Some time back, when I lamented the fact that I had essentially written all that I intended for my book "The Lab Rats' Guide to the Brain," I asked readers for suggestions for topics. Little did I know that it would start a chain of topics that has added a whole new chapter to the book. On the other hand, it has provided an opportunity to explore many new topic areas of interest to me - and I hope of interest to readers as well.
Mirror neurons are neurons - mainly in prefrontal lobe, but may also exist in parietal lobe - which respond not just to planning/performing an action, but also when observing that same action by others. The location of these neurons is important, as I will discuss later, but right up front I need to state one huge caveat.
So far, mirror neurons have only been identified in nonhuman primates (NHPs) - that is, in monkeys. Very few studies can record neurons in human brain, and that only when probing for the damaged areas in brain of epilepsy patients. Any "discovery" of mirror neurons in humans is inferential from imaging, limited in number, or in brain that is known to not be normal.OK. With that out of the way, what are mirror neurons? How do they work? What do they imply about how we learn and perceive others?
First, mirror neurons were first observed in the 80's/90's in rhesus macaques. It should first be noted that rhesus monkeys are true monkeys, not Great Apes (gorillas, chimpanzees, orangutans, bonobos) which show human-like intelligence. Second, the ability of monkeys to learn by watching varies with species. Clearly Great Apes learn by watching, but most monkeys learn by doing. In 1992, researchers at the University of Parma, Italy, were recording neurons in the "ventral premotor" area of the frontal lobe. They specifically targeted neurons which responded to the motor act of picking up a piece of food and bringing it to their mouths. Surprisingly, those same neurons also responded when one of the human experiments picked up a piece of food.
To explain a bit more background - the premotor are of the frontal/prefrontal cortex is involved in planning of complex muscle and limb movements. Researchers can record neurons in motor cortex that signal movement, but neurons in premotor cortex respond before such movements begin. Much the same as sensory association cortex combines fine details of sight and sound into pictures or melodies, the premotor cortex specializes in combinations of movements which constitute an action. As I implied earlier, it is important that mirror neurons have been found in parietal lobe near the association areas for senses and the frontal lobe near the "association" are for movement.
One of the most profound implications of mirror neuron function is in the copying of facial expression. babies - whether human, ape or monkey - tend to copy the facial expressions of people around them. Researchers have identified mirror neurons that control the muscles of facial expression, thus implying that we "learn" facial expression by first copying it. Again, this is important because premotor cortex is involved in planning movement and does not require muscle activation to do so. Thus even just "thinking" about movement activates neurons in premotor cortex. So mirror neurons may have a significant role in "practicing" or "rehearsing" movements that mimic observed movements - such as facial expressions.
So what is the purpose of mirror neurons?
Ah, here we get into controversy, for many of the "roles" hypothesized for mirror neurons cannot be inferred from monkeys, we cannot record from the brains of great apes, and we are very limited in what we can observe or record from humans. Many of these theories must wait for confirmation and testing until we have sufficient evidence of human mirror neurons, but here are a few:
- Mirror neurons help us learn by mimicry - this one is easy, and likely true based on current evidence.
- Mirror neurons are essential for developing a sense of self-identity and the external world (i.e. self vs. nonself) - this one is harder to judge, and gets into many of the same metaphysics as the mind-brain problem - but again, there is merit to the theory.
- Mirror neurons create empathy - like the previous item, this is somewhat abstract, but based on functional imaging results which show increased activation of the presumed "mirror system" in humans with increased empathetic responses - it may in fact account for ability to read "body language."
- Mirror neurons help us learn language - this one is more of a stretch. The mirror systems can assist in developing speech and even vocabulary, but would do nothing for the development syntax and meaning.
- Mirror neurons are impaired with autism - much of this theory relies on the above four items being true and measurable. The idea that autism results from "broken mirrors" is highly contested and way too simplistic to explain the complexities of the autism spectrum. Impaired connectivity goes much further to define autism (see Autism Update blog).
- Mirror neurons are responsible for what we think of as the Mind - Again, this is quite metaphysical (see A Problem of Mind and Brain blog) and there is little way to prove or disprove it. If Dr. Travis S. Taylor's quantum theories are correct, the presence or absence of mirror neurons would have absolutely nothing to do with The Mind. [On the other hand, they might also explain how mirror neurons work!]
Why do we think all of these things? What is our basis for the presumption? Well, first, we know from our own motivation, that if we were reaching for cookies, it would be because we were hungry. Second, the cookies were placed out of reach of the child, yet the child knew where they were and endeavored to reach them. Finally, the fact that the cookies were placed out of reach was likely due to parent, babysitter or guardian deliberately placing them out of reach.
In other words, we infer motivation of others from our own motivation. We see the action, mimic it with our mirror neurons, and run that rehearsal of events through our "minds" to recall similar contexts when we reach (or reached) for cookies. Thus it very well may be the case that without mirror neurons we could learn and would not be able to infer motivation or understand the actions of others.
Pretty heady stuff - and all because of "monkey see, monkey do" neurons!