A mirror neuron is characterized by its activation both when an animal performs an action and when it observes the same action executed by another. This phenomenon implies that the neuron "mirrors" the observed behavior, as if the observer were performing the action itself. While not always physiologically distinct from other neuronal types, mirror neurons are primarily identified by their unique response patterns. These neurons have been directly observed in humans, other primates, and birds.
In humans, neural activity indicative of mirror neuron function has been identified across several brain regions, including the premotor cortex, supplementary motor area, primary somatosensory cortex, and inferior parietal cortex. The precise role of this mirror system in human cognition remains a subject of extensive debate. Similarly, birds exhibit imitative resonance behaviors, with neurological data suggesting the existence of a mirroring system. Currently, no universally accepted neural or computational models adequately explain how mirror neuron activity contributes to cognitive functions.
The topic of mirror neurons continues to provoke considerable discussion within the scientific community. For instance, in 2014, Philosophical Transactions of the Royal Society B dedicated an entire special issue to research on mirror neurons. Some scholars hypothesize that mirror systems facilitate the simulation of observed actions, thereby contributing to theory of mind capabilities, while others link mirror neurons to linguistic aptitudes. Prominent neuroscientists, including Marco Iacoboni, have posited that human mirror neuron systems are instrumental in comprehending the actions and intentions of others. Furthermore, Iacoboni has contended that mirror neurons form the neural substrate for human emotional capacities, such as empathy.
Discovery
During the 1980s and 1990s, neurophysiologists Giacomo Rizzolatti, Giuseppe Di Pellegrino, Luciano Fadiga, Leonardo Fogassi, and Vittorio Gallese, affiliated with the University of Parma, conducted studies involving electrode placement in the ventral premotor cortex of macaque monkeys. Their objective was to investigate neurons specialized in governing hand and mouth movements, such as grasping and manipulating objects. In these experiments, monkeys were permitted to retrieve food items, while recordings from individual neurons in their brains measured responses to specific movements. The researchers observed that certain neurons activated both when the monkey watched a person retrieve food and when the monkey performed the same action. This initial discovery was submitted to Nature but faced rejection due to a perceived "lack of general interest" before ultimately being published in a less prominent journal.
Subsequently, the same research group published an additional empirical study, which explored the mirror-neuron system's role in action recognition and posited that the human Broca's area is homologous to the monkey's ventral premotor cortex. While these initial publications documented mirror neurons responsive to hand actions, a later investigation by Pier Francesco Ferrari and colleagues identified mirror neurons that responded to mouth movements and facial expressions.
Subsequent experiments corroborated that approximately 10% of neurons within the monkey's inferior frontal and inferior parietal cortex exhibit "mirror" characteristics, demonstrating comparable responses to both executed and observed hand actions. In 2002, Christian Keysers and his team further reported that the mirror system in both humans and monkeys also activates in response to the auditory cues of actions.
Extensive research on mirror neurons has been published and validated, confirming their presence in both the inferior frontal and inferior parietal regions of the brain. Recent functional neuroimaging data strongly indicate the existence of analogous mirror neuron systems in humans. Researchers have pinpointed specific brain regions that activate during both the performance and observation of actions. These identified regions include those previously observed in macaque monkeys. Nevertheless, functional magnetic resonance imaging (fMRI), which allows for whole-brain examination, suggests that a considerably broader network of brain areas exhibits mirror properties in humans than initially hypothesized. These supplementary regions encompass the somatosensory cortex and are believed to facilitate the observer's subjective experience of performing the observed movement.
Origin
A prevalent implicit assumption posits that the mirroring function of mirror neurons is primarily attributable to heritable genetic determinants, and that this genetic predisposition evolved to facilitate action comprehension. Conversely, several theoretical frameworks contend that mirror neurons may arise solely from learned associations, such as Hebbian theory, associative learning theory, and canalization. An alternative perspective elucidates neurophysiological processes within particular interpersonal dynamics where mirror neurons are observed. These specific interpersonal dynamics align with the mother-fetus neurocognitive model, wherein localized neuronal oscillations synchronize via interference with the maternal heart's low-frequency electromagnetic field.
