Rapid eye movement (REM) is the period when we see dreams, but a new study shows that this happens when we forget, according to ZME Science.
In a study on mice a group of researchers from Japan and the USA found that the brain may use REM sleep to actively forget the extra information. The authors also suggest a group of neurons deep inside the brain that control this process of forgetting during sleep.
“Have you ever wondered why we forget many of our dreams?” — asks Thomas Kilduff, Ph. D. and senior author of the study.
“Our results show that the start-up of a particular group of neurons during REM sleep controls whether the brain learns new information after a good night’s sleep.”
REM is one of several stages of sleep that our bodies undergo every night. It usually begins about 90 minutes after we fall asleep, and got its name from the rapid, high-pitched movements of our eyes during this phase. For it is also characterized by palpitations, the fixed limbs, the dream and the patterns of brain waves resembling awake state.
The role of sleep in memory storage have been studied in the past — especially his role in helping our brain form new memories. However, the researchers did not examine whether it can help the brain to get rid of the extra information stored during the day.
Recent studies in mice showed that during sleep, including during REM sleep, some synaptic connections involved in learning to selectively “cut” that effectively destroys the memory that they hold.
This is the first study of such a plan.
“Understanding the role of sleep in oblivion can help researchers better understand a broad range of storage-related diseases such as post-traumatic stress disorder and Alzheimer’s disease,” says Janet He, Ph. D., program Director at the National Institute of Neurological disorders and stroke (NINDS).
“This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to keep”.
Team doctor Kilduff together with a team of doctors of Sciences Akihiro Yamanaka of Nagoya University in Japan has spent years studying the role of hormone hypocretin/orexin in the control of sleep and narcolepsy.
Narcolepsy is a disorder that causes people to feel excessive sleepiness during the day and sometimes experience a feeling resembling a quick night’s sleep, including loss of muscle tone in the limbs and hallucinations. Narcolepsy may be associated with loss of neurons that produce the hormone in the hypothalamus, a region the size of a peanut, found deep inside the brain.
For the present study Dr. Kilduff collaborated with members of the Hokkaido University in Sapporo, Japan, to look at cells adjacent to the neurons that secrete melanin-concentrating hormone (MCH), which is involved in appetite control and sleep.
They found that the majority (52.8 per cent) of MCH cells in the hypothalamus worked when the mice were asleep fast sleep, about 35% worked only when the mouse was awake, and about 12% worked both times — this is consistent with previous data on this issue.
Electrical brain recordings and experiments on the tracking also showed that many cells of the hypothalamus MCH sends a deny message in the hippocampus, the memory center of the brain via long axons.
“From previous studies carried out in other laboratories, we already knew that the cells MCH was active during REM sleep. By opening this new scheme, we thought that these cells can help the brain to store memories,” said Dr. Kilduff.
To test this idea, the researchers switched on and off MCH neurons in mice during the memory tests. They were particularly interested in the role these cells play in the retention, that is, the period between learning something and its storage (consolidation) in long-term memory — a kind of “uncertainty” of memory.
The team reported that the activation of MCH cells during storage has worsened long-term memory consolidation, and disabling them improved it. For example, activation of cells reduced the time that mice spent, sniffing new objects compared with familiar, but off cells had the opposite effect.
Further experiments have shown that MCH neurons perform this task during REM sleep. The mouse is better behaved in tests of memory when the MCH neurons were turned off during REM sleep, and off neurons when the mouse did not sleep or were in other States of sleep did not affect memory.
“These results show that MCH neurons help the brain to actively forget a new, perhaps unimportant information,” said Dr. Kilduff.
“Since it is believed that dreams mostly occur during REM sleep, the stage of sleep when include MCH cells, activation of these cells can prevent users from saving content in the hippocampus of sleep — hence the dream is quickly forgotten”.