Mirror neuron

  A mirror neuron is a premotor^[1] neuron which fires both when an animal acts 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 itself acting. These neurons have been directly observed in primates, and are believed to exist in humans and in some birds. In humans, brain activity consistent with mirror neurons has been found in the premotor cortex and the inferior parietal cortex. Some scientists consider mirror neurons one of the most important findings of neuroscience in the last decade. Among them is V.S. Ramachandran^[2], who believes they might be very important in imitation and language acquisition. However, despite the popularity of this field, to date no plausible neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation. Furthermore, it is generally accepted that no single neurons can be responsible for the phenomenon. Rather, a whole network of neurons (neuronal assembly) is activated when an action is observed.

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Introduction

In the macaque monkey, mirror neurons are found in the ventral premotor cortex (probably the equivalent of the inferior frontal gyrus in humans) and in the anterior inferior parietal lobule. These neurons are active when the monkeys perform certain tasks, but they also fire when the monkeys watch or hear 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, ^{[3] [4]} while others relate mirror neurons to language abilities.^[5] It has also been proposed that problems with the mirror system may underlie cognitive disorders, in particular autism.^[6][7]

Research into all of these possibilities is ongoing.

Discovery

In the 1980s and 1990s, Giacomo Rizzolatti was working with Luciano Fadiga, 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.^[8]

This work has since been published ^[9] and confirmed ^[10] with mirror neurons found in both inferior frontal and inferior parietal regions of the brain. Recently, evidence from fMRI, TMS and EEG and behavioral strongly suggest the presence of similar systems in humans, where brain regions which respond during both action and the observation of action have been identified. Not surprisingly, these brain regions closely match those found in the macaque monkey.^[11]

More recently Keysers and colleagues have shown that both in humans and monkeys, the mirror system also responds to the sound of actions.^{[12] [13]}

Mirror neurons in monkeys

The only animal where mirror neurons have been studied individually is the macaque monkey. In these monkeys, mirror neurons are found in the inferior frontal gyrus (region F5) and the inferior parietal lobule.^[14]

Mirror neurons are believed to mediate the understanding of other animal's behavior. For example, a mirror neuron which fires when the monkey rips a piece of paper would also fire when the monkey sees a person rip paper, or hears paper ripping (without visual cues). These properties have led researchers to believe that mirror neurons encode abstract concepts of actions like 'ripping paper', whether the action is performed by the monkey or another animal.^[15]

The function of mirror neurons in macaques is not known. Adult macaques do not seem to learn by imitation. Recent experiments suggest that infant macaqes can imitate a human's face movements, though only as neonates and during a limited temporal window.^[16] However, it is not known if mirror neurons underlie this behaviour.

In adult monkeys, mirror neurons may enable the monkey to understand what another monkey is doing, or to recognise the other monkey's action.^[17]

The mirror neuron system in humans

It is not normally possible to study single neurons in the human brain, so scientists can not be certain that humans have mirror neurons. However, the results of brain imaging experiments using fMRI have shown that the human inferior frontal cortex and superior parietal lobule is active when the person performs an action and also when the person sees another individual performing an action. Therefore, these brain regions are likely to contain mirror neurons and have been defined as the human mirror neuron system ^[18].

Several indirect measures have been used to study the mirror neuron system in humans. For example, when a person observes another person's action, their motor cortex becomes more excitable ^[19]. This excitability can be measured by recording the size of a motor evoked potential (MEP) induced by TMS. The changes in MEP size are taken as a measure of mirror neuron system activity, because MEPs come from primary motor cortex which is closely connected to the mirror neuron regions of the brain. Recent data suggests that these changes in MEP size can be strongly influenced by training on different stimulus-response mappings, with the strongest enhancement for well-learnt mappings ^[20]

Eye tracking provides another indirect measure that may reflect mirror neuron processing. As a person moves their hand, their eyes move ahead of the hand to look at the object the person will grasp. Similarly, when watching someone else's action, your eyes are likely to anticipate what the other person will do ^[21].

