How the brain feels false limbs

Which limbs are real?

Scientists have shown how the brain can be fooled into feeling sensations in a fake limb.

They recorded changes in brain activity during an experiment in which volunteers were made to think a rubber hand was their own limb.

The University College London team hope their work will shed light on self-perception disorders such as schizophrenia and stroke.

Their work is published in Science Express Online.

In the study, funded by the Wellcome Trust, volunteers hid their right hand under a table and a rubber hand was put in front of them at an angle to make it look like part of their body.

The rubber hand and hidden real hand were stroked simultaneously with a paintbrush while the volunteer's brain was scanned using magnetic resonance imaging.

You could argue that the bodily self is an illusion being constructed in the brain. Dr Henrik Ehrsson

It took just 11 seconds for volunteers to start feeling the rubber hand was their own.

The stronger the feeling, the greater the activity recorded in the brain.

Volunteers were later asked to point towards their right hand. Most pointed towards the rubber hand instead of the real one, showing how the brain had readjusted.

The researchers found one area of the brain, called the premotor cortex, recognises the body by accepting information from three different senses - vision, touch and proprioception (position sense).

Mixed messages

But if there are inconsistencies, the brain "believes" visual information as it is the strongest sense.

Lead researcher Dr Henrik Ehrsson said: "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.

Greg Vines, spokesman for the Stroke Association, welcomed the research.

He said: "It's a great step forward. We want to look at widening research out and testing on more cases and developing rehabilitation services to help make people more aware of spatial perception."