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That may overstate the case. Even enthusiasts agree that there are limits to how much mirror neurons can explain. At the same time, says Christian Keysers, scientific director of the neuroimaging center at University Medical Center Groningen in the Netherlands, their discovery provides sharp insight into the mechanisms by which humans communicate their innermost desires and feelings. "When you sit in a chair and watch a movie," Keysers observes, "you don't have to think to yourself, 'Now the hero has this expression on his face, so he must be afraid.' Or, 'Now he is smiling, so he must be happy.' You don't have to build up theories about how the hero feels because through the mirror system, you just know it."
HOW WE RECOGNIZE INTENT
MIRROR NEURONS OPERATE ON A subconscious level; their activity is reflexive and involuntary. Yet their firing patterns may be capable of encoding not just movements but also the meaning behind the movements. Consider one of the tests Rizzolatti and his team devised. First they trained their monkeys to pick up a morsel of food and either eat it or put it into a container. Then they had the monkeys watch a researcher doing the same things. In both instances, mirror neurons in an area of the monkeys' parietal cortex, or inferior parietal lobule, fired more strongly when the goal of the grabber was to eat rather than to set the food aside. UCLA neuroscientist Marco Iacoboni and his colleagues recorded a similar response in 23 human volunteers when they watched a series of videos, one showing a hand reaching for a brimming teacup next to a plate full of cookies, another showing a hand reaching for an empty cup surrounded by crumbs and a crumpled-up napkin.
The links that are emerging between movement and meaning have inspired some scientists to see the mirror-neuron system as the biological foundation on which human language is constructed. Such speculation is supported indirectly by the fact that Broca's area--a critical language center in the left hemisphere of the human brain--appears to be a close analogue of the premotor mirror region in monkeys. Broca's area, it turns out, is important for sign language as well as spoken language, and its connection to the mirror system has led Rizzolatti and U.S.C. neuroscientist Michael Arbib to propose that language traces its roots to hand gestures and facial expressions that, over time, became increasingly complex.
WHY OUR SKIN CRAWLS
IN MACAQUES, MIRROR NEURONS HAVE THUS FAR BEEN LOCALIZED to just two brain areas (the parietal and premotor cortexes) that exercise control over voluntary movement. In humans, however, evidence suggests that neurons with mirror properties may be more widely distributed. For example, a recent experiment conducted by Keysers and his colleagues revealed that a discrete patch of the somatosensory cortex lit up when the human subjects felt their legs brushed by a glove and when they watched a video in which an actor's legs were brushed.