IF THERE IS ANYTHING about your self of which you can be sure, it is that it is anchored in your own body and yours alone. The person you experience as you is here and now and nowhere else.

But even this axiomatic foundation of your existence can be called into question under certain circumstances. Your sense of inhabiting your body, it turns out, is just as tenuous an internal construct as any of your other perceptionsand just as vulnerable to illusion and distortion. Even your sense of owning your own arm is not fundamentally differentin evolutionary and neurological termsfrom owning your car (if you are Californian) or your shotgun (if you are Sarah Palin).

Outlandish as such a notion may seem, what you think of as your self is not the monolithic entity that youand itbelieve it to be. In fact, it is possible to pharmacologically manipulate body ownership with a drug called ketamine, which reliably generates out-of-body experiences in normal people. Patients on ketamine report the sensation of hovering above their body and watching it. If someone gives them a sharp poke, they might say, My body down below is feeling the pain, but I dont feel it myself. Because in such patients the I is dissociated from the body it inhabits, they do not experience any agony or emotional distress (for this reason, ketamine is sometimes used as an anesthetic).

Your sense of body ownership, and of being a distinct entity, seems to derive in part from a network of brain cells known as mirror neurons. Located in the premotor cortex, they interact with your prefrontal cortex, the part of the brain that makes plans and decisions. Ordinarily, when you move your hand to, say, reach for a pen (a motion that is accompanied by your sense of having free will), certain motor-command neurons in the motor cortex fire. Intriguingly, as Giacomo Rizzolatti of the University of Parma in Italy and his colleagues Marco Iacoboni and Vittorio Gallese have demonstrated, some of these neurons also fire when you merely watch another person perform the same action.

Mirror neurons allow you to put yourself in another persons shoes. Your brain says, in effect, The same neurons are firing as when I move my hand, so I know what he is feeling and what he is up to. In addition, neurons we might loosely call touch mirror neurons fire when you are touched or watch someone else being touched. That humans have these abilities made intuitive sense to Charles Darwin, who noted that when you watch a javelin thrower about to release the spear, your leg muscles flinch unconsciously and that when a child watches his mother use a pair of scissors, he clenches and unclenches his jaws in uncontrollable mimicry. In this phenomenon we see an evolutionary prelude to the ability to imitate and emulatethe basis of cultural transmission of knowledge.

Yet as you grow to adulthood, you no longer irresistibly mime the actions of whomever you happen to be looking at; your self doesnt feel like a puppet controlled by others. You preserve your sense of free will and agency (although patients with Tourette syndrome do sometimes engage in unconscious mimicry).

The tendency to unconsciously mimic the person you are with is normally inhibited by your prefrontal cortex (the most evolutionarily advanced part of the brain, which is pronounced in humans). We recently suggested in an essay on the Edge Foundation Web site (www.edge.org) that interactions between the mirror neuron system and feedback from the prefrontal cortex is what gives the self its peculiar dual character of simultaneously maintaining individuality and reciprocity with others.

Derangements in this system would lead to out-of-body experiences, which may explain the mechanism of ketamine. Under its influence you empathize with your body the same way you empathize with other people, and you are able to simultaneously detach yourself from itjust as you detach yourself from others.

Parlor Tricks to Lose Yourself In
You dont need ketamine to produce such dissociations, however; if you have the money, you can do it with immersive virtual-reality technology. For the rest of us there are some simple optical tricks.

For example, try looking at a Halloween mask through a shiny pane of glass, so that you see a reflection of your face superimposed on the mask. By changing the relative illumination of the mask and your face, you can optically blend the two to produce a strange hybrid creature. Now make odd facial expressions, and you will get the impression that the creature is mimicking your contortions in perfect synchrony. The experience should give you a momentary sense of decapitationan inkling of what it must feel like to take ketamine.

The illusion will be enhanced if you place two panes of glass at right angles. Shift your head until the reflection of the center of your nose is exactly on the corner of the two panes (and superimposed on the mask behind). If you now wink your right eye, the reflection will wink its right eye (the double reflection violates an ordinary reflections left-right reversal). The result is an even more compelling illusion that you occupy the mask.

If you go to the next levelwhich involves a combination of lighting, makeup, mannequins and a hall-of-mirrors effect created when you stand between two body-length mirrors that face one another, producing an endless number of optical clones of yourselfyou start to approximate the effects of ketamine. In the mid-1990s we showed (with William Hirstein and Eric L. Altschuler of the University of California, San Diego) that punching the mask under these conditions produces instant fright. We measured subjects fear objectively by monitoring changes in their skin resistancethat is, how much they sweated. If I threatened any old mask you were looking at (without using optics to help you identify with it), you would not flinch. Its the sense of merging with the other head that does it.

