Adolescence is a Sensitive Period for Fear Learning
Adolescence can be a time of vulnerability to illness. Anxiety disorders increase during this period, and three-quarters of adults with anxiety disorders trace the illness back to their childhood or adolescence. The most common treatments for anxiety disorder are based on the idea of fear extinction. A certain stimulus, like a social situation or seeing a spider, provokes a fear reaction in the brain. Through gradually increasing exposure to the stimulus and extinction training, the person becomes desensitized to the stimulus. New research on rodents presented by Francis S. Lee at the 2015 meeting of the Society for Biological Psychiatry suggests that the extinction process is diminished during adolescence.
At specific stages of maturation, neural circuits related to particular abilities can become flexible. Brain and behavior become sensitive to and are increasingly shaped by experience. Studies of rodents and humans have shown that adolescence is a time when the neural circuitry for fear extinction is in flux. In mice, this period falls around their 29th day of life. Lee reported that around this time, the mice begin to exhibit resistance to extinction of fear learning.
In adolescent rodents, there is a surge of contextual fear learning and retrieval that is mediated by hyper-connectivity of the ventral hippocampus and the amygdala to the prelimbic part of the prefrontal cortex. In contrast, the pathway from the amygdala to the infralimbic cortex mediates the extinction of this type of learning. Because the prelimbic pathway for fear learning is overactive, the infralimbic pathway for extinction learning is less effective.
Adolescent mice temporarily lose their ability to retrieve memories related to cue-dependent (as opposed to context-dependent) fear learning. Remarkably, when these animals proceed into adulthood, the fear learning associated with cues returns and becomes accessible again.
This could help explain how teenagers can lose fear conditioning to cues (for example, speeding through a red light) they learned in childhood. The fear is forgotten (or becomes inaccessible) in adolescence, but then what had been learned is again “remembered” (retrieved) in adulthood.
The adolescent brain prunes back 100,000 synapses per second, perhaps accounting for some of the chaos and recklessness that can characterize adolescence. Adolescents’ entire brains are being rewired, in a process that typically replaces excitatory synapses with inhibitory ones. Now it appears that teens may just not be able to recall previously well-learned lessons linking certain cues to fear responses that would tell them to avoid certain situations and behaviors. In a few years, access to the appropriate cue-dependent fear learning will return and more mature choices and responses will likely re-emerge.