In Monkeys
The macaque monkey was the initial animal model where researchers conducted individual studies of mirror neurons. Within these primates, mirror neurons are localized to the inferior frontal gyrus (specifically region F5) and the inferior parietal lobule.
Mirror neurons are hypothesized to facilitate the comprehension of actions performed by other animals. For instance, a mirror neuron activated when a monkey tears paper also exhibits activity when the monkey observes a human tearing paper or perceives the sound of paper tearing (even in the absence of visual stimuli). Such characteristics suggest to researchers that mirror neurons encode abstract action concepts, such as 'ripping paper,' irrespective of whether the action is executed by the monkey itself or by another organism.
The precise function of mirror neurons in macaques is still undetermined. Adult macaques do not appear to acquire skills through imitation. However, recent studies by Ferrari and colleagues indicate that infant macaques can mimic human facial movements, albeit exclusively during their neonatal stage and within a restricted developmental period. Although not yet empirically substantiated, it has been posited that mirror neurons underlie this imitative behavior and other related phenomena. Overall, the extent of imitative behavior exhibited by monkeys remains poorly understood.
In adult monkeys, mirror neurons might facilitate the comprehension of another monkey's actions or the recognition of specific behaviors.
In Rodents
Numerous investigations have demonstrated that rats and mice exhibit indicators of distress when observing a conspecific receiving footshocks. Christian Keysers's research team recorded neuronal activity in rats experiencing pain or observing pain in others, thereby identifying 'pain mirror neurons' within the rat's anterior cingulate cortex. These neurons respond both during an animal's own pain experience and when witnessing the pain of others. Inactivating this specific cingulate cortex region resulted in diminished emotional contagion among rats, with observer rats displaying less distress when witnessing another rat's pain. The analogous anterior cingulate cortex in humans has been linked to empathy for pain, implying a homologous relationship between the neural systems underlying emotional contagion in rodents and empathy/emotional contagion for pain in humans.
In Humans
Direct investigation of individual neurons in the human brain is typically unfeasible; consequently, most evidence supporting the existence of mirror neurons in humans is indirect. Functional magnetic resonance imaging (fMRI) studies have demonstrated activation in the human inferior frontal cortex and superior parietal lobe both during action execution and during the observation of another individual performing an action. These brain regions are hypothesized to harbor mirror neurons and are collectively termed the human mirror neuron system. Further recent experiments, utilizing fMRI scans of individual participants, have revealed increased activity across extensive areas comprising multiple fMRI voxels during both the observation and execution of actions.
Neuropsychological investigations examining lesion sites associated with deficits in action knowledge, pantomime interpretation, and biological motion perception have indicated a causal relationship between the structural integrity of the inferior frontal gyrus and these specific behaviors. Transcranial magnetic stimulation (TMS) studies have corroborated these findings. Collectively, these findings suggest that activation in mirror neuron-related areas is unlikely to be merely epiphenomenal.
A study published in April 2010 documented the presence of single neurons exhibiting mirror properties within the human brain. Mukamel et al. (Current Biology, 2010) conducted recordings from the brains of 21 patients undergoing treatment for intractable epilepsy at Ronald Reagan UCLA Medical Center. These patients had previously received intracranial depth electrodes for the purpose of identifying seizure foci for potential surgical intervention. The placement of these electrodes was determined solely by clinical criteria; however, researchers, with patient consent, utilized the same electrodes to conduct their research. The investigators identified a limited number of neurons that exhibited peak activity or fired both when an individual performed a task and when they observed the same task. Conversely, other neurons displayed anti-mirror characteristics, responding when the participant executed an action but becoming inhibited upon observing that action.
