Are Our Short-term Memories Linked to Our Long-term Memories?

How do you think we get our long-term memories? If you answered “from our short-term memories”, your answer would fit that of the general belief about long-term memories. For decades, neuroscientists believed that our brain records short-term memories and then eventually those memories are transferred to another part of the brain to become long-term memories.

This was mainly a result of the study of brain injuries, which informed our current understanding of memory formation.


Brain research
The link between brain injury and memory

Brain injury and memory

Our knowledge about how our brains process memories are mainly a result of studying brain injuries in the past. One man, in particular, named Henry Molaison, played a key role in the understanding of our brain’s hippocampus. In 1953, doctors operated on him to control his epileptic seizure. They ended up removing his hippocampus.

The hippocampus is a seahorse-shaped brain structure that can be found on each side of the brain.

So whatever happened to Henry Molaison? After the surgery, he couldn’t store any new memories at all! That’s when the doctors realized the hippocampus’ role in creating memories. Even though Molaison couldn’t store any new memories, he could still walk, talk, etc. He could even recall memories prior to his surgery.

For years, researchers believed that the hippocampus was responsible for creating memories of recent events, while other areas of the brain were responsible for memories of events that happened in the past.

Scientists believe that memories are stored in the neocortex. The neocortex is important for attention and planning. Until recently, it was believed that short-term memories moved from the hippocampus to the long-term storage in the neocortex, leaving no trace behind.

Until a research team at MIT made a discovery.


Brain research
Our brains are incredibly effective

New brain research

What are short-term and long-term memories? When you have new experiences, such as meeting someone new or watching a new movie, your brain record these events as short-term memories. Then these memories become long-term memories that are available to your brain for recall, later on, even many years in the future. This was the understanding of how our memories work.

That is, until now.

Not according to MIT neuroscientist Susumu Tonegawa and his colleagues. In 2017, they discovered that short-term memories don’t become long-term memories. According to their research, both short-term and long-term memories are actually formed almost at the exact time!

Tonegawa and his team’s research, published in the journal Science in 2017, identified “engram cells“. These cells contained specific memories, which they tracked down using a technique called optogenetics (wherein they use light to turn certain cells on or off). The scientists labeled memory cells of mice’s brain: the hippocampus, the prefrontal cortex (this is part of the neocortex, located at the very front of the brain) and basolateral amygdala (this is responsible for storing the emotional context of memories). These mice were then exposed to mild electric shocks (aka a painful event). The scientists observed if the mice behaved fearfully when they recreated these painful events. If the mice froze then that meant that the researchers activated the right memory. After a day, the scientists found that the painful memories were being stored in the hippocampus and the prefrontal cortex.

What does this mean?

The mouse could recall the memories stored in the hippocampus naturally, whereas the memories in the prefrontal context had gone “silent”. The “silent” memories could be activated artificially but didn’t initialise during normal memory recall. Two more weeks later, those “silent” memory cells eventually matured until they were triggered by recalling a painful event. By this time, the memory cells in the hippocampus became silent but didn’t completely disappear.

So whatever happens to the memory cells in the emotion-centered basolateral amygdala? The memory is retained and gets communicated with the other regions of the brain during each recall. So if a person has a fear of rats and mice, s/he will always be scared at the sight of those creatures – every single time.

What is the implication of Tonegawa’s study for us?

Their research would indicate that our brains keep a “backup” of every memory we form.

The scientists also believe that memories never fully disappear. That’s good news for people have memory problems brought about by a partially-damaged brain. They may be able to reactivate memories that are found in other parts of the brain.


Brain specialisations
We are still learning about the brain

Closing words

We are always learning more about our brain, which is an incredibly fascinating, and complex, organ. Studies like this enable us to not just better understand how our brains work, they also take us closer to creating cures for neurodegenerative diseases like Alzheimer’s, etc.

What are your thoughts on this brain and memory research?

Share your thoughts and suggestions in the comments below. 

If you want to learn more about the brain and how it keeps improving, check out this article about growing new brain cells. And if you want to keep your brain healthy for years to come, watch how much you drink. You can read here to find out about the 20 reasons not to drink alcohol, brain health being a big part of that.

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