The mirror test is considered a classic indicator of self-knowledge: Does an animal recognize that its reflection is not a member of its species, but itself? So far, only a few animal species have passed this test, including great apes, dolphins and ravens. A new study now shows that mice can also do this under certain conditions. The researchers also managed to identify which neurons in the mice’s brains are involved in self-recognition.
When animals see themselves in the mirror, they usually assume that they are confronting a member of their own species. Only a few animals are able to recognize themselves in a mirror. To test this ability, researchers developed the so-called mirror test in the early 1970s. A colored dot is unnoticed and painted on an individual’s face, which they can only see in the mirror. If, after seeing your own reflection, you try to examine the spot on yourself more closely or remove it, this is considered a sign of self-knowledge. Toddlers take the test from around the age of two. Great apes, dolphins, elephants and some corvids are also capable of this. Dogs, cats, pigs and most other animals, on the other hand, lack this ability to know themselves.
Splashes of color on the mouse’s face
A team led by Jun Yokose from the University of Texas at Dallas has now successfully carried out the mirror test with mice. To do this, the researchers painted a white spot on the foreheads of black mice and then gave them the opportunity to look at themselves in the mirror. In fact, the animals that saw themselves in the mirror spent more time brushing their heads to remove the paint. However, this only applied if the white spot was relatively large, meaning the mice could not only see the color in the mirror, but also feel it on their fur.
In order to rule out that the tactile stimulus was primarily triggering the grooming behavior, the researchers carried out the same experiment with black paint, which was barely visible on the mice’s fur. Although this color felt the same as white, the mice painted this way did not show increased grooming behavior. On the other hand, if the researchers painted a small white dot on a large blob of black paint, the mice groomed themselves as much after seeing their reflection as if they had a large blob of white paint on their heads. “The mice required significant external sensory cues to pass the mirror test,” says Yokose. “We have to apply a lot of paint to their heads and then the tactile stimulus allows the animals to visually match the paint on their heads in the mirror.”
Social experience as a basis
According to the study, in addition to the size of the marking, other requirements must also be met for mice to pass the mirror test. Only animals that had had the opportunity to experience mirrors before the test were able to control their own appearance in the mirror. In addition, social contacts are apparently also important. Animals that were raised in isolation from an early age and did not see any other animals showed no signs of self-awareness in the mirror test. The same was true if all of the tested mouse’s cage mates looked different from themselves, for example if a black mouse was raised among white mice. “These results suggest that visual self-image can be developed in mice through social experience and habituation to the mirror,” the team concludes.
The researchers also examined which areas in the mice’s brains are involved in self-recognition. They discovered a subset of neurons in the ventral hippocampus that were activated when the mice recognized themselves in the mirror. “Previous studies in humans suggest that this region plays a role in processing and storing visual features of the self,” explain the researchers. To find out whether these neurons are really essential for the mice’s self-recognition, the team gave some mice drugs that turned off these neurons. And in fact: If the corresponding neurons were deactivated, the animals were no longer able to recognize themselves in the mirror.
Neurons for self-recognition
Further tests revealed that the neurons required for self-recognition are also involved in certain social situations: “A subset of these neurons was also activated when the mice encountered other individuals of the same strain,” reports Yokose’s colleague Takashi Kitamura. “This is consistent with previous research in humans that showed that some hippocampal cells fire not only when a person looks at themselves, but also when they look at familiar people, such as a parent.”
In further studies, the researchers want to investigate in more detail what significance the visual and tactile stimuli have for the mice’s self-recognition and which additional brain regions may be involved in recognition. “Now that we have this mouse model, we can manipulate or monitor neuronal activity to comprehensively study the neural circuit mechanisms behind the triggering of self-recognition-like behavior in mice,” says Yokose.
Source: Jun Yokose (University of Texas Southwestern Medical Center, Dallas) et al., Neuron, doi: 10.1016/j.neuron.2023.10.022