Memories are stored in assemblies of neurons, called engram-bearing cells. When we recall past events, our brain reconstructs them with these data-filled cells, but just the act of accessing a memory distorts it.
In a previous study last year, MIT researchers detected a single memory in the brain, genetically tagged the brain cells housing that memory with a light-sensitive protein, and used pulses of light to "turn on" the memory at any given moment.
Scientists at the RIKEN-MIT Center for Neural Circuit Genetics have built on that work by altering the memory to change its contents -- creating a false memory.
Researchers isolated the memory of a safe environment, Box A, and tagged it for light-sensitivty. Next, they placed the mouse in the completely different environment of Box B, and engaged their memory of Box A while exposing their feet to electric shocks.
Later, when the mouse returned to the safe environment of Box A, they cowered in fear. Researchers then put the mouse in a completely different Box C, activated the memory of Box A, and elicited the same fear response.
"Human studies utilizing behavioral and fMRI (functional magnetic resonance imaging) techniques have not been able to delineate the hippocampal subregions and circuits responsible for generating false memories," said study author Susumu Tonegawa, Picower Professor of Biology and Neuroscience and director of the RIKEN-MIT Center for Neural Circuit Genetics.
"Our experiments provide the first animal model in which false and genuine memories can be investigated at the memory engram level."
Almost three-quarters of the first 250 people to be exonerated by DNA evidence in the US were victims of faulty eyewitness testimony.
Tonegawa, who won the Nobel Prize for Physiology in 1987, said that only humans have false memories naturally.
"Humans are the most amazing, imaginative animals," he said. "Just like our mice, an aversive or appetitive event could be associated with a past experience one may happen to have in mind at that moment, hence a false memory is formed," Tonegawa said.