Mind & Body

Short-Term Memories Don't Turn Into Long-Term Memories — It's Weirder Than That

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When you go on vacation or meet a new love interest, your brain stores that event as a short-term memory. For decades, neuroscientists believed that your brain eventually transferred those short-term memories into another part of the brain responsible for long-term storage, enabling you to recall them years later. In 2017, MIT neuroscientist Susumu Tonegawa and his colleagues made a startling discovery: short-term memories don't turn into long-term memories. Instead, they're both formed at the exact same time.

Memory Myth

Many of the things we know about the brain today were discovered due to brain injuries: if damage to one part of a patient's brain results in certain behavioral changes, that tells scientists that that part of the brain must be important for that behavior. In 1953, doctors performed brain surgery on a man named Henry Molaison in order to help control his epileptic seizures. They ended up removing his hippocampus, a seahorse-shaped brain structure that sits on each side of the brain.

When he woke up, he was no longer able to store new memories, and the physicians quickly realized that the hippocampus must be essential for making memories. Since then, Molaison's case has become the most studied in medical history. Interestingly, however, Molaison wasn't entirely without memories: he still could learn physical skills, and could even recall memories formed before the surgery.

That — and studies since — taught researchers that the hippocampus is mostly responsible for the recall of memories of recent events, while other structures are responsible for memories of events long past. Scientists now believe those memories are stored in the neocortex, a region also central to attention and planning. Until recently, the theory was that short-term memories were first formed in the hippocampus, then gradually transferred to long-term storage in the neocortex as they disappeared from the hippocampus completely.

But the MIT research has turned that theory on its head.

Total Recall

For a study published in the journal Science in 2017, Tonegawa and his team used a technique he developed in 2012 to label so-called "engram cells," which contain specific memories, to trace memory storage and retrieval in lab mice. They also relied on a technique called optogenetics that uses light to turn certain cells on or off. The team labeled memory cells in three parts of the brain: the hippocampus, the prefrontal cortex (the part of the neocortex at the very front of the brain), and the basolateral amygdala, which is responsible for storing the emotional content of memories. Then, they placed the mice in a special chamber and gave them mild electric shocks. (Being a lab mouse is no picnic). The idea was to create memories of a fearful event that they could reactivate to see if the mice behaved fearfully. If they froze, the researchers knew they'd activated the right memory.

A day after the electric shocks, the team found that the fearful memories were being stored not only in the hippocampus, but in the prefrontal cortex, too. There was a difference, however: while the memories in the hippocampus could be recalled by a mouse naturally, the memories in the prefrontal cortex were "silent." The researchers could activate them artificially, but they didn't fire during normal memory recall. Over the next two weeks, those "silent" memory cells gradually matured until they played an essential part in recalling the fearful event. By that time, the memory cells in the hippocampus had gone silent — but they didn't disappear. The researchers could still activate them, and the mice still froze as a result. Throughout the process, the memory cells in the emotion-centric basolateral amygdala stayed the same. They retained the memory and communicated with the other regions during each recall.

This is great news for memory. Your brain essentially keeps a "backup" of every memory you form, and the researchers have reason to believe that those memories never fully disappear. This means that if a person suffers from memory problems because of damage to one part of the brain, they may be able to reactivate those memories in another part. Your memories may be safer than you realized.

Controlling the Brain With Light to Reactivate Lost Memories

Written by Ashley Hamer March 8, 2018

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