mirror neuron 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. thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. such neurons have been directly observed in human and primate species, and in birds. mirror systemidentifiersmeshd059167anatomical terms of neuroanatomy 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. the function of the mirror system in humans is a subject of much speculation. birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. to date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions. the subject of mirror neurons continues to generate intense debate. in 2014, philosophical transactions of the royal society b published a special issue entirely devoted to mirror neuron research. some researchers speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. neuroscientists such as marco iacoboni (ucla) have argued that mirror neuron systems in the human brain help us understand the actions and intentions of other people. in a study published in march 2005 iacoboni and his colleagues reported that mirror neuron activity could predict whether another person who was picking up a cup of tea planned to drink from it or clear it from the table. in addition, iacoboni has argued that mirror neurons are the neural basis of the human capacity for emotions such as empathy. there are scientists who express skepticism about the theories being advanced to explain the function of mirror neurons. in a 2013 article for wired, christian jarrett cautioned that: ...mirror neurons are an exciting, intriguing discovery – but when you see them mentioned in the media, remember that most of the research on these cells has been conducted in monkeys. remember too that there are many different types of mirror neuron. and that we're still trying to establish for sure whether they exist in humans, and how they compare with the monkey versions. as for understanding the functional significance of these cells … don't be fooled: that journey has only just begun. discovery in the 1980s and 1990s, neurophysiologists giacomo rizzolatti, giuseppe di pellegrino, luciano fadiga, leonardo fogassi, and vittorio gallese at the university of parma placed electrodes in the ventral premotor cortex of the macaque monkey to study neurons specialized in the control of hand and mouth actions; for example, taking hold of an object and manipulating it. during each experiment, the researchers allowed the monkey to reach for pieces of food, and recorded from single neurons in the monkey's brain, thus measuring the neuron's response to certain movements. they found that some neurons responded when the monkey observed a person picking up a piece of food, and also when the monkey itself picked up the food. the discovery was initially submitted to nature, but was rejected for its "lack of general interest" before being published in a less competitive journal. a few years later, the same group published another empirical paper, discussing the role of the mirror-neuron system in action recognition, and proposing that the human broca's region was the homologue region of the monkey ventral premotor cortex. while these papers reported the presence of mirror neurons responding to hand actions, a subsequent study by pier francesco ferrari and colleagues described the presence of mirror neurons responding to mouth actions and facial gestures. further experiments confirmed that about 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. in 2002 christian keysers and colleagues reported that, in both 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 strongly suggests that humans have similar mirror neurons systems: researchers have identified brain regions which respond during both action and observation of action. not surprisingly, these brain regions include those found in the macaque monkey. however, functional magnetic resonance imaging (fmri) can examine the entire brain at once and 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. origin many implicitly assume that the mirrorness of mirror neurons is due primarily to heritable genetic factors and that the genetic predisposition to develop mirror neurons evolved because they facilitate action understanding. in contrast, a number of theoretical accounts argue that mirror neurons could simply emerge due to learned associations, including the hebbian theory, the associative learning theory, canalization and exaptation. in monkeys neonatal (newborn) macaque imitating facial expressions the first animal in which researchers have studied mirror neurons 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' behaviour. 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 remains unknown. adult macaques do not seem to learn by imitation. recent experiments by ferrari and colleagues suggest that infant macaques can imitate a human's face movements, though only as neonates and during a limited temporal window. even if it has not yet been empirically demonstrated, it has been proposed that mirror neurons cause this behaviour and other imitative phenomena. indeed, there is limited understanding of the degree to which monkeys show imitative behaviour. in adult monkeys, mirror neurons may enable the monkey to understand what another monkey is doing, or to recognize the other monkey's action. 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 are 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. neuropsychological studies looking at lesion areas that cause action knowledge, pantomime interpretation, and biological motion perception deficits have pointed to a causal link between the integrity of the inferior frontal gyrus and these behaviours. transcranial magnetic stimulation studies have confirmed this as well. these results indicate the activation in mirror neuron related areas are unlikely to be just epiphenomenal. a study published in april 2010 reports recordings from single neurons with mirror properties in the human brain. 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 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 performed an action but were inhibited when the participant saw 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, no one 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 that 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. another study has suggested that human beings don't necessarily have more mirror neurons than monkeys, but instead that there is a core set of mirror neurons used in action observation and execution. however, for other proposed functions of mirror neurons the mirror system may have the ability to recruit other areas of the brain when doing its auditory, somatosensory, and affective components. in rodents a number of studies have shown that rats and mice show signs of distress while witnessing another rodent receive footshocks. the group of christian keysers recorded from neurons while rats experienced pain or witnessed the pain of others, and has revealed the presence of pain mirror neurons in the rat's anterior cingulate cortex, i.e. neurons that respond both while an animal experiences pain and while witnessing the pain of others. deactivating this region of the cingulate cortex led to reduced emotional contagion in the rats, so that observer rats showed reduced distress while witnessing another rat experience pain. the homologous part of the anterior cingulate cortex has been associated with empathy for pain in humans, suggesting a homology between the systems involved in emotional contagion in rodents and empathy/emotional contagion for pain in humans. doubts concerning mirror neurons although many in the scientific community have expressed excitement about the discovery of mirror neurons, there are scientists who have expressed doubts about both the existence and role of mirror neurons in humans. according to scientists such as hickok, pascolo, and dinstein, it is not clear 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. in 2008, ilan dinstein et al. 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 scientists have argued that the measurements of neuron fire delay seem not to be compatible with standard reaction times, and pointed out that nobody has reported that an interruption of the motor areas in f5 would produce a decrease in action recognition. (critics of this argument have replied that these authors have missed human neuropsychological and tms studies reporting disruption of these areas do indeed cause action deficits without affecting other kinds of perception.) in 2009, lingnau et al. carried out an experiment in which they compared motor acts that were first observed and then executed to