How Memories Stabilize When We Sleep

The hippocampus reactivates itself spontaneously by generating activity similar to that while we are awake, it sends information to the cortex which reacts in turn, this exchange is often followed by a period of delta wave silence then by sleep spindle rhythmic activity. This is when cortical circuits reorganize to form stable memories, but the role of delta waves in the formation of new memory is still not fully understood. 

While examining what happens during delta waves themselves it was discovered that the cortex is not entirely silent, a few neurons remain active and form assemblies such as a small coactive set that codes information; this surprising observation suggests that the small number of neurons that activate while others stay quiet can carry out important calculations while being protected from possible disturbances. 

Hippocampus spontaneous reactivations determine which cortical neurons remain active during delta waves and reveal transmission of information between cerebral structures; the activated assemblies are formed by neurons that have participated in learning a spatial memory task from the day. When taken together this suggests that the processes are involved in memory consolidation. 

Artificial delta waves were caused in rats to demonstrate their findings by either isolate either neurons associated with reactivation in the hippocampus or random neurons; when the right neurons were isolated the rats were able to stabilise their memories and succeed in spatial testing the following day.

Findings change understandings of the cortex; delta waves are therefore a means of selectively isolating assemblies of chosen neurons which send information between periods of hippocampo cortical dialog and reorganisation of cortical circuits to form long term memories.