In a recent experiment, mice were able to recover suppressed memories after exposure to blue light pulses. This was the first time researchers have retrieved specific memories in an animal, and suggests that similar memory loss in humans could be reversible.
In the case of retrograde amnesia — where the sufferer can’t recall events that happened before the trauma or brain injury — scientists are still uncertain whether the memory is permanently lost or just laying dormant, unable to be recalled.
“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,” said one of the researchers, Susumu Tonegawa, in a press release.
In an attempt to resolve this question, researchers at the RIKEN-MIT Center trained a group of mice to remain still in a specific enclosure, called Chamber A, by giving them an electric shock when they were put there. Eventually the mice would freeze when placed in Chamber A even without receiving the shock.
To induce amnesia in the mice, one group was injected with anisomycin, a compound that interferes with memory formation, and the control group with saline. The latter predictably froze when placed into Chamber A, but the amnesiac mice did not.
When the mice were still being trained to fear the shock in Chamber A, researchers identified which neurons were active in the Pavlovian process, and labeled them with a protein sensitive to blue light, which was delivered to the neurons by a engineered virus.
The term for using blue lights to activate certain cells is “optogenetics,” which has already been used in other memory experiments on mice, notably the implantation of false memories.
To jog the memories of the amnesiac mice, the researchers placed them in a neutral environment, called Chamber B, then shined blue light on the neurons active in the training. When these neurons, which the scientists call “memory engrams,” were stimulated by the blue light, the mice froze in place just as before.
“Our conclusion is that in retrograde amnesia, past memories may not be erased, but could simply be lost and inaccessible for recall,” said Tonegawa. “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.”
In their writeup of the experiment in “Science,” the researchers speculated that the memory of the Pavlovian training had formed engrams, or memory circuits, in the hippocampus and the amygdala, the parts of the brain associated with memory and fear. The connections were likely suppressed, but not eliminated, by the memory-inhibition solution.
A practical upshot of the study is if memory recovery is possible, doctors might one day find a cure for retrograde amnesia, which currently eludes effective treatment . But don’t count your eggs before they hatch, the researchers say.
“It’s very difficult to be doing this in humans, partly for the ethical reasons — the work is invasive — but also because we tag the memories in the brain before they’re learned,” Tomas Ryan, a co-author of the study, told the Verge.