From Science Daily, July 26, 2005

Happy, sad, angry, scared: Some of us are good at hiding these everyday emotions, while others are unable to disguise them. Whether subtle or intense, facial expressions are the key to how we identify human emotion.

Most studies of how we recognize facial expressions have used static models of intense expressions. But new research indicates that facial motion—seeing the range of movement in the arching of an eyebrow or the curve of a smile—is in fact an extremely important part of what makes subtle facial expressions identifiable.

A recent study by Zara Ambadar and Jeffrey F. Cohn of the University of Pittsburgh and Jonathan W. Schooler of the University of British Columbia, examined how motion affects people’s judgment of subtle facial expressions. Their report, “Deciphering the Enigmatic Face: The Importance of Facial Dynamics in Interpreting Subtle Facial Expressions,” is in the May 2005 issue of Psychological Science, a journal of the American Psychological Society.

Two experiments demonstrated robust effects of motion in facilitating the perception of subtle facial expressions depicting six emotions, anger, disgust, fear, happiness, sadness and surprise. For the initial experiment, participants viewed a series of subtle facial expressions, displayed by faces on a computer screen. The faces varied in the way they were displayed; in the single static display, only the final expression was shown, while the dynamic displays showed the emergence of a subtle expression through several images, ending at the identical final expression.

Expressions that were not identifiable in static presentations suddenly became apparent in dynamic displays. “The participants were more accurate and more confident judging facial expressions in the conditions in which they perceived motion (dynamic) than in conditions in which motion perception was prevented.”

The possible roles of timing and perception of change were investigated in an additional experiment. While the second experiment was conducted in a format similar to the first, it added a new condition called “first-last mode,” that contained only the first and last images of each sequence. “This new condition substantially changed the temporal characteristics of the facial expressions while preserving the perception of motion.”

The results of both studies confirmed that, overall, motion improves perception of facial expressions. The studies ruled out several possible explanations for the results, including the idea that the dynamic display provided more facial information inherent in multi-frame sequences. The idea that this beneficial effect of motion is due to its role in revealing the temporal characteristics of each emotion was not supported in the final experiment, as participants continued to identify the emotions more accurately during the dynamic sequence as well as during first-last mode, than when they were merely shown the final, static facial expression.

“Given the absence of evidence for any of the alternative roles of motion considered in the current studies, it seems quite likely that the benefits of motion observed here stem from its ability to enhance individuals’ perception of the way in which expressions have changed. The present findings thus suggest that motion’s role in the detection of change … is critical in mediating individuals sensitivity to the communication of emotion.”