Field of Science


In Adolescence, Fears Are Harder to Forget

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, St├ęphanie 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


  1. Perhaps, it has something to do with the increasing amount of hormones in adolescents. Hormones are the cause of many emotional responses and how someone would respond to stress. We often experience stress in the face of our fears. I don't know, but maybe the endocrine system and the nervous system are somehow interacting to hold on to this fear.

  2. I think that this is very interesting because the prefrontal association cortex is involved in the planning of voluntary movements. In the study the mice were watched to see how they reacted to the sound and this proves that the prefrontal cortex was the part of the brain that this was happening in. This same area is associated with perceptions and memories. In the study the mice had the memory that they got shocked when they heard that noise, so when they heard the noise again they froze because they were scared they were going to expereince another shock. The humans had the memory that they heard a loud noise when they saw a certain color and the noise hurt their ears and they did not want to experience that again. I feel that fears are harder to forget as a adolescence because your brain is still expanding and gaining more neurons so you are able to communicate quickly through the neurons.

  3. In this experiment the body is triggered to react from your sensory neurons. Sensory neurons send information such as light or sound from specialized receptor cells in the sense organs to the brain. Then there is the excitatory message which is the likeliness that it will activate and generate the action. With the excitatory message playing a roll on the sensory neurons it binds to a receptor which controls whether or not to have a reaction to the sound it is hearing. The reaction that follows the sound is what comes from retrieval cue which is what brings back the memory for instance when seeing the boxes on the screen, they immediately are drawn back to the screeching sound that followed the image. The age difference comes in to play because the younger in age the less neurons there are to bind to which causes less memory.

  4. I think that hormones play a big part in the emotional behavior of teens. Hormones are in involved in emotional responses and the way you respond to stress. Therefore, the human test performed put more stress on the adolescents because they have higher hormones. They had to perform a memory test in which each color had a specific loud noise response. Possibly, the noise hurt their ears so before they pushed another button they instantly became stressed out because they did not want to hear that loud noise again.
    The prefrontal cortex is located just behind the forehead and is responsible for the regulation of cognitive, emotional, and behavioral functioning. The experiment that involved mice proved that prefrontal cortex was the brain region in which reaction to sound took place. The mice placed in the box heard loud tones followed by electrical shocks. The study of mice could also deal with your motor neurons. Since the mice were so fearful of another electrical shock, once they heard a loud tone, their bodies completely froze. Fears in general are hard to shake, but I believe that the younger a person is, the more likely the fears they have experienced will impact their behavior and responses to certain situations.

  5. As stated in various comments, I do believe that increased levels in hormones for adolescence contribute to the excessive fear. But perhaps too, the energy level helps them to react more quickly. Also, us old folks were born in the "Rock-N-Roll" era where it was cool to blare loud music...our hearing is not so great. Good blog, thanks for the info! Now I will be experimenting on my children of various ages...bring on the haunted houses.


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