The identified mirror neurons were situated in the supplementary motor area and medial temporal cortex; other brain regions were not sampled in this investigation. For pragmatic reasons, these locations differ from those where mirror neurons have been recorded in monkeys. Researchers in Parma focused on the ventral premotor cortex and the associated inferior parietal lobe, areas where epilepsy is uncommon, thus precluding routine single-cell recordings in humans. Conversely, the supplementary motor area or the medial temporal lobe in monkeys has not yet been investigated for mirror neurons. Consequently, this evidence does not imply distinct mirror neuron locations in humans and monkeys. Instead, it suggests that both species may possess mirror neurons in the ventral premotor cortex and inferior parietal lobe (where they have been recorded in monkeys), as well as in the supplementary motor areas and medial temporal lobe (where they have been recorded in humans). This interpretation is further supported by detailed human fMRI analyses, which indicate activity consistent with mirror neuron presence across all these regions.
Another study posits that humans do not necessarily possess a greater number of mirror neurons than monkeys; rather, a core set of mirror neurons is employed for action observation and execution. Nevertheless, for other proposed functions of mirror neurons, the mirror system may be capable of recruiting additional brain areas to process its auditory, somatosensory, and affective components.
Development
Data from human infants, obtained through eye-tracking measures, indicate that the mirror neuron system develops prior to 12 months of age and may facilitate infants' comprehension of others' actions. A key inquiry addresses how mirror neurons acquire their mirror properties. Two closely related models propose that mirror neurons are trained through Hebbian or associative learning. However, if premotor neurons require action-based training to develop mirror properties, it remains unclear how neonates can mimic the facial gestures of another person (imitation of unseen actions), as suggested by the work of Meltzoff and Moore. One hypothesis suggests that the visual perception of tongue protrusion triggers an innate releasing mechanism in neonates. Careful analysis indicates that the 'imitation' of this singular gesture might account for nearly all reported instances of facial mimicry by newborn infants.
Possible functions
Understanding Intentions
Numerous studies associate mirror neurons with the comprehension of 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 responsible for integrating sensory information. The monkeys observed an experimenter either grasping an apple and bringing it to his mouth or grasping an object and placing it into a cup.
- Specifically, 15 mirror neurons fired vigorously when the monkey observed the "grasp-to-eat" motion but remained inactive during the "grasp-to-place" condition.
- Conversely, for 4 other mirror neurons, the opposite pattern was observed: they activated in response to the experimenter ultimately placing the apple in the cup but not to the act of eating it.
Neuron activity was solely determined by the action type, rather than the kinematic force employed by models manipulating objects. Furthermore, a notable observation was the neuronal firing preceding the monkey's observation of the human model initiating the subsequent motor act, such as bringing an object to the mouth or placing it into a cup. Consequently, inferior parietal lobule (IPL) neurons encode identical actions, like grasping, distinctly based on the ultimate objective of the broader action sequence. This mechanism potentially provides a neural foundation for anticipating others' subsequent actions and deducing their intentions.
The comprehension of intention can be delineated into distinct phases, including body perception and action identification. These phases correspond to specific brain regions; for instance, the perception of body parts and shapes is associated with the extrastriate and fusiform body areas. The mirror neuron system is instrumental in identifying and facilitating the action itself. Action comprehension operates across two distinct processing hierarchies: the mirror neuron system and the mentalizing system. While anticipated actions are predominantly processed by the mirror neuron system, unanticipated actions engage a combined processing effort from both the mirror neuron and mentalizing systems.
Facilitation of Learning
An additional hypothesized function of mirror neurons involves the facilitation of learning. Mirror neurons encode a concrete representation of an action, specifically the neural pattern that would activate if the observer were to perform the action. This mechanism enables the implicit internal simulation of observed actions, thereby accumulating personal motor programs for these actions and preparing for their subsequent reproduction. This process constitutes a form of implicit training. Consequently, observers can explicitly execute the action with enhanced agility and finesse. This phenomenon is attributed to associative learning processes, where the strength of a synaptic connection increases proportionally with its activation frequency.
