Know your brain: Mirror neurons

Where are mirror neurons?

Image from: Gross L. Evolution of neonatal imitation. PLoS Biol. 2006 Sep;4(9):e311. doi: 10.1371/journal.pbio.0040311

Mirror neurons were first identified in the premotor cortex of monkeys in 1992, and since that time they have also been found in several other areas of the monkey brain, including the primary motor cortex, inferior parietal lobulefrontal cortex, and the area surrounding a sulcus called the intraparietal sulcus.

There is very little conclusive evidence that mirror neurons exist in the human brain, although there is evidence from neuroimaging studies that indicates there are neurons in the human brain that display patterns of activity similar to the mirror neurons identified in the monkey brain. There is, however, one study to date that directly recorded the activity of purported mirror neurons in the brains of human patients who were being prepared for neurosurgery. Although this study could only explore certain areas of the brain (which didn't include the regions most frequently associated with mirror neurons in monkeys), investigators found neurons in the supplementary motor area and temporal lobe that displayed properties of mirror neurons. This, combined with the neuroimaging data mentioned above, suggests that mirror neurons likely exist in the human brain as well as the monkey brain.

What are mirror neurons and what do they do?

In 1992, a group of researchers at the University of Parma in Italy were recording the activity of individual neurons in the brain of a macaque monkey. They were observing neurons in the premotor cortex---specifically a region of the premotor cortex called area F5. Previous research had found neurons in this area to be active during goal-directed hand movements (e.g. grasping, holding, etc.). The investigators at the University of Parma were attempting to further understand this type of neural activation when they observed something surprising.

They noticed that neurons in the F5 region of the monkey's brain were activated not only when the monkey moved its hands, but also when the monkey observed an experimenter using his or her hands (e.g. to pick up a food reward and place it in the testing area). Four years later, they named these neurons mirror neurons because they seemed to be active not only when monkeys performed a particular action, but also when they saw someone else perform a similar action.

As mentioned above, since their initial discovery mirror neurons have been found in various other regions of the monkey brain (as well as in the brains of other species like songbirds), and there is evidence to suggest that mirror neurons exist in human brains. 

The discovery of mirror neurons has generated a type of excitement both in and outside of the scientific community that is not often seen in response to scientific findings. Some have interpreted the activity of mirror neurons as the basis for our ability to understand the actions of others---a deduction thought by some to be unjustified, and one that has led to other (perhaps even less justifiable) extrapolations. For example, some have claimed that mirror neurons provide the necessary neural machinery for empathy, complex social interactions, language---and even that they are responsible for the rapid cultural advancement of the human race that led to us becoming modern humans.

Based on these proposed roles for mirror neurons, researchers began to speculate that impaired functioning of mirror neurons may be the basis for certain psychiatric disorders. For example, some have argued that dysfunctional mirror neurons underlie autism spectrum disorders (ASD). This hypothesis, sometimes called the "broken mirror hypothesis," suggests that individuals with ASD have abnormalities in mirror neuron networks that cause them to have an impaired ability to experience empathy, difficulty understanding the actions of others, and deficits in various aspects of social interaction ranging from eye contact to language. 

Indeed, in the few decades since their discovery, mirror neurons have been credited or blamed for a long list of things ranging from simple feats like helping us to enjoy watching sports to complex emotions like compassion to disorders like ASD and schizophrenia.

The problem with these claims, however, is that they are mostly unsubstantiated. The first caveat to speculation about mirror neurons is that the vast majority of concrete evidence we have to support the existence of mirror neurons comes from studies in monkeys (concrete evidence in this case refers to evidence obtained from monitoring the activity of individual neurons---something that is difficult to do in humans except in rare circumstances like the example cited above where mirror neurons were explored in patients preparing for neurosurgery). Therefore, at this point we cannot with confidence attribute behaviors to mirror neurons unless we have been able to record mirror neuron activity while monkeys have exhibited such behaviors. Thus, we do not yet have the evidence to consider complex human emotions and behaviors (which can only be roughly approximated in studies of non-human primates) as attributable to mirror neurons. 

Similarly, the evidence to point to dysfunctional mirror neurons as the main causal factor in human psychiatric disorders is lacking. Let's take the hypothesis that dysfunctional mirror neurons contribute to ASD as an example. This hypothesis was initially supported by two highly-cited studies from the early 2000s. One was a neuroimaging study that found reduced activity in autistic patients in a part of the brain thought to be heavily populated with mirror neurons. The other used electroencephalography to measure electrical activity believed to be indicative of mirror neurons; again, individuals with ASD appeared to display abnormalities in this activity.

Each of these studies, however, failed to replicate multiple times. Additionally, critics of the "broken mirror hypothesis" have been quick to point out that there is not good evidence that individuals with ASD even have deficits in understanding the intentions of others. Thus, the "broken mirror hypothesis" has been found to be wanting, and other hypotheses that attribute psychiatric abnormalities to mirror neurons are similarly in need of more support to make them tenable.

