Researchers studied veterans from Iraq who survived incidents like being in a Humvee blown up by a bomb to find out why fearful memories are so persistent in the brain and how they can be erased.

RENEE MONTAGNE, host:

This is MORNING EDITION from NPR News. Good morning. I'm Renee Montagne.

ARI SHAPIRO, host:

And I'm Ari Shapiro. The memory of a terrifying event can be almost impossible to erase. That's one reason it's so hard to treat people who suffer from post-traumatic stress disorder. Now, new research shows that scientists are beginning to figure out why fearful memories are so persistent and how to eliminate them. NPR's Jon Hamilton has the story.

JON HAMILTON: Some veterans come home haunted by memories they can't escape. Kerry Ressler of Emory University treats a few of these veterans. He says a typical story goes something like this.

Dr. KERRY RESSLER (Emory University): When I was in the Humvee in Iraq the bomb blew up, and I have all these awful memories from that.

HAMILTON: Death, dismemberment, chaos. The vets come home and find that even a drive to the supermarket can trigger those bad memories. Ressler says behavioral therapies can help PTSD patients cope with their fears. They learn to believe that a car ride doesn't have to end in violence.

Dr. RESSLER: When I'm driving my car down a street in Atlanta, it's a different context, it's a different place. I don't need to be afraid.

HAMILTON: But the traumatic memory is still there and can be set off again by almost any emotional event. That's because fear comes from a part of the brain called the amygdala. Ressler says the amygdala isn't logical. It just reacts.

Dr. RESSLER: Before we are even consciously aware of something, the amygdala has activated the fight-or-flight reflex and activated the fear system.

HAMILTON: So to understand why fearful memories are so persistent, researchers have been studying the amygdala. And a Swiss team seems to have found some clues — at least in rats.

Andreas Luthi of the Miescher Institute for Biomedical Research in Basel worked with rats who'd learned to associate a certain sound with an electric shock. Even after the scientists stopped giving them the shocks, the sound would cause a freezing response.

Dr. ANDREAS LUTHI (Miescher Institute for Biomedical Research): It simply means they stop moving.

HAMILTON: Usually for several seconds. Luthi and his team showed that adult rats never lost their fear of that sound. But it was a different story for baby rats. They did eventually forget. The memory was erased. Luthi thinks he knows why.

When rats are about three weeks old, the brain cells in the amygdala acquire a molecular sheath that seems to protect memories made after that time. And when Luthi's team injected adult rats with a drug that dissolved the sheath, voila, the adult rats forgot their fear. The scientists restored their earlier ability to erase fearful memories.

Dr. LUTHI: This juvenile mechanism is not lost during development but still exists. It's still like dormant in adult animals. That is, you can recover the ability of these animals to erase fear memories.

HAMILTON: But what about people? Joseph LeDoux of New York University is one of the nation's most prominent fear researchers. He says when it comes to fear, rats and people have a lot in common. He says you can see that in a video taken during the 1996 Olympics in Atlanta. There's a crowd of people watching a concert. Then a bomb explodes.

(Soundbite of explosion)

(Soundbite of screaming)

Unidentified Man: Whoa.

Dr. JOSEPH LEDOUX (New York University): The bomb goes off and you see the camera shake and the camera's still pointing at the crowd and everyone is, you know, hunkered down in a freezing posture.

HAMILTON: For several seconds, just like rats. LeDoux says he's confident that rat experiments like the one done by Luthi's team will lead to better treatments for humans. Human brain cells have sheaths just like those in rats, and these could become a target for new drugs.

This sort of memory research has some people worried about the potential for misuse by governments or the military. But LeDoux says it would be wrong to halt experiments that could help people with phobias, panic disorder, or PTSD.

Dr. LEDOUX: Any PTSD patient you talk to or at least that I've talked to has been willing to sacrifice a few normal memories for the bad ones that they may get rid of if these experiments are successful.

HAMILTON: The new study appears in the journal Science.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.