What We’ve Learned About Mirror Neurons

Post by Shireen Parimoo

Think back to a time when you picked up the cup in front of you and took a sip of hot coffee. Now, think about a time when you watched somebody do the same thing. The same set of neurons in the brain – dubbed mirror neurons (MNs) – were likely activated in both of those instances. Mirror neurons were initially discovered in the monkey brain in 1992 when the same set of neurons in the ventral premotor cortex would fire when the monkey performed an action and when the monkey watched somebody else perform the same action. This finding was followed by a slew of research studies that sought to identify the role of mirror neurons in cognitive abilities like empathy or decision-making. When mirror neurons were initially discovered, there was considerable debate regarding their function and origin. In the last 30 years, substantial progress has been made in better understanding the role of mirror neurons, yet some questions still remain.   

What are mirror neurons, exactly?

Early studies using single-cell recordings initially identified what are known as strictly congruent MNs that fire when an animal performs an action and observes someone else performing exactly the same action. Though they were initially found in premotor regions of the frontal cortex, MNs have since been located in other frontal areas as well as parietal and medial temporal areas in the brain, including in humans. Subsequent studies have also identified a broadly congruent type of MN that would fire when executing an action or observing a similar enough action, as well as mirror-like neurons that are active while observing but not performing an action. This finding is interesting because as the name suggests, motor areas in the brain are predominantly involved in preparing for and performing actions, but mirror-like neurons do not appear to be associated with any motor output. In addition to actions, these cells are also responsive to information about emotions, space, decisions, and beliefs.

Population-level activity in different regions of the MN system has also been shown to represent action sequences during observation and execution. Thus, there are multiple types of neurons distributed across the brain that exhibit MN or mirror-like properties, both at a single-cell level and as a neuronal population.

What is the function of mirror neurons? 

The existence of both MNs and mirror-like neurons, especially in motor regions of the brain, begs the question: what are they useful for? Mirror neurons were initially thought to serve a range of functions, including action understanding, speech perception, and imitation. In fact, they were even implicated in autism as researchers speculated that dysfunction of the MN system might explain many of the social and emotional symptoms associated with autism. Although initial studies hinted at atypical recruitment of regions containing MNs in individuals with autism during action observation, this hypothesis was not confirmed by subsequent experiments. Similarly, there is no clear evidence that MNs are involved in understanding others’ actions, that is, in inferring the intentions behind actions performed by others. This is because most of the evidence supporting this idea cannot distinguish between the role of MNs in low-level cognitive processes (e.g., recognizing an action) and higher-level processes (e.g., inferring others’ goals from their actions). Nonetheless, many neuroimaging and patient studies in humans do provide support for the notion that regions containing MNs are important for speech perception and imitation – both of which are important for social cognition and learning.

Do mirror neurons have an evolutionary origin?

Humans are social creatures. Much of what we learn – as children and as adults – is through observation and imitation. So, it’s not surprising to consider the possibility that our brains evolved in tandem to optimize learning from others, in turn guiding our behavior in social situations. Two major perspectives on the origins of MNs emphasize the role of (i) genetics (we come with MNs hardwired!), or (ii) associative learning. Initial evidence for the idea that MN development is genetically determined came from studies with newborn babies and monkeys. Unlike young infants and toddlers, newborn monkeys have not had as much opportunity and experience to learn from their surroundings. Yet, they were able to spontaneously imitate the facial expressions of their caregivers. However, the genetic explanation has been questioned because some of the neonatal imitation findings have not been reliably replicated or suffer from limitations in the research methods used.

Instead, some speculate that MNs develop from sensorimotor associative learning. Essentially, this means that as we learn to associate actions (e.g., reaching for a cup of coffee) with visual input (watching our hands reach for the cup), motor neurons form stronger connections with the visual neurons. Over time, MNs develop as the motor neurons begin selectively responding to both performing the action and to watching someone else perform the action. The engagement of the MN system in response to both biological (e.g., grasping) and artificial or unnatural (e.g., robotic) stimuli lends further support to the associative over the genetic account. Nevertheless, some researchers have provided alternative explanations for these findings and the debate regarding the origin of MNs has not yet been fully resolved.

What’s next?

Since 1992, the diversity of neuron types in the MN system, as well as the discovery of MNs in emotion-processing areas (outside of motor areas), has led to the idea that by integrating sensorimotor representations about the self with sensorimotor representations about others, the MN system facilitates behavior during social interactions. The diversity of neuronal types within the MN system may each contribute to a different aspect of social learning. For instance, unlike the strictly congruent MNs found in the premotor cortex, mirror-like neurons in the monkey motor cortex that regulate motor output are sometimes inhibited during action observation. These findings illustrate how the MN system enables us to learn through observation without imitation or necessarily producing actions of our own. Moreover, the extended MN system has been linked to both self-related emotional regulation as well as emotional perception in others. This may be unsurprising since emotional expression is closely tied to actions – including facial expressions, gestures, and body language – that MNs are responsive to. However, more research is required to understand the role that MNs and mirror-like neurons play in emotional processing and traits like empathy and prosocial behavior. Together, these findings highlight the need to further investigate the extended MN system at both the neuronal and network level to understand its role in social behavior.

References +

Bonini et al (2022). Mirror neurons 30 years later: implications and applications. Trends in Cognitive Sciences, in press.

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Jerjian et al (2020). Movement initiation and grasp representation in premotor and primary motor cortex mirror neurons. eLife, 9, e54139.

Kraskov et al (2009). Corticospinal neurons in macaque ventral premotor cortex with mirror properties: a potential mechanism for action suppression? Neuron, 64.

Mazurek et al (2018). Mirror neuron populations represent sequences of behavioral epochs during both execution and observation. Journal of Neuroscience, 38(18).

Papadourakis & Raos (2018). Neurons in the macaque dorsal premotor cortex respond to execution and observation of actions. Cerebral Cortex, 29(10).

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