Nicole Zhao ’20

Imagine having the ability to never forget. This would come in handy if one needed to memorize a textbook or lecture slides for an exam. However, being able to remember every single moment of your life in snapshots does have its drawbacks. This is exactly what happened to a man known as subject S. who was known for his unforgettable memory in 1929 (1). Although he could recite a book that he learned decades earlier, he had trouble understanding abstract concepts or figurative language (1). He also had trouble recognizing faces because he only had memories of them at an exact point and was unable to understand the concept of aging (1). Therefore, it was believed that forgetting might just be as important as remembering. In a new study, researchers at Nagoya University in Japan have identified a group of neurons that may be responsible for helping the brain to forget (2). 

These brain cells, known as melanin-concentrating hormone (M.C.H.) neurons are located in the hypothalamus and are most active during rapid eye movement (R.E.M.) sleep (2). The hypothalamus is an area of the brain involved in producing hormones that maintain homeostasis, including those involved in sleep and arousal (3). R.E.M. sleep is a cycle of sleep characterized by a state resembling a wakefulness and vivid dreams. Researchers found that the M.C.H. neurons in the hypothalamus suppressed neurons in the hippocampus, a brain region that is responsible for the consolidation of memory (2). 

To determine to role of M.C.H. neurons and their effect on memory consolidation, researchers performed a controlled experiment in which they activated or suppressed M.C.H. neurons before performing memory exams on mice (2). First, the mice were allowed to see and sniff two items before researchers artificially activated or suppressed their M.C.H. neurons. After, the mice in the two groups would be presented with the same objects in addition to different ones and the rate of approach would determine whether they remembered or not. It was found that activating M.C.H. neurons during R.E.M. sleep worsened their memory and the mice would approach the new items with the same frequency as they did with the old items which they have seen and smelled (2). Researchers further validated these results with another experiment in which mice formed bonds with items which they liked to play with and activating M.C.H. neurons would change this behavior (2). 

The results of these experiments suggest that hypothalamic M.C.H. neurons actively help the brain forget information that is not important. Moreover, it is important to note that both experiments worked only when M.C.H. neurons were activated or suppressed during R.E.M. sleep. Their high activity during this sleep cycle may explain why we forget our dreams when we wake up. Although activation of M.C.H. neurons may not be the only explanation to why we forget, it plays a role in doing so and contributes to the necessary balance between forgetting and memory consolidation. 

References:

1. R. Johnson. The mystery of S., the man with an impossible memory. The New Yorker (2017). 
2. S. Izawa, et al., REM sleep–active MCH neurons are involved in forgetting hippocampus- dependent memories. Science 365, 1308-1313 (2019). doi: 10.1126/science.aax9238.
3. J. Johnson. Hypothalamus: function, hormones, and disorders. Medical News Today, (2018). 
4. Brain basic: understanding sleep. National Institute of Neurological Disorders and Stroke, (2019). 
5. Image retrieved from: https://www.flickr.com/photos/nihgov/25873052091