Scientists have discovered the secret of how memories are made – the brain makes two copies of every event, in a discovery they described as “beautiful”.
Researchers said even they were surprised when they realised the secret of how recollections are created and stored.
They found that the brain “doubles up” by simultaneously making two memories of events.
One is for the present and the second is for the long-term, they found.
It had been thought that all memories start as a short-term memory and are then slowly converted into a lifetime version.
Experts said the findings from MIT in the US and a team from Japan were “beautiful and convincing”.
Two parts of the brain are involved in collecting and storing personal experiences.
The hippocampus collects short-term memories while the cortex retains long-term memories.
That discovery was made in the 1950s after the case of man whose hippocampus was damaged as a result of epilepsy surgery.
Henry Molaison was no longer able to make new memories – ones from before the operation remained intact.
Scientists decided then that memories must be formed in the hippocampus and then moved to the cortex where they are “banked”.
The new experiments, by the Riken-MIT Center for Neural Circuit Genetics, carried out on mice, have established a very different theory.
They involved watching the way memories were formed as brain cells responded to a shock.
Light was then beamed into the brain to control the activity of individual neurons, switching memories on and off.
The results, published in the journal Science, found that in fact, memories were formed simultaneously in the hippocampus and the cortex.
Researchers said the cortex’s long-term memory did not seem to be used in the first few days after memories were formed, when it was “immature or silent”.
When scientists turned off the short-term memory, the shock event was forgotten.
Yet the mice could be made to remember by manually switching the long-term memory on.
Prof Susumu Tonegawa, the director of the research centre, said: “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.”
He said the discovery was “surprising”.
The study also found the long-term memory never matured if the connection between the hippocampus and the cortex was blocked – suggesting that over time, the balance of power shifts to the cortex.
Dr Amy Milton, who researches memory at Cambridge University, said the study was “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 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,” she said.
Scientists said the finding could also help to uncover how diseases such as dementia work.
One of Prof Susumu Tonegawa’s previous studies showed mice with Alzheimer’s were still forming memories but were not able to retrieve them.
“Understanding how this happens may be relevant in brain disease patients,” he said.