Scientists say they've been able to restore "lost memories" in the brains of mice simply by stimulating neurons in their brains with pulses of light.
There has long been debate about what the fate of "lost memories" is, whether they get erased entirely or somehow remain, hidden and just out of reach.
A new study suggests memories thought lost in certain forms of amnesia are in fact still intact and may be recoverable, researchers at the Massachusetts Institute of Technology report in the journal Science.
Although their success in retrieving memories has so far been limited to mice, the study could have implications for humans as well, they say.
In an experiment, mice were subjected to a small shock within a chamber, and the memory of that caused them to freeze in fearful anticipation of another shock when placed back in the same chamber later.
However, when they were given a drug preventing them from consolidating a memory of the shock, they showed none of the same fearful behavior when placed back in the chamber — the memory of the unpleasant experience appeared well and truly "lost."
However, when the researchers used pulses of light — a technology known as optogenetics — to stimulate neurons in which memories are encoded, the mice suddenly reverted to their "fearful" behavior, indicating that the memory of the shocks had been retrieved and restored.
The findings could have human applications, the researchers suggest, in cases of retrograde amnesia characterized by an inability to recall established memories.
Retrograde amnesia can be the result of traumatic brain injury, Alzheimer's disease or other neurological conditions.
"Brain researchers have been divided for decades on whether amnesia is caused by an impairment in the storage of a memory, or in its recall," says study leader Susumu Tonegawa, working at the RIKEN-MIT Center for Neural Circuit Genetics.
The mice study is evidence old memories do remain in the amnesia brain, and the cellular pathways underpinning them can be reactivated, allowing the retrieval of the lost memories, he says.
"Our conclusion," he explains, "is that in retrograde amnesia, past memories may not be erased, but could simply be lost and inaccessible for recall. These findings provide striking insight into the fleeting nature of memories, and will stimulate future research on the biology of memory and its clinical restoration."