More recently, scientists have used video cameras to produce similar disembodiment illusions, in which people feel they are projecting their body to some outside location. These spooky experiences are of the kind that might occur after, say, a stroke damaged the right parietal lobe. This is the area of the brain that seems to be partly responsible for creating body image, a sense of inhabiting ones own form.

Patients with right parietal lobe damage sometimes feel they are seeing themselves from the outside (as with ketamine), or they may experience a doppelgnger. A few years ago we saw a patient with a right frontoparietal brain tumor who was mentally normal in every respect except that he felt a phantom twin attached to the left side of his body that mimicked his every action. If he was touched, he also felt the twin being touched a few seconds later. Stimulating the vestibular canals in the patients inner ear made him feel like he was twirling around and caused the phantom to shrink and shift. (The vestibular system, which contributes to balance and spatial orientation, connects to the right parietal lobe.)

The great English neurologist MacDonald Critchley described many other patients whodepending on the parts of the parietal lobe involvedfelt like giants or pygmies; experienced their body parts as distorted or swollen; disowned an arm, claiming it belonged to their mother; or even hated a particular limbclaiming, for example, that my hand is a communist. We suggest that the sense of ownership of even external objects (wedding rings, tennis rackets) that is so ubiquitous in our species (Gandhi being a notable exception) may have exaptedin other words, developed as a secondary usefrom neural systems that originally evolved for body ownership.

The Mirror Cure
We mentioned earlier that one reason you do not mimic someone or literally feel anothers touch sensations when you watch her being touched is that your prefrontal cortex inhibits your mirror neuron output. A second reason may be that when you watch someone else being touched, even though your touch mirror neurons are active, your skin receptors report the fact that they are not being touched, and this null signal prevents the mirror neuron activity from reaching the threshold of conscious experience.

But guess what would happen if someone were to numb your hand using an anesthetic? Astonishingly, we have found (in collaboration with U.C.S.D. graduate student Laura Case) that the patient now quite literally feels touch sensations in his anesthetized hand when he merely watches another person being poked. Or if the other person handles an ice cube, the patient feels the cold freezing his hand! Once you remove the touch signals from the intact hand, the patient does not merely empathize with othershe feels what they touch. The same thing happens in patients with phantom limbs. Watching another persons hand being massaged seems to relieve pain in the patients absent arm or leg.

Clinically it is known that visual feedback using mirror reflections can help alleviate phantom pain and stroke paralysis, perhaps by tapping into mirror neurons. We are currently exploring whether illusions of disembodiment produced with mirrors can also be used to mimic the effects of ketamine and treat chronic pain syndromes by allowing a patient to detach from his body and the pain it experiences.

Extraordinarily, even real pain in a real arm can be cured through optical feedback. In particular, there is a cruel disorder called reflex sympathetic dystrophy in which a trifling injury leads to permanent excruciating pain, swelling and paralysis of an arm, a condition we have dubbed learned pain and paralysis. In 1995, in a lecture at the Society for Neuroscience meeting in San Diego, we suggested using mirrors to treat this disorder, and several large-scale clinical trials have since confirmed their efficacy. Even the swelling subsidesa remarkable example of mind-body interaction.

The strangest of body-image disturbances is one in which a perfectly healthy person desires to have an arm or leg amputated. In conjunction with our U.C.S.D colleagues David Brang and Paul McGeoch, we have found that touching the skin of the affected limb produces an abnormal sweating response, whereas touching the normal limb does not. Further, our brain-imaging studies indicate an impoverished representation of the affected limb in the right parietal lobe (the body image area), although the areas for touch in the somatosensory cortex remain normal. This discrepancy between accurate sensory input from the arm and a lack of arm representation in the brain creates a curious abhorrence of the limb [see Amputee Envy, by Sabine Mueller; Scientific American Mind, December 2007/January 2008].

Thus, studying people with brain abnormalities or manipulating sensory input in normal people using mirrors and other optical tricks can provide key insights into the way the right parietal lobe of the brain creates a vibrant image of ones body that endures in space and time.

These observations have important implications, both theoretical and clinical. They suggest that what we call touch sensation, pain, the body or even the self results from a dynamic interplay of signals from three sources: sensory signals from the skin, muscles and gut; inhibitory signals from the prefrontal cortex; and input from mirror neurons, which respond to behavior that originates in neurons in other peoples brains! From this fluctuating mosaic of brain activity emerges your sense of an embodied self that is distinct from others and all your own.