The Mind

A special class of brain cells reflects the outside world, revealing a new avenue for human understanding, connecting and learning By Giacomo Rizzolatti, Leonardo Fogassi and Vittorio Gallese

John watches Mary, who is grasping a flower. John knows what Mary is doing—she is picking up the flower—and he also knows why she is doing it. Mary is smiling at John, and he guesses that she will give him the flower as a present. The simple scene lasts just moments, and John's grasp of what is happening is nearly instantaneous. But how exactly does he understand Mary's action, as well as her intention, so effortlessly?

A decade ago most neuroscientists and psychologists would have attributed an individual's understanding of someone else's actions and, especially, intentions to a rapid reasoning process not unlike that used to solve a logical problem: some sophisticated cognitive apparatus in John's brain elaborated on the information his senses took in and compared it with similar previously stored experiences, allowing John to arrive at a conclusion about what Mary was up to and why.

Although such complex deductive operations probably do occur in some situations, particularly when someone's behavior is difficult to decipher, the ease and speed with which we typically understand simple actions suggest a much more straightforward explanation. In the early 1990s our research group at the University of Parma in Italy, which at the time included Luciano

54 SCIENTIFIC AMERICAN NOVEMBER 2006

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ACTION PERFORMED by one person can activate motor pathways in another's brain responsible for performing the same action. The second understands viscerally what the first is doing because this mirror mechanism lets her experience it in own her mind.

Fadiga, found that answer somewhat accidentally in a surprising class of neurons in the monkey brain that fire when an individual performs simple goal-directed motor actions, such as grasping a piece of fruit. The surprising part was that these same neurons also fire when the individual sees someone else perform the same act. Because this newly discovered subset of cells seemed to directly reflect acts performed by another in the observer's brain, we named them mirror neurons.

Much as circuits of neurons are believed to store specific memories within the brain, sets of mirror neurons appear to encode templates for specific actions. This property may allow an individual not only to perform basic motor procedures without thinking about them but also to comprehend those acts when they are observed, without any need for explicit reasoning about them. John grasps Mary's action because even as it is happening before his eyes, it is also happening, in effect, inside his head. It is interesting to note that philosophers in the phenomenological tradition long ago posited that one had to experience something within oneself to truly comprehend it. But for neuroscientists, this finding of a physical basis for that idea in the mirror neuron system represents a dramatic change in the way we understand the way we understand.

Instant Recognition our research group was not seeking to support or refute one philosophical position or another when we first noticed mirror neurons. We were studying the brain's motor cortex, particularly an area called F5 associated with hand and mouth movements, to learn how commands to perform certain actions are encoded by the firing patterns of neurons. For this purpose, we were recording the activity of individual neurons in the brains of macaques. Our laboratory contained a rich repertoire of stimuli for the monkeys, and as they performed various actions, such as grasping for a toy or a piece of food, we could see that distinct sets of neurons discharged during the execution of specific motor acts.

Then we began to notice something strange: when one of us grasped a piece of food, the monkeys' neurons would fire in the same way as when the monkeys themselves grasped the food. At first we wondered whether this phenomenon could be the result of some trivial factor, such as the monkey performing an unnoticed movement while observing our actions. Once we managed to rule out this possibility and others, including food expectation by the monkeys, we realized that the pattern of neuron activity associated with the observed action was a true representation in the brain of the act itself, regardless of who was performing it.

Often in biological research, the most direct way to establish the function of a gene, protein or group of cells is simply to eliminate it and then look for deficits in the organism's health or behavior afterward. We could not use this technique to determine the role of mirror neurons, however, because we found them spread across important regions on both sides of the brain, including the pre-motor and parietal cortices. Destroying the entire mirror neuron system would have produced such broad general cognitive deficits in the monkeys that teasing out specific effects of the missing cells would have been impossible.

So we adopted a different strategy. To test whether mirror neurons play a role in understanding an action rather than just visually registering it, we assessed the neurons' responses when the monkeys could comprehend the meaning of an action without actually seeing it. If mirror neurons truly mediate understanding, we reasoned, their activity should reflect the meaning of the action rather than its visual features. We therefore carried out two series of experiments.

First we tested whether the F5 mirror neurons could "recognize" actions merely from their sounds. We recorded the mirror neurons while a monkey was observing a hand motor act, such as ripping a sheet of paper or breaking a peanut shell, that is accompanied by a distinctive sound. Then we presented the monkey with the sound alone. We found that many F5 mirror neurons that had responded to the visual observation of acts accompanied by sounds also responded to the sounds alone, and we dubbed these cell subsets audiovisual mirror neurons.

Next we theorized that if mirror neurons are truly involved in understanding an action, they should also discharge when the monkey does not actually see the action but has sufficient clues to create a mental representation of it. Thus, we first showed a monkey an experimenter reaching for and grasping a piece of food. Next, a screen was positioned in front of the monkey so that it could not

Overview/Meeting of Minds i Subsets of neurons in human and monkey brains respond when an individual performs certain actions and also when the subject observes others performing the same movements. i These "mirror neurons" provide a direct internal experience, and therefore understanding, of another person's act, intention or emotion. i Mirror neurons may also underlie the ability to imitate another's action, and thereby learn, making the mirror mechanism a bridge between individual brains for communication and connection on multiple levels.

The pattern of activity was a true representation in the brain of the act itself, regardless of who was performing it.

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