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A mirror neuron is a neuron that fires both when an animal performs an action and when the animal observes the same action performed by another (especially conspecific) animal. Thus, the neuron "mirrors" the behavior of another animal, as though the observer were himself performing the action. These neurons have been observed in primates and in some birds. In humans, they have been found in Broca's area and the inferior parietal cortex of the brain. Some scientists consider mirror neurons one of the most important findings of neuroscience in the last decade. See for example, an essay by Ramachandran on their potential importance in imitation and language.
In humans, mirror neurons are found in the inferior frontal cortex, close to Broca's area, a language region. This has led to suggestions that human language evolved from a gesture performance/understanding system implemented in mirror neurons. However, like many theories of language evolution, there is little direct evidence either way.