Scientists Trace Memories of Things That
Never Happened

By James Gorman
July 25, 2013
The vagaries of human memory are notorious. A friend insists you were at your 15th
class reunion when you know it was your 10th. You distinctly remember that another
friend was at your wedding, until she reminds you that you didn’t invite her. Or, more
seriously, an eyewitness misidentifies the perpetrator of a terrible crime.
Not only are false, or mistaken, memories common in normal life, but researchers have
found it relatively easy to generate false memories of words and images in human
subjects. But exactly what goes on in the brain when mistaken memories are formed has
remained mysterious.
Now scientists at the Riken-M.I.T. Center for Neural Circuit Genetics at the
Massachusetts Institute of Technology say they have created a false memory in a mouse,
providing detailed clues to how such memories may form in human brains.
Steve Ramirez, Xu Liu and other scientists, led by Susumu Tonegawa, reported Thursday
in the journal Science that they caused mice to remember being shocked in one location,
when in reality the electric shock was delivered in a completely different location.
The finding, said Dr. Tonegawa, a Nobel laureate for his work in immunology, and
founder of the Picower Institute for Learning and Memory, of which the center is a part,
is yet another cautionary reminder of how unreliable memory can be in mice and
humans. It adds to evidence he and others first presented last year in the journal
Nature that the physical trace of a specific memory can be identified in a group of brain
cells as it forms, and activated later by stimulating those same cells.
Although mice are not people, the basic mechanisms of memory formation in mammals
are evolutionarily ancient, said Edvard I. Moser, a neuroscientist at the Norwegian
University of Science and Technology, who studies spatial memory and navigation and
was not part of Dr. Tonegawa’s team.
At this level of brain activity, he said, “the difference between a mouse and a human is
quite small.” In both, memories form in an area of the hippocampus called the dentate
gyrus.
“What I find fascinating about this,” Dr. Moser said, “is that you actually can point to a
physical substrate to memory,” what the researchers call an engram. Neuroscientists
have long talked about engrams, but Dr. Moser said the recent research is the closest
they have gotten to pointing to a spot in the brain and saying, “That is the memory.”
In the research reported Thursday, Dr. Tonegawa’s team first put mice in one
environment and let them get used to it and remember it. They identified and
chemically labeled the cells in the animals’ brains where that memory was being formed.
The mice were not shocked in that environment.
A day later, in a completely different environment, the researchers delivered an electric
shock to the mice at the same time that they stimulated the previously identified brain
cells to trigger the earlier memory.
On the third day, the mice were reintroduced to the first environment. They froze in
fear, a typical and well studied mouse behavior, indicating they remembered being
shocked in the first environment, something that never happened. The researchers ran
numerous variations of the experiment to confirm that they were in fact seeing the mice
acting on a false memory.
The tools of optogenetics, which are transforming neuroscience, were used to locate and
chemically label neurons, as well as make them susceptible to activation by blue light
transmitted by a fiber optic cable. With these techniques the researchers were able to
identify and label which neurons were involved in forming the initial memory of the first
environment, and to reactivate the labeled cells a day later with light.
Dr. Tonegawa said that because the mechanisms of memory formation are almost
certainly similar in mice and humans, part of the importance of the research is “to make
people realize even more than before how unreliable human memory is,” particularly in
criminal cases when so much is at stake.
That unreliability, he said, prompts a question about evolution: “Why is our brain made
in such a way that we form false memories?”
No one knows, he said, but he wonders if it has to do with the creativity that allows
humans to envision possible events and combinations of real and imagined events in
great detail. That rich internal experience fuels work in the arts and sciences and other
creative activities, he said. “Unless you have that kind of ability, there is no civilization,”
he said.
But it could also provide raw material for false memories — a possible “tradeoff for this
tremendous benefit.”
A version of this article appears in print on July 26, 2013, Section A, Page 3 of the New
York edition with the headline: “Scientists Trace Memories of Things That Never
Happened.”
“Scientists Trace Memories of Things That Never Happened” by James Gorman (New
York Times, 2013) https://www.nytimes.com/2013/07/26/science/false-memory-planted-ina-mouse-brain-study-shows.html