You're having a casual conversation- suddenly, your friend's eyes bug out at something behind you. Do you continue speaking as if nothing is wrong, or jump to follow his gaze?  

The likely reaction is obvious, and a new study helps explain why. Researchers at the University of Toronto have detailed evidence that widening eyes, a universal human facial expression of fear, serve two major sensory and social functions: to enlarge the visual field in order to identify threats, and to orient observers to the source of danger.

Lead researcher Daniel Lee, a graduate student at UToronto, worked on the two-part study with his advisor Adam Anderson and Joshua Susskind of the University of California, San Diego, who have collaborated on previous research on the evolutionary function of facial expressions of fear and disgust.

The results were published online last week in the journal Psychological Science.

In the first experiment, they measured how much eye-widening fear expressions enhanced research participants' visual encoding of objects in their peripheral vision. 28 college students with normal vision looked at complex flashing stimuli on a computer screen while making fearful, neutral, or disgusted facial expressions.

The results showed that when participants made wide-eyed fearful expressions, their visual field expanded peripherally by 10 percent.

In the second experiment, they examined the signaling benefits of fearful faces for observers. 26 different participants were asked to judge the direction of eye gazes across several pairs of isolated eyes, which ranged in size from narrow-eyed expressions of disgust to wide-eyed expressions of fear.

They also had to judge how much the isolated eyes expressed the six basic emotions (anger, disgust, fear, happiness, sadness, and surprise), then follow the direction of the gaze towards particular visual targets.

As the researchers expected, participants could more easily judge the direction of a pair of eyes as the eyes widened, and more accurately follow the gaze of wider eyes.

Their ability to follow the gaze had no correlation with the perception of the eyes as fearful, which may indicate that the evolution of eye morphology was driven more by social signaling than the perception of emotion.

"Emotional expressions look the way they do for a reason," said Lee in a statement. "They are socially useful for communicating emotional states, but they are also useful as raw physical signals. In the case of widened eyes, they help send a clearer gaze signal that tells observers to 'look there.'"

Widening eyes are apparent among mammals, including dairy cows, and the researchers write that enlarging the visual field for enhanced sensory perception was likely an early evolutionary adaptation.

The shape of human eyes is uniquely suited to social signaling, they go on to say. Unlike other primates, humans have a high contrast between the sclera, or whites of the eyes, and the colored iris, allowing an incredible range of expressivity with minute shifts.

The "prominent flashing of white sclera" visible when someone's eyes widen is a powerful cue for observers to follow the gaze, allowing them to quickly orient themselves toward potential threats.

"Our eyes are important social signals," concludes Lee in the statement. "This research really shows how social we are wired to be."