Anyone who watches TV courtroom dramas knows that memory can't be trusted. Eyewitnesses believe that their recall is complete and perfect, but in truth, memories are, at best, sensory and emotional impressions blurred by imagination, belief, ambiguity, and time. As convincing as juries may find the testimony of witnesses, good prosecutors know that human memory is, more often than not, the least reliable source of evidence.
That's true for several reasons. For one, attitudes and beliefs can affect the memories we form. Scientists at Cornell University told college students a story about a man who walked out on a restaurant bill. Half the participants were told that the man "was a jerk who liked to steal." Half were told that the man left without paying because he received an emergency phone call.
“One week later the people who were told he was a jerk remembered a higher bill—10 to 25 percent more than the bill actually was. Those who were told he had an emergency phone call remembered a slightly lower bill,” says investigator David Pizarro. “Negative evaluations,” he concludes, “are capable of exerting a distorting effect on memory."
It is even possible to remember something that never really happened. In one experiment, researchers showed volunteers images and asked them to imagine other images at the same time. Later, many of the volunteers recalled the imagined images as real. Using fMRI, the researchers were able to determine which parts of the brain formed the false memories and which formed the real ones. “We think parts of the brain used to actually perceive an object and to imagine an object overlap,” says Northwestern University scientist Kenneth Paller. “Thus, the vividly imagined event can leave a memory trace in the brain that’s very similar to that of an experienced event.”
The memory trace is, of course, chemical. Memories are stored with the formation of particular proteins in the brain. Each time a memory is recalled, the proteins can be reformed or modified. How this process works is a research question of great interest to neuroscientists. This week, researchers affiliated with a project at MIT reported a big step forward toward explaining how external stimuli can distort mental representations to produce brand new, seemingly accurate—but completely false—memories.
Steve Ramirez and his colleagues used a combination of optical and genetic techniques to control the activity of individual neurons in the brains of specially bred experimental mice. The researchers studied a group of brain cells in the hippocampal region of the mouse brain. They found that they could create false associations between events and environments by artificially stimulating the neurons.
Specifically, Ramirez and his team identified particular cells that were activated by foot shocks in a particular environment. Then they moved the mice to a shock-free environment and stimulated those same neurons. This reactivation of the neurons that fired when the mice were shocked caused the mice to freeze (a natural response to fear) even when no shock was given. So strong were the implanted false memories that the mice froze, even when the hippocampal cells weren’t stimulated.
These findings demonstrate that memories can be induced by artificial means, and they provide a model for studying the mechanisms of false memory formation in humans. A member of the MIT team, Susumu Tonegawa, commented on the significance of the research in Science magazine's weekly podcast:
"Independent of what is happening around you in the outside world, humans constantly have internal activity in the brain. So just like our mouse, it is quite possible that we can associate what we have in our mind with a bad or good high valence online event. In other words, there could be a false association of what you have in your mind rather than what is happening to you, so this is a way we believe that at least some form of strong force memory observed in humans could be made. Because our study showed that the false memories and the genuine memories are based on very similar, almost identical, brain mechanisms, it is difficult for the false memory bearer to distinguish between them. We can study this because we have a mouse model now."
Pizarro quoted in "Bad Judgments about People Can Affect Memories of Them, Cornell Study Finds." Cornell Chronicle, March 13, 2006.
Brian Gonsalves, Paul J. Reber, Darren R. Gitelman, et al., “Neural Evidence That Vivid Imagining Can Lead to False Remembering,” Psychological Science (October 2004), 655.
Richard G.M. Morris, Jennifer Inglis, James A. Ainge, et al., “Memory Reconsolidation: Sensitivity of Spatial Memory to Inhibition of Protein Synthesis in Dorsal Hippocampus during Encoding and Retrieval,” Neuron (May 2006), 479-489.
S. Ramirez, X. Liu, P.-A. Lin, J. Suh, M. Pignatelli, R.L. Redondo, T.J. Ryan, and S. Tonegawa. "Creating a False Memory in the Hippocampus." Science (July26, 2013), 387-391.