Mirror neuron

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A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another.^[1] Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primates, and are believed to occur in humans and other species including birds. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex and the inferior parietal cortex.

Some scientists consider mirror neurons one of the most important recent discoveries in neuroscience. Among them is V.S. Ramachandran, who believes they might be very important in imitation and language acquisition.^[2] However, despite the popularity of this field, to date no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions such as imitation.^[3]

The function of the mirror system is a subject of much speculation. Many researchers in cognitive neuroscience and cognitive psychology consider that this system provides the physiological mechanism for the perception action coupling (see the common coding theory). These mirror 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 theory of mind skills,^[4][5] while others relate mirror neurons to language abilities.^[6] It has also been proposed that problems with the mirror system may underlie cognitive disorders, particularly autism.^[7][8] However the connection between mirror neuron dysfunction and autism remains speculative and it is unlikely that mirror neurons are related to many of the important characteristics of autism.

Possible functions

Understanding intentions

Many studies link mirror neurons to understanding goals and intentions. Fogassi et al. (2005)^[33] 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. It was also significant that 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".^[33] They may furnish a neural basis for predicting another individual’s subsequent actions and inferring intention.^[33]

Empathy

Stephanie Preston and Frans de Waal,^[34] Jean Decety,^[35][36] and Vittorio Gallese^[37][38] have independently argued that the mirror neuron system is involved in empathy. A large number of experiments using functional MRI, electroencephalography and magnetoencephalography have shown that certain brain regions (in particular the anterior insula, anterior cingulate cortex, and inferior frontal cortex) are active when a person experiences an emotion (disgust, happiness, pain, etc.) and when he or she sees another person experiencing an emotion.^{[39][40][41][42][43][44][45]} 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, Christian Keysers at the Social Brain Lab and colleagues have shown that people who are more empathic according to self-report questionnaires have stronger activations both in the mirror system for hand actions^[46] and the mirror system for emotions^[44], providing more direct support for the idea that the mirror system is linked to empathy.

Language

In humans, functional MRI studies reported that areas homologous to the monkey mirror neuron system have been found in the inferior frontal cortex, close to Broca's area, one of the hypothesized language regions of the brain. 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.^[47]. This hypothesis is supported by some cytoarchitectonic homologies between monkey premotor area F5 and human Broca's area ^[48]. Rates of vocabulary expansion link to the ability of children to vocally mirror nonwords and so to acquire the new word pronunciations. Such speech repetition occurs automatically, fast^[49] and separately in the brain to speech perception.^[50][51] Moreover such vocal imitation can occur without comprehension such as in speech shadowing^[52] and echolalia.^[53]

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.^[7] Also, children with autism have less activity in mirror neuron regions of the brain when imitating.^[8] 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.^[54] 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.

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 girl reaching into a jar labeled 'cookies,' you might assume that she wants a cookie (even if you know the jar is empty) and believes that 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 empathize with the person we're observing and, accounting for relevant differences, imagine what we would desire and believe in that scenario.^[55][56] 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).^[57] More recently, Theory of Mind and Simulation have been seen as complementary systems, with different developmental time courses.^[58][59][60]

Gender differences

The issue of gender differences in empathy is quite controversial and subject to social desirability and stereotypes. However, a series of recent studies conducted by Yawei Cheng, using a variety of neurophysiological measures, including MEG,^[61] spinal reflex excitability,^[62] electroencephalography,^[63][64] have documented the presence of a gender difference in the human mirror neuron system, with female participants exhibiting stronger motor resonance than male participants.

Criticism

Although many in the scientific community have been excited about the discovery of mirror neurons, there are some researchers who express skepticism in regards to the claims that mirror neurons can explain empathy, theory of mind, etc. Greg Hickok, a cognitive neuroscientist at UC Irvine, has stated that "there is little or no evidence to support the mirror neuron=action understanding hypothesis and instead there is substantial evidence against it."^[65] Hickok also published a detailed analysis of these problems in his paper, "Eight problems for the mirror neuron theory of action understanding in monkeys and humans."^[66] The eight problems he refers to are:

• There is no evidence in monkeys that mirror neurons support action understanding.
• Action understanding can be achieved via non-mirror neuron mechanisms.
• M1 contains mirror neurons.
• The relation between macaque mirror neurons and the “mirror system” in humans is either non-parallel or undetermined.
• Action understanding in humans dissociates from neurophysiological indices of the human “mirror system.”
• Action understanding and action production dissociate.
• Damage to the inferior frontal gyrus is not correlated with action understanding deficits.
• Generalization of the mirror system to speech recognition fails on empirical grounds.