"When you see me perform an action -- such as picking up a baseball -- you automatically simulate the action in your own brain," said Dr. Marco Iacoboni, a neuroscientist at the University of California, Los Angeles, who studies mirror neurons. "Circuits in your brain, which we do not yet entirely understand, inhibit you from moving while you simulate," he said. "But you understand my action because you have in your brain a template for that action based on your own movements.

"When you see me pull my arm back, as if to throw the ball, you also have in your brain a copy of what I am doing and it helps you understand my goal. Because of mirror neurons, you can read my intentions. You know what I am going to do next."

He continued: "And if you see me choke up, in emotional distress from striking out at home plate, mirror neurons in your brain simulate my distress. You automatically have empathy for me. You know how I feel because you literally feel what I am feeling."

Mirror neurons seem to analyzed scenes and to read minds. If you see someone reach toward a bookshelf and his hand is out of sight, you have little doubt that he is going to pick up a book because your mirror neurons tell you so.

In a study published in March 2005 in Public Library of Science, Dr. Iacoboni and his colleagues reported that mirror neurons could discern if another person who was picking up a cup of tea planned to drink from it or clear it from the table. "Mirror neurons provide a powerful biological foundation for the evolution of culture," said Patricia Greenfield, a psychologist at the U.C.L.A. who studies human development.

Until now, scholars have treated culture as fundamentally separate from biology, she said. "But now we see that mirror neurons absorb culture directly, with each generation teaching the next by social sharing, imitation and observation."

Other animals -- monkeys, probably apes and possibly elephants, dolphins and dogs -- have rudimentary mirror neurons, several mirror neuron experts said. But humans, with their huge working memory, carry out far more sophisticated imitations.