Empathy
Researchers Stephanie Preston, Frans de Waal, Jean Decety, Vittorio Gallese, and Christian Keysers have independently posited the involvement of the mirror neuron system in empathy. Numerous experiments employing functional magnetic resonance imaging (fMRI), electroencephalography (EEG), and magnetoencephalography (MEG) have demonstrated activation in specific brain regions—notably the anterior insula, anterior cingulate cortex, and inferior frontal cortex—both during an individual's own emotional experience (e.g., disgust, happiness, pain) and when observing another person experiencing an emotion. David Freedberg and Vittorio Gallese have additionally proposed that this particular function of the mirror neuron system is critical for aesthetic experiences. However, a study by Soukayna Bekkali and Peter Enticott at Deakin University, which investigated mirror neuron activity in relation to empathy, produced divergent findings. Their analysis of the study's data led to two primary conclusions regarding motor and emotional empathy. Firstly, no correlation was found between motor empathy and mirror neuron activity. Secondly, only weak evidence supported mirror neuron activity in the inferior frontal gyrus (IFG), and no evidence linked emotional empathy to mirror neurons in critical brain regions such as the inferior parietal lobule (IPL). Consequently, a definitive conclusion regarding the precise role of mirror neurons in empathy and their essentiality for human empathy remains elusive. It is important to note that these brain regions differ from those involved in mirroring hand actions, and mirror neurons specifically associated with emotional states or empathy have not yet been identified in monkeys.
A recent 2022 study presented sixteen distinct hand actions for each experimental assignment. Each assignment depicted both an activity word phrase and an intended word phrase. Hand actions were presented in "trials," with each action introduced twice. One presentation involved a matching phrase, while the other featured a misleading word phrase. The action words were consistently presented as two-to-three-word phrases, each commencing with "to." For example, "to point" represented an action, and "to spin" represented an intention.
Participants were required to determine if the presented word phrase accurately corresponded to the depicted action or intended word. Responses to the word phrase were mandated within 3000 milliseconds, with a 1000-millisecond black screen interval separating each image. The black screen served to provide sufficient temporal separation between participant responses. Participants indicated their yes/no responses by pressing either the "x" or "m" key on the keyboard.
Research by Christian Keysers and colleagues at the Social Brain Lab indicates a direct correlation between self-reported empathy levels and heightened activation within the mirror system, encompassing both hand actions and emotional responses. This suggests a strong link between the mirror system and empathic capacity. Furthermore, observations reveal that the human mirror system is not merely a passive responder to observed actions but is actively modulated by the observer's mindset. The connection between mirror neurons and empathetic engagement has also been noted in the context of patient care.
Investigations in rats have identified pain-responsive mirror neurons within the anterior cingulate cortex, which activate during both direct pain experience and the observation of pain in others. Inhibiting this cortical region in rodents reduces emotional contagion and decreases aversion to inflicting harm, thereby providing causal evidence for a link between pain mirror neurons, emotional contagion, and prosocial behavior—phenomena associated with empathy. The observed association between brain activity in the homologous human region and individual differences in empathy suggests that a comparable mechanism may operate across mammalian species.
Human Self-Awareness
V. S. Ramachandran has theorized that mirror neurons could constitute the neurological foundation of human self-awareness. In a 2009 essay for the Edge Foundation, Ramachandran elucidated his theory, stating: "I also speculated that these neurons can not only help simulate other people's behavior but can be turned 'inward'—as it were—to create second-order representations or meta-representations of your own earlier brain processes. This could be the neural basis of introspection, and of the reciprocity of self awareness and other awareness. There is obviously a chicken-or-egg question here as to which evolved first, but... The main point is that the two co-evolved, mutually enriching each other to create the mature representation of self that characterizes modern humans."
