Mirror neuron

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][2] Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Such neurons have been directly observed in primates, humans and other species including birds. In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory 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.[3] 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.[4]

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,[5][6] while others relate mirror neurons to language abilities.[7] It has also been proposed that problems with the mirror system may underlie cognitive disorders, particularly autism.[8][9] 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.[4]

Contents

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.[10][11] The discovery was initially sent to Nature but was rejected for its “lack of general interest”.[12] 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.[13] 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.[14][15] Reports on mirror neurons have been widely published[16] and confirmed[17] 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.[1] 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 [18][19]

In monkeys

Neonatal (newborn) macaque imitating facial expressions

The first 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.[1]

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.[1]

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.[20] 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.[21]

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 most evidence for mirror neurons in humans is indirect. 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.[22] 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.[18]

A study published in April 2010 reports recordings from single neurons with mirror properties in the human brain.[23] Mukamel et al (Current Biology, 2010) recorded from the brains of 21 patients who were being treated at Ronald Reagan UCLA Medical Center for intractable epilepsy. The patients had been implanted with intracranial depth electrodes to identify seizure foci for potential surgical treatment. Electrode location was based solely on clinical criteria; the researchers, with the patients' consent, used the same electrodes to "piggyback" their research. The experiment included three parts: facial expressions, grasping and a control experiment. Activity from a total of 1,177 neurons in the 21 patients was recorded as the patients both observed and performed grasping actions and facial gestures. In the observation phase, the patients observed various actions presented on a laptop computer. In the activity phase, the subjects were asked to perform an action based on a visually presented word. In the control task, the same words were presented and the patients were instructed not to execute the action. The researchers found a small number of neurons that fired or showed their greatest activity both when the individual performed a task and when they observed a task. Other neurons had anti-mirror properties, that is, they responded when the participant saw an action but were inhibited when the participant performed that action. The mirror neurons found were located in the supplementary motor area and medial temporal cortex (other brain regions were not sampled). For purely practical reasons, these regions are not the same as those in which mirror neurons had been recorded from in the monkey: researchers in Parma were studying the ventral premotor cortex and the associated inferior parietal lobe, two regions in which epilepsy rarely occurs, and hence, single cell recordings in these regions are not usually done in humans. On the other hand, noone has to date looked for mirror neurons in the supplementary motor area or the medial temporal lobe in the monkey. Together, this therefore does not suggest that humans and monkeys have mirror neurons in different locations, but rather than they may have mirror neurons both in the ventral premotor cortex and inferior parietal lobe, where they have been recorded in the monkey, and in the supplementary motor areas and medial temporal lobe, where they have been recorded from in human - especially because detailed human fMRI analyses suggest activity compatible with the presence of mirror neurons in all these regions[18] .

Doubts concerning mirror neurons

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 [4]. Other reviews argued that the measurements of neuron fire delay seem not to be compatible with standard reaction times [24], and pointed out that nobody has ever reported that an interruption of the motor areas in F5 would produce a decrement in action recognition.[25] 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)[24], and whether mirror activity is a distinct type of response or simply an artifact of an overall facilitation of the motor system[25]. Indeed, there is limited understanding of the degree to which monkeys show imitative behavior in the first place.[4]

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.[26] A critical question concerns how mirror neurons acquire mirror properties. Two closely related models postulate that mirror neurons are trained through Hebbian[27] or Associative learning[28][29][30] (see Associative Sequence 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. One possibility is that the sight of tongue protrusion recruits an innate releasing mechanism in neonates. Careful analysis suggests that 'imitation' of this single gesture may account for almost all reports of facial mimicry by new-born infants[31].

Possible functions

Understanding intentions

Many studies link mirror neurons to understanding goals and intentions. Fogassi et al. (2005)[32] 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.

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".[32] They may furnish a neural basis for predicting another individual’s subsequent actions and inferring intention.[32]

Empathy

Stephanie Preston and Frans de Waal,[33] Jean Decety,[34][35] and Vittorio Gallese[36][37] 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.[38][39][40][41][42][43][44] 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[45] and the mirror system for emotions[43], 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.[46]. This hypothesis is supported by some cytoarchitectonic homologies between monkey premotor area F5 and human Broca's area [47]. 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[48] and separately in the brain to speech perception.[49][50] Moreover such vocal imitation can occur without comprehension such as in speech shadowing[51] and echolalia.[52] Further evidence for this link comes from a recent study in which the brain activity of two participants was measured using fMRI while they were gesturing words to each other using hand gestures - a modality that some have suggested might represent the evolutionary precursor of human language. Analysis of the data using Granger Causality revealed that the mirror neuron system of the observer indeed reflects the pattern of activity of the activity in the motor system of the sender, supporting the idea that the motor concept associated with the words is indeed transmitted from one brain to another using the mirror system [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.[8] Also, children with autism have less activity in mirror neuron regions of the brain when imitating.[9] 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, but others have argued against this.

