Forgetting Fear

"It's essentially new learning. Previously you've learned that something is something that you should be afraid of, and now by being exposed to it and not having adverse consequences, you're learning that there really is nothing to be afraid of in this situation," Phelps explains.

"The human amygdala and the rat amygdala work pretty much the same, at least in a general sense, so if you damage the rat amygdala, the rat loses its fear of cats and will climb all over a cat, something they would not normally do," says LeDoux, NYU and Henry and Lucy Moses professor of science at the Center for Neural Science.

The amygdala's job is to detect sensory information coming from the world around us, so it's receiving constant inputs, most of which go unnoticed. But, "If a bomb goes off in this room, the noise will go through our brain and activate the amygdala instantaneously and automatically and produces a defense response: freezing, blood pressure's going to rise, hormones release," he explains.

This initial response, which LeDoux calls the "low road," passes through the brain very quickly, so that we feel afraid even before we know what we're dealing with. As we begin to process the situation and come to realize that there was a bomb and other details about what happened, information is passing along the slower, more refined system of the "high road."

When you're momentarily frozen with fear, it's actually a very active process that involves many muscle contractions and requires a lot of energy on the part of the body, which is why our blood pressure rises. People who suffer from an anxiety or fear disorder have a normal fear response, but to situations that shouldn't normally causefear responses, like social situations. Or they may have fear in appropriate situations but in a much more exaggerated way than the average person.

The primitive, almond-shaped amygdala

So Phelps set out to find out how we acquire fears and what happens when we use techniques to get over our fears.

While scanning volunteers' brains with an MRI machine, Phelps and her team showed them colored squares on a screen. A blue square was paired with a mild shock to create a simple fear in the their minds. "We measured the fear response by looking at their body's response by how much they sweat. We did this with something called the skin conductance response, and this is just a natural response you have in a fearful situation," she explains. "So after pairing the blue square with the shock a few times the subjects learned the blue square predicted they might get a shock and now they're sweating a little bit more when they see the blue square."

Then the researchers taught the volunteers that something had changed, displaying the blue square a number of times without the shock. "Eventually they learned that something's different, that there's nothing to fear in this situation and we also saw that the skin conductance response diminished," she says.

The brain scans showed that both the amygdala and the more highly-evolved medial prefrontal cortex — front and center in the brain, behind the eyes — are involved in getting over fears.

Prefrontal cortex lies front and center in the brain, behind the eyes.

Phelps' group has since repeated the study using a tone paired with a shock rather than the colored squares, which correlate more closely with LeDoux's work in animals. "We can take this animal model and try to see in experiments in humans how far it will extend to situations that are typical in human experience," she explains. "So we've been able to show that in the laboratory this simple model explains a lot of more complex human behaviors."

One of the ways they've shown that this simple model of learning extends to more complex situations more typical of humans is by looking at using cognitive strategies to control a person's emotional responses.

"We all know that you can look at the glass half full or half empty," says Phelps. "You can imagine being in the presence of something that you view slightly fearful and focusing on that or trying to focus on something else about that stimulus that makes it calming for you. And this is the type of things that's done in cognitive therapy."

They've been able to show that using this type of strategy to control a learned fear — something most of us do in everyday life — relies on some of the same neural mechanisms as this learning through gradual exposure

But even if you manage to overcome your fears and extinguish them it doesn't mean they are gone for good.

"One of the things we know about fear is that you can always bring it back," explains LeDoux. "So if either a rat or a person, with the previously learned fear, is exposed to stress, the fear will return. And so if a patient with a fear of heights is doing fine because of the therapy, and then the patient's mother dies and it's very stressful and so the phobia comes back."