How do we know which hands are own own? Scientists zero-in on the explanation.
Sense of touch, vision and receptors in the muscles and joints help the brain keep track of the body.
Research shows the brain develops a map of what belongs to it.
The mystery of how the brain develops the sense of ownership that recognizes our body belongs to us is a step closer to being solved.
Australian researchers have shown that along with the sense of touch and vision, signalling receptors in the muscles and joints also play a critical role. The finding, published recently in the Journal of Physiology, will help in designing treatments for disorders of body ownership that can occur with conditions such as stroke and epilepsy.
Lead author Lee Walsh, of Neuroscience Research Australia, explained we instinctively know our body parts "belong" to us. However, how the brain develops that map of what belongs to it is still in part unknown.
"How do I know my hand is mine and not yours and that the telephone is not a part of my body," he said.
Previous research shows people can be deluded into claiming ownership of an artificial hand. This is done by simultaneously stroking the subject's hidden hand and a visible artificial rubber hand.
"Once the illusion of ownership of the hand is established, subjects have physiological responses to threats made against the rubber hand," Walsh and his colleagues wrote in the paper.
In this study the team was interested to see if other sensory channels could also be important in developing body ownership.
"We can use vision to see parts of the body, but we can also see other bodies, so vision alone cannot differentiate foreign body parts from those we own," Walsh said. "Muscle receptors can only signal things that are happening to the body so would seem to be an ideal candidate to signal ownership."
To test this theory, the team induced an illusion of ownership over a plastic index finger. The finger was used because it is feasible to block the sense of touch with local anaesthetic. The team found the sense of ownership still occurred when participants' fingers were anaesthetized. Walsh said the results show visual-tactile cues are not critical in establishing body ownership. Instead muscle-related signals coupled with vision are sufficient for the brain to recognize ownership.
"The results clearly show that muscle receptors contribute to the sense of body ownership," said Walsh. "This is the first study to show that the brain uses information from muscles to tell us what is our body and what isn't. This is fundamental to identifying how the brain and body interact to give us a sense of self."
The study also found that congruence between sensory stimuli was critical in manipulating the sense of body ownership. Walsh said in the study, the real finger was 'held" by a pipe designed to mimic the way the researcher held the plastic finger.
However at times during the experiment the visual stimulus did not perfectly match the tactile stimulus and participants' sense of ownership of the fake finger was reduced. Walsh said some stroke patients lose their sense of ownership over an affected hand or arm.
"This can make recovery tricky because how do you rehabilitate sensory stimulus?"
He said the research they are undertaking could answer this question.
"Once we understand body ownership better we can look at it in a stroke patient and see what is different between them and healthy people."