Parlor Tricks to Lose Yourself In
You don’t 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 decapitation—an 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 reflection’s left-right reversal). The result is an even more compelling illusion that you occupy the mask.
If you go to the next level—which 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 yourself—you 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 resistance—that 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. It’s 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 one’s 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 doppelgänger. 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 patient’s 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 who—depending on the parts of the parietal lobe involved—felt 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 limb—claiming, 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 exapted—in other words, developed as a secondary use—from 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 another’s 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.