The brain continues to perform activity whether we are sleep or awake. During the period we sleep, our brain is believed to be involved in stabilising long term memories. Different regions of the brain are involved in different activities, and even while sleeping key regions of the brain—involved in long term memory formation—remain active. There have been a plethora of studies dedicated to deciphering brain activities during sleep that stabilise memories.
The brain also emanates certain waves at different stages, known as brain waves. During sleep, in the resting period, the characteristic brain wave is the delta wave. The brain waves are the collective behaviour of large numbers of neurons put together at work. The different characteristic brain waves show different activity patterns of the brain while performing different tasks.
Hippocampus and cortex are two key areas of the brain that are involved in the process of long term memory consolidation. During sleep, the hippocampus reactivates itself spontaneously and generates activity pattern similar to that while we are awake. Also, the hippocampus and cortex cross talk, with the cortex reacting to the signals sent by hippocampus. This exchange of information is followed by a long period of quiescence or silence which is the delta wave. And the delta wave is then followed by a rhythmic activity called a sleep spindle. It is during the period of the sleep spindle that the cortex reorganises its circuits to form stable memories.
What remains a puzzle is the delta wave. Why does the period of silence represented by the delta wave interrupt the sequence of information exchange between the hippocampus and the cortex, and the functional organisation of the cortex?
A recent study published in Science looked into what happens during the delta wave. The researchers found that delta waves generated in the brain during sleep are not generalised periods of silence that the cortex rests, which has been the explanation for decades. In other words, the new finding breaks the decades-old perception about the delta waves. The study suggests that cortex actually isolate assemblies of neurons that play an essential role in long term memory formation.
In the period of the delta wave, in the cortical area, few neurons remain active and also form assemblies—through inter neuronal connections. The assembly of small fraction of neurons code information. This is a surprising finding. Because, during the delta wave, most of the cortical neurons remain silent and this is expected. But the small numbers of neurons that are active, as the research proposes, can perform important calculations while protected from possible disturbances. The discoveries go even further. It says that spontaneous reactivation of neurons in the hippocampus decides which neurons in the cortex would remain active.
During the day’s activities, the brain learns many things and tends to make memories of many of them. The neurons that participate in the formation of a spatial memory are also the neurons that form the assembly in the cortex during the period of delta wave. It is amazing how the brain solves so many complexities in memory formation for a long term.
The scientists caused artificial delta waves in rats to isolate neurons associated with reactivation in the hippocampus or random neurons. The result was—when the right neurons were isolated, the rat was found to stabilise its memory and succeed at the spatial test next day. These results significantly change our understanding of the cortex.