A mirror neuron is a neuron that belongs to a class of neurons that have the property of firing both 1) when an animal performs a particular action and 2) when that animal merely observes the same action being performed by another animal. For that reason, the neuron is said to "mirror" the actions of the animals being observed.
Mirror neurons have been conclusively identified in macaque monkeys and other non-human primates. Although there is no conclusive evidence for mirror neurons in humans, there is evidence suggesting that neurons with "mirroring" properties exist.
Because there is almost no conclusive data about the functional role of mirror neurons, and because there is so much disagreement among the scientific community about how to interpret the data that does exist, the precise nature and function of mirror neurons, as well as their broader importance for the mind, remains unclear.
Discovery & Controversy
Mirror neurons were first identified during research being done in the early 1990's by Giacomo Rizzolatti and colleagues in Parma, Italy. Working with Luciano Fadiga, Leonardo Fogassi and Vittorio Gallese, Rizzolatti was trying to determine if there are specific neurons in the brain of the macaque monkey that specialize in the control of hand actions by recording and studying activation patterns of the macaque brain, in particular of the inferior frontal cortex.<ref>Vittorio Gallese, et al. (1996) "Action Recognition in the premotor cortex" Brain, 119, pp. 593-609. [PMID: 8800951 http://pmid.us/8800951]</ref>
Since the time that they were discovered, there has been much debate within the neuroscientific community about exactly what sort of conclusions can be inferred about the functional role of mirror neurons from the fact that mirror neurons exhibit this property. Some scientists think they will turn out to play very important roles in the mechanisms that underlie a number of very significant cognitive phenomenona, including the ability to understand others' intentions, autism, and language acquisition. For instance, V.S. Ramachandran has claimed that mirror neurons may be so central to our higher cognitive capacities that "mirror neurons will do for psychology what DNA did for biology." <ref>Ramanchandran, V.S. (2000) "Mirror neurons and imitation learning as the driving force behind "the great leap forward" in human evolution" Edge Issue 69, Edge Foundation.</ref>
However, many others disagree, and - partly because of the strong claims made by Ramachandran and others - many scientists firmly insist that the evidence of the functional role of mirror neurons still remains very inconclusive. For instance, in a recent review article of mirror neuron research, the authors expressed justified skepticism towards any claims made about the nature and function of mirror neurons that purport to be conclusive, "Mirror neurons are exceptionally interesting neurons, which may underlie certain social capabilities in both animals and humans. However, the study of mirror neurons and the 'human mirror system' in particular has been characterized by much speculation and relatively little hard evidence."<ref>Ilan Dinstein, et al. (2008) "A mirror up to nature" Current Biology, 18(1):R13-R18. [doi:10.1016/j.cub.2007.11.004 http://dx.doi.org/10.1016/j.cub.2007.11.004]</ref>
In Non-Human Primates
During their experiments, Rizzolatti and the other researchers realized that approximately 17% of the 532 neurons in area F5 of the macaque's premotor cortex that they recorded electrical activity from exhibited the "mirroring" property. The neurons of F5 were known to selectively discharge with both hand and mouth movements are performed, but the new evidence suggested that they were selective for both the performance and observation of an action. Thus, for instance, a subset of the mirror neurons fired when the monkey grasped a piece of food or when the monkey observed another agent grasping a piece of food. The neurons did not fire when the monkey observed only a hand or only an object (e.g. a piece of food). Even when the monkey observed a hand mimicking the action while the object involved in the action was nearby the neurons did not activate; they only activated when the actual action was performed.<ref>Vittorio Gallese, et al. (1996) "Action Recognition in the premotor cortex" Brain, 119, pp. 593-609. [PMID: 8800951 http://pmid.us/8800951]</ref> Also, it is interesting to note that mirror neurons do not lack any of the motor properties exhibited by the other, non-mirror or "canonical" neurons that share their region in the brain.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, p. 191. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref>
Moreover, the researchers noticed that approximately 30% of the mirror neurons responded selectively to one and only one specific movement (e.g. grasping, or placing, or holding) either when performed by the monkey or observed by the monkey. The remaining two thirds of the mirror neurons each responded selectively in varying degrees to several similar movements. In other words, "different subpopulations of mirror neurons respond selectively to different movements... and each subpopulation respondeds similarly to its preferred movement either when it is observed or when it is executed."<ref>Ilan Dinstein, et al. (2008) "A mirror up to nature" Current Biology, 18(1): 1. [doi:10.1016/j.cub.2007.11.004 http://dx.doi.org/10.1016/j.cub.2007.11.004]</ref> '
As of January 2008, only three other electrphysiology studies addressing the topic of mirror neurons in monkeys have been published in peer-reviewed journals. Two of the studies replicated and verified the results initially reported by Rizzolatti et al.. Remarkably, one of these two studies reported evidence that a small set of neurons in area F5 responded selectively not only when the monkey performed an action or visually observed another agent perform that action, but also when the monkey heard a sound closely associated with that action.