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 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]

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Rizzolatti, Giacomo; Craighero, Laila (2004). "The mirror-neuron system". Annual Review of Neuroscience 27: 169–192. doi:10.1146/annurev.neuro.27.070203.144230. PMID 15217330. http://web.mit.edu/hst.722/www/Topics/Language/RizzolattiReview2004.pdf 
  2. ↑ Keysers, Christian (2010). "Mirror Neurons". Current Biology 19 (21): R971–973. doi:10.1016/j.cub.2009.08.026. PMID 19922849. http://www.bcn-nic.nl/txt/people/publications/2009_Keysers_CurrentBiology.pdf. 
  3. ↑ V.S. Ramachandran, "Mirror Neurons and imitation learning as the driving force behind "the great leap forward" in human evolution". Edge Foundation. http://www.edge.org/3rd_culture/ramachandran/ramachandran_p1.html. Retrieved 2006-11-16. 
  4. ↑ 4.0 4.1 4.2 4.3 Dinstein I, Thomas C, Behrmann M, Heeger DJ (2008). "A mirror up to nature". Curr Biol 18 (1): R13–8. doi:10.1016/j.cub.2008.01.044. PMID 18177704. 
  5. ↑ Christian Keysers and Valeria Gazzola, Progress in Brain Research, 2006, [1]
  6. ↑ Michael Arbib, The Mirror System Hypothesis. Linking Language to Theory of Mind, 2005, retrieved 2006-02-17
  7. ↑ Hugo Théoret, Alvaro Pascual-Leone, Language Acquisition: Do As You Hear, Current Biology, Vol. 12, No. 21, pp. R736-R737, 2002-10-29
  8. ↑ 8.0 8.1 Oberman LM, Hubbard EM, McCleery JP, Altschuler EL, Ramachandran VS, Pineda JA., EEG evidence for mirror neuron dysfunction in autism spectral disorders, Brain Res Cogn Brain Res.; 24(2):190-8, 2005-06
  9. ↑ 9.0 9.1 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
  10. ↑ Di Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V., & Rizzolatti, G. (1992). Understanding motor events: a neurophysiological study. Experimental Brain Research, 91, 176-180.
  11. ↑ Giacomo Rizzolatti et al. (1996) Premotor cortex and the recognition of motor actions, Cognitive Brain Research 3 131-141
  12. ↑ Rizzolatti G, Fabbri-Destro M. (2010). Mirror neurons: from discovery to autism. Exp Brain Res. 200(3-4):223-37. doi:10.1007/s00221-009-2002-3 PMID 19760408 [
  13. ↑ Gallese, V.; Fadiga, L.; Fogassi, L.; Rizzolatti, Giacomo (1996). "Action recognition in the premotor cortex". Brain 119 (2): 593–609. doi:10.1093/brain/119.2.593. http://brain.oxfordjournals.org/cgi/content/abstract/119/2/593 
  14. ↑ Kohler et al., Science, 2002 [2]
  15. ↑ Gazzola et al., Current Biology, 2006 [3]
  16. ↑ Gallese et al., Action recognition in the premotor cortex, Brain, 1996
  17. ↑ Fogassi et al., Parietal Lobe: From Action Organization to Intention Understanding, Science, 2005
  18. ↑ 18.0 18.1 18.2 Gazzola, V.; Keysers, C. (2009). "The observation and execution of actions share motor and somatosensory voxels in all tested subjects: single-subject analyses of unsmoothed fMRI data". Cereb Cortex 19 (6): 1239–1255. doi:10.1093/cercor/bhn181. PMID 19020203 
  19. ↑ Keysers, Christian; Kaas, John; Gazzola, Valeria (2010). "Somatosensation in Social Cognition". Nature Reviews Neuroscience 11 (6): 417–28. doi:10.1038/nrn2833. PMID 20445542. http://www.bcn-nic.nl/txt/people/publications/2010_NRN2833_Keysers_Kaas_Gazzola.pdf. 
  20. ↑ Ferrari, P. F.; Visalberghi, E.; Paukner, A.; Fogassi, L.; Ruggiero, A.; Suomi, SJ (2006). et al.. "Neonatal Imitation in Rhesus Macaques". PLoS Biology 4 (9): e302. doi:10.1371/journal.pbio.0040302. PMID 16953662 
