Image: Courtesy of Julian Keenan

Nearly everyone has seen a dog bark at its image in the mirror--or a cat swat at its reflection. Indeed, only we humans and the apes seem capable of self-recognition. But how? Scientists have long wondered about the ability because it appears to be an important piece in the puzzle of human consciousness. Now they have a new clue, thanks to research published in today's issue of Nature. By way of two different experiments, Julian Keenan and colleagues at Beth Israel Deaconess Medical Center and Harvard Medical School have demonstrated that the right hemisphere plays an essential part in making self-identifications.

The first experiment enlisted the help of five epilepsy patients undergoing pre-operative testing to determine whether their right or left hemispheres were dominant for speech and memory. During the test, doctors anesthetized each half of each patient's brain for up to three minutes--and during those times, Keenan and colleagues presented each patient with an image of his or her face morphed with that of a famous person. Women's faces were combined with either Marilyn Monroe or Princess Diana; men's faces were mixed with Albert Einstein or Bill Clinton, as was Keenan's face in the example shown here.

When the anesthesia wore off, the scientists asked the patients to remember who they saw. All of the patients readily recalled having seen their own face when their left hemispheres were asleep. But after their right hemispheres awoke, four out of five only remembered seeing the famous person. To follow up, the scientists then did a second test on 10 healthy people working in the Medical Center's neurology department. They asked each subject to look at pictures of themselves morphed with famous people--and of their colleagues morphed with famous people--and measured the brain activity in each hemisphere using transcranial magnetic stimulation. Again self-recognition involved a significantly greater amount of activity on the right side of the brain. "It's not an all or nothing phenomenon," Keenan says, "but recognizing one's own face appears to be a preferential ability of the right hemisphere."