Why Young Brains Lack Caution

CAMBRIDGE, MASSACHUSETTS--"No fear." It's the brazen brand name of youth-oriented fitness gear. Now brain scans have revealed some truth behind the hype: Teenage brains, it appears, have not fully developed the structures that are thought to comprehend emotions like fear and temper--instinctive "gut" reactions. The findings were presented here today at the Brain and Psyche seminar at the Whitehead Institute for Biomedical Research.

Teenagers seem to ride an emotional roller coaster and often have difficulty dealing with their emotions. To map the brain regions responsible for these emotions, neuropsychologist Deborah Yurgelun-Todd, of Harvard-affiliated McLean Hospital in Belmont, used functional magnetic resonance imaging, which measures changes in blood flow as a proxy for neuron activity. She flashed 40 faces showing expressions of fear to 16 adolescents aged 11 to 17.

Young brains reacted to fear with heightened activity in the amygdala, an almond-shaped structure near the center of the brain that triggers the primitive "fight or flight" instinct. But the older the teenager, the larger the portion of their frontal lobe--charged with reasoning--that is also activated when processing fear. In previous work, adults showed greater frontal lobe and less amygdala activity compared to adolescents.

The apparent shift of emotional and cognitive processing from the instinctive to the cognitive regions corresponds with rapid growth in the adolescent brain, marked in part by expanding support tissue in the frontal lobes. "You have to learn to be afraid, and it's probably a pretty extensive process," says Yurgelun-Todd. "But if you don't have the right brain tissue in place, you may not be ready yet to make use of the experiences."

The findings are "remarkably consonant with our understanding of the fear system, as discovered through studies of animal brains," says New York University neuroscientist Joseph LeDoux. "The amygdala is the key to the processing of danger signals and the expression of fear responses, whereas the frontal cortex is involved in the inhibition of fear."