Mice’s facial expressions can reveal a wide range of emotions

Although it’s tricky for us humans to see, mouse feelings are written all over their furry little faces.

With machine learning tools, researchers reliably spotted mice’s expressions of joy, fear, pain and other basic emotions. The results, published in the April 3 Science, provide a field guide for scientists seeking to understand how emotions such as joy, regret and empathy work in animals other than humans (SN: 11/10/16; SN: 6/9/14; SN: 12/8/11).

Using machine learning to reveal mice’s expressions is “an extraordinarily exciting direction,” says Kay Tye, a neuroscientist at the Salk Institute for Biological Studies in La Jolla, Calif. The findings “lay the foundation for what I expect will be a game changer for neuroscience research on emotional states.”

Neuroscientist Nadine Gogolla of the Max Planck Institute of Neurobiology in Martinsried, Germany, and colleagues gave mice experiences designed to elicit distinct emotions. Sugar water evoked pleasure, a shock to the tail triggered pain, bitter quinine water created disgust, an injection of lithium chloride evoked a nauseated malaise, and a place where shocks previously had been delivered sparked fear. For each setup, high-speed video cameras captured subtle movements in the mice’s ears, noses, whiskers and other parts of the face.

Observers can generally see that something is happening on the mouse’s face, Gogolla says. But translating those subtle clues into emotions is really hard, “especially for an untrained human being,” she says.

Machine learning techniques handle the job beautifully, the researchers found. The methods were able to spot subtle face movements that came with good or bad experiences. For instance, on the face of a mouse drinking sweet water — and presumably happy about it — the ears move forward and fold at the back toward the body, and the nose moves down toward the mouth. A mouse tasting bitter quinine sends its ears straight back, and the nose curls slightly backward, too.

Activity of nerve cells in the mice’s brains also changed with distinct emotions, other analyses showed. These cells reside in the insular cortex, a deeply buried spot known to be involved in human emotions, too.

By prodding these cells to fire signals, the researchers could prompt the mice to display certain facial expressions. These connections among brain activity and facial expressions may lead to insights about the neural basis of emotions, and what goes awry in disorders such as anxiety, the researchers suggest.