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If you suffer from social anxiety, you may have been told it's all in your head. Now, scientists scientists have zeroed in on a key brain function that explains how you interpret the world around you.

By measuring electrical activity in the brain while it processes emotion in other people, researchers found that socially anxious subjects were wired to have a stronger response to a key emotion: fear.

In the study to be published in the January issue of the Journal of Cognitive Neuroscience, subjects were outfitted with electrodes on their heads and shown a series of pictures. A face displaying either a happy or fearful expression was flashed for a fraction of a second - a period far too brief to be noticed - and then subjects were shown a surprised face, this time for longer.

Volunteers who reported higher levels of social anxiety were more likely to rate the surprised faces as frightened if they had been flashed the fearful face. Those who did not report suffering from social anxiety, defined in this case as the tendency to feel anxious in social situations, were also affected by the fearful faces, but not to the same degree.

"Evaluating ambiguous cues as negative is a very hallmark symptom of social anxiety," says Wen Li, lead author of the study at Northwestern University's medical school in Evanston, Ill. "What we found might be one of the underlying mechanisms."

The results show that an unconsciously perceived signal of threat, such as a brief facial expression of fear, can still bubble up and unwittingly influence social judgments and how we act, Dr. Li says.

"We live in an ocean of sensory input we are not necessarily aware of," she says.

The findings have direct implications for understanding psychiatric disorders such as phobias, obsessive-compulsive disorder and generalized anxiety disorder, Dr. Li says.

It's possible that socially anxious people might be able to retrain themselves not to overemphasize the negative and to "pay more attention to neutral and pleasant cues rather than unpleasant," she says.

The subliminal flash of fear was imperceptible to all of the participants, who only reported seeing the surprised faces. But Dr. Li's electrodes recorded brains processing information as early as 130 milliseconds after volunteers saw the subliminal images, even though they weren't conscious of anything they'd seen until about 800 milliseconds had passed. These are all tiny measures of time. Even 800 milliseconds is still under the one-second mark.

The fact that the socially anxious subjects were more sensitive to the flashes of fear point to subtle processing differences in their brains. The parts of the brain behind visual processing are more enhanced in these people, Dr. Li says. "The visual cortex works more."

One theory Dr. Li and colleagues have explored in previous research is that those who are socially anxious are both hyper-vigilant and hyper-sensitive.

They tend to avoid any clear threats they perceive in their environment. But if any subliminal threat "leaks" through, they are particularly ready to read it and act to protect themselves. This, in turn, may lead to even more anxiety, however.

Dr. Li, who has also recently studied the effects of subliminal odours, says there is much work to be done looking at the emotional and cognitive processes carried out below our conscious awareness.

"Maybe, gradually, we can probably do something to the part that is under the water."