How the sleeping brain decides which memories are worth keeping

Marie Kondo could get down with this.


In the quest to improve memory, humans exercise physically and train mentally, but one fact remains: The brain may simply be built to forget.

New research on mice shows that a specific type of neuron in the brain is fundamentally involved in helping the brain forget memories during that most seductive of sleep periods, rapid eye movement, or REM, sleep.

Researchers presented evidence in the journal Science on Thursday that a type of neuron in the brain’s hypothalamus — melanin-concentrating hormone-producing neurons, or MCH neurons for short — are especially active while mice are in REM sleep. Those same neurons aren’t nearly as active during non-REM sleep.

They also learned that inhibiting the MCH neurons during REM sleep improved the mice’s memory abilities, whereas activating MCH neurons impaired the mice’s memories.

Previous research has shown that sleep rebalances synapses in the brain, shrinking these spaces between neurons to prepare our brains for learning again the next day.

Other work has shown that sleep may play an important role in clearing toxins from the brain.

With this latest study, the team sheds light on a crucial function of sleep: helping the brain forget unimportant memories while we sleep.

Akihiro Yamanaka, Ph.D., a professor at Nagoya University’s Research Institute of Environmental Medicine in Japan and the paper’s corresponding author, tells Inverse why he and his team pursued this study: They wanted to reveal the neural mechanism that regulates sleep vs. wakefulness.

Melanin-concentrating hormone (MCH)-producing neurons are somewhat active during wakefulness, inactive during NREM sleep, and highly active during REM sleep. Researchers concluded that these neurons are crucial to our brain's ability to forget unnecessary memories while we sleep.

“Melanin-concentrating hormone (MCH)-producing neurons are located in the hypothalamus which is thought to be a center of sleep/wakefulness regulation,” he says. “We focused on MCH neurons to reveal their physiological function.”

And indeed they seem to have done just that.

To investigate the MCH neurons, the team used genetically altered mice whose MCH neurons could be activated and inhibited by either shining a light on the brain or introducing a specific chemical into the brain. These two techniques both achieved the same effect: When the MCH neurons were turned off during REM sleep, the mice scored significantly better at multiple different memory tests. Conversely, when the MCH neurons were turned on during REM sleep, the mice scored significantly worse. Altering the neurons’ activity at other times did not show significant effects.

Under normal circumstances, the MCH neurons, which are only found in the brain’s hypothalamus, are slightly active during waking hours, inactive during non-REM sleep, and highly active during REM sleep. Therefore, the researchers conclude that these neurons play a central role in the brain forgetting unnecessary memories during REM sleep.

But why do we need to forget?

“Probably, to forget is important for the selection of important memories and not important memories,” says Yamanaka. “In addition to this, forgetting saves memory resource in the brain.”

And the difference between an important and unimportant memory, he says, likely comes down to our emotional connection to these memories.

REM sleep is the phase of sleep when MCH neurons activate in the brain and help us forget the unnecessary memories from that day.

Unsplash / Alexandra Gorn

“This is an important point to reveal,” he says. “Important memories are formed with emotional input, which comes from the center of emotion, the amygdala. MCH neurons give pressure to erase all memory, [but] important memories could survive this pressure.”

In other words, a memory that has formed a connection among different brain regions is more likely to survive this nightly housecleaning process.

Yamanaka explains that while the exact function of REM sleep is still not clear, his team’s data shows that part of its importance has to do with forgetting.

“Without this process, memory ability would be decreased, for example [we] cannot retrieval memories at appropriate situations,” he says.

He’s careful to point out that this research does not open the door to manipulating people’s memories. The type of genetic alterations done to the mice that enabled scientists to switch their MCH neurons on and off are not done in humans, and likely won’t be done on humans in the foreseeable future.

What this research does do is emphasize the importance of REM sleep. So while there may not be any kind of technological brain hack that helps you hold onto memories, there is a good old-fashioned one you can employ to ensure that your brain forgets unnecessary memories and frees up resources for new ones: a good night’s sleep.

Abstract: The neural mechanisms underlying memory regulation during sleep are not yet fully understood. We found that melanin concentrating hormone–producing neurons (MCH neurons) in the hypothalamus actively contribute to forgetting in rapid eye movement (REM) sleep. Hypothalamic MCH neurons densely innervated the dorsal hippocampus. Activation or inhibition of MCH neurons impaired or improved hippocampus-dependent memory, respectively. Activation of MCH nerve terminals in vitro reduced firing of hippocampal pyramidal neurons by increasing inhibitory inputs. Wake- and REM sleep– active MCH neurons were distinct populations that were randomly distributed in the hypothalamus. REM sleep state–dependent inhibition of MCH neurons impaired hippocampus-dependent memory without affecting sleep architecture or quality. REM sleep–active MCH neurons in the hypothalamus are thus involved in active forgetting in the hippocampus.
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