They have been implicated in empathy, language acquisition and even consciousness itself. Now individual mirror neurons have been directly observed in people for the first time.
Mirror neurons are brain cells that are activated both when a person performs an intentional action and when he or she sees someone else performing that same action. First discovered in macaque monkeys 11 years ago, this new class of cells generated a booming field of research. Yet until now, evidence for human mirror neurons could only be inferred from functional MRI studies, which measure general patterns of brain activity, and presumed similarities between humans and other primates.
Marco Iacoboni of the Ahmanson Lovelace Brain Mapping Center at the University of California, Los Angeles, described his team鈥檚 observations of mirror neurons last week at the annual Society for Neuroscience conference in San Diego, California. His team recorded the actions of 286 individual neurons in people with epilepsy whose brains were already being wired up by a surgeon seeking the starting point of their seizures. Electrodes were inserted into the frontal lobes of the subjects鈥� brains and they were then asked to perform simple actions and to observe short films of others executing the same actions. The study identified 34 mirror neurons, which were activated by both performance and observation.
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鈥淣ow we鈥檝e got concrete proof that individual human mirror neurons exist,鈥� says V. S. Ramachandran, director of the Center for Brain and Cognition at the University of California, San Diego. 鈥淢ore importantly, we have the beginning of a basis to study in a fine-grained, single-neuron way, how they work.鈥�
鈥淲e鈥檝e got concrete proof that individual human mirror neurons exist and a basis to study how they work鈥�
Significantly, Iacoboni鈥檚 team has identified several new types of mirror neuron not previously seen in macaques or any other primates. For example, they found mirror neurons that had a paradoxical reaction to the stimulus 鈥� their firing becoming suppressed in response to watching an action being performed, rather than increased. 鈥淭he suppression may act as a kind of brake mechanism,鈥� says Iacoboni. 鈥淭hat could help explain why we don鈥檛 mimic observed behaviours all the time and why we don鈥檛 confuse someone else鈥檚 behaviour for our own.鈥�
The team also described 鈥渟uper鈥� mirror neurons. These fire more when they are involved in performing an action, but are suppressed when one is observed. 鈥淭hey do it all,鈥� says Iacoboni.
The presence of more complex kinds of mirror cells that are more widely distributed in the brain is not surprising, he says. While primates engage in imitation, more complex human traits such as empathy, predicting the intentions of others and using abstract or metaphorical thinking would require a more highly developed system.
The Human Brain 鈥� With one hundred billion nerve cells, the complexity is mind-boggling. Learn more in our cutting edge special report.
