Studies on rats show that the brain stimulation seems to make them forget that they should be frightened, said study co-author Gregory Quirk, associate professor of physiology at the Ponce School of Medicine in Puerto Rico. "We've fooled the brain into thinking that it's safe."
Like humans, rats learn to be scared of events or things that they relate to pain. When faced with something frightening, rats will freeze while their heart rate and blood pressure go up.
"It's a classic fight-or-flight response," Quirk said. "It's a hard-wired way that organisms deal with danger. That's true in people, rats, birds, and lizards -- it's true across many species."
In his study with colleague Mohammed Milad, Quirk taught rats to associate an audio tone with a mild shock to their feet. They froze each time they heard the tone, much like Pavlov's famous dog drooled each time the scientist rang a bell announcing dinnertime.
Then the researchers tried to make the rats forget about their fear by playing the tone without the shock. They report their findings in tomorrow's issue of the journal Nature.
"The freezing starts out very high, but the rat learns that the tone no longer predicts the shock," Quirk said. "You might be tempted to think that that caused the fear memory to be erased," but in fact it remains buried in the brain, waiting to be resurrected as time passes.
"You can teach the rat not to be afraid by giving the tone without the shock, but if you wait a long time, the freezing is back immediately," he said.
But there seems to be an important exception to the rule. The rats did a better job of forgetting their fear -- and not reacting to the tone -- when researchers electrically stimulated a part of the brain that's associated with learning that something is no longer scary.
The fear didn't go away, but merely got covered up, Quirk said. "It's like throwing a blanket on the fear that's already there. It makes the rat look like nothing happened, even though it's not true."
The findings could help people who can't learn to stop being afraid, such as those who suffer from posttraumatic stress disorder, Quirk said. "In the current therapies, you find what the person is afraid of, and keep showing it to them again and again to extinguish their fear. But with time, the fear response slowly recovers, and that's a problem."
The proposed brain stimulation may be too broad to work effectively in humans, said James L. Olds, a neuroscientist and director of the Krasnow Institute for Advanced Study at George Mason University. It would turn on thousands or millions of nerve cells in an important part of the brain for higher mental skills, he said.
But the findings are still "remarkable" and provide much insight into how the brain works, he said.
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