Language
Functional MRI studies in humans have identified regions within the inferior frontal cortex, near Broca's area—a proposed language center—that are homologous to the monkey mirror neuron system. This discovery has prompted the hypothesis that human language may have evolved from a gesture-based performance and comprehension system mediated by mirror neurons. Mirror neurons are posited to facilitate action understanding, imitation learning, and the simulation of others' behaviors. Further support for this hypothesis stems from cytoarchitectonic homologies observed between monkey premotor area F5 and human Broca's area. Furthermore, the rate of vocabulary acquisition in children correlates with their capacity to vocally mimic non-words, thereby facilitating the learning of new word pronunciations. This form of speech repetition is automatic, rapid, and distinct from speech perception in the brain. Notably, vocal imitation can occur independently of comprehension, as evidenced in phenomena like speech shadowing and echolalia.
Additional evidence supporting this connection emerges from a recent fMRI study that measured brain activity in two participants engaged in a game of charades, communicating words via hand gestures—a modality proposed by some as an evolutionary precursor to human language. Granger Causality analysis of the data demonstrated that the observer's mirror-neuron system accurately mirrored the activity patterns in the sender's motor system. This finding reinforces the proposition that the motor concepts associated with words are effectively transmitted between brains through the mirror system.
Conversely, the mirror neuron system appears inherently insufficient to account for syntax, a defining characteristic of human languages. This is because the hierarchical recursive structure fundamental to syntax is linearized into sequential phonemes, rendering the underlying recursive organization inaccessible to direct sensory detection by the mirror system.
Imitation
This term typically describes instances where an individual, upon observing a body movement, inadvertently replicates it or modifies their own execution of a similar movement. Overt execution of matching responses is seldom observed in automatic imitation. Instead, its effects are generally manifested in reaction time variations, rather than accuracy, and in discrepancies between compatible and incompatible experimental trials. Research indicates that automatic imitation, a covert form of imitative behavior, is distinct from spatial compatibility. Furthermore, findings suggest that while automatic imitation is susceptible to input modulation by attentional mechanisms and output modulation by inhibitory processes, it is primarily mediated by learned, long-term sensorimotor associations that are not directly modifiable by intentional processes. A significant number of researchers posit that the mirror neuron system mediates automatic imitation. Moreover, evidence demonstrates that postural control is compromised when individuals listen to sentences describing actions performed by others. For instance, individuals exhibit diminished postural stability when tasked with maintaining posture while simultaneously hearing action-related sentences, such as "I get up, put on my slippers, go to the bathroom." This phenomenon might be attributable to the activation of motor cortex regions during action perception, which mirrors the activation observed when an individual performs the same action, suggesting involvement of the mirror neuron system.
Motor Mimicry
In contrast to automatic imitation, motor mimicry is observed in (1) naturalistic social contexts and (2) through assessments of action frequency within a session, rather than measures of speed or accuracy within individual trials.
Integrating research on motor mimicry and automatic imitation could potentially reveal compelling evidence that these phenomena rely on shared psychological and neural processes. However, preliminary evidence stems from studies demonstrating that social priming exerts comparable effects on motor mimicry.
Nevertheless, the observed similarities among automatic imitation, mirror effects, and motor mimicry have prompted some researchers to hypothesize that automatic imitation is mediated by the mirror neuron system and represents a rigorously controlled laboratory analogue of motor mimicry observed in naturalistic social environments. If this hypothesis holds true, automatic imitation could serve as a valuable tool for investigating the mirror neuron system's contribution to cognitive functioning and how motor mimicry fosters prosocial attitudes and behaviors.
Meta-analyses of human imitation studies indicate sufficient evidence of mirror system activation during imitation to suggest probable mirror neuron involvement, despite the absence of published studies recording the activity of individual neurons. However, this activation is likely insufficient for comprehensive motor imitation. Research demonstrates that regions within the frontal and parietal lobes, extending beyond the classical mirror system, are also activated during imitation. This implies that other brain regions, in conjunction with the mirror system, are critical for imitative behaviors.
