Mirror neurons are cells that fire during both the execution and observation of a specific action. They have been linked to many behaviours and abilities, from empathy to learning by imitation, as well as implicated in conditions such as autism. Mirror neurons were discovered in monkeys, but it's still not clear whether they also exist in the human brain.
Mirror neurons were identified in the brains of macaque monkeys by a team of Italian researchers during experiments performed in the 1990s. The researchers, who were studying how the brain controls hand and mouth movements, implanted microelectrodes into the monkeys' brains in order to monitor the activity of single cells while the animals reached for pieces of food and put them into their mouths. These experiments revealed that the activity of certain cells increased when the monkeys performed this action.
The cells in question are located in the premotor cortex, a part of the brain involved in planning and executing movements, so the finding was not in itself particularly surprising. By chance, however, the researchers discovered that a few of the same cells also fired weakly when the animals merely observed the researchers putting food into their own mouths, and fired more strongly when they saw other monkeys performing the same action. Subsequently, the same team of researchers identified mirror neurons in several other regions of the monkey brain. They also located cells that fire when monkeys observe an action as well as when they hear the sound related to it.
But what does it all mean? The precise role of the mirror neuron system in monkeys is still not known, though the researchers who discovered them believe that they perform two functions. First, that mirror neurons are involved in understanding the actions of others – observing an action triggers the mirror neuron system to generate a motor representation of it. This corresponds to the activity produced by the action itself: in other words, the mirror neuron system transforms the visual information into knowledge of the intention of the others' actions. The second proposed function is imitation – or learning to perform an action by observing others.
Casting a long reflection
The discovery of mirror neurons was greeted with a great deal of excitement, and some have hailed it as one of the most important discoveries of modern neuroscience. Since these neurons were discovered in monkeys, researchers have speculated that the human brain may also contain mirror neurons.
In human beings, as in monkeys, mirror neurons are hypothesized to play an important role in imitation and understanding the actions of others. Some researchers argue that they are critical for many aspects of social interactions. These include understanding the intentions of others, and inferring their mental state from their behaviour (an ability referred to as theory of mind); empathy, or putting oneself 'into another's shoes'; self-awareness; and the evolution of, and the ability to learn, language.
Given their purported role in social cognition, one prominent neuroscientist has proposed that a defective mirror neuron system is what causes autism, a neurodevelopmental condition characterized primarily by impairments in social interaction and communication. The same researcher argues that the discovery of mirror neurons is 'the single most important "unreported" story of the decade', and has even referred to the cells as 'the neurons that shaped civilization', because human culture involves the transfer of complex skills and knowledge from person to person.
Box: The broken mirror hypothesis
In the late 1990s, two groups of researchers independently proposed the so-called broken-mirror hypothesis, which states the social impairments characteristic of autism are caused by defects in the mirror neuron system. The broken mirror hypothesis has received considerable attention in the mass media, but has been the subject of severe criticism by many autism researchers. It is based on assumptions that mirror neurons are involved in understanding action, imitation and language acquisition, and that people with autism are insensitive to the emotions and intentions of others. Critics say both that the first assumption is actually false, and also that there is evidence that people with autism are in fact overly sensitive to others' emotions and intentions. What's more, the broken-mirror hypothesis does not attempt to explain how the mirror neuron system is defective, or how the defects might arise.
But do we have mirror neurons?
Mirror neurons have proven to be highly controversial. A handful of brain- scanning studies show that several regions of the brain are activated during both action execution and observation, and it has been suggested that these areas constitute the human mirror system. But while hundreds of other studies attempt to explain their results by alluding to mirror neurons, very few actually provide hard evidence.
So there is, as yet, very little convincing direct evidence that mirror neurons exist in the human brain. In fact, a number of studies have failed to find evidence of human mirror neurons altogether. In 2009, for example, Harvard researchers exploited a phenomenon called adaptation, whereby neurons reduce their activity in response to the same repetitive stimulus. The researchers showed their participants a film clip of hand gestures and asked them to mimic the action. The scans showed that the cells adapted when the gestures were observed and mimicked, but not when they were mimicked first and then observed.
One of the difficulties is that researchers rarely get the opportunity to examine the working human brain directly. In 2010, though, one research group who had just such an opportunity, while evaluating the brains of conscious epileptic patients about to undergo neurosurgery, claimed that they had obtained the first direct evidence of human mirror neurons. Some of these cells fired both when the patients performed and observed an action, but the activity of almost as many cells decreased during execution and observation, raising doubts that they are indeed mirror neurons. Furthermore, the cells were located in the hippocampus, an area involved in memory formation, and not previously thought to be part of the presumed mirror neuron system.
The researchers who originally discovered mirror neurons in the monkey brain have recently refined their claims, and now suggest that the cells have a far more restricted role than was originally thought. Instead of being involved in understanding the actions of others, they suggest that the cells play a role in helping us to understand, 'from the inside', actions that we already know how to perform. Critics argue that this confirms the alternative theory that mirror neurons are involved instead in selecting and controlling actions.
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