Individuals respond most quickly to a fearful expression
Individuals react more quickly to a fearful expression than to faces showing other emotions such as joy, a study in the journal Emotion found.
Researchers from Vanderbilt University found the same speedy reaction to fear when only the eyes were visible.
It is thought the brain has evolved to react more quickly to potentially threatening situations
The brain responds very quickly to all facial expressions - at a speed of less than 40 milliseconds.
So to assess if certain emotions prompt a faster reaction, the researchers had to slow down the speed at which volunteers became aware of facial expressions.
Volunteers looked through a viewer which flashed a black and white, quick-changing pattern to one eye and a static image of a face to the other eye.
The flashing image had the effect of slowing down the speed at which the individual noticed the face.
Participants became aware of a fearful expression far faster than a neutral or happy face.
Reaction to happy faces was consistently slower than for the other expressions looked at.
The fast reaction to fear was the same if the whole face was visible or just the eyes.
It is thought an area of the brain called the amygdala can process simple visual signals bypassing the normal visual processing pathway.
Dr David Zald, associate professor of psychology at Vanderbilt University in Nashville, Tennessee said: "We believe that the brain can detect certain cues even before we are aware of them, so that we can direct our attention to potentially threatening situations in our environment."
He added there was other evidence showing the eyes were an important part of the picture.
"Fearful eyes are a particular shape, where you get more of the whites of the eye showing.
"That may be the sort of simple feature that the amygdala can pick up on, because it's only getting a fairly crude representation."
He added that the brain may react to happy faces slowly because they signal safety and do not require immediate attention.
The team are now planning to do a similar study to look at the response to anger.
Dr Bahador Bahrami, associate researcher in the UCL Institute of Cognitive Neuroscience in London said the findings were very interesting but not unexpected.
"It's quite well accepted that fearful faces have a special significance.
"And other imaging studies have shown the brain responds more strongly to fear, so this is consistent with that finding."