Hold me tight! Don't let me go.

(female narrator) Why do people cry at movies? Why do sports fans get so excited during a game? Why do they feel anxious if their team is losing? The answer-- mirror neurons, a set of brain cells on either side of the head that might explain empathy. We found the mechanism that underlies something, which is absolutely fundamental to the way we see other people.

(narrator) The mirror neuron was first discovered at the University of Parma in Italy. Researchers there identified a neuron that fires both when a monkey picks up a peanut and when the monkey watches someone do it. The same neurons-- one neuron-- fire both when the monkey observes something and does it. It's almost unbelievable.

(Glaser) It was surprising because this cell involved with motor planning for the monkey turned out to be interested in the movements of others.

(narrator) The brain uses the mirror neurons to mirror the movements it sees. This happens in people too.

(Glaser) If you use the years of training you've done-- learning to crawl, then walk, then eat-- this is an incredibly rich set of knowledge to apply to seeing what's going on. The mirror system is the way you harness your own abilities and project them out into the world.

(narrator) Sports fans picture themselves playing when they watch a game. What makes them feel pain when their player drops the ball? The idea was to figure out how the emotional system and this motor system are connected.

(narrator) To find out, Marco Iacoboni looked at the brain scans of individuals when presented with a series of faces. In the first round, he asked people to imitate the faces. In the second round, he asked people to simply look at the faces. As expected, he found that the mirror neurons were activated during both rounds. But in addition, he found that the happy area of the brain lit up whenever people either imitated or looked at happy faces. So he thinks the mirror neurons can send messages to the emotional system in our brain. That's the mechanism that allows us to feel empathy with other people. Yeah, yeah! We strongly believe that's a unifying mechanism that allows people to connect at a simple level.

(narrator) So what happens if the mechanism is broken?

(man) It's been known for some time that children with autism could be intelligent, but have a profound deficit in social interaction.

(narrator) Christian likes to play, but like many with autism, prefers to play alone. He has a hard time communicating and does not always understand questions. Can you tell me what you did in school today? Doing well.

(narrator) Dr. Ramachandran and his graduate student hope to gain understanding of autism by studying the brain waves of children like Christian. We're going to read your brain waves with this.

(narrator) When mirror neurons are working correctly, they fire both for doing an action and seeing the same action. So for most people, the brain waves for doing and seeing look the same. But for Christian, the waves look different. Their brains may be different in that regard, and they may have deficits in the mirror neuron system. We don't know for sure. Additional work is needed using brain imaging.

(narrator) We do know healthy humans are very social. Everybody's interested-- what makes humans unique? You can say humor-- the laughing biped. Language, certainly. But another thing is culture. A lot of culture comes from imitation-- watching your teachers do something.

(narrator) So the mirror neuron is a big part of what makes us human.

(Glaser) There'd be little point in a mirror system living on your own. There'd be a point to having a digestive system or having a movement system or a visual system, but no point to a mirror system. The mirror system is the basic social brain system, a system which is pointless if you don't interact or relate to others.