Why should emotions be contagious? Why should seeing someone laugh make us want to laugh as well? Or cry, for that matter?
On a seemingly unrelated topic, why on earth do we yawn when others yawn?
Some researchers believe the answers to questions like these will be found in the study of "mirror neurons." In the 1980s and 1990s, a group of Italian neurophysiologists at the University of Parma were studying neuronal activity by placing electrodes directly on the cortex of macaque monkeys. The monkey would reach for food, and a nerve cell would fire. Interestingly, the researchers found that these cells also fired when the monkey saw a human pick up a piece of food. This led to further experiments that found such "mirror" activity in about ten percent of neurons in the inferior frontal, and interior parietal, cortices of monkeys.
Measuring electrical activity directly off the surface of the brain is more challenging than doing so in macaques. With the advent of functional magnetic resonance imaging, the study of similar networks became possible in humans. Functional neuroimaging studies have demonstrated that there are areas of overlap between regions activated by watching others experiencing emotions, or performing certain actions, and brain regions that activate when we undergo those experiences ourselves. For example, part of the inferior parietal lobe may light up both when we move, or when we watch someone else move.
In 2010, researchers were able to directly record electrical activity off the surfaces of brains in people undergoing brain surgery. Mirror neuron activity was again detected, which supported the findings of fMRI studies.
There's a lot of speculation about the significance of mirror neurons. Some researchers have argued that mirror neuron systems help us better understand the intentions of other people, which can both help us predict the actions of others, and may be crucial to empathizing with others' emotions. Some have speculated that disorders in mirror neuron systems may be involved with autism, though the reality of this purported connection remains to be seen.
On the other hand, many researchers have cautioned that many claims made about mirror neurons are not sufficiently backed by science at this point. They argue that mirror neurons may just be signs of a partially stimulated motor system — a kind of extension of more mundane neurological processes — and a byproduct of everyday thought, rather than a driver of empathy. Various points questioning the quality of mirror neuron research have also been raised. The idea that mirror neurons may facilitate the understanding of actions has been particularly challenged. One of the major points of contention is the idea that there is something unique or special about the neurons engaged in this mirroring. Instead of saying "mirror neurons," it may make more sense to say mirror networks, as there's nothing about an individual neuron that is capable of itself experiencing something as complex as empathy.
The idea of a network that contributes to empathy has been referred to as shared representations, or also as a "mirror" neuron system, which seems to predominantly involve regions in the frontal lobes (including the premotor cortex and supplementary motor area) and parietal lobes (including the primary somatosensory cortex and the inferior parietal cortex) in humans. Other work has suggested that humans who watch another human being in pain, particularly if it's someone close to them, also have neurons fire in the anterior insula and anterior cingulate cortex — regions that are themselves associated with pain.
In a way, the ability of one brain to imitate another is nothing new. In fact, it was probably essential to our learning, especially when we were very young. Infants love to imitate their parents, and in order to, say, pretend to sweep the floor like Mommy, similar neurons have to fire to move those arms and legs. It's not too hard to imagine the brain having a similar mechanism to support the understanding of language or emotion. Perhaps, in the end, "mirroring" is actually the way that most neurons in the brain are able to do their jobs of learning and adapting, based on what they see others do in the world around them.
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