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The Gift Of Mimicry

Friday, Jan. 19, 2007

For 20 minutes Andrea McColl, a research assistant at the University of Southern California, has been repeating the same string of nonsense syllables, changing her intonation on cue. When a smiling cartoon face pops up on the screen in front of her, she tries to sound happy. When a frowning face pops up, she sounds sad. And then, again on cue, she falls silent, listening via a headphone as an actress runs through a similar da-da-da-da-da routine.

It's an exercise that would seem trivial, even silly, were McColl not lying on her back inside a brain-scanning machine. She's one of the first participants in a research project designed by Lisa Aziz-Zadeh, a neuroscientist at U.S.C.'s Brain and Creativity Institute, to test an intriguing question at the heart of a new field of brain research: Do areas of gray matter respond to the emotional contours of speech produced by others in the same way they do when we ourselves are speaking?

The question is one few researchers would have thought of asking a decade ago. But that was before University of Parma neuroscientist Giacomo Rizzolatti and his colleagues began publishing the eyebrow-raising results of experiments they had been conducting with macaques. The Italian scientists were monitoring the monkeys' brain activity--observing how neurons in the premotor cortex buzzed with activity as the animals grasped a piece of food--when something strange kept happening. The monkeys would be sitting still, doing nothing in particular, and one of the researchers would pick up some raisins or sunflower seeds in order to place them on a tray. At that point, the same neurons started buzzing again, in just the same pattern. The scientists couldn't explain it; they thought that perhaps the monkeys were subtly moving in anticipation of being fed. Through a series of experiments, however, they finally established that the neurons started firing whenever the monkeys saw a person grasp an object. It was as if the monkeys were mentally mirroring the action they observed.

Brain scans have now indirectly established the presence of similar monkey-see-monkey-do neurological activity in human subjects, and mirror neurons, to use the term the University of Parma team coined, have emerged as a compelling biological explanation for a broad range of brain activity, from a newborn's instant response to a mother's smile to a movie audience's gasps during a particularly effective chase scene.

Indeed, there are multiple if still tenuous lines of evidence to suggest that neural networks with mirror properties may be responsible for the empathetic response that forms the root of social behavior. They may also help explain how human language emerged from the more primitive communication systems of monkeys and apes. Almost seven years ago, Vilayanur Ramachandran, head of the Center for Brain and Cognition at the University of California at San Diego, went so far as to declare that "mirror neurons will do for psychology what DNA did for biology: they will provide a unifying framework and help explain a host of mental abilities."

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."


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.


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.

To Keysers, this makes sense. The somatosensory cortex, after all, is the region of the brain that responds to sensations registered by the skin--among them tickling, itching, tingling and burning. It's why, says Keysers, our skin seems to crawl when we watch James Bond in Dr. No and see the tarantula creeping up Sean Connery's chest. Likewise, our emotional reactions to such scenes may well be crafted by mirror responses in still other brain regions. Among these, one of the most interesting is the insula, a complex structure that integrates sensory with visceral information. For example, as Keysers and his colleagues recently demonstrated, exposure to nauseating smells--rancid butter, rotten eggs--activates the same area of the insula as watching a video of an actor who sniffs a glass, then reacts with a grimace that conveys his disgust.

Static images found in photographs, paintings and sculptures can also evoke mirror responses, says the University of Parma's Vittorio Gallese, one of the researchers who participated in the original macaque experiments. Gallese is now collaborating with Columbia University art historian David Freedberg on a project that will explore the link between mirror activity and aesthetic experience. "Go to the Borghese Gallery and look at Bernini's Rape of Proserpina," Gallese suggests. "Even though the statue is made of marble, one of the coldest materials on earth, it conveys a vivid impression of carnality. Or look at Goya's Disasters of War, with its excruciating images of lacerated bodies. The powerful emotional resonance we get is due, in part, to our empathetic reaction to pain."

That mirroring of pain does occur seems clear. A study undertaken by researchers at University College London recently showed that the mere thought that a loved one's hand is receiving an electric shock lights up many of the same brain areas as shocks that are directly experienced.

It may even be, as Gallese and others have proposed, that mirroring is a general mechanism for grasping the feelings of others and sharing their moods. "It's why, even though we are trapped in ourselves, we can have a good understanding of other people," says Iacoboni. Once again, it appears that the cues can be auditory as well as visual. For example, prosody, the melodic component of human speech, is one of the ways people convey information about their continuously shifting emotional states. Aziz-Zadeh's da-da-da-da-da experiment, in fact, is designed to pinpoint the brain areas that house the prosody mirror circuitry.


NEURONS WITH MIRROR PROPERTIES ARE NOT SPECIAL IN ANY obvious sense. Under a microscope, they look like other neurons. What makes them special is the web of connections that link neurons in the motor and sensory systems to the limbic centers that process visceral and emotional reactions. And while some of these connections may well be in place at birth, they are, neuroscientists think, vastly expanded through experience. A baby smiles. Her mother smiles back. Click. The brain sets up a circuit linking the motor system that turns up the corners of the baby's mouth to the visual image of the smiling mother to the emotional state we call happiness.

But while human mirror systems are similar, they are not identical. Individuals vary widely, for example, in their capacity to resonate with the emotional state of others--something that can be measured by psychological tests. In a sequel to their rancid-butter experiment, Keysers' team found that subjects with higher empathy scores on such tests also exhibited stronger mirror reactions to facial expressions of both disgust and pleasure.

An even more provocative result comes from a study undertaken in 2005 by UCLA developmental psychologist Mirella Dapretto and her colleagues. They found that autistic children, compared with other children, showed depressed activity in their premotor cortex while imitating or observing facial expressions--and the more severe the autism, the more depressed the activity was. The results did not surprise Dapretto. A central problem in autism, after all, is an impaired ability to understand the feelings of others, and it seems plausible, if far from proven, that a deficiency in the mirror-neuron system could be involved.

Whether insights into the mind's hall of mirrors will actually lead to a better understanding of autism is another question, of course, and it's only one of many that remain unanswered. But as a number of researchers see it, this is a strength rather than a weakness. For the mirror-neuron system has provided neuroscientists with a powerful new probe into the biological roots of the human psyche and prompted them to take a fresh look at old questions. Indeed, says Parma's Gallese, that's what makes the research so exciting--it's still in an early phase, and the fun has just begun.