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CHICAGO (Reuters) - Smiles may take a while, but a horrified expression is a sure-fire attention getter, U.S. researchers said on Sunday, based on a study of how fast people process facial expressions.
They believe fearful facial expressions make a beeline to the alarm center of the brain known as the amygdala, cuing humans to potential threats.
"We think what is happening with fear is that this is a critical threat signal for us," said David Zald, associate professor of psychology at Vanderbilt University in Tennessee.
"Fear tells you something is wrong and you need to pay attention," Zald said in a telephone interview.
To find out which images get the brain's attention first, Zald, graduate student Eunice Yang and colleagues used a technique called visual flash suppression that slows down the brain's responses to facial expressions -- which typically register in less than 40 milliseconds.
The researchers had people look into a special viewer that allowed each eye to see different images at the same time. "If you present different images to the two eyes, usually you will only perceive one of them at a time," Zald explained.
The image that registers with the brain typically depends on which eye is dominant for that person.
"But if one of the eyes is presented by a dynamic, changing stimulus, it will basically suppress perception from the other eye," he said.
Using this approach, they showed people a static image of a face in one eye and a series of rapidly presented, random images in another, creating a type of visual "noise." People then were asked to indicate when they first registered the static face in the first eye.
The team found people became aware of fearful expressions much faster than neutral or happy faces. "We were seeing it pretty much universally," Zald said.
He thinks it has something to do with the eyes.
"If you compare the amounts of the whites showing with a fearful face versus a neutral face, the difference is really quite striking," he said.
People who were shown images only of eyes consistently picked out the fearful ones first.
Zald said fear appears to be such important information that these images shortcut the normal pathway for processing visual images through the visual cortex.
The study, which appears in the journal Emotion, also reveals how little interest the brain has in smiling faces.
"The happy expressions were slower to be detected than even the neutral expressions," he said, suggesting things that signal a lack of threat need decreased attention.
Zald said facial expressions represent a crucial way for people to convey social information.
"We see the effects on people who can't do this well. Autistic children, for instance, cannot read emotional expressions in others very well. They have devastating social consequences because of this," he said.