Smiling might seem the obvious way to catch a person’s attention, but looking terrified works more quickly.

Our brains process fearful faces more immediately than those showing other emotions, research suggests. Happy expressions take the longest for us to become aware of them.

Scientists believe that our rapid reaction to frightened faces may be an instinct that evolved as a “threat radar” millions of years ago. The whites of the eye become particularly visible when a person is scared, possibly indicating a potential threat.

Researchers think that such cues may take a short cut through the brain to the amygdala, a centre for processing emotional reactions.

David Zald, a psychologist from Vanderbilt University in Nashville, who co-authored the study, said: “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.

“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.”

The scientists set out to determine if people became aware of fearful, neutral or happy expressions at the same speed.

Measuring response times is normally very difficult, because processing facial information usually takes the brain less than 40 milliseconds. The team therefore asked a group of volunteers to look at images of faces through a special viewer, similar to the eyepieces on a microscope and designed to confuse the brain.

Multiple images were rapidly presented to one eye, while a normal, static image was shown to the other. The multiple images serve as visual “noise”, slowing down the speed at which participants became aware of what they were seeing with the other eye.

“If you present different images to the two eyes, usually you will only perceive one of them at a time,” Dr Zald said. 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.

The researchers asked the volunteers to state when they first became aware of the static image, and could then measure whether the expression on the face had any impact on how quickly the volunteer became aware of it.

The findings, reported in the journal Emotion, showed that people became aware of fearful faces more quickly than of other expressions.

The team believe that the shape of frightened eyes played a crucial role in this, because they represented a visual cue which could take a shortcut through the brain to the amygdala. Dr Zald said: “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.”

Fearful eyes, he said, are a particular shape. “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.”

The scientists also discovered, to their surprise, that their volunteers processed smiling faces more slowly than any other emotion. “What we believe is happening is that the happy faces signal safety,” Dr Zald concluded. “If something is safe, you don’t have to pay attention to it.”

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