For decades, we’ve thought that memories were formed in two distinct stages—short-term first, then long-term later. We might be wrong. New research suggests that our brains make two copies of each memory in the moment they are formed. One is filed away in the hippocampus, the center of short-term memories, while the other is stored in cortex, where our long-term memories reside. These findings, published yesterday in the journal Science, upend more than 50 years of accepted neuroscience, and they’re being hailed by other neuroscientists. Here’s James Gallagher, reporting for BBC News:

Dr Amy Milton, who researches memory at Cambridge University, described the study as “beautiful, elegant and extremely impressive”. She told the BBC News website: “I’m quite surprised. “The idea you need the cortex for memories I’m comfortable with, but the fact it’s so early is a surprise. “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.”

Famous patient Henry Molaison, also known as H.M., helped solidify the prevailing theory of memory formation and storage. After a brain surgery to treat his epilepsy damaged his hippocampus, he could no longer form new memories. But those memories he made before his surgery still existed, leading neuroscientists to believe that the hippocampus was key to forming new memories. And in the new theory, it still is. Though the research suggests that the link between the hippocampus and the cortex is as linear as once thought. When the link is blocked, the mice in the experiment didn’t develop long-term memories, just like H.M.

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But when the connection is present, memories form in both the cortex and the hippocampus simultaneously. For the first few days, though, the neurons in the hippocampus are the only ones that fire during memory retrieval. Eventually, the memories in the cortex mature, and the neurons involved light up when the memory is recalled. The researchers, based at the Riken-MIT Center for Neural Circuit Genetics in Japan, used a relatively new technique known as optogenetics to both trace memory formation and test the roles of the hippocampus and cortex. Optogenetics involves genetically engineering animals—in this case mice—to express a light-sensitive protein in certain neurons. By implanting fine fiber-optic cables into their brains, researchers are effectively able to turn specific neurons on and off, which allows them to probe their function.

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Image credit: Anastasia Yendiki, Ph.D., Viviana Siless, Ph.D., MGH/Harvard, Boston Adolescent Neuroimaging of Depression and Anxiety (BANDA)