His double is always a transparent, full figure that is slightly smaller than life size. It often wears different clothing than the patient and does not share the patient’s thoughts or emotions. The double is usually engaged in an activity that the patient feels he should be doing, and he says, “that guy is my guilty conscience.”
Embodiment seems to be the surest thing in the world, the one irrefutable fact. We think of ourselves as being in our bodies, and of our bodies as belonging to us, and us alone: thus we look out on the world with our own eyes, walk with our own legs, shake hands with our own hands. We have a sense, too, that consciousness is in our own head. It has long been assumed that the body image or body schema is a fixed and stable part of one’s awareness, perhaps in part hardwired, and largely sustained and affirmed by the continuing proprioceptive feedback from joint and muscle receptors regarding the position and movement of one’s limbs.
There was general astonishment, therefore, when Matthew Botvinick and Jonathan Cohen showed in 1998 that a rubber hand, under the right circumstances, could be mistaken for one’s own. If a subject’s real hand is hidden under a table while the rubber hand is visible before him, and both are stroked in synchrony, then the subject has the convincing illusion, even though he knows better, that the rubber hand is his — and that the sensation of being stroked is located in this inanimate though lifelike object. As I found when I looked through the “eyes” of a robot, knowledge in such a situation does nothing to dispel the illusion. The brain does its best to correlate all the senses, but the visual input here trumps the tactile.
Henrik Ehrsson, in Sweden, has developed a great range of such illusions, using the simplest equipment — video goggles, mannequins, and rubber arms. By disrupting the usual unity of touch, vision, and proprioception, he has induced uncanny experiences in some people, convincing them that their bodies have shrunk or grown enormous, even that they have swapped bodies with someone else. I experienced this for myself when I visited his laboratory in Stockholm for a number of experiments. In one, I was convinced that I possessed a third arm; in another, I felt embodied in a two-foot-high doll, and as I looked through “its” eyes via video goggles, normal objects in the room appeared enormous.
It is evident, from all of this work, that the brain’s representation of the body can often be fooled simply by scrambling the inputs from different senses. If sight and touch say one thing, however absurd, even a lifetime of proprioception and a stable body image cannot always resist this. (Individuals may be more or less susceptible to such illusions, and one might imagine that dancers or athletes, who have an exceptionally vivid sense of where their bodies are in space, may be harder to fool in this way.)
The body illusions Ehrsson is exploring are very much more than party tricks; they point to the ways in which our body ego, our sense of self, is formed from the coordination of senses — not just touch and vision but proprioception and perhaps vestibular sensation, too. Ehrsson and others favor the idea that there are “multisensory” neurons, perhaps at a number of places in the brain, which serve to coordinate the complex (and usually consistent) sensory information coming into the brain. But if this is interfered with — by nature or experiment — our seemingly unassailable certainties about the body and the self can vanish in an instant.
1. The term “out-of-body experience” was introduced in the 1960s by Celia Green, an Oxford psychologist. While there had been stories of out-of-body experiences for centuries, Green was the first to systematically examine a large number of firsthand accounts, from more than four hundred people whom she located by launching a public appeal through the newspapers and the BBC. In her 1968 book,
2. Several of Celia Green’s subjects described similar feelings. “My mind was clearer and more active than before,” one wrote; another spoke of being “all-knowing and understanding.” Green wrote that such subjects felt they “could obtain an answer to any question they chose to formulate.”
3. August Strindberg noted, in his autobiographical novel
