Relationship between Consolidation of Memories and Sleep
A research group found that if the cerebral neocortex is reactivated, memories can be improved without necessity of sleep.
Sign of RIKEN
REM Sleep and Non-REM Sleep
There are two types of sleep: shallow sleep, which is referred to as REM (Rapid Eye Movement) sleep, and deep sleep, which is referred to as non-REM sleep. When a person falls asleep, the non-REM sleep occurs, followed by the deepest non-REM sleep within one hour. After an elapse of one or two hours, the REM sleep gradually occurs. Thereafter, the non-REM sleep and the REM sleep alternately occur. The REM sleep lasts for about 20 to 30 minutes nearly every 90 minutes. In a typical sleep of 6 to 8 hours per night, the REM sleep occurs four to five times.
When a person falls asleep, perceived experiences are consolidated as memories. While the person is sleeping, the memories are processed in the brain. So far, it was unknown what part of the brain handles consolidation of memories. However, it is known that the high-level secondary motor cortex (M2) in the cerebral neocortex is connected to the low-level primary somatosensory cortex (S1) so as to form a top-down circuit. A research group led by Masanobu Murayama, the group leader of RIKEN Brain Science Institute in Japan, found that perceived memories are consolidated when information is input to the top-down circuit that transmits information from M2 to S1.
The research group used mice that had just learnt perceived experiences. When information that had been input to the top-down circuit was suppressed, the perceived memories were prevented from being consolidated. Thus, immediately after the mice had learnt perceived experiences, if the mice fell asleep, the perceived experiences were consolidated as memories. In addition, immediately after the mice had leant perceived experiences and they had fallen asleep in the non-REM sleep state, when both M2 and S1 were optically stimulated at the same time, the mice could store the learnt perceived memories longer than those that regularly fell asleep. As a result, if the perception circuit of the brain was activated while the mice were falling asleep, the memories were improved.
Unless humans and animals fall asleep for long hours, their memories are prevented from being consolidated. However, if mice that have just leant perceived experiences are kept awaking and M2 and S2 are optically stimulated at the same time, they can store the perceived memories longer than those that regularly fell asleep. Even if a person does not have enough sleep, if the cerebral neocortex is stimulated at an appropriate timing, the perceived memories can be improved.
The cerebral neocortex containing the sensory cortex optically stimulated is located on the surface of the brain. In recent years, transcranial magnetic stimulations and transcranial DC stimulations have been clinically studied.
If the stimulation pattern of the cerebral neocortex using mice is clinically applied, memory disorder due to sleep disorder could be cured.
This research was published in the online edition of scientific journal "Science" on May 26, 2016.