One Experience, Two Memories

Scientists have long believed the human brain stores memories following a two-step process. Memories are first stored in the brain's "short-term" memory, and are later moved to the "long-term" memory. New research from the Riken-MIT Center for Neural Circuit Genetics have proven that this may not be the case. Their study, published in the journal Science, can change the way we approach neurodegenerative diseases that attack memory.

Previously, scientists thought that the memory is first stored as a short-term location in the brain's hippocampus. It then gets converted into a long-term memory, and stored in the brain's cortex. Now, the new study from Riken-MIT revealed that the brain simultaneously makes two memories: one is a present, albeit temporary, version and the other version goes into long-term — even lifetime — storage.

"This was surprising," research director Susumu Tonegawa told BBC News. "This is contrary to the popular hypothesis that has been held for decades. This is a significant advance compared to previous knowledge, it's a big shift."

Retrieving Lost Memories

The U.S. and Japanese team of researchers arrived at this conclusion after conducting experiments on mice. They observed specific memories forming, as a cluster of connected brain cells, after a shocking stimulus. The researchers were then able to switch memories on and off, using light beamed into the brain which can control individual neural activity.

If the researchers turned off the short-term memory in the hippocampus, the mice forgot about the shock. But when they turned on the long-term memory stored in the cortex, the mice were able to remember — despite the long-term memory being infrequently used in the first few days after its formation in the cortex. Researchers also discovered that if the connection between the cortex and the long-term memory "bank" gets blocked, those memories get blocked, too.

The researchers are hopeful their study can provide new insights into the fight against diseases that induce memory loss, such as Alzheimer's and dementia. "Understanding how this happens may be relevant in brain disease patients," Tonegawa said, who previously discovered that mice with Alzheimer's could still form memories, the memories were just rendered inaccessible.

Cambridge University researcher Amy Milton, who wasn't part of the study, found the results not only  surprising but "beautiful, elegant and extremely impressive." She told BBC News: "This is [just] one study, but I think they've got a strong case, I think it's convincing and I think this will tell us about how memories are stored in humans as well."

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