Autism
It has been hypothesized that dysfunctions within the mirror neuron system may contribute to cognitive disorders, particularly autism. Nevertheless, the link between mirror neuron dysfunction and autism remains tentative, and the precise relationship between mirror neurons and many key characteristics of autism requires further elucidation.
Certain researchers propose a correlation between mirror neuron system deficiencies and autism spectrum disorder. Electroencephalography (EEG) recordings from motor regions exhibit suppression when an individual observes another person's movement, a phenomenon potentially linked to the mirror neuron system. This correlation can also be quantified by combining eye-movement tracking of biological motions with EEG recordings to derive a mu suppression index. Children diagnosed with autism demonstrated reduced mu suppression. While several research teams have replicated these observations, other investigations have not substantiated evidence of a dysfunctional mirror neuron system in autism. In 2008, Oberman et al. published a study presenting contradictory EEG findings. Oberman and Ramachandran observed typical mu-suppression in response to familiar stimuli but not to unfamiliar stimuli, leading them to infer that the mirror neuron system in children with Autism Spectrum Disorder (ASD) was functional yet less sensitive compared to neurotypical children. Given the inconsistent evidence from mu-wave suppression experiments, Patricia Churchland has advised against using mu-wave suppression results as a definitive indicator for assessing mirror neuron system performance. More recent scholarly inquiry suggests that mirror neurons may not have a significant role in autism.
...conclusive evidence for a fundamental mirror system deficit in autism remains elusive. Behavioral investigations have demonstrated that individuals with autism exhibit a proficient comprehension of action objectives. Moreover, two distinct neuroimaging studies have indicated typical functioning of the parietal component of the mirror system in autistic individuals.
Anatomical distinctions have been identified in brain regions associated with mirror neurons in adults diagnosed with autism spectrum disorders, in contrast to neurotypical adults. These cortical areas exhibited reduced thickness, and the extent of this thinning correlated with the severity of autism symptoms, a relationship predominantly confined to these specific brain regions. Consequently, certain researchers hypothesize that autism originates from impairments within the mirror neuron system, resulting in deficits in social skills, imitation, empathy, and theory of mind.
Numerous researchers have asserted that the "broken mirrors" theory of autism is excessively simplistic, contending that mirror neurons alone cannot account for the observed variations in individuals with autism. Primarily, as previously indicated, none of the aforementioned studies directly measured mirror neuron activity; thus, fMRI activity or EEG rhythm suppression do not unequivocally serve as indices for mirror neurons. Dinstein and colleagues, utilizing fMRI, observed typical mirror neuron activity in individuals with autism. Furthermore, deficits in intention understanding, action comprehension, and biological motion perception—functions considered central to mirror neurons—are not consistently present in autistic individuals or are contingent on specific tasks. Presently, a consensus suggests that an absolute dysfunction of the mirror system is unlikely to be the sole underlying cause of autism. Instead, "additional research needs to be done, and more caution should be used when reaching out to the media."
A 2010 study concluded that autistic individuals do not manifest mirror neuron dysfunction, though the limited sample size constrains the generalizability of these findings. A subsequent review contended that insufficient neurological evidence exists to substantiate the "broken-mirror theory" of autism.
Theory of Mind
Within the philosophy of mind, mirror neurons have emerged as a central tenet for simulation theorists addressing the concept of "theory of mind." "Theory of mind" denotes the cognitive capacity to deduce another individual's mental states, such as beliefs and desires, based on their experiences or observed behavior.
