Faces express a wide range of emotions, from the raised eyebrows of interest to the gaping mouth of shock. Now, a new study suggests that the whites of our eyes alone can convey fear. The findings may explain how the brain uses simple visual cues to quickly assess danger.
Fearful signals activate an ancient structure buried deep within the brain known as the amygdala. When researchers show subjects pictures of people looking scared, the amygdala lights up in brain scans. Since wide eyes, which display more of the white sclera region of the eyeball, are a key feature of fearful faces, University of Wisconsin, Madison neuroscientist Paul Whalen and colleagues wondered if large eye whites alone were enough to activate the amygdala.
To test this, the team placed subjects in a scanner and showed them a series of image sets on a video monitor. The first image in each set was a pair of eyes against a black background; the second was an entire face with a blank expression. The subjects were only aware of the blank faces, however, because the eye images flashed onscreen for a mere 17 milliseconds.
Yet, these transient images were enough to stimulate the amygdala. When wide eyes with large whites flashed across the screen, subjects' amygdalas showed 4 times more activation than when narrow eyes with small eye whites appeared, the team reports in the 17 December issue of Science. However, when the researchers inverted the eye pictures—-so that the pupils were white and the scleras were black—the amygdala responded the same to wide and narrow eyes, indicating that it's the amount of white--not just the amount of eye—that signals fear.
There's a good reason why the brain apparently focuses on just one aspect of fearful expressions. "Dangerous events have to be rapidly recognized, and the less processing the brain has to do, the faster it will respond," says New York University neuroscientist Jospeh LeDoux. He adds that the study is "fascinating" and calls it "an important initial find."