Mirror neurons are cells in the brain which have been found to fire when an action is performed by the individual and also when that same action is observed in another individual. Research on mirror neurons originally focused on monkeys and it was excitedly claimed shortly after their initial discovery that mirror neurons may play a key role in imitating, or mirroring, the actions of others. Neuroscientists then proceeded to stick humans into brain scanners in the hope of finding evidence of mirror neurons. Unsurprisingly they were successful, and found that two main areas were active for both observation and execution of different motor behaviours (e.g. finger or hand movements) and these were located in the premotor area (located at the front of the head) and the inferior parietal lobule (located around the crown of the head). These were similar to the areas found in studies using monkeys and hinted at a brain network behind our ability to understand others’ movements.
Human studies also identified associations between activity in mirror neuron systems and performance in social cognition tasks, which hinted at their role in empathy and social functioning. This suggested that simulating the actions of others through specific neural pathways might underlie our ability to function successively as social creatures. Moreover, that a breakdown of the functioning of mirror neuron systems would lead to problems in social functioning characteristic of psychiatric illnesses such as schizophrenia and autism. If an individual is unable to understand the actions of others, then it stands to reason they will not always be able to act appropriately on them either. And there is evidence to back these claims up, although there is considerable debate here (see the Wired article, “A calm look at the most hyped concept in neuroscience” below in extra reading).
You may also have noticed that I have shifted to talking about ‘mirror neuron systems’ as opposed to ‘mirror neurons’. It may seem like a small difference but it is an important distinction when comparing animal and human research in this field. In animal studies, this type of research typically looks at recording from a small number of neurons via an electrode placed at a specific brain region. This allows for the precise recording of brain cells but comes with the problem that it is an invasive procedure which could not be replicated with a healthy human participant (there are rare exceptions which are covered in the extra reading). In human studies, brain imaging is used as a non-invasive way to look at what’s going on in the brain, but it comes with its own disadvantages: only larger areas can be imaged and we’re less able to unpick what a single brain cell is up to. This is an important distinction, because if we are to understand how mirror neurons work, we need to be able to understand how different mirror neurons with different properties work individually and as part of wider neural circuits.
To add additional complexity to the issue, mirror neurons can be very picky. Work done with monkeys showed that certain groups of mirror neurons would only fire if there was a reward associated with the action they saw. Others would only fire to very specific actions or would only fire depending on whether they saw the action in real life or on a television screen. By contrast, there are groups of mirror neurons which fire more freely and are considerably less picky about what movements they like. As you can see, mirror neurons are not some single unified group of cells all performing the same job, but a varied and interesting bunch who all contribute to a shared goal in their own important way. This creates a problem for research in humans. By looking a wide brain regions without effective ways to separate out different types of brain cells, there are limits on what we can understand about how mirror neurons work. Hopefully, you can now start to understand why it is so hard to study the exact role of mirror neurons in humans using a brain scanner!
The important question, and the one which you specifically asked about, was how mirror neurons actually work. Well, as stated above, it was initially hypothesised that these mirror neurons enabled us to imitate the movement of others by simulating the brain activity required to make said movement. However, it is unlikely that mirror neurons or mirror neuron systems complete such a complex task all by themselves. Remember the qualities of brain scans earlier? Evidence from these types of studies will show overall activation within mirror neuron systems, not necessarily from mirror neurons alone. This would require the invasive procedures used in animal experimentation, and suggests that other types of neurons within the imaged systems may play a role. There are many animal studies which support the role of other brain cells other than mirror neurons in imitation. Therefore, our ability to imitate the actions of others or understand the purpose of their actions, whether emotionally or physically, may be served by mirror neurons but most likely involving other types of cell as well.
Finally, mirror neurons in monkeys are complex, and increasingly complex as you look in more detail at them. With the addition of empathy, social cognition and language this complexity is far from reduced when you turn your eye to the human brain. So, simply put we don’t fully know how mirror neurons work but we have a reasonable idea of what they’re involved in and how they might contribute to our everyday lives. Their existence raises more questions than answers at this point, but hopefully this little introduction should help you understand what those questions might be!
An introduction to the hype of mirror neurons:
And a great debunking of the hype behind mirror neurons:
Guardian Article which goes into what I’ve said in a bit more detail:
An open access review on evidence for mirror neurons:
Single cell recordings in human (more direct mirror neuron evidence):