Mirror Neuron System: function, dysfunction, and rehabilitation proposals

Definition of the mirror neuron system

Neuroanatomy of the mirror neuron motor/imitation system:

There are two main neural networks that make up the mirror neuron system (Cattaneo & Rizzolatti, 2008): one formed by areas of the parietal lobe and the premotor cortex, as well as the caudal part of the inferior frontal gyrus, and another formed by the insula and the anterior medial frontal cortex.

We will now focus on the first system, which involves learning based on observation and imitation. The anatomical organization of the first system follows a somatotopic hierarchy in the ventral premotor cortex: motor acts of the legs are located dorsally, facial behaviors ventrally, and hand movements in between. Proximal motor acts (moving the hand toward a point) are represented dorsally, while the simple act of grasping produces ventral activity in the premotor cortex. The observation of motor acts also produces differential activation in the parietal cortex.

Observing transitive acts produces activation of the intraparietal sulcus, as well as activation of the parietal convexity adjacent to that area. Observing intransitive acts, whether symbolic acts or mimed repetition, produces specific activity in the posterior part of the supramarginal gyrus, extending to the angular gyrus. Finally, observing acts performed with tools specifically activates the most rostral part of the supramarginal gyrus.

The mirror neuron system produces an evocation of the observed motor act within the observer’s own premotor cortex. This activity is coordinated at the same time in the parietal lobe. The sequence of observation processes must be differentiated in order to correctly define the neuroanatomy of the first system described (frontoparietal). In this system, we are talking about observed behaviors that involve visuomotor priming for carrying out, or not carrying out, an action.

We will therefore exclude the concept of motor-visual priming, which involves predicting consequences while planning actions. The activity of this frontoparietal system (and this is the key point) occurs when the behavior potentially exists in the subject’s repertoire. In other words, a human observing a dog barking does not activate the premotor and parietal areas, since that behavioral repertoire does not exist in their cortex.

In addition, the system’s activity is proportional to the observer’s experience with the behavior they are observing. The functional connectivity of the frontoparietal mirror neuron system follows a sequence during observation. The sequence originates in the occipital lobe, where the main features of the observed stimuli are registered. All the information is sent to the integration areas in a series of steps lasting 20 to 60 ms, in this order: first the superior temporal sulcus, then the inferior parietal lobe, then the inferior frontal gyrus, and finally the primary motor cortex.

Iacoboni et al. (1999) propose that the frontal areas that are activated involve a computation of the goals to be achieved, while parietal activity corresponds to the activation of the motor representations of the acts being observed. However, Iacoboni’s group makes a functional distinction in the system’s neural activity, focusing on the pars opercularis of the left inferior frontal gyrus. For them, the dorsal part of the pars is activated both when the act is observed and when it is imitated, but ventral activity only occurs when it is imitated. In fact, Iacoboni et al. (2005) functionally analyzed the activations described above.

For them, the mirror neuron system is essential for learning through imitation. The activation sequence would be completed as follows:

• (i) first, the superior temporal sulcus is activated, where the representations of the observed movements are found (ventral pathway).
• From there, (ii) action goals are encoded through the frontoparietal system, in which the dorsal prefrontal cortex computes the different aspects of the action, such as the goal itself or its meaning, storing this information, sending information to the parietal lobe, and correcting spatial computations.

The Anterior Cingulate Cortex: Executive Control and Emotional Control

This efferent information would then be sent from the frontoparietal mirror neuron system, through the pars opercularis, back to the superior temporal sulcus. At this point, a computation of the fit between the predicted consequences of the planned imitative action and the visual description of the observed action would take place. In short, the frontoparietal mirror neuron system is a feedback-based learning system.

In fact, what is transferred from the visual areas to the motor areas is not a detailed motor program but a prototype of the action, a meaningful action processed in the pars opercularis of the inferior frontal gyrus, which then guides motor planning according to a precise, detailed representation of the observed action, represented in the superior temporal sulcus and the inferior parietal lobe. When the observed action is new, before the execution period, the frontoparietal mirror neuron system is activated, along with Brodmann area (BA) 46 and the anterior medial cortex.

