Mice Just Passed the Mirror Test. Here’s What That Says About Our Sense of Self

Here’s a fun test: Dab some blush onto the forehead of a six-month-old baby and plop them in front of a mirror. They might look at their reflection with curiosity but ignore the rouge. Redo the experiment at two years old. Now they’ll likely furrow their brows, touch the blush, and try to wipe it off.

In other words, with a few years of life experience, they’ve learned to see the person in the mirror as “me.”

The so-called mirror test has been a staple in cognitive science to gauge self-recognition—the ability to realize that a reflection of you is you and learn how you differ from other people. It’s a skill that naturally comes to babies, but how this works in the brain has long baffled scientists.

This week, a study in Neuron suggests that mice may also have a rudimentary sense of self.

When the scientists dabbed white ink on the foreheads of mice with black fur, they readily groomed it off when looking at themselves in a mirror, but ignored the stain if it matched their fur tone. Like when we peer in the mirror and see a pimple, the mice “recognized” their reflection and realized something was wrong. Similar to other species—including humans—they could better “recognize” themselves when raised with other mice.

The scientists then used gene mapping technologies to hunt down the neurons involved in self-recognition. Buried in the hippocampus, a brain region associated with memory and the regulation of emotions, cells lit up when the mice saw their reflections in the mirror and also seemed related to their grooming behavior. The mice ignored the white blob on their foreheads when these cells were dampened—as if they no longer recognized themselves.

These lowly rodents join an elite group of animals that has passed the mirror test, including our closest evolutionary cousin, the chimpanzee. Because we can readily record the electrical chatter in their brains, the mice could help unveil the neural circuits behind self-recognition.

To study author Dr. Takashi Kitamura at the University of Texas Southwestern Medical Center, self-recognition isn’t about vanity, it’s about constructing a sense of self.

As we go about our lives, the brain stores information “about where, what, when and who, and the most important component is self-information,” he said in a press release. “Researchers usually examine how the brain encodes or recognizes others,” but how the brain constructs a model of the self is a mystery. These mice may finally crack the black box of self-recognition.

Mirror, Mirror, on the Wall

Glance at a mirror, and you’ll immediately recognize yourself. We take the skill for granted.

Under the hood, constructing a visual sense of “me” takes complex cognitive gymnastics. A dramatic new haircut or pair of glasses can make your reflection strange or even unrecognizable. The brain must gradually recalibrate how you see yourself and still know that it’s you. It’s thought self-recognition relies on high-level cognitive processes, but because it’s based on an internal “sense,” the mechanism has been difficult to gauge objectively.

Here’s where the mirror test comes in. Developed by Dr. Gordon Gallup Jr. in the 1970s, it became a staple among scientists testing self-recognition in an array of species, from killer whales to magpies.

Here’s how it works. Put a mark onto the face of any cooperating species and place them in front of a mirror. Do they recognize that the mark on the face in the mirror is a mark on their own face? Gallup tried it with chimps. “What they did was to reach up and touch and examine the marks on their faces that could only be seen in the mirror,” Gallup told NPR in 2020.

Over the decades, the test was used widely to study childhood development and self-recognition in animals. But because it requires heavy cognitive power, mice were written off.

Not so fast, the new study says.

A Social Reflection

The team first tested mice with glossy black fur to see how they reacted to a mirror.

The mice happily roamed around an “apartment” with two rooms. One side of the “wall” had a mirror, the other did not. To make things more challenging, the mirror wall was moved around every day. When first faced with their reflection, most mice reared up in an aggressive attacking pose—suggesting they didn’t realize they were looking at themselves. Two weeks later, they mostly ignored the reflection.

But is it because they learned to recognize themselves, or that they were happy to live with a strange doppelgänger?

For an answer, the team squeezed a dab of either white or black ink directly onto the mice’s foreheads and set them loose in the chamber. Using deep learning software to detect different types of behavior, the team found that larger white ink stains—but not ones that matched their fur color—caused a grooming frenzy when they saw themselves in the mirror.

The mice furiously pawed at the inkblots but groomed other body parts—whiskers and tails—as usual (despite their reputation, mice love to clean themselves). It’s like finding a sauce splatter on your forehead after seeing yourself in the mirror. You recognize yourself, see the stain, and try to brush it off.

Not all mice behaved the same way. Those raised by foster mice with lighter fur—or those raised alone without social interactions—didn’t mind the white ink blot. Previous studies in gorillas reported similar results, showing that social experiences are critical for self-recognition, explained the team.

Who Am I Inside

To be very clear: The study isn’t saying the mice are self-aware or conscious.

But the setup could help us track down the neurons supporting our sense of self. In one test, the team mapped gene expression changes in the whole brain after the mirror test to see which neurons were activated and then traced their connections.

A small part of the hippocampus, a brain region that encodes and retrieves memories, lit up. When the team dampened these neurons’ activity, the mice no longer groomed the white ink blob in front of the mirror.

Surprisingly, these neurons also sparked to life when the mice saw peers that looked like them. The brain network seems to not only support self-recognition, but also recognition of others that look like us—like a parent.

The study is just a first step toward unraveling the mechanisms behind self-recognition.

And it has flaws. For example, the mirror test doesn’t account for behaviors specific to different species. The urge to wipe off a stain is a very primate-like response and relies on vision. Some species, such as Asian elephants or dogs, both of which have tried the mirror test, may not care about a stain, or they may heavily rely on other senses. Many animals also avoid eye contact—including when looking at themselves in the mirror—as it can be a sign of hostility. While the mice showed signs of self-recognition, they needed far more training and visual cues than a human baby.

But to the authors, the results are a start. Next, they plan to see if mice can recognize themselves with virtual filters—like puppy face ones in social media apps—and hunt down other potential brain regions allowing us to build a visual image of “me.”

Image Credit: Nick Fewings / Unsplash

Shelly Fan
Shelly Fanhttps://neurofantastic.com/
Shelly Xuelai Fan is a neuroscientist-turned-science writer. She completed her PhD in neuroscience at the University of British Columbia, where she developed novel treatments for neurodegeneration. While studying biological brains, she became fascinated with AI and all things biotech. Following graduation, she moved to UCSF to study blood-based factors that rejuvenate aged brains. She is the co-founder of Vantastic Media, a media venture that explores science stories through text and video, and runs the award-winning blog NeuroFantastic.com. Her first book, "Will AI Replace Us?" (Thames & Hudson) was published in 2019.
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