What role does rapid eye movement (REM) sleep play in forming human memories?

Just as getting deep sleep is integral to better mental and physical health, the REM stage of sleep helps the brain turn everyday experiences into emotional and contextual memories, a new study revealed.

But sufficient and uninterrupted REM sleep is so vital that research involving REM sleep deprivation would be deemed unethical. Studies have shown that lack of deep sleep could result to increased irritability, anxiety, lack of focus and stress.

Because of this, scientists resort to performing studies on lab animals. Although the results cannot be as simply translated to humans, it can provide insight on the beginnings of memories.

Formation Of Emotional And Contextual Memories

Emotional memories are long-lasting. They can either enrich or deepen our lives or haunt us with a paralyzing effect. These memories are often linked to strong emotions such as nurturance and fear.

Contextual or spatial memories, meanwhile, help us identify people, objects, places and remember their significance. Merging these two kinds of memories is extremely important to navigating our social and physical worlds.

In the new study, a team of researchers from Switzerland and Canada applied optogenetics to examine the process of how these two types of memories are consolidated in mice during REM sleep. Optogenetics is the use of light and chemicals to switch certain neurons on or off.

For humans, slipping into REM sleep could take more than an hour. It is characterized by the rapid twitching of the eyes under our lids, which scientists say is the loss of voluntary muscle movement and an apparent sign of vivid dreaming.

Inside the brain, REM sleep is marked by subtle changes. In the brain regions central to memory, learning and emotions — neocortex, hippocampus and amygdala — specific neurons start to synchronize in a distinct pattern. This theta rhythm might possibly be the key to memory consolidation.

Scientists used optogenetics to disrupt the hum of theta rhythm in sleeping mice while being careful not to wake them up. They focused on several neurons located in the medial septum that act as pacing mechanisms to begin and halt the theta rhythm during REM sleep.

After selectively switching these neurons off, the brains of the mice were prevented from entering the REM stage, but they were still asleep.

How Lack Of REM Sleep Affected Mice

Among mice, the emotional and contextual memories are laid down in a survival manner.

The lab mice received a foot shock whenever it stepped on a specific spot. Researchers then tried to see whether the mice, which has committed its dread and fright of another shock to memory, will avoid the spot.

Scientists also took advantage of the mice's impulse to explore a new object or a familiar object placed in a new spot in order to test contextual memories. If one of the lab mice fails to favor a newly placed object over a familiar one for exploration, researchers assume that the mice did not make a contextual memory.

In the end, the research team found that although the REM sleep-deprived mice were well-rested, they were less likely to have formed memories of conditioning experiments they went before sleeping.

These REM sleep-deprived mice were less likely to avoid the place where they experienced shock and unlikely to demonstrate any memory that they have seen an object before.

What's interesting, researchers said, is that it was only during REM sleep that the memories of the lab mice were impacted. When they tried to impair theta rhythm outside of REM, the memories were not affected.

The findings of the study featured in the journal Science need to be replicated in a human trial in the future for further affirmation. It was conducted by scientists from McGill University and Inselspital University Hospital.

Photo: Andrew Roberts | Flickr

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