This activation reflects an executive control mechanism, probably as part of Shallice’s supervisory mechanism, on which Baddeley (2000) based his working memory model. In our case, this system could involve a computation of top-down movement planning, in which working memory handles the observed content and plans the movement based on it, producing frontoparietal activity that corresponds to the mirror neuron mechanism.

The mirror neuron system should not be seen as a separate neural module but as an intrinsic mechanism underlying most areas related to motor movements. In fact, as we will see later, disruption of this system does not cause a selective deficit in focal lesions. Rather, the involvement of this system can be seen in developmental disorders of the nervous system and in frontal lobe lesions. Let’s look at the latter below.

Dependency and hierarchy

As mentioned, the mirror neuron system overlaps with other systems, and the control system is no exception, since it suppresses spontaneous imitation behaviors. Frontal lesions cause a series of deficits characterized by impulsive behaviors triggered by external stimuli. Imitative behavior is especially relevant to the mirror neuron system and can be part of “environmental dependency syndrome.” The condition usually results from a bilateral lesion, although it can also be due to a unilateral lesion, which is less common. Observing other people’s behavior can trigger activation of the premotor and parietal areas that depend on the mirror neuron system.

In healthy people, this activation does not show because the frontal lobe suppresses it. Damage to the frontal lobe destroys these mechanisms, turning potential acts into actual acts. Echopraxia is the forced, uncritical imitation of observed behaviors, usually with perseveration. Although it is usually associated with damage to the basal ganglia, it can also result from frontal damage, which causes disinhibition of the mirror neuron system.

Functions of the frontoparietal mirror neuron system

Imitation and learning

Imitation is a fundamental learning task that leads to the development of some basic social skills, especially the ability to identify gestures and postures, and allows the development of understanding of other people’s intentions.

These neurons fire when the subject performs goal-directed behaviors, but especially when they observe these behaviors in others, distinguishing between the different components of the action according to how relevant they are from the point of view of intention, even with objects that are not present.

It follows that mirror neurons handle not only content related to motor or visual patterns but also abstract content, both in terms of the sensory modality of the contingency (a meaningful sound) and in terms of elements that are not present or are abstract. In terms of learning, these are related to intention, a reality in which understanding other people’s motives plays an important role.

The integrated motor information has significant processing characteristics:

• processing of movement,
• of body parts,
• tracking of an external subject’s goal-directed action,
• etc.

The proximity to frontoparietal systems that support various types of sensorimotor integration suggests that the action coding implemented in the mirror neuron system is linked to some form of sensory integration. Imitation is one of the many forms of this type of integration. In this integration, the observer compares the information in the primary areas (visual inputs) with the observed behavior, as explained above.

The literature on imitation behavior stresses that an essential aspect in this area is distinguishing between various forms of imitation or contagion and true imitation, that is, adding something new to one’s own motor repertoire after observing others perform the action. This distinction can be seen at the neural level, separating the interactions between the mirror neuron system and prefrontal and parietal execution-preparation structures during imitation learning from the interaction between the mirror neuron system and the limbic system during emotional contagion. As we will discuss later, the mirror neuron system in autism probably also allows this distinction to be made, with one of the interaction systems more damaged than the other.

Memory: definition, types, exercises, and evaluation

Mirror neurons have individual properties:

They are activated in the imitated action, but also in the action being observed, even without imitating it. They have two levels of congruence:

• strict, in which the neurons are activated only by substantially identical actions and observations;
• and broad congruence, in which they are activated in response to observing an action that is not necessarily identical to the executed action but achieves the same goal.

The activation thresholds are defined by the logic of the action, not by the object or the distance of the action. These properties suggest that they handle abstract content from the observed actions. But how abstract is this coding? Highly abstract, as shown in experiments with prior “hiding” conditions, in which the neurons are activated based on an initial situation of presence or absence, distinguishing between situations.

There is sensory recognition of sound-producing actions (auditory inputs) in the mirror neuron system. This lays a foundation for understanding speech and language as a code that is learned, at least in the early stages, through physical and gestural imitation.

Functional hierarchy of the frontoparietal system in motor processing

As mentioned above, there is a functional hierarchy in the mirror neuron system when the subject observes a motor action in order to learn it. The basic levels of motor processing have been studied extensively. However, the mirror neuron system follows a hierarchy in which movement processing is high-level, producing computations between the consequences of the action and the goals.

