Mirror Neurons After a Quarter Century: New light, new cracks

by JohnMark Taylor
figures by Youngeun Kaitlyn Choi

What about the human brain allows a person to perform such feats as learning guitar through imitation, empathizing with anothers’s pain, or intuiting where a fencer will strike next? Nearly twenty-five years ago, scientists discovered a special kind of cell called a mirror neuron that many both in science and the popular press came to believe might enable social skills like these, skills that underlie much of what makes us uniquely human. However, after a quarter century, dozens of experiments, and reams of popular articles, the true significance of these cells has become increasingly controversial. What have mirror neurons really told us so far about the human mind, and what remains to be learned from them?

What are mirror neurons?

The story of mirror neurons began simply enough. In 1992, a team of neuroscientists led by Giacomo Rizzolatti inserted tiny electrodes into the brains of macaque monkeys, hoping to better understand how the brain orchestrates the delicate interplay of muscles involved in moving the hand. Using these electrodes, the researchers monitored the activity of neurons, the cells that constitute the smallest processing units of the brain, analogous to microchips in computers. A neuron can be specialized to perform any of a dizzying number of functions, from perceiving a face to regulating sleeping and waking.

Rizzolatti’s team, examining neurons in a part of the macaque brain involved in controlling the muscles of the hand, expected to find neurons that fired specifically when the monkey performed particular actions, such as reaching for or grabbing something. They indeed found neurons that fired when the monkey performed these actions, but it turned out that this was only half the story. One day, when the experimenters ate lunch in the same room as the monkeys, they observed something entirely unexpected: some of these neurons also fired when the monkey observed an experimenter performing the same action (in this case, bringing food to one’s mouth). In short: these neurons fired both when monkey see, and when monkey do.

Figure 1: Mirror neurons in action. A mirror neuron fires an electrical pulse, or action potential, when the monkey either observes or executes a specific action. In this case, the mirror neuron responds to grasping actions. The graph at the bottom shows what the action potentials (each depicted as a hump) would look like when measured with an electrode, as used by the researchers. — **Figure 1:** Mirror neurons in action. A mirror neuron fires an electrical pulse, or action potential, when the monkey either observes or executes a specific action. In this case, the mirror neuron responds to grasping actions. The graph at the bottom shows what the action potentials (each depicted as a hump) would look like when measured with an electrode, as used by the researchers.

Mirror neurons’ great potential

For nearly a decade, these neurons, termed “mirror neurons,” remained relatively unknown to the public. However, their reputation began to change in 2000, when the famous neuroscientist and popularizer of science V.S. Ramachandran wrote an edge.org article speculating that “mirror neurons would do for psychology what DNA did for biology: they will provide a unifying framework and help explain a host of mental abilities that have hitherto remained mysterious and inaccessible to experiments.”

In a series of elegant, compelling proposals, Ramachandran theorized that mirror neurons might help explain a wide variety of human social abilities. For example, how, biologically, do people imitate the actions of others, an ability that in part enables the spread of culture? Ramachandran proposed that mirror neurons translate an observed action into a series of commands for the muscles to execute. How do people understand the intentions behind another’s actions? Mirror neurons may run a sort of virtual reality simulation of what it would be like for oneself to perform that action. Why are autistic individuals impaired when it comes to understanding the thoughts of others? Perhaps they have deficient mirror neurons (an idea that came to be called the “broken mirror” hypothesis). Within a year, the use of the phrase “mirror neurons” more than doubled, and over the next decade, mirror neurons captured the public imagination, being touted as able to offer insight into everything from empathizing with therapy clients to international diplomacy, how children learn music, and how people appreciate art. Not bad for a finding that was initially rejected from the top science journal, Nature, for “lacking public interest.”

As interest in mirror neurons exploded among the public, scientists remained divided regarding their significance. Some scientists, such as Rizzolatti and Ramachandran, were optimistic that mirror neurons would prove crucial for many of humans’ social abilities, while others thought that their importance was overblown. For some time, skeptics had one particularly effective arrow in their quiver: despite claims that mirror neurons might underlie much of what makes humans unique (such as language and culture), until 2008, they had never once been decisively identified in humans. Even as of 2016, only one study, using electrodes implanted into the brains of epilepsy patients, has successfully identified human neurons with properties similar to those found in the macaques.

An updated perspective

Accordingly, over the past ten years, the pendulum of scientific opinion has begun to swing towards the skeptics. Many of the more prominent theories regarding the function of mirror neurons have not survived scrutiny. First, it was seen as increasingly implausible that mirror neurons alone could explain the human capacity for imitation; adult macaques, it became increasingly clear, did not engage in mimicry despite having mirror neurons, and so mirror neurons could not explain this ability by themselves. Second, the theory that the ability to mentally simulate others’ actions (putatively enabled by mirror neurons) is necessary to understand others’ actions has become increasingly shaky. For instance, some patients with brain damage that prevent them from performing certain actions (such as brushing one’s teeth) are nonetheless able to understand the meaning of these actions when performed by others.

Finally, the theory that mirror neuron defects might underlie autism—the “broken mirror” hypothesis—has proven most dubious of all. An exhaustive recent review of 25 different studies presents a wide array of behavioral and neurological evidence that deficient mirror neurons probably do not lie at the core of autism. For example, while the broken mirror hypothesis predicts that autistic individuals should show severe impairments in understanding and imitating actions such as reaching, several studies have found no such impairments. Moreover, while many studies have reported differences between the brains of autistic and non-autistic individuals, these differences do not appear to lie in parts of the brain thought to contain mirror neurons.

Using magnetic resonance imaging (MRI), several studies have examined the cortical thickness (the size of the sheet of neurons covering the brain) of various brain areas, and have found only sparse evidence that structural differences in mirror neuron regions might be involved in autism. Rather, structural differences between autistic and non-autistic individuals appear to extend widely throughout the brain, and differences in mirror neuron regions do not appear to show reproducible patterns between subjects. Additionally, these mirror neuron regions appear to show similar activity in autistic and non-autistic individuals when they view or perform various actions, suggesting that the neural basis of autism probably lies elsewhere.

Counterarguments like these have pushed mirror neuron proponents to fine-tune their claims. For example, Rizzolatti, the original discoverer of mirror neurons in macaques, now suggests that mirror neurons might only be required for understanding the actions of others from a first-person perspective. He explains that this internalization of behaviors we see might provide us a deeper level of understanding about another person’s goals, but cedes that mirror neuron activity might only constitute one among several ways of comprehending others’ behavior.

Mirror neurons have begun to assume a humbler identity than was initially theorized, but it is important to remember that despite recent criticism, their activity may still play an important role in many behaviors. For instance, even Gregory Hickok, perhaps the most prominent critic of the hype surrounding mirror neurons, accepts that they probably play a role in enabling imitation, given that there must be some mechanism in the brain that converts an observed action to a series of muscle commands. Much research remains to be done; for instance, there has yet to be a study that specifically disables mirror neurons (an experiment that recent technological advances may make possible in monkeys), which would help to elucidate what exact behaviors rely on these neurons. Now that the hype around mirror neurons has begun to dissipate, it will be interesting to see what role remains for these curious cells.

JohnMark Taylor is a PhD student in the Harvard University Psychology Department.

For more information:

Read the original study in which mirror neurons were discovered here. For a detailed, critical look at theories regarding mirror neurons, see this recent book by psychologist Gregory Hickok. Rizzolatti, the original discoverer of mirror neurons, writes a response to Hickock’s criticisms here, to which Hickok replies here.