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A new study offers a small advance in our understanding of the body ownership illusion, but it is a good opportunity to review this cool and important neurological phenomenon. It also has practical implications. The body ownership illusion is the subjective sense that you occupy or own your body and its parts. There is also a separate phenomenon that gives you the subjective sense that you control your body parts.

What I find extremely interesting about these phenomena is that we initially didn’t know they needed to exist. People don’t generally wonder what creates the sense that they own and control their own bodies. It’s not even a question people think to ask (unless they are somehow involved in neuroscience). At first it may seem obvious – we are our bodies, and we do control our bodies, so why shouldn’t we have that sense? But like everything you experience and feel, this is not a passive or automatic sensation. It is an active construction of your brain. There are dedicated circuits in the brain whose specific function is to create these sensations.

As with most neurological phenomena we first suspected their existence by encountering patients who have suffered brain damage (like from a stroke) and therefore have a lack of some subset of these functions. For example, there is alien-hand syndrome. This is the sense that a body part is acting “on it’s own” without your conscious control. There are also cases that demonstrate the separation of body parts from the sense that we own those body parts. Phantom limbs, for example, occur when a limb is removed but the brain circuitry that creates the sense of ownership of the limb is still there. There can even be supernumerary phantom limbs – an illusion of an extra limb that was never there. This can happen when a stroke paralyzes a limb but spares the circuitry that creates the sense of ownership. There are also cases of the apparent opposite – when a limb is present and functioning, but the person has the uncomfortable sense that it does not belong to them. We also have evidence from certain drug-induced and other states of feeling entirely separated from our bodies.

We have a partial understanding of how our brains create these sensations. They all involve circuits that compare two or more inputs. When they are synchronous we have the subjective sense of ownership or control. For the sense of control the circuits compare our intention to move and our actual movements. When they match, we feel we have control. For ownership the primary circuit involves “visual-motor synchronization” or visual-sensory synchronization. If what we see of ourselves matches what we feel, that synchronization produces the sense of ownership. Obviously, we don’t immediately have an out-of-body experience when we close our eyes. There are other sensory inputs that act redundantly – proprioception, vestibular function, tactile sensation, and muscle feedback. It’s a robust system, which is why the illusion rarely breaks in day-to-day life. You have to disrupt the main parts of the brain producing this sensation with drugs, oxygen deprivation, or something else.

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