Scientists at the RIKEN Center for Brain Science in Japan have discovered how sleep helps strengthen memories linked to positive emotions, such as happiness and joy. Their study, published in Neuron, challenges the long-held belief that emotional memories are mainly processed during dream-filled REM sleep.
Instead, the researchers found that non-REM sleep, which occurs earlier in the sleep cycle, plays the most important role in making these memories last.
This discovery could help explain why certain memories—like a song, a smell, or a texture—become strongly tied to emotional events. It may also lead to new treatments for conditions like addiction, where emotional memories can trigger relapses.
How Do Emotional Memories Last Longer?
Scientists already know that sleep is essential for turning new experiences into lasting memories. However, they wanted to understand why emotional events seem to create much stronger and longer-lasting memories compared to neutral experiences.
To explore this, researchers led by Masanori Murayama developed a way to study emotional memory in mice. First, they let male mice explore a smooth surface. The next day, they introduced both smooth and grooved textures to see which the mice preferred.
Normally, mice like new environments, so they spent more time exploring the grooved texture, suggesting they didn’t remember the smooth one very well.
But when the researchers paired the smooth texture with a positive experience—letting the mice interact with a female mouse—their memory of the texture became much stronger. Even four days later, the mice still preferred the smooth texture, showing that emotional experiences help preserve memories for a much longer time.
The Brain Circuit That Strengthens Emotional Memories
By studying brain activity, the researchers identified a key circuit responsible for strengthening emotional memories. The amygdala, the brain’s emotional center, plays a major role in this process.
It connects to a pathway between the motor cortex (which controls movement) and the sensory cortex (which processes touch and other sensory information). This three-part brain circuit strengthens emotionally linked perceptual memories.
The researchers discovered that this brain circuit was activated when the mice first experienced the texture and the positive emotion. Later, during non-REM sleep, the same brain areas reactivated, reinforcing the memory.
To test whether this non-REM sleep reactivation was essential, the scientists temporarily blocked signals from the amygdala during this sleep stage. When they did this, the mice forgot the smooth texture within a few days—despite having associated it with a positive experience.
However, blocking these signals during REM sleep had no effect, confirming that non-REM sleep is the key sleep stage for strengthening emotional memories.
A New Understanding of Emotional Memory and Addiction
For years, REM sleep was thought to be the most important stage for processing emotional memories. This study challenges that idea, showing that non-REM sleep plays a bigger role in strengthening positive perceptual memories.
These findings could have important medical applications, especially for treating addiction. Many people with addiction experience flashbacks—sudden, intense memories of past drug use or other emotional experiences that trigger cravings. These flashbacks are thought to be caused by strong connections between sensory memories and emotions.
By targeting the amygdala and its related brain circuits during non-REM sleep, scientists may be able to weaken these unwanted memories. This could help prevent addiction relapses by reducing the power of emotional triggers.
Future Research and Possible Treatments
The researchers plan to explore how these findings could be used to treat other conditions, such as memory loss in aging. “It will be important to examine whether we can recover or even strengthen memories in older mice,” said Murayama.
Their ultimate goal is to use this knowledge to develop treatments that improve mental health and help people with memory-related conditions. By better understanding how sleep shapes our emotional memories, scientists may be able to create therapies that enhance positive experiences while weakening harmful ones.
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The research findings can be found in Neuron.
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