

Mice in pain have facial expressions that are very similar to human facial expressions, according to scientists who have developed the "mouse grimace scale." The pain expressions of mice could help researchers gauge the effectiveness of new drugs.

People have been using similar facial-expression coding systems in babies and other humans who are unable to verbally express their pain. "No one has every looked for facial expression of pain in anything other than humans," said Jeffery Mogil of McGill University, co-author of the study published on May 9 in Nature Methods.

Most pain drugs fail in human trials, because pain-drug effectiveness in rodent trials is based on sensitivity to touch, which is not a good indicator of spontaneous pain, Mogil says. The mouse grimace scale adds another way to catalog pain and pain mitigation in laboratory animals.

"This is a true measure of spontaneous pain, a measure that was derived from the analogous human scale," said Mogil. "If pain researchers would adopt this, we could get more accurate translations [of drug effectiveness] to humans."

Mogil first noticed that mice can sense the pain of other mice in 2006. He saw that mice were communicating their pain visually, which had to be either by interpreting each other's facial expressions or body movements. Mogil wondered if we could see whatever the mice were seeing.

To test for facial expressions of the mice, Mogil put them through mild to moderate pain tests (similar to a headache or swollen finger, easily treated with Tylenol or aspirin) and used high-definition cameras to monitor their expressions. Pictures from before and after the pain stimulus were shown to technicians at the lab of colleague Kenneth Craig.

The technicians used five facial expressions to determine if the mice were in pain: eye squinting, nose bulge, cheek bulge, ear position and whisker changes. While two of the expressions, whisker and ear movement, are impossible for humans to create, the other three were taken directly from the human facial expression of pain scale.

"It suggests that this is all a matter of evolution," Mogil said.

The problem with pain is that it is both an emotional and a physical response. In humans, one area of the brain is associated with the emotional aspect of pain. When that area is destroyed by a stroke, patients report feeling sensations, but they don't describe it as pain.

If this area is damaged in the mice, you can block most of the pained facial expressions without reducing other pain responses. "What we are seeing in the pain face is the emotional reaction, it could actually be the 'I'm not happy face,'" Mogil said.

Mogil also tested his mouse grimace scale on mice who have migraines. His team could see changes in their facial expressions when they were probably having a migraine, and could see the expressions diminish when they were treated with migraine drugs.

To pet owners and Cute Overload readers, the discovery of facial expressions in other mammals won't come as a surprise. It also wouldn't have surprised Charles Darwin, who predicted that all mammals express emotion through their faces in his 1872 book The Expression of the Emotions in Man and Animals. His book also theorizes that these facial expressions are evolutionarily conserved.

Psychologist Amanda C de C Williams of the University College London thinks this study highlights the evolutionary hardwiring of facial expressions. "These expressions predate when mice and humans split on the evolutionary scale," she says. "If there are characteristic expressions, then all mammals should share them."

For a long time, scientists thought facial expressions were culturally and behaviorally determined. Studies of tribes in Papua New Guinea done by Paul Eckman (who is loosely the basis for the character Cal Lightman in the show Lie to Me) in 1972 found universally recognized facial expressions in completely isolated cultures and overthrew this theory. While Eckman didn't do research on pain expressions, the mouse grimace scale beefs up his argument that facial expressions are not only ubiquitous across human cultures, but also all mammals.

"It suits us to think that animals don't have a real depth of feeling or emotion, so it's OK to treat them badly," Williams said. "Farming practices aren't very sensitive to animals' feelings. It's convenient to just hope they aren't feeling these things."

*Images: *Jeffery Mogil

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Citation: "Coding of Facial Expression of Pain in the Laboratory Mouse" by Dale J Langford, Andrea L Bailey, Mona Lisa Chanda, Sarah E Clarke, Tanya E Drummond, Stephanie Echols, Sarah Glick, Joelle Ingrao, Tammy Klassen-Ross, Michael L LaCroix-Fralish, et al. Nature Methods, early online publication, May 9, 2010. doi:10.1038/nmeth.1455