Think about the last time you were home alone and you heard a loud, unexpected noise. Chances are, your eyes widened as you listened for more information. In a new paper, researchers have detailed why this happens, as well as why our eyes narrow when something disgusts us.
The researchers, led by Prof. Adam Anderson of Cornell University's College of Human Ecology, published their paper in the journal Psychological Science, where they suggest these opposite facial expressions are based on emotional responses that use the way our eyes gather and focus light to detect unidentified threats.
When our eyes open wider - as they do in fear - the researchers say this increases sensitivity and expands our field of vision to identify surrounding danger.
Alternatively, when our eyes narrow in disgust, this blocks light and sharpens focus to determine the source of our displeasure.
The team says their findings suggest that human facial expressions came about from adaptive reactions to stimuli in our environment, not as social communication signals, which supports Charles Darwin's theories on the evolution of emotion from the 19th century.
Commenting on the findings, Prof. Anderson says:
"These opposing functions of eye widening and narrowing, which mirror that of pupil dilation and constriction, might be the primitive origins for the expressive capacity of the face. And these actions are not likely restricted to disgust and fear, as we know that these movements play a large part in how perhaps all expressions differ, including surprise, anger and even happiness."
Emotions trigger facial expressions that harness useful light properties
Emotions filter our reality, says the team, and they shape what we see before any light actually reaches the inner eye.
Researchers say our eyes widen when we are fearful to allow more light and a wider visual field so that we can identify the cause of our fear.
Prof. Anderson says though we think of perception as a process that happens after the brain receives an image, "in fact emotions influence vision at the very earliest moments of visual encoding."
The narrowing of the eye in disgust results in the greatest visual acuity, they note, which involves less light and better focus, whereas wide-eyed fearful expressions create the most sensitivity, allowing more light and a wider visual field.
"These emotions trigger facial expressions that are very far apart structurally, one with eyes wide open and the other with eyes pinched," Prof. Anderson says, noting that this allows the eye "to harness the properties of light that are most useful in these situations."
He and his team are currently studying how such contrasting eye movements could explain how facial expressions in humans have evolved to support nonverbal communication across different cultures.
"We know that the eyes can be a powerful basis for reading what people are thinking and feeling," he says, "and we might have a partial answer to why that is."
Speaking with Medical News Today, Prof. Anderson said:
"We are now examining how the optical origins of eye widening and narrowing in emotional expressions may now be used to communicate scrutiny or acceptance of ideas or people. For instance, we might narrow our eyes when scrutinizing an idea, as if to bring it into focus, or show wide-eyed acceptance of it."
In 2013, we reported on a study that suggested oxytocin - "the love hormone" - has a self-perpetuating effect, whereby the hormone can make us fearful during future stressful situations if a social encounter is negative.