Mirror neurons are a class of neurons that become active both when an individual performs a particular action and when he observes another individual performing the same action. They were discovered in monkeys in the early 1990s by an Italian research group led by Giacomo Rizzolatti at the University of Parma, Italy: a classic tale of serendipity in science.

While studying the activation of neurons associated with goal-directed hand movements in monkeys, such as grasping, holding and tearing, the researchers incidentally observed that those neurons would also fire when the monkeys observed the experimenters performing similar actions, without any movement by the monkey itself.

Switching the focus of their research, they found that there were neurons that would respond to both the observation and the execution of a movement, as well as other neurons that would only be activated by the observation of movements by the experimenter that coincided or were very similar to those performed by the monkey. This was the first evidence of the existence of mirror neurons and was regarded as an indication that the perception and the execution of a movement could share the same neuronal circuits.

Since their discovery, many studies in monkeys have aimed at understanding the characteristics and functional significance of mirror neurons. The existence of a human mirror neuron system has raised much debate and enthusiasm in the neuroscientific community, but its existence is controversial.

Evidence from functional magnetic resonance imaging (fMRI) studies in humans has shown that many brain regions become active both when an action is performed or observed, and these areas have been suggested to integrate the human mirror neuron system. Although its existence is still disputed by some scientists, research has been moving forward and the mirror neuron system has been associated with numerous putative functions such as action and intention understanding, imitation, language, empathy, and self-awareness.

However, many of the possible roles attributed to mirror neurons in humans are still mostly speculative, with little scientific evidence being available mainly due to the obvious difficulties of directly studying the human brain in vivo.

The mirror neuron system has also been proposed to play a part in the neural mechanisms of autism spectrum disorders. The broken mirror theory of autism claims that a dysfunction of the mirror neuron system may be a cause of poor social interaction and cognition in individuals with autism. Many of the behavioral characteristics of autism are coherent with the putative roles of the mirror neuron system. It is speculated, for example, that mirror neuron system dysfunction may be the cause of difficulties in imitation, language, empathy, in the comprehension of actions and emotions, and in understanding the intention of an action.

However, there is little evidence to support this view and the assumptions of this theory have been widely criticized both in what concerns the hypothetical roles of mirror neurons and the characteristics of autism. Also, the broken mirror hypothesis does not suggest an explanation as to how the presumed dysfunctions in the mirror neuron system arise. In fact, a recent study on the role of the mirror neuron system in children with autism spectrum disorder has actually challenged the broken mirror theory, suggesting that impaired neural mirroring is not a distinctive feature of the disorder.

Regardless of all the controversy, mirror neurons were one of the most important discoveries in neuroscience in last two decades. But even with all the hype and excitement surrounding mirror neurons, one must bear in mind that this is still a blurry subject.

References

Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V., & Rizzolatti, G. (1992). Understanding motor events: a neurophysiological study Experimental Brain Research, 91 (1) DOI: 10.1007/BF00230027

Gallese, V., Fadiga, L., Fogassi, L., & Rizzolatti, G. (1996). Action recognition in the premotor cortex Brain, 119 (2), 593-609 DOI: 10.1093/brain/119.2.593

Hamilton, A. (2013). Reflecting on the mirror neuron system in autism: A systematic review of current theories Developmental Cognitive Neuroscience, 3, 91-105 DOI: 10.1016/j.dcn.2012.09.008

Rizzolatti G, & Craighero L (2004). The mirror-neuron system. Annual review of neuroscience, 27, 169-92 PMID: 15217330

Rizzolatti G, Fadiga L, Gallese V, & Fogassi L (1996). Premotor cortex and the recognition of motor actions. Brain research. Cognitive brain research, 3 (2), 131-41 PMID: 8713554

Rizzolatti G, & Sinigaglia C (2010). The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations. Nature reviews. Neuroscience, 11 (4), 264-74 PMID: 20216547

Ruysschaert L, Warreyn P, Wiersema JR, Oostra A, & Roeyers H (2014). Exploring the role of neural mirroring in children with autism spectrum disorder. Autism research : official journal of the International Society for Autism Research, 7 (2), 197-206 PMID: 24515797

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