  21. ↑ Rizzolatti, Giacomo; Arbib, Michael A. (1998). "Language within our grasp". Trends in Neurosciences 21 (5): 188–194. doi:10.1016/S0166-2236(98)01260-0. PMID 9610880 
  22. ↑ Iacoboni, Marco; Woods, Roger P.; Brass, Marcel; Bekkering, Harold; Mazziotta, John C.; Rizzolatti, Giacomo (1999). "Cortical Mechanisms of Human Imitation". Science 286 (5449): 2526–2528. doi:10.1126/science.286.5449.2526. PMID 10617472 
  23. ↑ Keysers, Christian (2010). "Social Neuroscience: Mirror Neurons recorded in Humans". Current Biology 20 (8): R353–354. doi:10.1016/j.cub.2010.03.013. http://www.bcn-nic.nl/txt/people/publications/2010_KeysersGazzolaMirrorNeuronsRecordedInHumans.pdf. 
  24. ↑ 24.0 24.1 Pascolo PB, Ragogna R, Rossi R, (2009). "The Mirror-Neuron System Paradigm and its consistency". Gait Posture 30 (Suppl. 1): 65. doi:10.1016/j.gaitpost.2009.07.064. 
  25. ↑ 25.0 25.1 Hickok G (2009). "Eight problems for the mirror neuron theory of action understanding in monkeys and humans". J Cogn Neurosci 21 (7): 1229–1243. doi:10.+1162/jocn.2009.21189. PMID 19199415. 
  26. ↑ Terje Falck-Ytter, Gustaf Gredebäck & Claes von Hofsten (2006), Infants predict other people's action goals[4], Nature Neuroscience 9 (2006)
  27. ↑ Keysers & Perrett, Trends in Cognitive Sciences 8 (2004)
  28. ↑ Heyes, C. M. (2001) Causes and consequences of imitation. Trends in Cognitive Sciences, 5, 253-261
  29. ↑ Brass, M., & Heyes, C. Trends in Cognitive Sciences 9 (2005)
  30. ↑ Heyes, C. M. (2010) Where do mirror neurons come from? Neuroscience and Biobehavioural Reviews, 34, 575-583
  31. ↑ Anisfeld, M. (1996). Only tongue protruding modeling is matched by neonates. Developmental Review, 16(2), 149-161
  32. ↑ 32.0 32.1 32.2 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.
  33. ↑ Preston, S. D., & de Waal, F.B.M. (2002) Empathy: Its ultimate and proximate bases. Behavioral and Brain Sciences, 25, 1-72.
  34. ↑ Decety, J. (2002). Naturaliser l’empathie [Empathy naturalized]. L’Encéphale, 28, 9-20.
  35. ↑ Decety, J., & Jackson, P.L. (2004). The functional architecture of human empathy. Behavioral and Cognitive Neuroscience Reviews, 3, 71-100.
  36. ↑ Gallese, V., & Goldman, A.I. (1998). Mirror neurons and the simulation theory. Trends in Cognitive Sciences, 2, 493-501.
  37. ↑ Gallese, V. (2001). The “Shared Manifold” hypothesis: from mirror neurons to empathy. Journal of Consciousness Studies, 8, 33-50.
  38. ↑ Botvinick, M., Jha, A.P., Bylsma, L.M., Fabian, S.A., Solomon, P.E., & Prkachin, K.M. (2005). Viewing facial expressions of pain engages cortical areas involved in the direct experience of pain. NeuroImage, 25, 312-319.
  39. ↑ Cheng, Y., Yang, C.Y., Lin, C.P., Lee, P.R., & Decety, J. (2008). The perception of pain in others suppresses somatosensory oscillations: a magnetoencephalography study. NeuroImage, 40, 1833-1840.
  40. ↑ Morrison, I., Lloyd, D., di Pellegrino, G., & Roberts, N. (2004). Vicarious responses to pain in anterior cingulate cortex: is empathy a multisensory issue? Cognitive and Affective Behavioral Neuroscience, 4, 270-278.
  41. ↑ Wicker et al., Neuron, 2003 [5]
  42. ↑ Singer et al., Science, 2004 [6]
  43. ↑ 43.0 43.1 Jabbi, Swart and Keysers, NeuroImage, 2006 [7]