Even when it comes to just the basics of mirror neuron function, we are still searching for answers. For example, some researchers argue that the evidence that mirror neurons are involved with something as abstract as understanding actions is inadequate. According to this perspective, even if mirror neurons may be involved with functions like recognizing basic movements, selecting movements to make, etc., the evidence isn't conclusive to suggest mirror neurons are involved with a type of higher-level cognition like understanding the behavior of others. This is a critically important point as the idea that mirror neurons help us to understand others' actions is essential to the interpretation of mirror neurons as being involved in behaviors like empathy and social interaction---and indeed is the basis for much of the enthusiasm about mirror neurons in general.

Thus, while mirror neurons have been lauded for their ability to explain a variety of uniquely-human behaviors and accomplishments, it seems that we may have jumped the gun a bit on our interpretation of the activity of these cells. Much more research still needs to be done before we can say with confidence what mirror neuron activation really means in terms of behavior---and indeed before we can be sure that mirror neurons are as prevalent in the human brain as they are in the monkey brain. As with all scientific discoveries, it is best to be conservative in our interpretations until they are the only logical ones to make based on the data.

References (in addition to linked text above):

Hickok G. Eight problems for the mirror neuron theory of action understanding in monkeys and humans. J Cogn Neurosci. 2009 Jul;21(7):1229-43. doi: 10.1162/jocn.2009.21189.

Kilner JM, Lemon RN. What we know currently about mirror neurons. Curr Biol. 2013 Dec 2;23(23):R1057-62. doi: 10.1016/j.cub.2013.10.051.

Mirror Neurons May Be Responsible For Global Warming & U.S. Economic Woes

Since their discovery in the 1990s, mirror neurons have experienced a degree of fanfare uncommon to findings in the field of neuroscience. Mirror neurons are so named because they are activated both when a primate participates in a task, and while watching another complete the same task, thus “mirroring” the behavior of the other animal. This unique activation pattern has led some to suggest that mirror neurons are integral not only to imitation, but also to understanding that others have their own mental states (theory of mind). By extension, it has been hypothesized that mirror neurons are necessary for language acquisition and social interaction. Dysfunctions in mirror neurons have even been offered as a possible cause of autism.

Thus, they have come to be viewed as a very special kind of neuron, with a versatility and importance to brain function that is unrivaled by other types of brain cells. But, do mirror neurons deserve the exalted status that some have ascribed to them? In short: probably not.

Mirror neurons do seem to play an interesting role in cognition. Primate studies have found mirror neurons to be activated in correlation with focusing on a particular goal or intention of movement. They are also activated in a selective fashion, with specific groups corresponding to different goals of an action, e.g. grasping to move vs. grasping to eat. Of additional interest, they have been found to respond to sounds associated with an observed action.

fMRI studies in humans have revealed specific activity in areas where mirror neurons are thought to be located, such as the ventral premotor (vPM) and anterior intraparietal sulcus (alPS), during observation and imitation of movement.

But, while these findings in humans and non-human primates are intriguing, they don’t support the rampant speculation that has followed about the role of mirror neurons in overall cognitive function. Experiments with monkeys to date haven’t assessed the ability to imitate, experience empathy, display theory of mind, or use language. Of course, it is debatable to what extent some of these attributes even exist in non-human primates, or if they can be studied if they do.

As for humans, neuroimaging experiments have allowed scientists to determine which regions of the brain are active during imitation or observation of an action. The specific neurons that are utilized, however, and any physiological characteristics that make them unique, cannot be assessed with current imaging technology.

Thus, the roles attributed to mirror neurons in the last decade since their discovery may have been an overly ambitious attempt to describe their function. By extension, implying that their malfunction is critical in autism could really be jumping the gun.

In an essay in last week’s Nature, Antonio Damasio and Kaspar Meyer discuss the exaggerated claims about mirror neurons, and suggest a rational hypothesis for how they may work. Twenty years ago Damasio proposed a theory known as “time-locked multimodal activation” to explain the development of complex memories. The theory is based on the proposed existence of groups of neurons that, during the encoding of memories, receive input from a number of different sites. Damasio termed these neuronal groups convergence-divergence zones (CDZ). He suggested there are two types of CDZs: local CDZs, which collect information from areas close to a sensory cortex (e.g. the visual cortex), and non-local CDZs, which are higher-order structures of the brain where the information from local CDZs converges.

According to this theory, when a memory is formed—Damasio and Meyer use the example of a monkey opening a peanut shell—all the information about the event converges on a non-local CDZ. Then, if the monkey hears a peanut shell opened in the future, this would activate a local auditory CDZ, as well as the non-local CDZ where memories associated with the noise are stored. Signals are sent out from the non-local CDZ to all local CDZs that were involved in the original experience of the event, activating these sites and resulting in a sort of recreation of the original peanut-cracking.