Multiple competing models endeavor to elucidate the mechanisms underlying theory of mind; among these, simulation theory is particularly pertinent to mirror neurons. Simulation theory posits that theory of mind is accessible because individuals subconsciously empathize with observed persons and, by accounting for pertinent distinctions, project their own desires and beliefs onto that scenario. Mirror neurons have been conceptualized as the neural substrate facilitating this simulation of others, thereby enhancing comprehension. Consequently, their discovery has been interpreted by some as corroborating simulation theory, which predated the identification of mirror neurons by approximately a decade. More recent perspectives view Theory of Mind and Simulation as complementary systems, each exhibiting distinct developmental trajectories.
A 2015 neuronal-level investigation by Keren Haroush and Ziv Williams, involving primates engaged in an iterated prisoner's dilemma game, identified specific neurons within the anterior cingulate cortex. These neurons exhibited selective prediction of an opponent's unrevealed decisions or internal mental states. Termed "other-predictive neurons," these cells distinguished between self and other-initiated decisions and demonstrated unique sensitivity to social contexts, yet they did not encode observed opponent actions or reward reception. Consequently, these cingulate cells could significantly augment mirror neuron function by furnishing supplementary, non-observable, or previously unknown data regarding other social agents.
Sex-Based Differences
Recent research by Yawei Cheng, employing diverse neurophysiological assessments such as magnetoencephalography (MEG), spinal reflex excitability, and electroencephalography, has documented a gender-based disparity within the human mirror neuron system, wherein female participants consistently display more robust motor resonance compared to their male counterparts.
Further investigation corroborated sex-based distinctions in mirror neuron mechanisms, revealing heightened empathetic capacity in females compared to males. During face-to-face emotional social interactions, females demonstrated superior emotional perspective-taking abilities. Nevertheless, the study also indicated that all participants exhibited comparable proficiency in recognizing others' emotions, with no significant divergence between male and female subjects.
Sleep Paralysis
Baland Jalal and V. S. Ramachandran have posited that the mirror neuron system plays a crucial role in the genesis of intruder hallucinations and out-of-body experiences during episodes of sleep paralysis. Their theory suggests that sleep paralysis induces a disinhibition of the mirror neuron system, thereby facilitating the emergence of hallucinations depicting human-like shadowy figures. This proposed mirror neuron disinhibition is attributed to the deafferentation of sensory information during sleep paralysis. The researchers further proposed a method to empirically evaluate their hypothesis concerning the mirror neuron system's involvement:
"These ideas could be explored using neuroimaging, to examine the selective activation of brain regions associated with mirror neuron activity, when the individual is hallucinating an intruder or having an out-of-body experience during sleep paralysis ."
Mirror Neuron Function, Psychosis, and Empathy in Schizophrenia
Contemporary research, utilizing mu-wave suppression as a metric, indicates a positive correlation between mirror neuron activity and psychotic symptoms; specifically, elevated mu suppression and mirror neuron activity were most pronounced in individuals exhibiting greater severity of psychotic symptoms. Investigators concluded that "higher mirror neuron activity may be the underpinning of schizophrenia sensory gating deficits and may contribute to sensory misattributions particularly in response to socially relevant stimuli, and be a putative mechanism for delusions and hallucinations."
Skepticism Regarding Mirror Neurons
While the discovery of mirror neurons initially generated considerable enthusiasm within certain segments of the scientific community, other researchers have voiced reservations concerning both their existence and their precise function in humans. Current perspectives often suggest that the significance attributed to these "mirror neurons" may be substantially exaggerated. According to scholars including Hickok, Pascolo, and Dinstein, it remains ambiguous whether mirror neurons constitute a genuinely distinct cellular class—rather than an infrequent observation in cells serving alternative functions—or if mirror activity represents a unique response type as opposed to a mere byproduct of generalized motor system facilitation.
