Mice display different facial expressions depending on their mood, say researchers writing in Science, who found pleasure, disgust, nausea, pain and fear all provoke different reactions in the rodents. It is thought to be the first time researchers have been able to ascribe emotions to different facial expressions in mice.
Changes in the facial expressions of mice can be hard for the human observer to make out, so researchers at the Max Planck Institute of Neurobiology used machine learning. The algorithm was fed images of mice reacting to different stimuli—including quinine to provoke disgust, sucrose for pleasure, lithium chloride for malaise, escape to spark active fear and freezing for passive fear.
"With our automated face recognition system, we can now measure the intensity and nature of an emotion on a timescale of milliseconds and compare it to the neuronal activity in relevant brain areas," Nejc Dolense, who co-led the study, said in a statement.
The results suggest each emotion prompted a unique facial expression—except one. Fear resembled multiple facial expressions, including those for pain and those provoked by bitter tastes. The researchers say this may be because the facial expression captures a range of different emotions at the same time.
Not only were the researchers able to identify different emotional states from specific facial reactions, they were able to determine the extent of those emotional states. A mouse drinking the sucrose solution when thirsty appeared happier than a mouse drinking the sucrose solution when not.
The researchers investigated further to find out whether these facial expressions were automatic responses to certain stimuli or if they really did reflect the rodent's inner emotional state. To do so, they analyzed neural activity in the brain. Specifically, they looked at a region called the insular cortex, which is known to play a role in emotions, using a flourescent imaging technique called 2-photon microscopy.
The facial expressions of mice were recorded when scientists stimulated certain neurons associated with different emotions using light. The researchers found the neurons and the facial expressions reacted at the same time and with the same level of intensity, supporting the idea that these are responses to internal emotions as opposed to reflex-like reactions.
"The authors showed that changes in facial expression are not reflex-like reactions but reflected some of the properties of emotions, such as valence (positive or negative), scalability (graded nature of emotional intensity), and flexibility (ability to flexibly regulate emotions)," said neuroscientists Benoit Girard of Sorbonne Université and Camilla Bellone of the University of Geneva in an accompanying editorial. Neither were involved in the original research.
Girard and Bellone say the research has practical implications in our understanding of social behavior and interpersonal relationships, but prompts other questions. For example, humans are able to recreate facial expressions to pretend to feel a certain emotion but can animals do the same?
The researchers hope to delve into the neuronal mechanisms behind emotions in animals like mice.
"This is an important prerequisite for the investigation of emotions and possible disorders in their processing, such as in anxiety disorders or depression," Nadine Gogolla, who co-led the study, said in a statement.
