This article was a front-page feature on Medium.
Have you ever wondered whether your arm actually belongs to you? I’d bet most people answer with a resounding “no”—because who would ask themselves such a question? But believe it or not, body ownership is something people take for granted far too often.
With many perceptual and cognitive functions, the brain operates outside conscious awareness. Minds don’t need to be burdened with or use working memory capacity on processes that work perfectly well unconsciously. And body and limb ownership is exactly one of those unconscious processes. Considering whether our arm belongs to us isn’t something we need to do—except when that process has a problem.
In typical everyday life, most of us experience an inner self inside our bodies (an intuition not unlike what many religions describe as a “soul”). Although it’s scientifically implausible for an immaterial self to exist separate from a biological body, it is difficult to shake this natural intuition. But when the experience of self versus body goes awry, the oddity of it all is in full view.
Out-of-body experiences are characterized by a person feeling as though they are seeing their own body from the outside, e.g., from the ceiling looking down or from across the room. They experience their inner self as projected out of their bodies and flung across space. These experiences have occurred in a range of neurological and psychiatric patients, but around 10% of healthy people have also reported such encounters once or twice in their lifetime.
Strange vestibular (balance and spatial orientation) sensations are common during out-of-body experiences and include a feeling of floating or flying in space separate from the body. Under normal circumstances, our senses are fully integrated, creating a coherent perception of our body in the world. But when vestibular sensations fail to integrate with visual sensations, this creates a conflict between how we perceive our body in space and how we perceive the outside world. Add in conflicts between proprioceptive (body position) and tactile (touch) senses, and our sense of our own body can be so confused it produces an out-of-body experience.
These conflicts between senses can be caused by sudden disruptions of the temporoparietal junction (TPJ) in the brain, which can occur during a struggle to remain conscious. The TPJ is known to be involved in exactly the kinds of integrated sensations that give a unified sense of self and body. In fact, if what’s going on in the TPJ is temporarily interrupted with transcranial magnetic stimulation—or when virtual reality causes a conflict between senses—how someone experiences their body becomes warped, and they’ll potentially feel something resembling an out-of-body experience.
A simple trick called the “rubber hand illusion” can create a distorted sense of body ownership. If you want to try it, first, grab a fake hand (a stuffed rubber glove works) and sit in front of a table with both hands resting on it. Place the rubber hand on the table immediately to the right of your left hand and then hide your real left hand from view (ideally with a screen between the fake hand and your real hand). Ask a friend to repeatedly stroke the individual fingers of the visible rubber hand at exactly the same time and in exactly the same way they stroke the fingers of your real hidden hand. As you look at the rubber hand, you might begin to feel that it’s your own real hand.
This happens because of the conflict between your visual and tactile senses. You can feel a stroke on your left hand through your tactile sense, and you can see only one hand on the table roughly where your real hand normally would be. The brain makes sense of this by shifting its representation of left hand to the fake hand. Reactions to this illusion can be entertaining to watch.
In the most extreme neurological cases, the sense of bodily awareness and ownership can fully hit the wall. I’ve previously described a phenomenon known as anosognosia for hemiplegia, where patients with a paralyzed limb believe they have no paralysis at all. Somatoparaphrenia is another bodily delusion where patients deny they even own their limb. For example, one patient with a paralyzed left arm was both unaware of her arm’s disability and firmly claimed the arm belonged to her granddaughter. Discussions and reasoning with doctors generally did not help, leading either to a confused response of “I do not know” or a stronger conviction in her delusional belief.
Although we never think about body ownership under normal circumstances, the brain actively represents our body as a unique entity belonging to us, separate from the bodies of everyone else we meet.
These symptoms are frequently associated with significant damage in the right hemisphere of the brain. In one study, researchers measured automatic bodily anxiety responses (specifically, skin conductance responses or SCRs) as a sharp needle approached patients’ paralyzed limbs. Unlike patients who have normal paralysis, patients with somatoparaphrenia actually elicited a weaker SCR when the needle approached their paralyzed and disowned hand than when it approached their normal right hand. In other words, their brain so completely disowned the limb, they no longer worried the limb would hurt when stabbed.
The study above may present the most compelling example of how a sense of body ownership can practically disappear when things go sufficiently wrong in the brain. It’s also a reminder that although we never think about body ownership under normal circumstances, the brain actively represents our body as a unique entity belonging to us, separate from the bodies of everyone else we meet.
At least for me, the general malleability and vulnerability of our body representations inspires a greater appreciation for what goes on behind the scenes of our awareness. We don’t need to wait for our senses to break down to consider how valuable they really are to us. Perhaps the next time you look down at your left arm, you’ll spend an extra few seconds pondering just how amazing it is that you know it’s there and that it is yours. One less thing to take for granted.