Mirror neurons
Mirror neurons are specialized brain cells that activate both when an individual performs an action and when they observe someone else performing the same action. Discovered in the early 1990s in primates, these neurons play a role in understanding motor actions and intentions, firing not just for physical movements but also for associated sounds and goal-directed behaviors. While evidence suggests that similar activity exists in humans, there are notable differences; human mirror-like responses occur across a broader range of stimuli, including emotional expressions and non-motor actions.
Research has linked mirror neurons to social cognition and imitation, which raises questions about their potential role in certain disorders, particularly autism. Individuals with autism often exhibit challenges in social interaction and communication, leading to theories that a malfunction in the mirror neuron system might contribute to these difficulties. However, the relationship is complex and research findings remain mixed, with some studies indicating reduced mirror neuron activity in autistic individuals while others point to additional brain regions involved in processing social cues.
The study of mirror neurons is still evolving, and debates continue regarding their precise functions and implications for social behavior. As research advances, it may provide insights into therapeutic approaches aimed at enhancing social skills through targeted training, though outcomes remain uncertain.
Mirror neurons
Anatomy
Definition: Neurons within the brain that fire during both the execution and observation of an action.
Anatomy or system affected: Brain
Key terms:
autism: a spectrum of disorders characterized by poor social interactions, deficits in communication, and stereotypical motor behaviors
theory of mind: the ability to infer the mental states of others
Structure and Functions
Mirror neurons were first discovered in the early 1990s in the premotor cortex and inferior parietal lobe of primates. These neurons fired when a monkey made a particular motor movement, such as reaching out to grasp an object, as well as when he observed another monkey make the same movement. Thus, these neurons were coined “mirror neurons” because they fire during both the execution and observation of the same movement. Mirror neurons can also respond to some nonmotor-based information. For example, some mirror neurons respond to sounds associated with particular actions, such as shells cracking when eating peanuts.
Whether or not mirror neurons fire is greatly dependent on the goal or intention of a motor action. An interesting phenomenon is that mirror neurons respond to goal-directed movements even when part of the motor sequence is hidden from view, e.g., seeing an object hidden behind a screen and then seeing a hand reach behind the screen as if to grasp the object. However, these same neurons do not respond to the same sequence when it is not goal-directed, e.g., a hand mimicking grasping a nonexistent object. Likewise, mirror neurons can differentiate between the same motor movement with two different end goals. For example, mirror neurons will fire differently for the action of grasping a piece of food when followed by eating the food compared to when followed by placing the food in a container.
Differences Between Humans and Primates
There is a variety of evidence from neuroimaging techniques such as functional magnetic resonance imaging (fMRI) that suggest there is mirror-like activity in humans. However, there are several differences between these systems in humans and primates. Imaging studies have found regions of mirror-like behavior in human brains that are outside the activity zones found in primates. Additionally, supposed human mirror neurons respond to a wider array of actions and stimuli such as meaningless movements, gestures, emotion, and pain.
These differences in humans and primates have caused debate regarding the classification of mirror neurons. In primates, mirror neurons are considered to be a sub-category of motor neurons. However, human studies tend to define mirror neurons more broadly as neurons which fire for a given state or behavior within an individual and when that same state or behavior is observed in another individual. These mirrored states may not be motor-related, e.g., neurons that fire when one feels disgusted and when observing someone else with an expression of disgust.
Disorders and Diseases
It has been proposed that a “broken” mirror neuron system in humans could underlie the symptoms of autism, which include poor social cognition, deficits in communication, and stereotyped motor behaviors such as flapping hands. People with autism are thought to have a poor theory of mind, which is the ability to infer the mental states of others. This leads to an inability to understand the intentions of others or empathize with others. Mirror neurons have been proposed as a neural mechanism for a theory of mind because their firing links the observed motor actions of others onto one's own motor repertoire. Likewise, mirror neurons may provide a neural mechanism for imitation, which may be a stepping stone to develop broader social skills. Autistic children are poor imitators, and their inability to imitate gestures may contribute to their underdeveloped communication skills. It is difficult to infer a direct relationship between these deficits and faulty mirror neurons, but this discussion has stimulated a new area of research in autism.
The neuroimaging data on mirror neurons and autism is mixed. Neuroanatomically, people with autism have less brain tissue in mirror neuron regions. Some studies report a correlation between social impairment and abnormal mirror neuron activity. However, people with autism also have reduced brain tissue in regions that process emotion and social cues but are not mirror neuron areas. Thus, a mirror neuron explanation may be too simplistic and dysfunction in these additional areas may better explain symptoms of autism.
Despite the inconclusive evidence, some proponents are proposing therapeutic techniques via sensorimotor training to improve mirror neuron function. It has been shown in primates that training can alter mirror neuron activity. Whether training will prove successful in autistic individuals is still to be seen.
Perspective and Prospects
It is important to consider that mirror neurons are a relatively new discovery. The function and ontology of these neurons is still not known and is widely debated. One theory of mirror neuron function is that they contribute to understanding the actions and intentions of others, which has led to the implication that mirror neurons may be involved in social cognition. However, as discussed, the data on mirror neurons and disorders such as autism is mixed.
It is also not clear if these neurons are hardwired from birth, and thus perhaps serve a function such as social cognition, or if they are learned from experience. If the latter is true, mirror neurons may simply reflect behavior rather than contribute to it. This may be a more accurate picture of mirror neurons as they are only a small percentage of all motor neurons (by one estimate in primates, approximately 6 percent).
Lastly, the differences between humans and primates raise questions on the definition of mirror neurons. In primates, mirror neurons are a subset or a particular firing pattern of motor neurons. However, in humans, mirror-like activity has been found in more regions of the brain and in response to a wider array of stimuli than in primates. This could be due to a more advanced system in humans. However, more research is necessary to further understand the function and ontology of these neurons.
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