Of Mice and Men

Charles Darwin’s The Expression of the Emotions in Man and Animals, published in 1872, describes how people of different cultures and animals of different species use highly similar behaviors to show what they feel. Darwin viewed emotions and their corresponding bodily and facial expressions as evolved traits conserved across the animal kingdom, especially among mammals and birds. Flip through Expression and you’ll find illustrations of furious swans and hens, a dog in a “humble and affectionate frame of mind,” and a sulking chimpanzee.

Today, psychologists agree that all people recognize the same facial expressions of certain core emotions — like surprise, fear and happiness — no matter where they grew up. Researchers have categorized the consistent facial patterns we use to show these core emotions — a useful tool for interpreting the emotional states of infants or for studies on people who cannot properly recognize the emotions of others. But no one paid much empirical attention to how nonhuman animals use their faces to show emotion.

Now, researchers have found that mice express pain on their faces in many of the same ways people do. The researchers even created a “mouse grimace scale” to accurately rate the intensity of the mice’s painful experiences. This is the first study on the facial expressions of pain in nonhuman animals. It appears in the May issue of the journal Nature Methods.

In their experiment, psychologists from McGill University and the University of British Columbia injected mice with inflammatory drugs that cause moderate pain akin to a headache or swollen finger and videotaped their behavior in Plexiglas observation cubicles. Technicians trained to recognize emotion in faces then analyzed still frames from the videotapes, looking in particular for five signs of pain: squinted eyes (called ‘orbital tightening,’) a bulged nose, bulged cheeks, ears drawn back, and whiskers pulled back or pushed forward. The first three behaviors — squinting eyes and bulging nose and cheeks — are classic indicators of pain in humans.

The technicians rated the intensity of the pain expressed in each image as 0 (not present), 1 (moderately visible) or 2 (severe) — the basis of the mouse grimace scale. Using this scale, technicians with one year of experience interpreted the intensity of the mice’s pain with 81% accuracy. When the researchers used high-definition video, the technicians’ accuracy increased to over 97%.

Not only does the study underscore Darwin’s idea that people and many animals use similar behaviors to express emotion, the mouse grimace scale could help scientists better evaluate the painful side effects of drugs tested on rodents.

Jeffery Mogil, a co-author of the Nature study and a neuroscientist who specializes in pain at McGill University, in Montreal, Quebec, Canada, is especially interested in how mice use their facial expressions of emotion in social behaviors. According to Wired, Mogil noticed in 2006 that mice can recognize their neighbors’ pain and wondered what behavioral cues they were reacting to.

The researchers also plan to test whether similar pain scales could be developed for other mammals.