My favorite region of the brain changes from year to year, but I'm currently fascinated by the rostral part of the ventral premotor area. Giacomo Rizzolatti at the University of Parma has elegantly explored the properties of neurons in this part of the brain -- the so-called "mirror" neurons, or "monkey see, monkey do" neurons. His research indicates that any given cell in this region will fire when a test monkey performs a single, highly specific action with its hand: pulling, pushing, tugging, picking up, grasping, etc. In addition, it appears that different neurons fire in response to different actions.

One might be tempted to think that these are motor "command" neurons, making muscles do certain things; however, the astonishing truth is that any given mirror neuron will also fire when the monkey in question observes another monkey (or even the experimenter) performing the same action! With knowledge of these neurons, you have the basis for understanding a host of very enigmatic aspects of the human mind: imitation learning, intentionality, "mind reading," empathy -- even the evolution of language. Anytime you watch someone else doing something (or even starting to do something), the corresponding mirror neuron might fire in your brain, thereby allowing you to "read" and understand another's intentions, and thus to develop a sophisticated "theory of other minds."

Mirror neurons may also help explain the emergence of language, a problem that has puzzled scholars since the time of Charles Darwin. Is language ability based on a specially purposed language organ that emerged suddenly "out of the blue," as suggested by Noam Chomsky and his disciples? Or did language evolve from an earlier, gesture-based protolanguage? No one knows for sure, but a key piece of the puzzle is Rizzolatti's observation that the ventral premotor area may be a homologue of "Broca's area" -- a brain center associated with the expressive and syntactic aspects of language. Rizzolatti and Michael Arbib of the University of Southern California suggest that mirror neurons may also be involved in miming lip and tongue movements, an ability that may present the crucial missing link between vision and language.

To test this exciting idea, my colleagues and I recently tested four Broca's aphasia patients -- individuals with lesions in their Broca's areas. We presented them with the sound of the syllable "da," spliced to a videotape of a person whose lips were actually producing the sound "ba." Normally, people hear the "da" as "ba" -- the so-called "McGurk effect" -- because vision dominates over hearing. To our surprise, though, we found that the Broca's patients did not experience this illusion; they heard the syllable correctly as "da." Even though their lesions were located in the left frontal region of their brains, they had a visual problem -- they ignored the lip movements. Our patients also had great difficulty with simple lip reading. This experiment provides a link between Rizzolatti's mirror neurons and the evolution of human language, and thus it calls into question the strictly modular view of language, which is currently popular.

We also studied three patients with anosognosia: they all vehemently denied that their left arms were paralyzed, when it was plainly the case. Remarkably, we found that two of these patients also denied the paralysis of another patient whose inability to move his arm was clearly visible to them. We suggest that this bizarre observation is best understood in terms of damage to Rizzolatti's mirror neurons. In addition, at the University of California-San Diego, Eric Altschuler and I have studied mu waves (brain waves associated with arm movements) in normal subjects. These waves are normally suppressed when the subject begins any voluntary movement, but we find now that they are also suppressed when the subject watches another person performing an action. Could mu wave suppression also reflect the activity of the mirror neuron system? These and other fascinating questions remain to be answered.

With such exciting developments, I predict that mirror neurons will do for psychology what DNA did for biology: they will provide a unifying framework and possibly even explain a host of mental abilities that have hitherto remained mysterious and inaccessible to experiments.

Vilayanur Ramachandran is a neurologist at the University of California-San Diego and co-author of Phantoms in the Brain: Probing the Mysteries of the Human Mind.