A look of fear can make an impression on someone faster than a smile, psychology experts say.

Researchers at Vanderbilt University in Nashville have found the human brain processes a fearful look faster than a neutral or happy expression. The faster detection is part of natural human survival instincts, the researchers 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," David Zald, associate professor of psychology at Vanderbilt University in Nashville, said in a release.

"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 findings, which will appear in the November edition of the journal Emotion, were based on tests that asked subjects to look into a microscope-like viewer, which used a technique known as continuous flash suppression to show different images to each eye. 

One eye was shown rapidly changing images, which acted as visual "noise" that suppressed the static faces being shown to the other eye. The subject would eventually overcome the noise shown to one eye and mentally block it out, thus recognizing the static image of the face displayed to the other eye.

Subjects indicated when they first became aware of seeing the faces, which showed different emotions, enabling researchers to gauge how long it took for the different expressions to register.

The researchers said an area of the brain called the amygdala, which shortcuts the normal brain pathway in processing visual images, is responsible.

The shape of the eye, particularly on fearful faces, may also play a key role in the mental processing of emotions.

"That may be the sort of simple feature that the amygdala can pick up on, because it's only getting a fairly crude representation," Zald said. "That fearful eye may be something that's relatively hard-wired in there."