To compute this knowledge, the components that make up the context of the action must be dissociated: first, the object itself that represents the goal. Existing studies were inconclusive until relatively recently. However, using neural suppression techniques such as transcranial magnetic stimulation, researchers have managed to dissociate high-level processing from purely kinematic processing. The goal object has been found to be identified in the anterior intraparietal sulcus (Hamilton & Grafton, 2006). Therefore, objects are processed differently even when the action is the same (for example, grasping). This dissociation also involves analyzing the expected consequences of the action, which sits at a higher level of the hierarchy than the previous one.

It’s very important to keep in mind that goal processing involves processing the movements needed to achieve that goal, but these aspects are processed at different levels, with the processing of the motor program (not its planning) at a lower level. Hamilton & Grafton (2007) showed that there is lateralization in the mirror neuron system that computes the consequences of action. They found that the consequences of an observed action are processed in the right inferior frontal gyrus and right inferior parietal lobe, as well as in the left postcentral sulcus and left anterior intraparietal sulcus.

Together, they have proposed a hierarchical model made up as follows: on the one hand, there is low-level (cognitive) processing that involves processing the motor pattern. The motor pattern is processed in a system that involves both a visual and a motor analysis of the action. The visual analysis takes place in lateral occipital areas, while the kinematic pattern is processed in the inferior frontal regions.

High-level processing, defined by an analysis of goals, takes place in a system involving two areas of the right hemisphere: the intraparietal lobe and, to a lesser extent, the inferior frontal gyrus. In this goal processing, goal objects are also processed in a lateralized way in the left inferior parietal cortex. Is there a neural hierarchy when the observed actions are executed? Yes, and the hierarchical level distinguishes the complexity of the actions, that is, whether they are simple, complex (made up of different steps), or driven by an intention. In this case, the lateralization of neural activity does not seem as evident.

There is evidence that the planning of simple acts takes place in the motor and premotor cortex, as well as in the left inferior parietal cortex. However, the right inferior parietal lobe seems to be involved in complex behaviors that require several steps, such as the Tower of London task (Newman et al., 2003). This area seems important for sending feedback about the consequences of the motor act and, together with the cerebellum, may compute corrections to movement in space or in planning.

Language

When imitating finger movements, activity increases in the rostral posterior parietal cortex and the inferior frontal gyrus, areas close to Broca’s area, which suggests that these mirror areas are involved in a phylogenetic language acquisition mechanism (Iacoboni & Dapretto, 2006).

This theory has been supported by several lines of evidence. First, a left lateralization of the mirror neuron system has been demonstrated. Second, the activation of the mirror neuron system in the macaque brain allows its areas to be mapped onto ours: the areas in the macaque would correspond to human BA 44, adjacent to Broca’s area. Based on the theory of semantic expression, which proposes that language is learned through a bottom-up process, and the motor theory of speech perception, which proposes that the target of speech analysis is the facial expressions associated with sounds rather than the sounds themselves, it has been found that the motor speech areas are activated during speech perception, and these coincide with the mirror neuron system.

It has also been found that processing linguistic material produces motor activation, and that the neural activity produced by processing linguistic material related to body parts and actions activates the somatotopic areas of the brain related to reading.

Social cognition and mirror neurons

Neuroanatomy of the limbic mirror neuron system

The second mirror system is the emotional one. As mentioned above, this system is involved in empathic behavior, but it doesn’t necessarily work separately from the first system, although this point will be discussed later. The mirror neuron system is also located in cortical areas that mediate emotional behavior. Observing someone else’s pain activates the cingulate cortex, the amygdala, and the insula. The insula is especially important in integrating sensory representations, both internal and external. It has an agranular structure and is cytoarchitectonically similar to the motor areas.