The third study reported the discovery of another set of neurons, located in the anterior intraparietal area PF/IPL, that exhibit the same mirroring property. Later scientists also found similar neurons in the macaque's inferior parietal lobule (IPL), which receives strong input from the cortex of the superior temporal sulcus (STS), a region known to code for biological motion, and directs its output to various areas in the premotor cortex, including area F5.<ref>Giacomo Rizzolatti and Maddalena Fabbri Destro (2008) Mirror neurons. Scholarpedia, 3(1):2055. http://www.scholarpedia.org/article/Mirror_neurons. Retrieved on 2008-8-26.</ref><ref>Ilan Dinstein, et al. (2008) "A mirror up to nature" Current Biology, 18(1):R13-R18. [doi:10.1016/j.cub.2007.11.004 http://dx.doi.org/10.1016/j.cub.2007.11.004]</ref>
Although it has been verified that within some non-human primates there exists a "mirror system", distributed over the two cortical regions of area F5 in the premotor cortex and in the inferior parietal lobule, that contains mirror neurons, claims about a homologous "mirror system" in the human brain are more tenuous. Nonetheless, since the initial report of mirror neurons in macaques, there has been experimental data from studies suggesting that there is a fronto-parietal circuit in the human brain that exhibits properties similar to those of the mirror system of macaque's and other monkeys. (For a good review of why trying to determine exactly whether mirror neurons exist in humans is problematic, see Dinstein et al. 2008.) The techniques used in the those studies include functional magnetic resonance imaging (fMRI), positron emissions tomography (PET), electroencephalography (EEG), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS).<ref>Ilan Dinstein, et al. (2008) "A mirror up to nature" Current Biology, 18(1): R3. [doi:10.1016/j.cub.2007.11.004 http://dx.doi.org/10.1016/j.cub.2007.11.004]</ref>
The first evidence indicating humans might have a "mirror system" came from a study by Fadiga et al.<ref>Luciano Fadiga et al. "Motor facilitation during action observation: a magnetic stimulation study" Journal of Neurophysiology, 1995;73:2608–11. http://jn.physiology.org/cgi/content/abstract/73/6/2608</ref> They hypothesized that if mirror neurons exist in humans and are activated in the same way as they are in macaques, then during the observation of an action TMS should induce an increase in the motor-evoked potentials recorded from the muscles that are active when the observed action is indeed executed. The hypothesis was confirmed; during the observation of various actions, in the muscles normally used to produce those actions there was a selective increase of motor-evoked potentials.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, Pages 188-194. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref> Although the experiment suggested humans may have a mirror system, it provided no evidence regarding the actual neural mechanisms that may come into play in such a system.
Evidence for such mechanisms was first produced by two PET experiments, one by Rizzolatti et al. (1996) and one by Grafton et al. (1996).<ref>Giacomo Rizzolatti et al. (1996) "Localization of grasp representations in humans by PET: 1. Observation versus execution" Experimental Brain Research, Vol. 111, No. 2, 246-252. [doi:10.1007/BF00227301 http://dx.doi.org/10.1007/BF00227301] </ref><ref>Scott Grafton et al. (1996) "Localization of grasp representations in humans by PET: 1. Observation compared with imagination" Experimental Brain Research, Vol. 112, No. 1, 103-111. [doi:10.1007/BF00227183 http://dx.doi.org/10.1007/BF00227183]</ref> Both studies showed selective activation for action observation in the superior temporal sulcus (STS), the inferior parietal lobe and the inferior frontal gyrus (area 45). As mentioned above, since the initial PET studies that tried to localize the mirror system in the human brain, similar studies using a wide variety of methodologies have been performed and purported to show strong evidence for the localization of the mirror system in the human brain. However, as also mentioned above, claims about what such studies actually demonstrate are not incontrovertible.
Since the discovery of mirror neurons numerous theories about their potential functional role have been proposed. The phenomena for which scientists have proposed mirror neurons might play a role in the underlying mechanisms range from the understanding of action and intentions, imitation (particularly its role in learning), emotions and empathy, theory of mind, autism, and the evolution and developmental acquisition of language.
