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Memories are stored in a region of the brain called the hippocampus, shown in red in this computer illustration. (Photo Researchers, Inc.)

How Our Brains Make Memories

Surprising new research about the act of remembering may help people with post-traumatic stress disorder

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Nader decided to revisit the concept with an experiment. In the winter of 1999, he taught four rats that a high-pitched beep preceded a mild electric shock. That was easy—rodents learn such pairings after being exposed to them just once. Afterward, the rat freezes in place when it hears the tone. Nader then waited 24 hours, played the tone to reactivate the memory and injected into the rat’s brain a drug that prevents neurons from making new proteins.

If memories are consolidated just once, when they are first created, he reasoned, the drug would have no effect on the rat’s memory of the tone or on the way it would respond to the tone in the future. But if memories have to be at least partially rebuilt every time they are recalled—down to the synthesizing of fresh neuronal proteins—rats given the drug might later respond as if they had never learned to fear the tone and would ignore it. If so, the study would contradict the standard conception of memory. It was, he admits, a long shot.

“Don’t waste your time, this will never work,” LeDoux told him.

It worked.

When Nader later tested the rats, they didn’t freeze after hearing the tone: it was as if they’d forgotten all about it. Nader, who looks slightly devilish in his earring and pointed sideburns, still gets giddy talking about the experiment. Eyes wide with excitement, he slaps the café table. “This is crazy, right? I went into Joe’s office and said, ‘I know it’s just four animals, but this is very encouraging!’”

After Nader’s initial findings, some neuroscientists pooh-poohed his work in journal articles and gave him the cold shoulder at scientific meetings. But the data struck a more harmonious chord with some psychologists. After all, their experiments had long suggested that memory can easily be distorted without people realizing it.

In a classic 1978 study led by Elizabeth Loftus, a psychologist then at the University of Washington, researchers showed college students a series of color photographs depicting an accident in which a red Datsun car knocks down a pedestrian in a crosswalk. The students answered various questions, some of which were intentionally misleading. For instance, even though the photographs had shown the Datsun at a stop sign, the researchers asked some of the students, “Did another car pass the red Datsun while it was stopped at the yield sign?”

Later the researchers asked all the students what they had seen—a stop sign or yield sign? Students who’d been asked a misleading question were more likely to give an incorrect answer than the other students.

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