The insula therefore functions as a communication hub between the limbic system and the somatotopic cortical activation associated with pain, both one’s own and other people’s, which is the evolutionary basis of empathy. However, this is not the only basis. The empathy system would be organized as follows:

• First, there must be a hub in the system, the amygdala, which is necessary for emotional arousal in individuals.
• Second, the areas of emotional expression and regulation. As mentioned, the area of emotional expression based on body schemas is made up of two structures: first, the insula, which, as we said, is the center for integrating interoceptive information. Second, the cingulate cortex, which is subdivided as follows: in contrast to the classic division of the cingulate cortex into cognitive/dorsal and emotional/rostral processes (Posner et al., 2007), it has recently been shown that there is a division between emotional (interoceptive) expression in the dorsal anterior cingulate cortex and an emotion-regulating function in the rostral anterior cingulate cortex (Etkin et al., 2010), which is consistent with an anteroposterior orbitofrontal control system (mainly orbitofrontal-cingulate-amygdala).
• Third, the high-level processing hub made up of the mirror neuron system. This system is formed by the insula and the anterior medial frontal cortex. In fact, the mirror neuron system of the insula overlaps with the interoceptive emotional expression system. The interaction of this system with emotion varies depending on the complexity of the emotional act.

How does the mirror neuron system work in social cognition?

The mirror neuron system works in two ways in social cognition:

• First, it is necessary for predicting and attributing thoughts (theory of mind).
• Second, it triggers mechanisms of affective recognition and expression. The first prediction system has already been explained: observed acts are computed in a frontoparietal mirror neuron system, together with their consequences.

This system serves as a predictive model that develops over time: from simple bottom-up behaviors and processes, the neurological system moves over the years to a top-down regulatory system in which observed motor schemas are compared with what has been learned over the years, with the aim of establishing statistical predictive patterns that minimize error (Kilner et al., 2007). This computation is also hierarchical, in the sense that the operations performed follow a hierarchical distribution along the brain’s theoretical axes. In this hierarchy, the frontal lobe computes the relationship between the observed behavior and the assumed mental state, while the motor cortex, the parietal lobe, and the superior temporal sulcus integrate visual information and stored motor schemas.

Next, we will focus on the second empathy system, which involves the limbic mirror neuron system (insula, cingulate cortex, and frontal lobe).

Mirror neurons and empathy

The role of mirror neurons in empathic behaviors, such as adopting facial gestures and postures in interactive imitative behaviors, is fundamental, along with emotional adoption (limbic system). As mentioned above, mirror neurons compute movements in terms of the consequences of execution and goals. This knowledge serves as the basis for social cognition, together with the second system of emotional integration. Empathy is not a single process. Although there is evidence that observing someone else being punished activates the amygdala, the anterior cingulate cortex, and the insula, as well as the thalamus and the cerebellum (Jackson et al., 2005), the complete process probably depends on a large-scale network, with high-level processing areas influencing or eliciting emotional responses.

In fact, this could be the role of mirror neurons in empathy. Empathy is supported by a large-scale neural network made up of the mirror neuron system, the limbic system, and the insula, which acts as a hub connecting both systems. Within this network, mirror neurons provide a simulation of the facial expressions and gestures observed in others to low-level processing areas via the insula, which triggers activity in those areas. Finally, this produces an emotional state in the observer that mirrors the observed behavior. In this way, the subject is provided with an alternative emotional system based on simulation, which partly enables social cognition.

This theory is called “simulation theory” (Gallese & Goldman, 1998; cited in Frith & Frith, 2006), and it proposes that we can understand the emotions we observe through the internal states they produce in us. Therefore, the most common way to empathize is to adopt the other person’s posture, literally, in order to simulate it internally. And again, when we try to adopt the posture of the person expressing their emotions, we do so facially, which activates the limbic system.

In short, mirror neurons provide a sensorimotor basis for empathy. When talking about this mirror neuron system and its relationship with empathy, an important distinction must be made: understanding and simulating emotions is not the only step in social cognition, since we must also consider the person’s stable personality in order to make predictions.

Here again, it’s interesting to make a distinction: at the neural level, is thinking about the likely behavior and emotions of a person similar to us the same as thinking about someone different? It isn’t. Thinking about someone similar to us usually activates areas of the ventral medial prefrontal cortex, especially BA 18, 9, 57, and 10, while thinking about the likely reactions and characteristics of others activates areas of the dorsal prefrontal cortex, BA 9, 45, and 42 (Frith & Frith, 2006).

