does a period of silence interrupt the sequence of information exchanges between the hippocampus and the cortex, and the functional reorganisation of the cortex," the researchers asked.

When the researchers looked more closely at what happens during delta waves, they discovered that the cortex was not entirely silent, but that a few neurons remained active and formed small, coactive sets that coded information. They suggested that the small number of neurons that remain active when all the others stay quiet can carry out important calculations while being protected from possible disturbances.

According to the study, spontaneous reactivation of the hippocampus determined which neurons in the cortex remained active during the delta waves, and revealed the transmission of information between the two brain regions. The neuron sets activated during the delta waves are formed of cells that participated in learning a spatial memory task during the day, the researchers said.

Putting these observations together, the researchers suggested that these processes are involved in memory consolidation. To prove this, the scientists caused artificial delta waves in rats to isolate either neurons associated with reactivation in the hippocampus, or random neurons.

When the right neurons were isolated, the study noted that the rats managed to stabilise their memories and succeeded at the spatial test the next day. Delta waves, the researchers said, are a means of specifically isolating assemblies of chosen neurons that sent crucial information between the hippocampus and cortical brain regions, and contributed to the reorganisation of brain circuits to form long-term memories.