Split-second facial expressions made by others — and the feelings they betray — might go unnoticed by your conscious mind, but apparently they do register subliminally.

Reading these subtle clues in faces can guide the brain, resulting in unconscious warnings, a new study suggests.

Although these hints might normally help warn a person of hidden threats, when kicked into overdrive they might exacerbate anxiety disorders, the scientists said.

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Student volunteers were shown 70 different faces on a computer screen, each face bearing a surprised expression.

Immediately before each surprised face was displayed, another face was flashed just for 30 milliseconds, faster than the conscious mind can register.

Half of these "micro-expressions" were happy, while the others were fearful.

During the experiment, Northwestern University cognitive neuroscientist Ken Paller and his colleagues recorded brain activity using electrodes placed on the scalps of the volunteers.

Although the volunteers were not aware they viewed subliminal emotional expressions, the recordings showed their brain detected them.

When asked to rate each surprised face as either positive, such as upon the sudden arrival of a friend, or negative, such as after experiencing a sudden act of violence, the volunteers rated surprised faces preceded by fearful micro-expressions more negatively on average than ones preceded by happiness.

"Our 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," said Paller's colleague Wen Li, also at Northwestern.

The researchers detailed their findings in an upcoming issue of the Journal of Cognitive Neuroscience.

"We can perceive a lot more than we are aware of perceiving, and doing so can influence behavior, although these influences can lead to just subtle differences and not make you do something out of the ordinary," Paller said.

Before the experiment, questionnaires given to the volunteers assessed how often they experienced anxiety, particularly in social affairs. Those with more anxious personalities had the strongest brain response to subliminal expressions of fear.

Paller suggested their brains may be highly sensitive to subtle hints of threat and "may thus cause excessive anxiety."

The findings "have direct implications for understanding psychiatric disorders such as phobia, obsessive-compulsive disorder and generalized anxiety disorder," Li said.

It is unclear whether susceptibility to micro-expressions has an effect on social judgments and behavior, Paller said.

"Most of the reason you like or don't like someone could be because of things you are aware of, not things you aren't," he said.

People do make facial micro-expressions in real life, and detecting them might normally help sense another's true intentions, he said.

Still, Paller noted that "valuable as this ability may be, when someone is paying you a compliment it is sometimes better to take it at face value rather than read more into it."

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