Today we’re talking bodies. Not how they look in skinny jeans or whether they can win a Tour de France without steroids. We’re talking about how it feels to have a body of your own, one that is (or seems to be) conveniently connected to your head and neck.

I’ve written about body ownership before in the context of pregnancy. Although I focused on how I dreamt of my body during sleep, I also mentioned that my ballooning physical dimensions affected my coordination. I’d bump into countertops or doorways with my big belly and sometimes struggled to locate my center of gravity. Yet as strange as my new body was, it always felt like it belonged to me. This was an enormous blessing, of course, but it’s somewhat surprising  as well. After all, before my pregnancy I’d lived with the same body since puberty. After more than a decade and a half of experience with that body, I suddenly had to adjust to my new body in a matter of months. Or rather days, because that new body kept growing larger still. Although my belly would feel surreal at times, overall I had remarkably little trouble adjusting to my metamorphosis. The body was still mine in all its lumpy glory.

I was reminded of this experience recently when I came across a scientific paper about body swapping. I know it sounds as if the only science in something called body swapping must come from the term science fiction. Actually, body swapping is a remarkable perceptual illusion that requires nothing more than a second person, a set of head mounted cameras and a set of head mounted displays. Someone facing you wears the cameras mounted on a helmet and you wear the visual displays (which are presented to your two eyes like goggles as part of a virtual reality-style headset). The camera footage, filmed from the visual perspective of the second person, is fed directly into your visual display. Thus, you see your own body from the second person’s perspective.

But we haven’t made it to Freaky Friday just yet. The illusion requires something more. You and the other person take each other’s hands and begin squeezing them simultaneously. Nothing fancy. But in the words of the write up by Valeria Petkova and Henrik Ehrsson, this simple setup alone “. . . evoked a vivid illusion that the experimenter’s arm was the participant’s own arm and that the participants could sense their entire body just behind this arm. Most remarkably, the participants’ sensations of the tactile and muscular stimulation elicited by the squeezing of the hands seemed to originate from the experimenter’s hand, and not from their own clearly visible hand.”

So after a lifetime in your own body, it only takes a video feed and a few hand squeezes for you to make yourself at home in someone else’s arms and legs. If this setup sounds familiar, it is a more impressive incarnation of the classic rubber hand illusion. And a new and remarkable twist on the illusion just appeared in the news: scientists in the same lab have made people feel as if they have an invisible hand. (For a great discussion of this new illusion, read this.)

In science, we tend to think about human perception in general and illusions in particular in terms of adaptations and optimizations. Lots of visual illusions are based on the statistical probability of objects and events in our environment. Our brains learn to predict and extrapolate information about our settings because they jump to the likeliest conclusions. In this way illusions, while technically errors, often reveal clever shortcuts our brain takes to help us understand or parse our surroundings faster, better, or at less of an energy cost.

But what about the body swap? Since we never actually swap bodies, why should we mentally be able to do it? What’s the advantage? Well, the advantage seems to come down to the very fact that we never actually swap bodies. In our ever-changing world, a rare given is that you will have the same body tomorrow that you had today and yesterday. So why should your brain waste precious time or energy soliciting proof from every finger and toe, curve and joint, flex and bend? Take a smidge of visual evidence (in this case, the video display) and a dab of tactile confirmation (hand squeezing) and you have a recipe for body ownership. How often in the natural world would this recipe ever lead you astray?

So in essence you only think that you feel that you own your body. In truth, your brain is creating that sensation on the fly all the time. You could think of it as a philosophical conundrum or cause for an existential crisis. I prefer to think of it as good news for pregnant ladies everywhere.

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Photo credit: Elizabeth Tan

Petkova VI, & Ehrsson HH (2008). If I Were You: Perceptual Illusion of Body Swapping PLOS One DOI: 10.1371/journal.pone.0003832