"There are reasons to believe that the brain has evolved mechanisms to detect things in the environment that signal threat. One of those signals is a look of fear," David Zald, associate professor of psychology and a co-author of the new study, 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.
To determine 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.
The analysis showed that participants became aware of a fearful expression far faster than a neutral or happy face. Researchers also found that the fast reaction to fear was the same if the whole face was visible or just the eyes.
The team believes that a brain area called the amygdala, which shortcuts the normal brain pathway for processing visual images, is responsible for the mechanism.
"The amygdala receives information before it goes to the cortex, which is where most visual information goes first. We think the amygdala has some crude ability to process stimuli and that it can cue some other visual areas to what they need to focus on," Zald said.
Zald and his colleagues believe the eyes of the fearful face play a key role.
"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. That fearful eye may be something that's relatively hardwired in there," he said.
A surprising finding, according to Zald, was that subjects perceived happy faces the slowest.
"What we believe is happening is that the happy faces signal safety. If something is safe, you don't have to pay attention to it," he said.
The team is now planning to explore how this information influences our behaviour.
The study will appear in the November 2007 issue of Emotion.