The human brain learns not only through direct experience, but also by observing. Witnesses to others' pain can learn to be weary or fearful of the cause of distress.

But such learning experience can also fuel irrational or debilitating fears. New research suggests the brain's endogenous opioid system can help people learn the proper lessons. The system, however, doesn't always work.

"Some people are over-sensitive to this form of social learning," Jan Haaker, an associated researcher at Karolinska Institute's Department of Clinical Neuroscience in Sweden, said in a news release. "Our study shows that the endogenous opioid system affects how sensitive we are and may explain why some people develop post-traumatic stress disorder, PTSD, merely by observing others who are experiencing traumatic events."

For example, people with over-sensitive endogenous opioid systems might become anxious in the wake of a terror attack, even if they weren't present or near the attack.

To test the role of the endogenous opioid system, researchers used drugs to the block the opioid system in the brains of 22 healthy subjects. Another 21 subjects were given a placebo pill.

Each group of study participants were made to watch a video of people being subjected to electrical shocks.

Previous research has showed surprise to trigger the brain's catalogue of fear. If a person isn't surprised, the brain won't update its log of fears. By blocking the subjects' opioid systems, researchers ensured each participant would remain surprised even though they knew the video subject was about to be shocked.

As a result, the brain continued to update its reservoir of fearful memories.

"When the people participating in the experiment were themselves subjected to threatening stimuli that they had previously associated with other people's pain, they perspired more and displayed more fear than those who had been given a placebo," said lead researcher Andreas Olsson, senior lecturer at Karolinska Institutute. "This enhanced learning was even visible three days after the social learning episode."

The researchers published their analysis of the endogenous opioid system in journal Nature Communications.

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