Mirror neurons are represented by the non-local CDZs. In this scenario, however, mirror neurons are not physiologically unique. They are normal neurons involved in a network that has less to do with “mirroring” than with integrating and syncing the various aspects of elaborate memories. This does not take away from the role and function of this network, but should detract a little from the aggrandized status attributed to individual mirror neurons, in favor of an appreciation of the holistic complexity of the brain.

The CDZ hypothesis, however, has not been tested, although research does indicate that networks involved in observing and imitating behavior spread beyond purported mirror neuron sites. Regardless of whether the specifics of the CDZ hypothesis come to be supported by future studies, I feel it represents a more sensible approach to mirror neurons. To credit mirror neurons alone with a function that carries such importance, like the ability to infer the mental states of others, seems to oppose much of what has been learned thus far about neuroscience. We have never found language neurons, love neurons, or fear neurons. Instead we have found networks that spread throughout brain regions that correlate with the ability to experience these aspects of cognition. I suspect we will soon say the same about mirror neuron networks and their involvement in social interaction.

 

Damasio, A., Meyer, K. (2008). Behind the looking-glass. Nature, 454 (7201), 167-168. DOI:10.1038/454167a

Dinstein, I., Thomas, C., Behrmann, M., & Heeger, D.J. (2008). A mirror up to nature. Current Biology, 18 (1), 13-17.

Human Flocking Behavior with a Shaky Segue into Mirror Neurons

On occasion, I will be in a public place like an airport, sports stadium, or bar/club, and I’ll pause to look at the sea of people that I’m part of. I then usually start to feel being human is a little less significant than we are inclined to think it is, as I get caught up making zoologically comparative observations. In the case of the airport or large event, I often consider how we resemble herds of cattle, moving in one direction or another with the urging of signs or velvet ropes instead of sheepdogs (or sometimes even with security guards barking at us much like a sheepdog would). When at a bar or club, I’m prone to make comparisons to the courtship displays of various animals, like the ostentatious demonstration of the male peacock, or the male fruit fly’s persistent pursuit of a mate.

A group of scientists at the University of Leeds recently published a study that inevitably leads one to similar comparisons. It focuses on flock-like behavior in humans. The researchers conducted their experiments in a large hall with groups of people that varied in size. A number of people in the group were given specific directions about which walking route to follow, the rest were left uninformed, to amble about on their own. They also weren't told that directions were given out to anyone. The participants in the group weren’t allowed to communicate with one another, by speech or gesture.

The study found that the uninformed individuals tended to follow those who had been given directions, even though they were a comparative few to the many. In fact, the researchers observed that as the number of people in the group was increased, the less informed participants were needed to create a following. The largest groups of 200 or more only needed about 10 people walking in a specific direction to cause the rest of the group to fall in behind them. The followers, when interviewed afterward, often didn’t seem to be aware they were being led.

You might be wondering what this all has to do with neuroscience and the answer is: no one knows. This was a behavioral study, and the authors didn’t speculate on the brain regions that might be involved. Although suggesting their involvement in this type of flocking behavior is purely speculative, it seems like as good a time as any to bring up the subject of mirror neurons.

Mirror neurons were first discovered in experiments with macaque monkeys. A group of researchers led by Giacomo Rizzolatti were using electrodes to measure brain activity while the monkeys engaged in motor tasks, like picking up pieces of food. They unexpectedly found that some of the neurons were activated not only when the monkeys picked up the food, but also when they saw someone else (experimenter or another monkey) pick it up. After this finding, areas were identified in the human brain that may play a similar role. They include regions of the inferior frontal cortex and parietal lobe.

Since this discovery much excitement has surrounded the concept of mirror neurons. As these neurons seem to be specifically activated when watching another person perform a goal-directed action, some have suggested they may underlie our ability to understand that other people have intentions. This could mean they are the foundation for empathy, imitation, and communication. Some studies have even indicated malfunctioning mirror neurons may contribute to autism.

The truth is (as is usually the case with the brain), mirror neurons aren’t going to be that easy to figure out. Nor will they be a magic bullet that will conveniently explain a panoply of human behavior. They are part of a complex system that we have a very vague understanding of at this point. We can only speculate about their involvement in much larger reactions like empathy and language. They do seem to play a role in perceiving intention, however, and thus have an intriguing potential when it comes to understanding human behavior, as so much of it is based on knowing that we are surrounded by other intentional agents.

So, to get back to the flocking study, it doesn’t have a specific connection to neuroscience—yet. Just remember, however, if you are standing in line among hundreds to get to your gate at the airport, or being herded through the turnstile at the sports stadium, or observing the human courtship rituals at a club with a sense of detachment, and these sights seem surreal or slightly dehumanizing to the comparative biologist in you, and you begin thinking how similar we are to cattle, or sheep, you may not be far off base—and you’re not alone.