Recently it has been pointed out by Rizzolatti that the question "What is the function of mirror neurons?" might be poorly formulated. Instead, he suggests, the appropriate question to ask would be "What is the mechanism in which mirror neurons play a role?" His answer to that questions is that "they represent a mechanism that maps the pictorial descriptions of actions carried out in the higher order visual areas onto their motor counterpart." According to him, once you reformulate the question to address the mechanism, it becomes clear that such a mechanism can itself play a role in a wide variety of higher-level functions.<ref>Giacomo Rizzolatti and Maddalena Fabbri Destro (2008) Mirror neurons. Scholarpedia, 3(1):2055. http://www.scholarpedia.org/article/Mirror_neurons. Retrieved on 2008-8-26.</ref>
Understanding of Actions and Intentions
The original paper by Rizzolatti and colleagues suggested two initial hypotheses for the functional role of the mirror neuron system. One was that of action understanding. He and his colleagues explained roughly how this might work:
- "When an individual emits an action, he "knows" (predicts) its consequences. This knowledge is most likely the result of an association between the representation of the motor act, coded in the motor centres [sic], and the consequences of the action. Mirror neurons could be the means by which this type of knowledge can be extended to actions performed by others. When the observation of an action performed by another individual evokes a neural activity that corresponds to that which, when internally generated, represents a certain action, the meaning of it should be recognized, because of the similarity between the two representations."<ref>Vittorio Gallese, et al. (1996) "Action Recognition in the premotor cortex" Brain, 119, pp. 593-609. [PMID: 8800951 http://pmid.us/8800951]</ref>
In other words, the mirror neurons provide a "link" by which the information encoded by the visual system during the observation of an action can be identified with the information stored in the motor cortex about the meaning of that action. More crudely, mirror neurons are the mechanism that lets monkeys see an action performed by another agent as that action, e.g. the result of a particular intention, and not just random movement or some other action. Thus, mirror neurons have been called a "observation-execution matching system." More recently, Rizzolatti has argued that this could serve as a "core mechanism," on top of which the other functions discussed below could be built.<ref>Giacomo Rizzolatti and Maddalena Fabbri Destro (2008) Mirror neurons. Scholarpedia, 3(1):2055. http://www.scholarpedia.org/article/Mirror_neurons. Retrieved on 2008-8-26.</ref>
The idea that mirror neurons play a role in understanding the intentions that motivate the actions of others has been brought to bear in the debate among cognitive scientists and philosophers of mind over the nature of mindreading or mentalizing, the ability to know and understand the mental states of other agents. Whether or not mirror neurons provide support for one side of the debate over the other still remains unclear. (For the initial use of mirror neurons of evidence in this debate see Gallese and Goldman 1998, and, for a comprehensive overview of the state of the debate see Goldman 2006.)<ref>Vittorio Gallese and Alvin Goldman (1998) "Mirror neurons and the simulation theory of mind-reading." Trends in Cognitive Sciences, 12:493-501. [doi:10.1016/S1364-6613(98)01262-5 http://dx.doi.org/10.1016/S1364-6613(98)01262-5]</ref><ref>Alvin Goldman (2006) Simulating Minds: The Philosophy, Psychology and Neuroscience of Mind-Reading Oxford University Press US. [doi:10.1093/0195138929.001.0001 http://dx.doi.org/10.1093/0195138929.001.0001]</ref>
In the initial report by Rizzolatti and his colleagues, they speculated that since it is well known that both children and adults learn by the imitation of others, the "imitation process could be based on an observation/execution matching mechanism similar to that represented by mirror neurons. Such a mechanism can, on the one hand, extract the essential elements describing the agent of the action (hand, arm, face) and, on the other, code them directly [onto] specific sets of neurons with motor properties like those of F5."<ref>Vittorio Gallese, et al. (1996) "Action Recognition in the premotor cortex" Brain, 119, p. 606. [PMID: 8800951 http://pmid.us/8800951]</ref>
However, a recent paper by leading primatologists argues that primatologists have long pointed out that human beings are unique in our ability for true imitation, and that in fact no primates other than humans actually imitate the behavior of their conspecifics. Instead, the authors of the paper provide evidence from within their own field of studying non-human primate social cognition for the theory mentioned above that proposes mirror neurons provided the mechanisms that allow for the capacity of non-human primates to understand the intentions of the agents whose behavior they observe.<ref>Derek E. Lyons et al. (2006) "Reflections of other minds: how primate social cognition can inform the function of mirror neurons" Current Opinion in Neurobiology,Volume 16, Issue 2, April 2006, pp. 230-234. [doi:10.1016/j.conb.2006.03.015 http://dx.doi.org/10.1016/j.conb.2006.03.015]</ref>
Another, somewhat related, function that has been proposed as a candidate for mirror neurons is empathy. In this context, empathy simply is the ability to experience the same emotions that are others are feeling, disregarding any positive or negative connotations that are normally associated with the notion of empathy.
