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A mirror neuron is a neuron which 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, in some birds,and in humans. 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 this (hyperbolic) essay by Ramachandran on their potential importance in imitation and language.
In the monkey, mirror neurons are found in the inferior frontal gyrus and inferior parietal lobule. These neurons are active when the monkeys perform certain tasks, but they also fire when the monkeys watch someone else perform the same specific task. Researches using fMRI, TMS, and EEG have found evidence of a similar system (matching observations with actions), in the human brain.
The function of the mirror system is a subject of much speculation. These neurons may be important for understanding the actions of other people, and for learning new skills by imitation. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to our theory of mind skills, while others relate mirror neurons to language abilities. It has also been proposed that problems with the mirror system may underlie cognitive disorders, in particular autism.
Research into all of these possiblities is ongoing.
Who discovered them?
In the 1980's and 1990's, Giacomo Rizzolatti was working with Leonardo Fogassi and Vittorio Gallese at the university in Parma, Italy. These scientists had placed electrodes in the inferior frontal cortex of the macaque monkey to study neurons specialised for the control of hand actions, for example, grabbing objects, picking items up etc. During each experiment, they recorded from a single neuron in the monkey's brain while the monkey was allowed to reach for pieces of food, so the researchers could measure the neuron's response to certain movements.
As with many other notable discoveries, mirror neurons were found by chance. Rizzolatti explains; “I think it was Fogassi, standing next to a bowl of fruit and reached for a banana, when some of the neurons reacted. How could this happen, when the monkey did not move. At first we thought it to be a flaw in our measuring or maybe equipment failure, but everything checked out OK and the reactions were repeated as we repeated the movement.”[How to reference and link to summary or text]
This work has since been published  and confirmed  with mirror neurons found in both inferior frontal and inferior parietal regions of the brain. Recently, evidences from fMRI, TMS and EEG and behavioral strongly suggest the presence of similar systems in human, where brain regions which respond during both action and the observation of action have been identified. Not surprisingly, these brain region closely match those found in the macaque monkey .
The significance of mirror neurons
Since the discovery of mirror neurons, grand claims have been made for their importance (e.g. by Ramachandran). In particular, there has been much speculation about the evolution of mirror neurons, and their relationship to language evolution. While mirror neurons are present in macaque monkeys, these monkeys do not imitate each other's behaviour, so it seems unlikely that mirror neurons evolved for imitation learning. Instead, they may allow the monkey to understand what another monkey is doing, or to recognise the other monkey's action.
In humans, mirror neurons are found in the inferior frontal cortex, close to Broca's area, a language region. This has lead to suggestions that human language evolved from a gesture performance / understanding system implemented in mirror neurons. Mirror neurons certainly have the potential to provide a mechanism for action understanding, imitation learning, and the simulation of other people's behaviour. However, like many theories of language evolution, there is little direct evidence either way.
- V.S. Ramachandran, MIRROR NEURONS and imitation learning as the driving force behind "the great leap forward" in human evolution
- Michael Arbib, The Mirror System Hypothesis. Linking Language to Theory of Mind, 2005, retrieved 2006-02-17
- Hugo Théoret, Alvaro Pascual-Leone, Language Acquisition: Do As You Hear, Current Biology, Vol. 15, No. 3, pp. 84-85, 2002-10-29
- Oberman LM, Hubbard EM, McCleery JP, Altschuler EL, Ramachandran VS, Pineda JA., EEG evidence for mirror neuron dysfunction in autism spectrum disorders, Brain Res Cogn Brain Res.; 24(2):190-8, 2005-06
- Mirella Dapretto, Understanding emotions in others: mirror neuron dysfunction in children with autism spectrum disorders, Nature Neuroscience, Vol. 9, No. 1, pp. 28-30, 2006-01
- Giacomo Rizzolatti et al., Premotor cortex and the recognition of motor actions, Cognitive Brain Research 3 131-141, 1996
- Gallese et al, Action recognition in the premotor cortex, Brain, 1996
- Fogassi et al, Parietal Lobe: From Action Organization to Intention Understanding, Science, 2005
- Rizzolatti G., Craighero L., The mirror-neuron system, Annual Review of Neuroscience. 2004;27:169-92
- Giacomo Rizzolatti and Michael A. Arbib, Language within our grasp, Trends in neurosciences, Vol. 21, No. 5, 1998
- Skoyles, John R., Gesture, Language Origins, and Right Handedness, Psycoloquy: 11,#24, 2000
- The others are inside you, article from “Ingeniøren” (original title: “De andre er inden I dig”).
- What do mirror neurons mean…?, workshop sponsored by The European Science Foundation.
- E. Bruce Goldstein (2002), Sensation and Perception, Wadsworth (p. 321-324)
- NOVA scienceNOW: Mirror Neurons -- including a 14 minute broadcast segment.
- Neuroscience And Bio Behavioural Reviews.
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