Mirror neurons in the human brain have been recorded for the first time by researchers.
It is believed that mirror neurons are what make us human-they are the cells in the brain that fire not only when we perform a particular action but also when we watch someone else perform that same action.
Neuroscientists have said that this "mirroring" is the mechanism by which we can "read" the minds of others and empathize with them. It's how we "feel" someone's pain, how we discern a grimace from a grin, a smirk from a smile.
But, until now, there was no proof that mirror neurons existed - only suspicion and indirect evidence.
Dr. Itzhak Fried, a UCLA professor of neurosurgery and of psychiatry and biobehavioural sciences, Roy Mukamel, a postdoctoral fellow in Fried's lab, and their colleagues have recorded both single cells and multiple-cell activity, not only in motor regions of the brain where mirror neurons were thought to exist but also in regions involved in vision and in memory.
They also showed that specific subsets of mirror cells increased their activity during the execution of an action but decreased their activity when an action was only being observed.
"We hypothesize that the decreased activity from the cells when observing an action may be to inhibit the observer from automatically performing that same action. Furthermore, this subset of mirror neurons may help us distinguish the actions of other people from our own actions," said Mukamel.
The researchers drew their data directly from the brains of 21 patients who were being treated at Ronald Reagan UCLA Medical Center for intractable epilepsy.
The patients had been implanted with intracranial depth electrodes to identify seizure foci for potential surgical treatment.
Electrode location was based solely on clinical criteria; the researchers, with the patients' consent, used the same electrodes to "piggyback" their research.
The researchers found that the neurons fired or showed their greatest activity both when the individual performed a task and when they observed a task.
The mirror neurons making the responses were located in the medial frontal cortex and medial temporal cortex, two neural systems where mirroring responses at the single-cell level had not been previously recorded, not even in monkeys.
This new finding demonstrates that mirror neurons are located in more areas of the human brain than previously thought.
Given that different brain areas implement different functions - in this case, the medial frontal cortex for movement selection and the medial temporal cortex for memory - the finding also suggests that mirror neurons provide a complex and rich mirroring of the actions of other people.
Because mirror neurons fire both when an individual performs an action and when one watches another individual perform that same action, it is believed that this "mirroring" is the neural mechanism by which the actions, intentions and emotions of other people can be automatically understood.
"The study suggests that the distribution of these unique cells linking the activity of the self with that of others is wider than previously believed," said Fried.
"It's also suspected that dysfunction of these mirror cells might be involved in disorders such as autism, where the clinical signs can include difficulties with verbal and nonverbal communication, imitation and having empathy for others. So gaining a better understanding of the mirror neuron system might help devise strategies for treatment of this disorder," said Mukamel.
The study was published in the latest edition of the journal Current Biology.