The discovery of mirror neurons has been hailed as one of the major recent breakthroughs in
neuroscience, with possible implications for the explanation of many important cognitive functions,
including action understanding, imitation, language, and empathy. Mirror neurons were first identified
in the 1990s by Giacomo Rizzolatti and his colleagues at the University of Parma. They found that the
monkey prefrontal cortex contains a particular class of neurons that discharge both when the monkey
does a particular action and when it observes another individual doing a particular action. Similar
classes of neurons have been found in humans, capable of mirroring not only physical actions but also
pain and disgust.
When a monkey grasps an object, there are neurons in area F5 of its premotor cortex that fire. Much
more surprising is the serendipitous discovery by Rizzolatti and his colleagues that the same region
contains neurons that fire both when the monkey grasps an object and when it observes another
monkey or a human grasping an object. There are mirror neurons in F5 for grasping both with hands
and with mouths, and another area, the superior temporal sulcus, contains mirror neurons for walking,
turning the head, bending the torso, and moving the arms. The observations represented by mirror
neurons are visual-motor, integrating the visual and motor experiences of monkeys.
Rizzolatti argues that the mirror neuron system is the basis for both action understanding and
imitation. Not only does a monkey's mirror neuron system give it a direct understanding of what
another monkey is doing when it moves; it also facilitates imitating those motions that might be useful
for its own goals, such as finding food. Mirror neurons can work with auditory-motor representations,
as well: there are neurons in the monkey premotor cortex that discharge when the animal performs a
specific action and when it hears the related sound.
The evidence for mirror neurons in monkeys comes from direct recording of single neurons, but
evidence for analogous systems in humans is largely indirect, from brain scanning. Many studies
show that the observation of actions done by others activates in humans a complex network formed by
visual and motor areas. Evidence that a mirror system exists in humans comes from many kinds of
brain experiments, including imaging and transcranial magnetic stimulation, which uses magnetic
pulses to affect neural activation in the cortex. Hence observing the physical actions of others
prepares people not only to understand what they are doing but also to imitate them.
In humans, mirror neurons may be relevant for how people understand emotions as well as actions.
A mirror-neuron system involving visceral-motor centers may enable people to understand each
other's emotions, just as one involving visual-motor centers enables people to understand each other's
actions. Investigators have used fMRI brain scans to compare how people react to disgusting smells
with how they react to video clips of people reacting to disgusting smells. They found that the brain's
anterior insula, which is known to collect information from various visceral centers, is activated both
during the emotion of disgust evoked by unpleasant odors and during the observation of facial
expressions of disgust. Additional overlap was found in the anterior cingulate cortex. Hence it
appears that these two cortical areas, the insula and the anterior cingulate, enable people to grasp
other people's emotions of disgust. Both these areas are part of the EMOCON model.
Similarly, neuroimaging found that perception of facial expressions of pain engages cortical areas
also engaged by the firsthand experience of pain, including the anterior cingulate and the insula, the
same areas that had been shown to mirror disgust. Another study found that the insular cortex and the
anterior cingulate cortex were activated both by the experience of pain and by the observation of a
loved one in pain. Further support for the mirroring of pain is found in the studies that used
transcranial magnetic stimulation to detect evidence for the presence of empathic appreciation of the
sensory qualities of the pain of others.