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FRIDAY, July 4 (HealthDay News) -- The brains of people who suffer from anxiety and severe shyness may respond more strongly to stress and show signs of being anxious even in situations considered safe by others, say researchers at the University of Wisconsin School of Medicine and Public Health.

They studied brain activity, anxious behavior and stress hormones in adolescent rhesus monkeys. Those with the most anxious temperaments showed higher activity in a part of the brain called the amygdala, which regulates emotion and triggers reactions to anxiety.

The anxious monkeys had more activity in the amygdala in both secure and threatening situations, the study found. When the monkeys were tested again 18 months later, the results were the same.

"The brain machinery underlying the stress response seems to be always on in these individuals, even in situations that others perceive as safe and secure," Dr. Ned Kalin, chairman of the department of psychiatry and HealthEmotions Research Institute, said in a prepared statement.

It has long been known that children with an anxious temperament are at increased risk for developing anxiety disorders, depression and substance abuse. The findings indicate a brain mechanism that's present early in life predisposes people to anxious temperament, and that it's difficult for someone with this temperament to be calm because their brain is wired in a way that keeps them tense and anxious.

The study was published July 2 in the online journal PLoS One.

More information

The Center for Mental Health Services has more about anxiety disorders.