In 2008, Dinstein et al. contended that the initial analyses lacked persuasiveness, attributing this to their reliance on qualitative descriptions of individual cellular characteristics and their failure to consider the limited quantity of highly mirror-selective neurons within motor regions. Furthermore, other researchers have posited that observed neuronal firing delays appear inconsistent with typical reaction times, noting the absence of reports indicating that disrupting motor areas in F5 diminishes action recognition capabilities. Conversely, proponents of the mirror neuron theory have countered these arguments by highlighting human neuropsychological and transcranial magnetic stimulation (TMS) studies, which demonstrate that disruptions in these specific areas do indeed lead to action deficits without impairing other perceptual modalities.
In 2009, Lingnau et al. conducted an experiment comparing motor actions that were initially observed and subsequently performed with those that were first executed and then observed. Their findings suggested a notable asymmetry between these two processes, leading them to conclude that mirror neurons are not present in humans. Specifically, they asserted, "Crucially, we observed no evidence of adaptation for motor acts that were initially executed and subsequently observed. The absence of cross-modal adaptation for executed and observed motor acts contradicts the fundamental premise of mirror neuron theory, which posits that action recognition and comprehension rely on motor simulation." Nevertheless, in the same year, Kilner et al. demonstrated that when goal-directed actions serve as stimuli, both the inferior parietal lobule (IPL) and premotor regions exhibit the repetition suppression effect between observation and execution, consistent with predictions from mirror neuron theory.
In 2009, Greg Hickok presented a comprehensive critique challenging the assertion that mirror neurons contribute to action understanding, titled "Eight Problems for the Mirror Neuron Theory of Action Understanding in Monkeys and Humans." He concluded, stating, "The initial hypothesis that these cells form the basis of action understanding is an intriguing and ostensibly plausible concept. However, despite its broad acceptance, this proposition has not been sufficiently investigated in monkeys, and in humans, substantial empirical evidence, derived from physiological and neuropsychological (double) dissociations, refutes this claim."
Vladimir Kosonogov identified an additional contradiction. Advocates of the mirror neuron theory of action understanding hypothesize that mirror neurons encode the objectives of others' actions, given their activation when an observed action is goal-directed. Yet, mirror neurons are exclusively activated when the observed action is goal-directed, such as an object-directed action or a communicative gesture, both inherently possessing an objective. This raises questions about how these neurons "recognize" a specific action as goal-directed and at what point in their activation they discern the presence or absence of a movement's objective. Kosonogov posits that the mirror neuron system can only become active subsequent to other brain structures attributing a goal to the observed action.
Neurophilosophers, including Patricia Churchland, have articulated both scientific and philosophical objections to the theory positing that mirror neurons are responsible for comprehending the intentions of others. In Chapter 5 of her 2011 publication, Braintrust, Churchland highlights that the assertion regarding mirror neurons' role in intention understanding (via simulating observed actions) rests on assumptions obscured by unresolved philosophical complexities. She contends that intentions are comprehended, or encoded, at a more intricate level of neural activity than that attributable to individual neurons. Churchland explicitly states, "A neuron, despite its computational complexity, remains merely a neuron. It does not function as an intelligent homunculus. For a neural network to represent something intricate, such as an intention [to insult], it necessitates appropriate input and correct placement within the neural circuitry."
Cecilia Heyes has proposed a theory suggesting that mirror neurons are a consequence of associative learning rather than an outcome of evolutionary adaptation. She posits that human mirror neurons arise from social interaction, not as an evolutionary adaptation specifically for action understanding. Specifically, Heyes refutes V.S. Ramachandran's theory that mirror neurons constituted "the primary impetus behind the significant advancement in human evolution."
Academic References
References
Scholarly Notes
- Hickok G, Poeppel D. "Talking Brains".
News and Views on the Neural Organization of Language
Ramachandran VS. "The Neurons That Shaped Civilization". TED TalksThomas B (2012). "What Is So Special About Mirror Neurons?". Scientific American Guest Blog. This article provides an overview of prominent research approaches, drawing from interviews with Iacoboni, Hickok, Heyes, and Gallese."Mirror Neurons". NOVA scienceNOW. January 2005.Source: TORIma Academy Archive