Development

Human infant data using eye-tracking measures suggest that the mirror neuron system develops before 12 months of age, and that this system may help human infants understand other people's actions.^[22]. A critical question concerns how mirror neurons acquire mirror properties. One model postulates that mirror neurons are trained through hebbian learning.^[23]. However, if premotor neurons need to be trained by action in order to acquire mirror properties, it is unclear how newborn babies are able to mimic the facial gestures of another person (imitation of unseen actions), as suggested by the work of Meltzoff & Moore (unless this is a specal type of imitation not supprted by mirror neurons).

Possible functions of the mirror neuron system

Many different functions for the mirror neuron system have been suggested. These include:

Understanding Intentions

Many studies link mirror neurons to understanding goals and intentions. Fogassi et al. (2005)^[24] 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 fired 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.

Empathy

Mirror neurons have been linked to empathy, because certain brain regions (in particular the anterior insula and inferior frontal cortex) are active when a person experiences an emotion (disgust, happiness, pain etc) and when they see another person experience an emotion. ^{[25] [26] [27]} However, these brain regions are not quite the same as the ones which mirror hand actions, and mirror neurons for emotional states or empathy have not yet been described in monkeys. More recently, Keysers and colleagues have shown that people that are more empathic according to self-report questionnaires have stronger activations both in the mirror system for hand actions ^[28] and the mirror system for emotions ^[29] providing more direct support to the idea that the mirror system is linked to empathy.

Language

In humans, mirror neurons have been 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. Mirror neurons have been said to have the potential to provide a mechanism for action understanding, imitation learning, and the simulation of other people's behaviour.^[30] However, like many theories of language evolution, there is little direct evidence either way.

Autism

Some researchers claim there is a link between mirror neuron deficiency and autism. In typical children, EEG recordings from motor areas are suppressed when the child watches another person move, and this is believed to be an index of mirror neuron activity. However, this suppression is not seen in children with autism ^[31]. Also, children with autism have less activity in mirror neuron regions of the brain when imitating ^[32]. Finally, anatomical differences have been found in the mirror neuron related brain areas in adults with autism spectrum disorders, compared to non-autistic adults. All these cortical areas were thinner and the degree of thinning was correlated with autism symptom severity, a correlation nearly restricted to these brain regions.^[33] Based on these results, some researchers claim that autism is caused by a lack of mirror neurons, leading to disabilities in social skills, imitation, empathy and theory of mind. This is just one of many theories of autism and it has not yet been proven.

Theory of Mind

In Philosophy of mind, mirror neurons have become the primary rallying call of simulation theorists concerning our 'theory of mind.' 'Theory of mind' refers to our ability to infer another person's mental state (i.e., beliefs and desires) from their experiences or their behavior. For example, if you see a person reaching into a jar labelled 'cookies,' you might assume that he wants a cookie (even if you know the jar is empty) and that he believes there are cookies in the jar.

There are several competing models which attempt to account for our theory of mind; the most notable in relation to mirror neurons is simulation theory. According to simulation theory, theory of mind is available because we subconsciously put ourselves in the shoes of the person we're observing and, accounting for relevant differences, imagine what we would desire and believe in that scenario. ^[34][35] Mirror neurons have been interpreted as the mechanism by which we simulate others in order to better understand them, and therefore their discovery has been taken by some as a validation of simulation theory (which appeared a decade before the discovery of mirror neurons).^[36]

Gender differences

Stronger MEG responses related to the mirror neuron system have been recorded in women compared to men ^[37]. This finding is consistent with the idea that women tend to be more empathetic, that the mirror neuron system is related to empathy, and that weak responses in the mirror neuron system could be linked to an extreme male brain theory of autism ^[38]. However, these ideas have not been tested in full.

Notes

References

• "The others are inside you," article from “Ingeniøren” (original title: “De andre er inden I dig”).
• E. Bruce Goldstein. (2002). Sensation and Perception, Wadsworth (p. 321-324)

Further reading

• Iacoboni M, Mazziotta JC (2007). "Mirror neuron system: basic findings and clinical applications". Ann Neurol. doi:10.1002/ana.21198. PMID 17721988.

See also

• Crosstalk (biology)
• Emotional contagion
• Molecular Cellular Cognition
• On Intelligence