- Short-term memories are stored in an area of the brain called the hippocampus
- These are later consolidated and transferred to long-term storage
- But a new study reveals that memories are formed in both areas simultaneously
- The findings could change how we understand and treat memory disorders
When we visit a friend or go to the beach, our brain stores a short-term memory of the experience in a part of the brain called the hippocampus.
Those memories are later consolidated and transferred to another part of the brain for long-term storage.
Now a new study has revealed, for the first time, that memories are actually formed simultaneously in the hippocampus and a long-term storage location in the brain called the cortex.
However, the long-term memories remain ‘silent’ for about two weeks before they ‘mature’ over time.
The researchers claim that their findings could change how we understand and treat memory disorders such as PTSD and amnesia.
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A new study reveals, for the first time, that memories are actually formed simultaneously in the hippocampus and a long-term storage location in the brain called the cortex (stock image)
PREVIOUS MEMORY STUDIES
Beginning in the 1950s, studies of the famous amnesiac patient Henry Molaison revealed that the hippocampus is essential for forming new long-term memories.
Molaison, whose hippocampus was damaged during an operation meant to help control his epileptic seizures, was no longer able to store new memories after the operation.
However, he could still access some memories that had been formed before the surgery.
This suggested that long-term memories of specific events are stored outside the hippocampus.
Scientists believe these memories are stored in the neocortex, the part of the brain also responsible for brain functions such as attention and planning.
The findings from researchers at the Massachusetts Institute of Technology may force a big change in how we understand memory, the researchers say.
The team found that memories transfer between ‘silent’ and ‘active’ states in the two memory areas.
‘In our study we identified that memory status naturally changes from active to silent in some parts of the brain, while other parts change from silent to active,’ lead author Dr Takashi Kitamura told MailOnline.
‘If we identified a mechanism for the overactive memory in PTSD to be switched back to silent, we may find a way to treat the disease.
‘Additionally, we could potentially activate the frozen, silent memories of amnesiac patients to retrieve their past.’
In the study, the team labelled memory cells in mice called ‘engram cells’ during a fear-conditioning event – a mild electric shock delivered when the mouse is in a particular chamber.
Then, they used light to activate the memory cells at different times and see if this provoked a behavioural response from the mice, namely freezing in place.
Just one day after the fear-conditioning event, the researchers found that fear memories were stored in cells in two areas of the brain associated with memory, known as the hippocampus and the prefrontal cortex.
However, the memory cells in the prefrontal cortex were ‘silent’.
Schematic model showing how memory is consolidated in the prefrontal cortex and how brain circuitry shifts as hippocampal memory cells (HPC) become silent and prefrontal cortex memory cells (PFC) become active
These cells could stimulate freezing behaviour when activated by UV light, but did not fire during natural memory recall.
Experts said the findings from MIT in the US and a team from Japan were ‘beautiful and convincing’
‘Already the prefrontal cortex contained the specific memory information,’ Dr Kitamura said.
‘This is contrary to the standard theory of memory consolidation, which says that you gradually transfer the memories. The memory is already there.’
Over the next two weeks, the ‘silent’ memory cells in the prefrontal cortex gradually matured and became active.
Eventually these cells became necessary for the animals to naturally recall the event.
By the end of the same two week period, the hippocampal memory cells became silent and were no longer needed for natural recall, while they remained active in the prefrontal cortex.
However, traces of the memory remained in the hippocampus: Reactivating those cells with light still prompted the animals to freeze.
The findings suggest that traditional theories of consolidation may not be accurate, because memories are formed rapidly and simultaneously in the prefrontal cortex and the hippocampus on the day of training.
‘They’re formed in parallel but then they go different ways from there. The prefrontal cortex becomes stronger and the hippocampus becomes weaker,’ coauthor Mr Mark Morrissey said.