That’s not my hand! How the brain can be fooled into feeling a fake limb

Scientists have made the first recordings of the human brain’s awareness of its own body, using the illusion of a strategically-placed rubber hand to trick the brain. Their findings shed light on disorders of self-perception such as schizophrenia, stroke and phantom limb syndrome, where sufferers may no longer recognize their own limbs or may experience pain from missing ones.

In the study published today in Science Express online, University College London’s (UCL) Dr Henrik Ehrsson, working with Oxford University psychologists, manipulated volunteers’ perceptions of their own body via three different senses – vision, touch and proprioception (position sense).

They found that one area of the brain, the premotor cortex, integrates information from these different senses to recognize the body. However, because vision tends to dominate, if information from the senses is inconsistent, the brain “believes” the visual information over the proprioceptive. Thus, someone immersed in an illusion would feel, for example, that a fake limb was part of their own body.

In the study, each volunteer hid their right hand beneath a table while a rubber hand was placed in front of them at an angle suggesting the fake hand was part of their body. Both the rubber hand and hidden hand were simultaneously stroked with a paintbrush while the volunteer’s brain was scanned using functional magnetic resonance imaging.

On average, it took volunteers 11 seconds to start experiencing that the rubber hand was their own. The stronger this feeling, the greater the activity recorded in the premotor cortex.

After the experiment, volunteers were asked to point towards their right hand. Most reached in the wrong direction, pointing towards the rubber hand instead of the real hidden one, providing further evidence of the brain’s re-adjustment.

Dr Henrik Ehrsson says: “The feeling that our bodies belong to ourselves is a fundamental part of human consciousness, yet there are surprisingly few studies of awareness of one’s own body.”

“Distinguishing oneself from the environment is a critical, everyday problem that has to be solved by the central nervous system of all animals. If the distinction fails the animal might try to feed on itself and will not be able to plan actions that involve both body parts and external objects, such as a monkey reaching for a banana.

This study shows that the brain distinguishes the self from the non-self by comparing information from the different senses. In a way you could argue that the bodily self is an illusion being constructed in the brain.”

Disorders such as schizophrenia and stroke often involve impaired self-perception where, for example, a woman might try to throw her left leg out of bed every morning because she believes the leg belongs to someone else. Misidentification or unawareness of a limb arising from damage to the premotor cortex from a stroke is not uncommon.

Phantom limb syndrome is a disorder which can arise after amputation. Remedies that trick the brain into believing the limb has been replaced, for example by using a mirror to reflect the opposite healthy limb onto the amputated limb, exploit the brain’s mechanism of self-perception.