sábado, 3 de abril de 2010
The Mirror Neuron System
A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. 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. However, despite the excitement generated by these findings, 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. 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, while others relate mirror neurons to language abilities. It has also been proposed that problems with the mirror system may underlie cognitive disorders, particularly autism. However the connection between mirror neuron dysfunction and autism is tentative and it remains to be seen how mirror neurons may be related to many of the important characteristics of autism. Contents [hide] 1 Discovery 2 In monkeys 2.1 Doubts on mirror neurons 3 In humans 3.1 Evidence against mirror neurons 3.2 Development 4 Possible functions 4.1 Understanding intentions 4.2 Empathy 4.3 Language 4.4 Autism 4.5 Theory of mind 4.6 Gender differences 5 Criticism 6 Notes 7 Further reading 8 See also 9 External links Discovery In the 1980s and 1990s, Giacomo Rizzolatti was working with Giuseppe Di Pellegrino, Luciano Fadiga, Leonardo Fogassi, and Vittorio Gallese at the University of Parma, Italy. These neurophysiologists had placed electrodes in the ventral premotor cortex of the macaque monkey to study neurons specialized for the control of hand and mouth actions; for example, taking hold of an object and manipulating it. 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. The discovery was initially sent to Nature but was rejected for its “lack of general interest”. They found that some of the neurons they recorded from would respond when the monkey saw a person pick up a piece of food as well as when the monkey picked up the food. A few years later, the same group published another empirical paper and discussed the role of the mirror neuron system in action recognition, and proposed that the human Broca’s region was the homologue region of the monkey ventral premotor cortex. Further experiments confirmed that approximately 10% of neurons in the monkey inferior frontal and inferior parietal cortex have 'mirror' properties and give similar responses to performed hand actions and observed actions. More recently Christian Keysers and colleagues have shown that both in humans and monkeys, the mirror system also responds to the sound of actions. Reports on mirror neurons have been widely published and confirmed with mirror neurons found in both inferior frontal and inferior parietal regions of the brain. Recently, evidence from functional neuroimaging and behavioral strongly suggest the presence of similar mirror neurons systems in humans, where brain regions which respond during both action and the observation of action have been identified. Not surprisingly, these brain regions include those found in the macaque monkey. However, due to the power of Functional magnetic resonance imaging to examine the entire brain at once human studies suggests that a much wider network of brain areas shows mirror properties in humans than previously thought. These additional areas include the somatosensory cortex and are thought to make the observer feel what it feels like to move in the observed way  In monkeys Neonatal (newborn) macaque imitating facial expressions The only animal in which 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. Mirror neurons are believed to mediate the understanding of other animals' 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. 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. 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. Doubts on mirror neurons There are continuing doubts concerning the evidence for the existence of mirror neurons in humans (see section below), but most researchers accept that the existence of mirror neurons in monkeys is well established. Their functional significance, however—specifically, their role in imitation—remains in dispute. One recent review argued that the original analyses were unconvincing because they were based on qualitative descriptions of individual cell properties, and did not take into account the small number of strongly mirror-selective neurons in motor areas . Other reviews argued that the measurements of neuron fire delay seem not to be compatible with standard reaction times , and pointed out that nobody has ever reported that an interruption of the motor areas in F5 would produce a decrement in action recognition. It is not clear, according to these reviews, whether mirror neurons really form a distinct class of cells (as opposed to an occasional phenomenon seen in cells that have other functions), and whether mirror activity is a distinct type of response or simply an artifact of an overall facilitation of the motor system. Indeed, there is limited understanding of the degree to which monkeys show imitative behavior in the first place. In humans Diagram of the brain, showing the locations of the frontal and parietal lobes of the cerebrum, viewed from the left. The inferior frontal lobe is the lower part of the blue area, and the superior parietal lobe is the upper part of the yellow area. It is not normally possible to study single neurons in the human brain, so scientists cannot be certain that humans have mirror neurons. However, the results of brain imaging experiments using functional magnetic resonance imaging (fMRI) have shown that the human inferior frontal cortex and superior parietal lobe is active when the person performs an action and also when the person sees another individual performing an action. It has been suggested that these brain regions contain mirror neurons, and they have been defined as the human mirror neuron system. More recent experiments