Mirror neurons are neurons that fire both when an individual performs an action and when they observe someone else performing that same action, such as reaching for a lever. These neurons respond to someone else's action just as if you yourself were doing it.
This response is not restricted to sight. Mirror neurons can also fire when an individual knows or hears someone else performing a similar action.
“The Same Action”
It's not always clear what's meant by “the same action.” Do mirror neurons code actions corresponding to the movement itself (you move your muscles a certain way to grab food), or, are they responsive to something more abstract, the goal that the individual is trying to achieve with the movement (grabbing food)?
It turns out that there are different types of mirror neurons, which differ in what they respond to.
Strictly congruent mirror neurons fire only when the mirrored action is identical to the performed action-so both the goal and the movement are the same for both cases.
Broadly congruent mirror neurons fire when the goal of the mirrored action is the same as the performed action's, but the two actions themselves are not necessarily identical. For example, you can grab an object with your hand or your mouth.
Taken together, strictly congruent and broadly congruent mirror neurons, which together comprised more than 90 percent of the mirror neurons in the study that introduced these classifications, represent what someone else did, and how they did it.
Other, non-congruent mirror neurons don't seem to exhibit a clear correlation between the performed and observed actions at first glance. Such mirror neurons may, for instance, fire both when you grasp an object and see someone else placing that object somewhere. These neurons could thus be activated at an even more abstract level.
The Evolution of Mirror Neurons
There are two main hypotheses for how and why mirror neurons evolved.
The adaptation hypothesis states that monkeys and humans-and possibly other animals as well-are born with mirror neurons. In this hypothesis, mirror neurons came about through natural selection, enabling individuals to understand the actions of others.
The associative learning hypothesis asserts that mirror neurons arise from experience. As you learn an action and see others performing a similar one, your brain learns to link the two events together.
Mirror Neurons in Monkeys
Mirror neurons were first described in 1992, when a team of neuroscientists led by Giacomo Rizzolatti recorded activity from single neurons in the macaque monkey brain and found that the same neurons fired both when a monkey performed certain actions, like grabbing food, and when they observed an experimenter performing that same action.
Rizzolatti's discovery found mirror neurons in the premotor cortex, a part of the brain which helps plan and execute movements. Subsequent studies have also heavily investigated the inferior parietal cortex, which helps encode visual motion.
Still other papers have described mirror neurons in other areas, including the medial frontal cortex, which has been recognized as important for social cognition.
Mirror Neurons in Humans
In many studies on monkey brains, including Rizzolatti's initial study and others involving mirror neurons, brain activity is directly recorded by inserting an electrode into the brain and measuring electrical activity.
This technique is not used in many human studies. One mirror neuron study, however, directly probed the brains of epileptic patients during a pre-surgery evaluation. Scientists found potential mirror neurons in the medial frontal lobe and the medial temporal lobe, which helps code memory.
Most studies involving mirror neurons in humans have presented indirect evidence pointing to mirror neurons in the brain.
Multiple groups have imaged the brain and shown that brain areas which exhibited mirror-neuron-like activity in humans are similar to the brain areas containing mirror neurons in macaque monkeys. Interestingly, mirror neurons have also been observed in Broca's area, which is responsible for producing language, though this has been the cause of much debate.
Such neuroimaging evidence seems promising. However, since individual neurons are not being directly probed during the experiment, it's difficult to correlate this brain activity to specific neurons in the human brain-even if the imaged brain areas are very similar to those found in monkeys.
According to Christian Keysers, a researcher who studies the human mirror neuron system, a small area on a brain scan can correspond to millions of neurons. Thus, the mirror neurons found in humans cannot be directly compared with those in monkeys to confirm whether the systems are the same.
Furthermore, it is not necessarily clear whether the brain activity corresponding to an observed action is a response to other sensory experiences rather than mirroring.
Possible Role in Social Cognition
Since their discovery, mirror neurons have been considered one of the most important discoveries in neuroscience, intriguing experts and non-experts alike.
