A paper in Nature Methods this week establishes a scale that can estimate how much pain mice are feeling based on facial expressions. The system, called the "mouse grimace scale," codes movements like squeezing the eyes shut or bulging the nose as indications of pain, similar to those displayed in humans. It's intended to allow us to study feelings of pain in mice when testing drugs or veterinary procedures.

Scientists often use facial expressions in humans as an indication of how much pain they are feeling from electrical shocks, topical acids, and so on; even newborn babies express pain this way. While there are established scales for measuring pain based on grimacing facial expressions in humans, there isn't one for the rodents that are so often used in experiments as human analogues.

To create their mouse grimace scale, a group of researchers exposed some mice to painful situations while recording their reactions with video cameras. They established cues like cheek bulge, nose bulge, ear position, whisker change, and eye squeeze as expressions of pain, and categorized them in levels of no pain, moderate pain, and severe pain.

When a mouse's facial expressions were recorded in high definition, researchers found that people presented with screen captures of the mice correctly identified mice in pain about 97 percent of the time. The system's developers plan to offer a training manual on the mouse grimace scale, and predict that it will be helpful for developments in veterinary care and drug testing.

Nature Methods , 2010. DOI: 10.1038/nmeth.1455 (About DOIs).