have shown that even at the level of single participants, scanned using fMRI, large areas containing multiple fMRI voxels increase their activity both during the observation and execution of actions. However, a recent study has shown that the movement-selective pattern of activity within these voxels is different across the observation and execution of a movement, suggesting that two separate neural populations are responding in each case rather than a single population of mirror neurons.. This result has put in question the conclusions of previous studies claiming that "mirror system" fMRI responses are generated by mirror neuron activity (but also see ). Overall, research in humans focuses on the "mirror neuron system" rather than "mirror neurons". Several indirect measures have been used to study the mirror neuron system in humans. For example, when a person observes another person's action, his motor cortex becomes more excitable. This excitability can be measured by recording the size of a motor evoked potential (MEP) induced by transcranial magnetic stimulation. The changes in MEP size are taken as a measure of mirror neuron system activity, because MEPs come from the primary motor cortex which is closely connected to the mirror neuron regions of the brain. Although indirect, MEPs modulation provides the evidence that in humans the motor system of the observer actively "resonates" on the basis of the various phases of the observed action (i.e. seeing a hand closing onto an object induces a facilitation of the observer's corticospinal channels leading to hand closure). 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-learned mappings. Eye tracking provides another indirect measure that may reflect mirror neuron processing. Upon moving his or her hand, a person's eyes move ahead of the hand to look at the object the person will grasp. Similarly, when watching someone else's action, a person's eyes are also likely to anticipate what the other person will do. Evidence against mirror neurons Three recent studies cast doubt on the importance of mirror neurons in the human brain. These fMRI studies suggested that the signal changes seen in 'mirror neuron regions' of the human brain are not necessarily due to the firing of mirror neurons themselves, but may reflect the responses of other neurons in these brain areas. The studies found evidence of movement selective activity for both observed and executed movements, i.e., for one population of visual neurons that respond selectively to observed movements and a separate population of motor neurons that respond selectively to executed movements, but there was no evidence of mirror neurons that would respond selectively to the same observed and executed movement. The authors of these papers concluded that although there may be movement-selective mirror neurons in the human brain, they make up only a minority of the neurons active during observation or execution of movement and do not dominate the fMRI responses in putative mirror system areas of the brain. Whereas these three papers failed to find evidence of mirror neurons that would respond selectively to the same observed and executed movement a more recent study has successfully demonstrated this effect . These authors argue that they were able to find the effect where others had previously failed because their study was optimally designed to find mirror neurons in the human inferior frontal gyrus. 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. A critical question concerns how mirror neurons acquire mirror properties. Two closely related models postulate that mirror neurons are trained through Hebbian or Associative learning. 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 and Moore (unless this is a special type of imitation not supported by mirror neurons). Possible functions Understanding intentions Many studies link mirror neurons to understanding goals and intentions. Fogassi et al. (2005) 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". They may furnish a neural basis for predicting another individual’s subsequent actions and inferring intention. Empathy Stephanie Preston and Frans de Waal, Jean Decety, and Vittorio Gallese have independently argued that the mirror neuron system is involved in empathy. A large number of experiments using functional MRI, electroencephalography (EEG) and magnetoencephalography (MEG) 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. 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 and the mirror system for emotions, 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.. This hypothesis is supported by some cytoarchitectonic homologies between monkey premotor area F5 and human Broca's area . 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 and separately in the brain to speech perception. Moreover such vocal imitation can occur without comprehension such as in speech shadowing and echolalia. 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. Also, children with autism have less activity in mirror neuron regions of the brain when imitating. 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. 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. 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). More recently, Theory of Mind and Simulation have been seen as complementary systems, with different developmental time courses. 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, spinal reflex excitability, electroencephalography, 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." 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." 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. The primary motor cortex (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.