Why the strong interest? It stems from the role mirror neurons may play in explaining social behavior. When humans interact with each other, they understand what other people do or feel. Thus, some researchers say that mirror neurons-which allow you to experience the actions of others-could shed light on some of the neural mechanisms underlying why we learn and communicate.
For example, mirror neurons may provide insights on why we imitate other people, which is critical to understanding how humans learn, or how we understand other people's actions, which could shed light on empathy.
Based on their possible role in social cognition, at least one group has also proposed that a “broken mirror system” may also cause autism, which is partly characterized by difficulty in social interactions. They argue that reduced activity of mirror neurons prevents autistic individuals from understanding what others are feeling. Other researchers have stated this is an oversimplified view of autism: a review looked at 25 papers focusing on autism and a broken mirror system and concluded there was “little evidence” for this hypothesis.
A number of researchers are much more cautious about whether mirror neurons are crucial to empathy and other social behavior. For example, even if you have never seen an action before, you are still capable of understanding it-for example, if you see Superman flying in a movie even if you can't fly yourself. Evidence for this comes from individuals who have lost the capacity to perform certain actions, like brushing teeth, yet can still understand them when others perform them.
Towards the future
Though much research has been conducted on mirror neurons, there are still many lingering questions. For example, are they only restricted to certain areas of the brain? What is their real function? Do they really exist, or can their response be attributed to other neurons?
Much more work has to be done to answer these questions.
- A calm look at the most hyped concept in neuroscience - mirror neurons, Christian Jarrett, Wired.
- Acharya, S., and Shukla, S. “Mirror neurons: Enigma of the metaphysical modular brain.” Journal of Natural Science, Biology, and Medicine, 2012, vol. 3, no. 2, pp. 118-124, doi: 10.4103/0976-9668.101878.
- Gallese, V., Fadiga, L., Fogassi, L., and Rizzolatti, G. “Action recognition in the premotor cortex.” Brain, 1996, vol. 119, pp. 593-609, doi: 10.1093/brain/awp167.
- Hamilton, A. “Reflecting on the mirror neuron system in autism: A systematic review of current theories.” Developmental Cognitive Neuroscience, 2013, vol. 3, pp. 91-105, doi: 10.1016/j.dcn.2012.09.008
- Heyes, C. “Where do mirror neurons come from?” Neuroscience and Behavioral Reviews, 2009, vol. 34, pp. 575-583, doi: 10.1016/j.neubiorev.2009.11.007.
- Keysers, C., and Fadiga, L. “The mirror neuron system: New frontiers.” Social Neuroscience, 2008, vol. 3, no. 3-4, pp. 193-198, doi: 10.1080/17470910802408513.
- Kilner, J., and Lemon, R. “What we currently know about mirror neurons.” Current Biology, 2013, vol. 23, no. 23, pp. R1057-R1062, doi: 10.1016/j.cub.2013.10.051.
- Kokal, I., Gazzola, V., and Keysers, C. "Acting together in and beyond the mirror neuron system." Neuroimage, 2009, vol. 47, no. 4, pp. 2046-2056, doi: 10.1016/j.neuroimage.2009.06.010.
- Miklósi, Á. Do dogs have mirror neurons? Scientific American Mind.
- Mirror neurons after a quarter century: New light, new cracks, JohnMark Taylor, Science in the News.
- Reflecting on mirror neurons, Mo Costandi, The Guardian.
- The mind's mirror, Lea Winerman, Monitor on Psychology.
- Uithol, S., van Rooij, I., Bekkering, H., and Haselager, P. “What do mirror neurons mirror?” Philosophical Psychology, 2011, vol. 24, no. 5, pp. 607-623, doi: 10.1080/09515089.2011.562604.
- What's so special about mirror neurons?, Ben Thomas, Scientific American Guest Blog.
- Yoshida, K., Saito, N., Iriki, A., and Isoda, M. “Representation of others' action by neurons in monkey medial frontal cortex.” Current Biology, 2011, vol. 21, no. 3, pp. 249-253, doi: 10.1016/j.cub.2011.01.004.