When it comes to fear, unlearning is as crucial as learning. Our growing brains learn to be afraid of scalding pans, oncoming traffic, and our parents calling us by our full names. But if we can't unlearn a fearful reaction, we may live our whole lives paralyzed by dentists' offices or barking chihuahuas. The ease of this unlearning may depend on our age: new research suggests that in both mice and humans, fears are hardest to dislodge in adolescence.
People suffering from PTSD or phobias are often un-taught their fears through "exposure therapy." By repeatedly facing the things that scare them while in a safe, controlled environment, they dilute the strength of the fearful association. This washing away of old associations is a classic psychological trick called extinction. But it doesn't always work.
Researchers at Cornell and New York University, reporting this week in PNAS, used human volunteers and mouse "volunteers" to assess how easy fear extinction is at different stages of development.
The mice were 23, 29, or 70 days old—representing childhood, adolescence, and adulthood. On the first day of the study, mice were put into a box where they repeatedly heard a long tone that was followed by an electric shock from the floor. A day later, the mice found themselves in a different box where they again heard the threatening tones, but received no shocks to the paws. These sessions went on for four days.
On the first day after their fearful experience in the shock chamber, mice demonstrated fear by freezing whenever they heard the tone. By the fourth day of their "therapy," this reaction was much weaker in both young and adult mice. But adolescent mice still reacted almost as fearfully as they had on the first day.
Human subjects were also split into groups based on their age: 30 kids (age 5-11), 28 adolescents (age 12-17), and 25 adults (age 18-28). On the first day, all the subjects completed a task that involved pressing computer keys while watching colored squares go by on a screen. Really, they were being taught to associate one color of square with a very loud and unpleasant noise that was sometimes blasted into their headphones.
On the second day, subjects again saw a sequence of colored squares, but this time were spared the loud noises. Meanwhile, researchers monitored their subjects' skin conductance. (This isn't exactly a measure of fear, but of general "arousal" or excitement. Or, really, sweatiness. It's the same factor measured in a lie detector test.)
Children and adults showed lower skin conductance, meaning they were less on edge, as the experiment went on and they learned not to expect loud noises anymore. But adolescents remained on edge, barely lessening their learned response to the colored squares.
To find out in more detail what was happening in the brains of their mouse subjects, the researchers dove right in. (Human volunteers were allowed to keep their brains.) They focused on an area of the prefrontal cortex that was already known to be involved in fear extinction, called the vmPFC (short for ventromedial prefrontal cortex, which is neuroscience-speak for "smack in the middle of the forehead").
Author B.J. Casey says that for the first time, her study showed that this brain region's role in fear extinction happens at the level of the connections between neurons. In young mice or adult mice, slices of fear-extinguished brains had more-active neurons in a subsection of this brain region, compared to slices of fearful brains. But adolescent brains all looked the same, whether they had undergone the fear-extinction training or not. The diminished activity in adolescent neurons, Casey says, matches adolescents' diminished ability to unlearn a fearful reaction. This region of the adolescent mouse brain—not unlike some adolescents—is relatively inactive and set in its ways.
Casey says her group's research might help explain teenage emotions in general. "Our findings are consistent with exaggerated emotional reactivity during adolescence in humans and rodents," she says, as well as "diminished ability to regulate these emotions."
In all mammals, adolescence—whether it happens at 29 days or 16 years—is a time to learn what threats you're facing and how to live independently. Holding on to learned fears more stubbornly might be helpful for an adolescent in evolutionary terms. But if the trait makes it harder for teens to get past crippling phobias and anxieties, therapists may need to learn a different way to talk to teenage patients and their stubbornly fearful brains.
Siobhan S. Pattwell, Stéphanie Duhoux, Catherine A. Hartley, David C. Johnson, Deqiang Jing, Mark D. Elliott, Erika J. Ruberry, Alisa Powers, Natasha Mehta, Rui R. Yang, Fatima Soliman, Charles E. Glatt, B. J. Casey, Ipe Ninan, & Francis S. Lee (2012). Altered fear learning across development in both mouse and human. PNAS : 10.1073/pnas.1206834109
Image: Drew Herron/Flickr