The Social Brain

Imitation, as the saying goes, is the sincerest form of flattery. But it even seems to form the heart of our humanity. Indeed, imitation appears to be a vital key to understanding human development, from behavior and language to empathy and social skills.

Compared with most other species, human beings are so immature at birth that they require an exceptionally long period of nursing and shelter. We spend our long infancy and childhood adapting to the widely varying and complex sociocultural environment that surrounds us. This allows us to interact and operate successfully within our environment throughout our lives.

From the outset, even before we develop the ability to use spoken language, nonverbal communication drastically shapes our personality and sense of self. Children learn by watching adults and other children, developing important skills under continuous social feedback. This feedback enables proper production and, later, understanding, of emotion-related facial expressions that the subject herself cannot see. Fortunately for human development, healthy children find imitation enormously gratifying. They enjoy both imitating and being imitated.

Imitation is so essential to mastering basic social skills and transferring cultural knowledge that it is likely to have an organic basis in the brain. Mirror neurons, first discovered in monkeys, may serve such a function, providing a common code between the sender and receiver of a nonverbal message. A monkey's mirror neurons are activated both when he himself is acting, say, taking a raisin from a tray, and when he is viewing another monkey (or the human experimenter) performing the same act.

Brain imaging studies have now shown that the human brain contains a similar mirror-neuron system (MNS). For example, merely viewing another person's movements activates our own motor cortex, which we normally use for precise control of our actual movements. As sports fans, musicians, and students of body language know, watching other people's movements and postures may facilitate the viewer's own motor schemes, sometimes resulting in unintended imitation.

One can copy movements and motor acts without understanding their meaning. This happens when a flock of geese suddenly takes flight from a lake, "blindly" following the flock's first frightened member. For humans, the MNS may provide a more sophisticated copying mechanism that helps us to understand the intentions of other people--in a sense, to read their minds. Humans compute other people's intentions and emotions continuously, constantly observing their movements, postures, and gaze. These mind-reading skills are essential for successful social interaction.

Researchers have found that a key part of the human MNS is Broca's region, an area of the brain that, when damaged, prevents the patient from speaking. Broca's region is the human counterpart of the monkey mirror-neuron area. As both human and monkey mirror neurons code hand manipulation and facial gestures, some interesting hypotheses have emerged. For example, the presence of mirror neurons in Broca's region suggests that human language co-evolved with hand and facial gestures rather than arising directly from vocalizations.

In humans, Broca's region is found in the left brain hemisphere, whose dominance for speech is well known. But the corresponding area is larger also in the left brain of great apes, further indicating that the Broca's region evolved first for gestural communication and only later for speech.

Behavioral studies, too, have shown that gesturing is closely related to speech production. To take a familiar example, we gesture even while speaking on the telephone, when others cannot see us. Indeed, congenitally blind persons gesture--even while speaking with people they know to be blind as well!

The existence of the human MNS means that the same brain areas may be activated when we perform a motor act and when we merely observe another person perform a similar act. This leads to an obvious question: how do we know that we actually performed a motor act rather than only seeing it? For most people, physical feedback provided by muscle and tendon sensors helps in resolving this, as does proper communication between brain areas. But misattribution of one's own acts does, in fact, occur in some psychiatric disorders.

Other disorders are associated with defective imitation skills. For example, autistic individuals imitate others less and in a different manner than healthy subjects. They also have poor mind-reading skills. Some patients, by contrast, suffer at the opposite extreme of dysfunction. They "echo," imitating almost all movements of other people.

Perhaps most intriguing, the MNS could provide a platform for mental simulation of future actions--one's own and those of others. It might also be part of a larger mechanism that allows intentions, emotions, and even the intensity of pain to be matched by gestures and communicated between individuals.

New imaging tools enable us to study these and other questions about human brain function more productively than ever before. We can now accurately follow in both time and space the brain's processing routes, which are determined not only by genes, but also by all the experience that an individual has gained during her entire lifetime.

In these studies, neurologists, psychiatrists, geneticists, and behavioral researchers all contribute to realizing their common interest in discovering how the human brain operates. Brain imaging, combined with the new conceptual framework implied by research on the mirror-neuron system, promises to uncover a more holistic "social" brain. This brain's functions and dysfunctions--its very structure--will broaden dramatically our understanding of the relationship between the self and others.