The term mirror neurons refers to neurons that show spike activity in response to the performance of an action and to the perception of the same type of action in an observed subject. These neurons, first found in the brain of macaque monkeys, are regarded as first evidence for a functional overlap between action and perception on a neuronal level.
Mirror neurons were first described in the1990s by Giacomo Rizzolatti’s research group at the University of Parma in Italy. Anecdotal evidence given by members of the group indicates that the discovery of mirror neurons was due to chance. Neurophysiologists were investigating neurons expected to respond to hand and mouth actions, such as manipulating objects or picking up food items for eating. Neuronal activity was measured from electrodes placed in the ventral premotor cortex of macaque monkeys. When the experimenter took a bite from his sandwich during the experiment, neurons from the macaque responded unexpectedly, indicating activity owing to the mere observation of the investigated action, this time performed by the human.
The first mirror neurons were identified in the F5 area of the inferior frontal gyrus of the macaque brain; later studies also identified mirror neurons in the inferior parietal lobule. Following their findings, a multitude of research groups engaged in the study of mirror neurons. Neurons with mirror properties were found in the premotor cortex, classically ascribed to the motor system, and in parietal areas involved in the processing of perceived information. Following the observation of neurons responding to the perception and performance of manual actions, neurons responding to actions involving the mouth, face, and body were investigated. Today, various types of mirror neurons have been described, including neurons that respond selectively to actions observed in extra or intrapersonal space, neurons that respond not to visual but to acoustic stimuli, and neurons that link actions to related acts of communication. Mirror neurons vary in their level of congruency; highly specialized neurons respond only to one type of action, whereas others generalize over actions that resemble each other, have the same goal, or follow each other in typical sequences.
Results from studies on mirror neurons provided strong support for theoretical perspectives that had so far lacked neuronal evidence. The overlap between action and perception processing in the brain had been addressed over decades by Wolfgang Prinz and colleagues through the common coding principle and the ideomotor theory; the concepts of embodiment and situatedness of cognitive processes are related to this work. These perspectives, together with the discovery of mirror neurons in monkeys, provided a strong motivation for the quest for neural structures with similar properties in the human brain. Studies investigating human brain areas with mirror like properties chiefly used neuroimaging techniques (i.e., functional magnetic resonance imaging [fMRI], positron-emission tomography [PET]), electroencephalography (EEG), transcranial magnetic stimulation (TMS), as well as behavioral paradigms. Areas with mirror properties were found in the ventral and dorsal premotor cortex and in parts of the parietal cortex, including the inferior parietal lobe, the superior parietal lobe, and the superior parietal sulcus. Numerous studies have corroborated that these brain regions are involved in and modulated by a large number of cognitive tasks, as well as social functions. Their activity has been claimed to underlie the planning and understanding of actions, imitation, language comprehension, empathy, social learning, and cultural transmission. Malfunctioning in these brain regions has been claimed to play a role in the emergence of disorders like autism. Because of their broad functional involvement, it has been argued that the term mirror system was misleading, and other terms have been suggested instead, such as action observation network (AON).
Neurocognitive research, often applying sports and dance-related paradigms, has shown that the relevant brain regions are more strongly activated during the observation of biological and especially human agents compared to artificial ones (e.g., robots) and that the degree of activation is modulated by the observer’s capability and experience of performing the observed actions. Furthermore, there is evidence that individual regions within the network are specialized on different aspects of action perception and performance. Beatriz Calvo Merino and colleagues studied brain responses to movement observation in capoeira and ballet experts. They found that the activation of relevant brain areas was more distinctive while the experts watched movements from their own discipline, and specifically movements belonging to their own active movement repertoire, compared to movements they had frequently watched but not performed themselves. Salvatore Aglioti and colleagues showed that expert basketball players performed better than experienced observers in predicting the success of a free throw during very early stages of the movement, and that their motor system and hand muscles were activated in a movement-specific way. Using a dance-training paradigm, Emily Cross and colleagues showed that the superior temporal cortex preferentially responded to the presence of a human model, whereas the ventral premotor cortex responded specifically to motor familiarity of the observed movement.
In recent years, critical views have been expressed regarding the interpretation of mirror neuron studies, specifically the existence and function of mirror neurons in the human brain. It has been argued that neurons with mirror properties might not represent a distinct class of cells but rather neurons in the motor system that have developed mirror properties as an artifact, additional to their original and more relevant functions. Based on empirical findings, Cecilia Heyes proposed that human mirror neurons might emerge as a byproduct of associative learning and social interaction. Gregory Hickok strongly argued against the claim that mirror neurons were crucial for action understanding, emphasizing the lack of evidence of this function in monkeys in which the neurons had been described. Arthur Glenberg and Patricia Churchland both pointed out that most of the phenomena the human mirror neuron system (MNS)has been associated with (including understanding intentions and actions, language comprehension, imitation, and disorders like autism) are not yet fully understood and that more research is needed to substantiate such claims.
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