News in Science

Muscles hold clue to what is 'you'

Self awareness Messages from muscles are all that is needed to persuade your brain that a body part belongs to you, find Australian researchers.

This is the finding from an experiment published this week in the Journal of Physiology, in which participants were tricked into believing that a silicone fake finger was really their own.

And it was messages from movement receptors in muscles that misled the brain, conclude the team, led by Professor Simon Gandevia of Neuroscience Research Australia.

Building on a 2011 study that showed muscles and vision together could bring a sense of ownership of a body part, the team set out to see whether muscle messages alone would suffice.

Nineteen subjects placed their right and left hands into a covered box, with the left hand resting 12 centimetres higher than the right.

Their fingers were anaesthetised, so that the sense of touch was eliminated from the experiment.

Although they couldn't see their hands, the participants could sense that they were not on a level, and reported that they thought they were about 8 centimetres apart.

Unbeknown to the participants, a silicone fake right index finger was also in the box, level with their left hand (and 12 centimetres above their right hand).

"Crudely it's like a real finger," says Gandevia. "It was taken from a prosthetic hand so it was slightly squishy, you could [feel] a fingernail and the centre of it was rigid as if there were a bone inside."

But the box hid a trick: a mechanical link between the fake finger and the real right index finger below. So when the fake finger moved, their right finger was made to move in an identical way.

"It was very important to literally and metaphorically keep the subjects in the dark" says Gandevia.

Guided by the experimenter's hand, the subjects were made to touch the fake finger with their left hands and wiggle its middle joint up and down.

Because of the trick, they felt their right index finger synchronously move in an identical way.

The participants immediately experienced an illusion - they reported that their two hands were suddenly much more on a level - only one centimetre apart.

"They have the strong illusion that the distance between the right and left index fingers has diminished," says Gandevia.

Taking ownership

Gandevia says the finding suggests that their brains are taking 'ownership' of the fake finger.

And when the participants were asked if they 'feel they are holding their right finger with their left hand', they agreed.

The experiment was designed to determine whether messages from muscles are all that is needed for the brain to feel a body part 'belongs'.

The anaesthetic not only blocked the sense of touch, it also prevented information from sensors in the finger joints from reaching the brain.

But messages from movement receptors in muscles in the hand and forearm that control the fingers could get through to the brain as normal, explains Gandevia.

Fundamental physiology

"It's the first time that this sense of knowing that you own bits of your body has been shown to be due to muscle receptors on their own," he says.

"This is fundamental physiology," says Gandevia. "We've gone about it from a bottom-up approach - asking what are the minimal conditions required to produce a sense of ownership."

He says the work may lead to a testing procedure for people who have a problem with their sense of ownership of parts of their body.

This can occur in people who have strokes and in schizophrenia, he adds.

"People who have strokes, for example, may not uncommonly deny that the right side of their body is their own."

He says this work is a step towards better understanding of these debilitating conditions.