  44. ↑ Lamm, C., Batson, C.D., & Decety, J. (2007). The neural substrate of human empathy: effects of perspective-taking and cognitive appraisal. Journal of Cognitive Neuroscience, 19, 42-58.
  45. ↑ Gazzola, Aziz-Zadeh and Keysers, Current Biology, 2006 [8]
  46. ↑ Skoyles, John R., Gesture, Language Origins, and Right Handedness, Psycholoqy: 11,#24, 2000
  47. ↑ Petrides, Michael, Cadoret, Genevieve, Mackey, Scott (2005). Orofacial somatomotor responses in the macaque monkey homologue of Broca's area, Nature: 435,#1235
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  53. ↑ Schippers, MB; Roebroeck, A; Renken, R; Nanetti, L; Keysers, C (2010). "Mapping the Information flow from one brain to another during gestural communication". Proc Natl Acad Sci U S A. 107 (20): 9388–93. doi:10.1073/pnas.1001791107. PMID 20439736. PMC 2889063. http://www.bcn-nic.nl/txt/people/publications/2010_SchippersKeysers_PNAS.pdf. 
  54. ↑ Hadjikhani and others; Joseph, RM; Snyder, J; Tager-Flusberg, H (2006). "Anatomical Differences in the Mirror Neuron System and Social Cognition Network in Autism". Cerebral Cortex 16 (9): 1276–82. doi:10.1093/cercor/bhj069. PMID 16306324. 
  55. ↑ Gordon, R. (1986). Folk psychology as simulation. Mind and Language 1: 158-171
  56. ↑ Goldman, A. (1989). Interpretation psychologized. Mind and Language 4: 161–185
  57. ↑ Gallese, V., and Goldman, A. (1998). Mirror neurons and the simulation theory of mindreading. Trends in Cognitive Sciences. 2: 493–501
  58. ↑ Meltzoff, A.N., & Decety, J. (2003). What imitation tells us about social cognition: A rapprochement between developmental psychology and cognitive neuroscience. The Philosophical Transactions of the Royal Society, London, 358, 491-500.
  59. ↑ Sommerville, J. A., & Decety, J. (2006). Weaving the fabric of social interaction: Articulating developmental psychology and cognitive neuroscience in the domain of motor cognition. Psychonomic Bulletin & Review, 13, 179-200.
  60. ↑ Keysers C and Gazzola V(2007) Integrating simulation and theory of mind: from self to social cognition. Trends in Cognitive Sciences 11(5):194-6[9]
  61. ↑ Cheng, Y., Tzeng, O.J., Decety, J., & Hsieh, J.C. (2006). Gender differences in the human mirror system: a magnetoencephalography study. NeuroReport, 17, 1115-1119.
  62. ↑ Cheng, Y., Decety, J., Hsieh, J.C., Hung, D., & Tzeng, O.J. (2007). Gender differences in spinal excitability during observation of bipedal locomotion. NeuroReport, 18, 887-890.
  63. ↑ Cheng, Y., Decety, J., Yang, C.Y., Lee, S., & Chen, G. (2008). Gender differences in the Mu rhythm during empathy for pain: An electroencephalographic study. Brain Research, in press.
  64. ↑ Cheng, Y., Lee, P., Yang, C.Y., Lin, C.P., & Decety, J. (2008). Gender differences in the mu rhythm of the human mirror-neuron system. PLoS ONE, 5, e2113.
  65. ↑ Greg Hickok (2009-02-10). "Eight problems for the mirror neuron theory of action understanding". Talking Brains blog. http://talkingbrains.blogspot.com/2009/02/eight-problems-for-mirror-neuron-theory.html. Retrieved 2010-08-29. 
  66. ↑ Hickok, Greg (2009). "Eight Problems for the Mirror Neuron Theory of Action Understanding in Monkeys and Humans". Journal of Cognitive Neuroscience (MIT Press Journals) 21 (7): 1229-1243. doi:10.1162/jocn.2009.21189. http://www.mitpressjournals.org/doi/abs/10.1162/jocn.2009.21189. 

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