Mirror neuron

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 thisV.S. Ramachandran, MIRROR NEURONS and imitation learning as the driving force behind "the great leap forward" in human evolution essay by Ramachandran on their potential importance in imitation and language. This essay has been criticised by some as hyperbolic.

Introduction

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. Researchers 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,Michael Arbib, The Mirror System Hypothesis. Linking Language to Theory of Mind, 2005, retrieved 2006-02-17 while others relate mirror neurons to language abilities.Hugo Théoret, Alvaro Pascual-Leone, Language Acquisition: Do As You Hear, Current Biology, Vol. 15, No. 3, pp. 84-85, 2002-10-29 It has also been proposed that problems with the mirror system may underlie cognitive disorders, in particular autism.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, 2005-06Mirella 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

Research into all of these possibilities is ongoing.

Discovery

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.Giacomo Rizzolatti et al. (1996). Premotor cortex and the recognition of motor actions, Cognitive Brain Research 3 131-141

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.”

This work has since been published Gallese et al, Action recognition in the premotor cortex, Brain, 1996 and confirmed Fogassi et al, Parietal Lobe: From Action Organization to Intention Understanding, Science, 2005 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 Rizzolatti G., Craighero L., The mirror-neuron system, Annual Review of Neuroscience. 2004;27:169-92.

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.Giacomo Rizzolatti and Michael A. Arbib, Language within our grasp, Trends in neurosciences, Vol. 21, No. 5, 1998

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.Skoyles, John R., Gesture, Language Origins, and Right Handedness, Psycoloquy: 11,#24, 2000 However, like many theories of language evolution, there is little direct evidence either way.

Studies also link mirror neurons to understanding goals and intentions. Fogassi et al. (2005)Fogassi, Leonardo, Pier Francesco Ferrari, Benno Gesierich, Stefano Rozzi, Fabian Chersi, Giacomo Rizzolatti. 2005. Parietal lobe: from action organization to intention understanding. Science 308: 662-667. recorded the activity of 41 mirror neurons in the inferior parietal lobe (IPL) of two rhesus macaques. The IPL has long been recognized as an association cortex that integrates sensory information. The monkeys watched an experimenter either grasp an apple and bring it to his mouth or grasp an object and place it in a cup. In total, 15 mirror neurons fired vigorously when the monkey observed the “grasp-to-eat” motion, but registered no activity while exposed to the “grasp-to-place” condition. For four other mirror neurons, the reverse held true: they activated in response to the experimenter eventually placing the apple in the cup but not to eating it. Only the type of action, and not the kinematic force with which models manipulated objects, determined neuron activity. Significantly, neurons discharged before the monkey observed the human model starting the second motor act (bringing the object to the mouth or placing it in a cup). Therefore, IPL neurons “code the same act (grasping) in a different way according to the final goal of the action in which the act is embedded” (664). They may furnish a neural basis for predicting another individual’s subsequent actions and inferring intention.

Footnotes

References

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)

External links

NOVA scienceNOW: Mirror Neurons (including a 14 minute broadcast segment)
Neuroscience And Bio Behavioural Reviews.

CNS neurons