A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in human and primate species, and in birds.
In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex, and the inferior parietal cortex. The function of the mirror system in humans is a subject of much speculation. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system.
To date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions. The subject of mirror neurons continues to generate intense debate. In 2014, Philosophical Transactions of the Royal Society B published a special issue entirely devoted to mirror neuron research. Some researchers speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. Neuroscientists such as Marco Iacoboni (UCLA) have argued that mirror neuron systems in the human brain help us understand the actions and intentions of other people. In a study published in March 2005 Iacoboni and his colleagues reported that mirror neuron activity could predict whether another person who was picking up a cup of tea planned to drink from it or clear it from the table. In addition, Iacoboni has argued that mirror neurons are the neural basis of the human capacity for emotions such as empathy.
There are scientists who express skepticism about the theories being advanced to explain the function of mirror neurons. In a 2013 article for Wired, Christian Jarrett cautioned that:
...mirror neurons are an exciting, intriguing discovery – but when you see them mentioned in the media, remember that most of the research on these cells has been conducted in monkeys. Remember too that there are many different types of mirror neuron. And that we're still trying to establish for sure whether they exist in humans, and how they compare with the monkey versions. As for understanding the functional significance of these cells … don't be fooled: that journey has only just begun.