There is some evidence of a neural population in humans that activates both when the person experiences disgust herself, and when the person observes the facial expression of disgust by another person. Similar evidence has been produced for the experience of pain.<ref>Giacomo Rizzolatti and Maddalena Fabbri Destro (2008) Mirror neurons. Scholarpedia, 3(1):2055. http://www.scholarpedia.org/article/Mirror_neurons. Retrieved on 2008-8-26.</ref>
It has been speculated - by Dapretto et al., among others - that there is a link between the dysfunction of the mirror neuron system of humans and autism spectrum disorder. According to these speculations, given that it quite likely the case that the mirror system plays a large role in the mechanism for understanding the actions of others, the reason children with autism are unable to understand and relate to people in ordinary ways probably has to do with the improper functioning of their mirror system.<ref>Mirella Dapretto, et al. (2006) "Understanding emotions in others: mirror neuron dysfunction in children with autism spectrum disorders" Nature Neuroscience 9, 28 - 30. [doi:10.1038/nn1611 http://dx.doi.org/10.1038/nn1611]</ref>
This reasoning however is not entirely valid. First, it has not been unequivocally demonstrated that mirror system plays a role in the neural mechanisms for understanding the actions and intentions of others. That idea still remains an unconfirmed hypothesis. Second, the reasoning rests upon the supposition that the primary deficit of individuals with autism is or is closely related to their inability to understanding the actions of others. However, it remains entirely possible that such a deficit is not the key component autism: the inability to understand the actions of others may be a surface level effect of a more fundamental cause involving the improper organization of mechanism in the regions of the brain involved with motor function, and therefore an deficit in the ability to organize one's own motor behavior.<ref>Giacomo Rizzolatti and Maddalena Fabbri Destro (2008) Mirror neurons. Scholarpedia, 3(1):2055. http://www.scholarpedia.org/article/Mirror_neurons. Retrieved on 2008-8-26.</ref>
Evolution of Language
Arbib and Rizzolatti have pointed out that there are a number of both structural reasons and functional reasons based upon recent experimental evidence to support the claim that the rostral part of the monkey ventral premotor cortex (area F5) is homologous to Broca's area in the human brain. First consider the structural reasons. For one, both area F5 and Broca's area are located within inferior area 6. Second, the location of each area within the agranular frontal cortex is similar. Third, there are strong cytoarchitectural similarities between the caudal part of Broca's area (area 44) and area F5.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, pp. 188-194. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref>
Next consider the functional reasons. Although F5 is normally thought to be an area for hand movements and Broca's area is thought to be an area for speech production, some recent experimental data supports the idea that the two areas are play functionally similar roles. First, F5 is somatotopically organized and as such its large ventral portion represents mouth and larynx movements. So, F5 is thought to also play a role the movement of the speech-producing organs. Second, Arbib and Rizzolatti report recent PET data suggesting that BRoca's area selectively activates not only for speech production but also for the execution of hand or arm movements, during the imagination of hand grasping movements, and during tasks that involve hand-mental rotations.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, pp. 188-194. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref>
Given all these reasons, it is plausible that area F5 in monkeys and Broca's area in humans are homologous. Thus, the area in the monkey brain largely responsible for action recognition and action production is likely homologous to the area in the human brain largely responsible for speech production. Arbib and Rizzolatti argue that this is not a coincidence. In fact, they argue that the mirror system in area F5 was at least instrumental, if not fundamental, for first the development of the capacity for early forms of intentional communication and then later on the development of speech.
- Our proposal is that the development of the human lateral speech circuit is a consequence of the fact that the precursor of Broca's area was endowed, before speech appearance, with a mechanism for recognizing actions made by others. This mechanism was the neural prerequisite for the development of inter-individual communication and finally of speech.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, p. 190. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref>
Of course, this leaves the gap between understanding the actions of others and speech as wide as ever. But, Rizzolatti and Arbib have a hypothesis about how to bridge it. First, they point out that during normal observation of action the pre-motor areas are activated, and, normally, there are strong inhibitory mechanisms responsible for ensuring that such pre-motor activity does not actually lead to motor behavior. Second, they indicate that in some cases, in particular when an action is of special interest to the observer, the pre-motor system is not entirely inhibited and actually does allow the emittance of a prefix of the action observed. Such an emittance could, of course, then be witnessed by the agent that performed the original action. From these facts, Rizzolatti and Arbib conclude that:
- the development of the capacity of the observer to control his or her mirror system is crucial in order to emit (voluntarily) a signal. When this occurs, a primitive dialogue [sic] between observer and actor is established. This dialogue forms the core of language.<ref>Giacomo Rizzolatti and Michael A. Arbib (1998) "Language within our grasp" Trends in Neurosciences Volume 21, Issue 5, 1 May 1998, p. 191. [doi:10.1016/S0166-2236(98)01260-0 http://dx.doi.org/10.1016/S0166-2236(98)01260-0]</ref>
This hypothesis is, of course, both bold and speculative, and there are many possible ways in which it could be criticized. Nevertheless, it does present a new and interesting perspective on the evolution of intentional communication and language, and, moreover, demonstrates yet another theory about the possible functional role of mirror neurons.