The Mirror Test: Mice Show Self-Recognition Ability and Evolved Cognitive Processes

Tuesday - December 5 2023, 20:15 UTC - 11 months ago

Mice have recently achieved self-recognition ability and joined an exclusive group of animals that includes great apes, bottlenose dolphins, and European magpies. Researchers marked the foreheads of black-furred mice with white and black ink marks, and upon observing their reflections in the mirror, the mice attempted to remove the ink stain from their foreheads. Through gene expression mapping, the researchers pinpointed a specific group of neurons in the ventral hippocampus that became active when the mice seemingly "recognized" themselves in the mirror. Additionally, the researchers discovered that a portion of these neurons responded not just when the mice looked in the mirror, but also when they watched other mice of the same strain, pointing to a more sophisticated understanding of self-recognition and cognitive functions.

Mice have recently joined a select group of animals that have passed the mirror self-recognition test (MSR) — a commonly recognized indicator of self-awareness. This exclusive club also includes great apes, bottlenose dolphins, and European magpies, to name a few. Recently, a team of researchers performed a set of experiments in which they marked the foreheads of black-furred mice with white and black ink marks. Upon observing their reflections in the mirror, the mice attempted to remove the ink stain from their foreheads. A statement by the researchers highlighted that "the mice only showed this self-recognition-like behavior if they were already accustomed to mirrors if they had socialized with other mice who looked like them, and if the ink spot was relatively large." .

Grooming behavior changed in size and color However, the authors emphasized that the mice’s ability to perceive changes in their appearance does not inherently imply self-awareness. While mice with white ink spent a lot of time trying to remove the spot from their heads, they didn't think about grooming other parts of their bodies in front of the mirror. On the other hand, mice with black spots similar to their fur did not at all groom their heads. This behavioral pattern was consistent when the black and white ink spots were small in size. The observed behaviors suggested a nuanced response to changes in appearance, highlighting that grooming reactions may be influenced by factors such as spot color and size rather than solely indicating self-awareness.

"The mice required significant external sensory cues to pass the mirror test—we have to put a lot of ink on their heads, and then the tactile stimulus coming from the ink somehow enables the animal to detect the ink on their heads via a mirror reflection. Chimps and humans don't need any of that extra sensory stimulus," stated Jun Yokose of the University of Texas Southwestern Medical Center, and the first author.

Gene expression mapping Through gene expression mapping, the researchers pinpointed a specific group of neurons in the ventral hippocampus that became active when the mice seemingly "recognized" themselves in the mirror. The researchers deliberately disrupted the function of these neurons to investigate the relationship between them and the observed grooming behavior. As a result, the mice were no longer groomed in response to the mirror and the presence of ink markings. Furthermore, the researchers discovered that a portion of these neurons responded not just when the mice looked in the mirror, but also when they watched other mice of the same strain. In response to mice with identical physical features and fur colors, these neurons were stimulated. This shows that these neurons are involved in identifying self and others based on physical likeness, which influences grooming behavior observed in the presence of mirrors and ink markings.

According to the authors, this gene mapping gives the first sight into the cerebral processes behind self-recognition, "something that was previously a black box in neurobehavioral research." .

"To form episodic meories, humans are known to use the hippocampus, and these findings suggest that such cognitive functions may be more ancient and evolutionarily preserved—shared by some organisms, such as mice, but absent in others," stated professor Jing Feng of the University of North Texas in a statement.