Scientists report they've successfully recovered "lost memories" from mice using just light, suggesting memories lost in retrograde amnesia because of brain injuries aren't gone but rather have become "hidden."
At the Massachusetts Institute of Technology researchers say they've recovered memories in mice by using pulses of blue light to stimulate neurons in their brains, evidence that memories believed destroyed in certain kinds of amnesia still exist, undamaged but inaccessible.
In an laboratory experiment, mice in a cage were given a small electric shock, the memory of which made them freeze out of fear in anticipation of receiving an additional shock when they were returned to the same enclosure later.
The experiment was then repeated, but the mice were administered a drug that prevented the consolidation of the memory. They displayed no similar signs of fear when returned to the cage as their memories of the disagreeable shock appear to have been completely "lost."
However, the researchers say they were able to restore that fear memory using a technique called optogenetics to directly stimulate the neurons where the memory was encoded, which caused the mice to suddenly revert to their "fear-based" behavior - they now remembered the shock experience.
In optogenics, researchers can target individual neurons in the brain and use an engineered virus to tag them with a special protein that makes them sensitive to light.
The sensitivity allows scientists to turn a particular set of neurons off and on as desired.
The study brings new insights into the long debate about what really happens to "lost memories," whether they are completely erased or somehow remain, hidden and just out of the reach of recall, they report in the journal Science.
The study results may have applications in treating humans, the researchers say, particularly in incidents of the retrograde type of amnesia in which previously established memories seem beyond recall.
Such amnesias can result from Alzheimer's disease, traumatic braini injuries or other neurological issues.
"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 at the RIKEN-MIT Center for Neural Circuit Genetics.
The new work strongly suggests previous memories still exist in a brain affected by amnesia and can be retrieved by activating the neural pathways that underpin them, he explains.
"Our conclusion," he says, "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."