In fact, there is a medial-lateral functional axis in the brain, in which the more central areas are related to the representation of the self and one’s own emotions, while the lateral regions involve a representation of the outside world and others. This medial-lateral axis hypothesis is based on the fact that medial areas tend to be more closely connected with limbic centers and proprioceptive sensory information, and are therefore more influenced by data, while the lateral areas are more reflective and dependent on representations of the outside world. Amodio & Frith (2006) mention a central hub in social cognition processing: the medial frontal cortex (BA 10).

The mirror neuron system and motor rehabilitation

Although we have explained the role of the mirror neuron system in motor learning, it’s worth noting its involvement in forming a motor memory bank. The strongest evidence comes from the studies by Stefan et al. (2007), in which the authors show that learning a motor sequence through observation enhances the formation of motor memories compared with learning alone. Observational learning has been found to mediate long-term neuroplasticity processes in the individual, and this effect is mediated by the mirror neuron system in the motor cortex.

In a study by Ertelt et al. (2008), two groups of patients with middle cerebral artery infarction and a paretic limb received two different therapies: one with audiovisual cues and one without. The group that trained with audiovisual examples of the exercises showed greater improvement in the paretic limb than the control group. In addition, mirror therapy has been proposed as an alternative that produces changes in plasticity. In this therapy, the patient practices with their healthy limb in front of a parasagittal mirror, which creates the visual illusion of moving the paretic limb. The results of this therapy show the generation of cortical plasticity.

Mirror neurons and therapy in autism spectrum disorders (autism and Asperger syndrome)

Development and dysfunction

There is indirect evidence of mirror neuron activity from the first year of life in predicting the goals of observed individuals (Falck-Ytter et al., 2006; cited in Iacoboni & Dapretto, 2006). In children under 11, this evidence, although less robust than in adults (which makes sense if we consider that the system is not yet fully mature in connectionist terms), shows signs of mirror neuron activation across several measures (mu rhythm suppression, EEG, near-infrared spectroscopy, functional magnetic resonance imaging) for imitation, social competence, and empathy activities. Therefore, although we don’t yet know for certain how involved the mirror neuron system is in social behavior, it clearly plays a central role.

One of the keys to establishing its importance is probably its dysfunction in children with autism and other communication disorders. In autism, a neural simulation deficit has been proposed in modeling observed behaviors, which prevents a correct “experiential understanding” of others. This deficit has been confirmed neurologically in the mirror neuron circuit, which shows structural abnormalities in individuals with autism spectrum disorders.

For example, this disorder prevents the correct identification of emotions in facial expressions, since the central circuit is not adequately activated. However, autistic individuals are able to identify the non-emotional act, although they don’t know its purpose, which suggests that the limbic mirror neuron circuit is more disrupted than the imitation mechanism. This deficit correlates with the severity of the disorder.

Data on Asperger syndrome confirm a similar but less severe deficit (involving difficulty and a delay in acquiring behaviors) related to imitation, which suggests that imitation may play an important role in therapy for these individuals. Therapy for people with autism spectrum disorders may involve the mirror neuron system. There is empirical evidence that the disorder involves, at least in part, a deficit in imitation and language production, and that the mirror neuron system is involved in the disorder (Wan et al., 2010).

Music therapy has been shown to help improve symptoms. Since the sensorimotor system is involved in language processing, and motor activity is also modulated during language processing, it seems logical that a way of activating the mirror neuron system could improve both symptoms. Music produces activity in the system, changing it (in a positive way) and providing neural plasticity, since music is also an act of motor expression and activates, among other areas, Broca’s area (BA 44). In fact, this type of therapy combined with singing has beneficial effects in patients with Broca’s aphasia, many of whom are able to say words embedded in a prosody different from normal.

Compared with speaking, singing produces bilateral activation of a frontotemporal network, and part of this network shares neurons with the mirror neuron mechanism. This overlap improves auditory-motor coordination schemas, an activation deficit seen in people with this type of communication disorder.

As for imitation, the degree of implementation and effectiveness varies in the same way as the experimental data described above. Children with Asperger syndrome seem to progress well, especially if therapy is initially carried out with people close to them, and even more so if it is done with videos of the children themselves. These data have been confirmed by mu wave suppression in the sensorimotor cortex, part of the mirror neuron system.

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“This article has been translated. Link to the original article in Spanish:”
Sistema de Neuronas Espejo: función, disfunción y propuestas de rehabilitación