“This study really highlights the power of optogenetics to probe fundamental questions about how the brain functions,” Boyden, who was not involved in the new research, said in an e-mail. “In this case, they were able to use light to activate a set of cells in a coordinated way, so that it could participate in a memory for things that had never behaviorally taken place. This helps reveal how neural circuits encode for memories, at a cellular level.”
This time, Tonegawa and colleagues started with mice in a a safe environment, Box A, and labeled the brain cells involved in creating the Chamber A memory using channelrhodopsin, the light-sensitive protein.
The next day, each mouse got into Box B, a different environment.
Here comes the tricky part: In Chamber B, each mouse in the experimental group got mild foot shocks at the same moment that scientists reactivated the memory of Chamber A, using light. That made the mouse associate its memory of Box A with the foot shocks in Chamber B.
On the third day, when the mouse was put back in Chamber A, where it had never received a shock, the animal displayed fear; it associated the shock it received in Chamber B with the memory of Chamber A. Researchers observed that eventually, the mouse froze up even when scientists were not activating the cells associated with the false memory.
The results indicate that the underlying brain mechanisms used in the recall of a false memory are very similar to those governing a real memory, Tonegawa said. This may be why our memories feel so real to us, even if they have been distorted.
“It’s not that false memory is formed just by some kind of forgetting or some kind of a simple mix-up, or what we call imagination,” Tonegawa said. “No, it really happened in the brain, as far as the brain is concerned.”
False memories in humans
The researchers have no plans to manipulate human memory using a similar technique in people.
“Ethically, one should not even pursue that,” Tonegawa said.
The mouse brain is also not a perfect model for the human brain; while mice have about 75 million neurons, humans have more than 1,000 times that — according a 2009 study, the human brain has about 86 billion neurons.
Still, researchers needed to start somewhere. And brain structures important to memory, such as the hippocampus and the amygdala, are present in both mice and humans.
The Science study could affect future exploration into the treatment of patients with psychiatric disorders in which patients have a false sense of reality, Tonegawa said. In schizophrenia, for example, patients may have hallucinations, which are sensory perceptions of events that are not real. Learning about how to alter information in the brain, says Tonegawa, may prove useful.
It has been shown in many studies that false memories can be “implanted” in people easily — no genetic alteration required.
Much of the research by Elizabeth Loftus, cognitive psychologist at the University of California, Irvine, shows that we are susceptible to change the way we remember events based on cues from other people.
A 1995 study of hers had seven out of 24 participants “remembering” false events that researchers told them were real.
In a different study, Loftus and her colleagues played videos of different incidents for volunteers and then asked them what they remembered. Merely asking about “THE broken headlight” led more people, on average, to say they remembered seeing it than those who were asked about “A broken headlight.” The catch: There was no broken headlight at all.
There’s evidence that eyewitness misidentification played a role in nearly 75% of the convictions that have been overturned because of DNA testing, according to the Innocence Project.
Such statistics suggest to Tonegawa that caution is necessary when considering eyewitness testimony in the courtroom.
“The use of testimony based on human memory should be really limited, restricted,” Tonegawa said. “I’m not saying it should be thrown out completely, but one should be very careful and very conservative about testimony-based evidence.”
It’s a controversial subject — but at least the mice don’t have to worry about it.
CNN’s Jacque Wilson contributed to this report.
Elizabeth Landau | CNN