Stressful events can often coincide with the creation of false memories, when people recall things that never happened, and scientists said Thursday they are learning more about this curious phenomenon.
A better understanding of false memories could help treat post-traumatic stress and possibly cut back on inaccurate eyewitness testimony that jails innocent people, experts say.
The latest advances in studying manipulated memories in the lab were reported in the journal Science by researchers in a US-Japanese partnership at the Massachusetts Institute of Technology.
They say they can make mice remember a traumatic event that never happened.
According to lead author Susumu Tonegawa of MIT, the method involves recognizing the brain cells that are changed physically and chemically during the formation of a memory, known as an engram.
"Whether it's a false or genuine memory, the brain's neural mechanism underlying the recall of the memory is the same," explained Tonegawa, a 1987 Nobel laureate 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."
Tonegawa and colleagues showed they could identify the cells for a specific memory in the hippocampus of mice and program the engram to respond to pulses of light, known as optogenetics.
Researchers placed the mice in a peaceful place, Box A, and isolated the animals' brain signature of that secure setting.
Then, they placed the mice in Box B and reactivated the secure memory while delivering a shock to the mice's feet.
When researchers returned the mice to Box A, they froze, exhibiting a common fear response, signifying they remembered something bad happening there, even though it had not.
Scientists also observed they could reactivate the false memory at will by manipulating the light pulses to the part of the brain where the memory was stored.
Even more, they could see the false memory aroused other parts of the brain, such as the amygdala, where active fear responses are based.
"To the animal, the false memory seems to have felt like a 'real' memory," said co-author Xu Liu.
Learning how to turn on false memories may also help scientists figure out how to turn off, or erase, bad ones, he added.
According to Bill Klemm, senior professor of neuroscience at Texas A&M University, the research could help treat conditions like post-traumatic stress.
"False memory is a big deal. It comes up in criminal trials and post-traumatic stress syndrome, all sorts of things.
"This paper is important in the sense that here is an animal model where you can do things you can't do in people," said Klemm, who was not involved in the research.
However, the study offers only a preliminary glance at how these processes may work in humans, since the lives of lab mice are much less complicated, and the memory span studied was short, he added.
"There was only a one-day interval in between each condition," Klemm told AFP.
"In a real world situation, like eyewitnesses at a crime or something, days or weeks or months may elapse between the different contexts and a lot of intervening things may happen."
Elizabeth Loftus, a distinguished professor of psychology at the University of California - Irvine, said she found the research "kind of exciting," because it appears to answer a key question raised by critics.
"When you work with humans you do have a concern about what we in psychology call demand characteristics -- the idea that humans are giving you the response they think you would like to hear," she told AFP.
"That is probably not true for mice."
Besides, not all false memories are harmful. Loftus said this flexibility allows humans to self-correct after misremembering something, and even helps plan for the future.
"Even without external influence, people remember that their grades were better than they were, or that they voted in elections that they didn't vote in, or that their kids walked and talked at an earlier age than they really did," she said.
"These self-enhancing memory distortions allow people to feel better about themselves."
MIT researchers said they are planning more study of how false memories are created, and whether they can extend to memories for objects, food, or even companions.