Notes on "Mirrors in the Mind", by Rizzolatti, Fogassi and Gallese, Scientific American Magazine, November 2006 issue, p30-37.
The need for deduction of the meaning of complex (human) behaviours is quite probable, however, the ease with which simple behaviours and actions are understood suggests that there is another, simpler, explanation. This line of thought (among others) has led to the discovery and characterisation of so-called 'mirror neurons'. This paper follows the line of research central to this new understanding, and is written by some of those who were, and still are, actively involved in its progress.
The first indications of these neurons came from primate studies. It was noticed that certain neurons fired in a monkey brain when the individual was engaged in simple goal directed behaviour. Even more interestingly, these same neurons fired when another individual was observed performing these same actions. It was because of this behaviour that they were called mirror neurons. They were studied through single unit recordings, and through behaviour analysis in monkeys, which provided strong evidence for the presence of these special neurons. A second possible method of analysing the functions of mirror neurons would be to remove the mirror neuron system - however, because of the very wide ranging cognitive deficiencies which would result, this method would not have provided any conclusive evidence. Instead, an alternative method was used. Given their supposed function in understanding, their activity should reflect the meaning of the action rather than the action itself. Two experiments were run on this basis. In the first, it was tested to see whether the activity of mirror neurons could be induced from only the sound of an action being performed - the results showed that a subset of mirror neurons which were activated for the sight and sound of an action being performed were also activated for the sound alone. This subset was dubbed audiovisual mirror neurons. In the next experiment, even fewer cues were provided for the monkey - merely sufficient for it to create an internal representation of what was going on. Again, the results confirmed that mirror neurons underlie the understanding of motor actions - when enough information is present for an action to be inferred, mirror neurons fire as if recreating the parts of the action which are not directly observed.
Based on these observations in monkeys, a natural progression of the research was to assess whether mirror neurons also exist in humans. Cortical signal measurements provided an indication that they were present, but the accurate localisation of the neurons in human subjects is more difficult than in monkeys (no invasive procedures). Brain imaging was thus used to address this question. Using PET (Positron Emission Tomography), further support for this view was obtained. A further question raised by this experiment was explore the possibility that mirror neurons allow one to understand the intention of an action by distinguishing a particular action from others which have different goals.
To address this question, monkeys were used for single unit recordings (of parietal neurons). In one experiment, its task was to grasp a piece of food and bring it to its mouth. Its next action was to grasp a piece of food, but this time place it into a container. For most of the neurons, there was a distinct difference in readings between the two cases, despite the similarities in actions, because of the different goals. This result "illustrated that the motor system is organised in neuronal chains, each of which encodes the intention of the act." The second part of the experiment then attempted to address the issue of whether this neural behaviour explains how we understand the intentions of others. Using a similar procedure as previously, a monkey was set to watch an experimenter perform one of two actions - grasp and then either bring food to the mouth, or place in a container - and single unit recordings taken. The results were quite striking (tome at least): "The patterns of firing in the monkey's brain exactly matched those we observed when the monkey itself performed the acts...". This was for both tasks, thus illustrating a link between the neural representations of motor actions and the capacity to understand the intention of those same actions in others. Using brain imaging, these results seem to be also confirmed in humans.
Further studies showed that emotions may also be communicated in this way. Given that the communication of emotions is of vital importance in a social network (such as the societies and interpersonal relationships in which humans persist), the action of mirror neurons may explain this - "Thus, when people use the expression 'I feel your pain' to indicate understanding and empathy, they may not realise just how literally true their statement could be". It is acknowledged that this mechanism does not explain all possible aspects of social cognition, but it does go a long way in providing a neural basis for interpersonal interactions. Deficiencies in this system may then be responsible for conditions such as autism spectrum disorders, in which interpersonal communication and difficulty with picking up social cues are characteristic. (V.S. Ramachandran, among others, has written on this subject.)
On a side note, philosophers have long held that one must experience something 'within oneself' in order to truly comprehend it - the finding of mirror neurons provides a physical basis for this view. More than this, this discovery has the potential to dramatically change "the way we understand the way we understand". More recent recent evidence has suggested that the mirror neuron mechanism plays an important part of how people learn new skills. Imitation is very important for humans when young and learning skills and knowledge, much more so than any of the other primates. Could the development of the mirror neuron system in our phylogenetic past be responsible for this? Studies are ongoing to address the question of whether they exist in other species, or whether they are a more recent evolutionary development. Whatever the answer, it is clear that mirror neurons have an important role to play in communication and learning - more detailed characterisation is the subject of ongoing research.