THURSDAY, July 1, 2004 (HealthDayNews) -- Most people have no trouble identifying one of their hands as their own, but how this happens in the brain was unknown until now -- thanks to a little sleight of hand.
British researchers have identified the parts of the brain that identify your own body parts. This feeling of body ownership appears to take place in the premotor cortex of the brain, they report.
While this may seem like a no-brainer, it's actually a complicated process that brings together many inputs from the senses that are processed in the brain to create the understanding that a body part belongs to you, according to a report in the July 2 online issue of Science.
"We think the brain solves this problem of distinguishing itself from the environment by looking at correlations between the different senses," said lead author Dr. H. Henrik Ehrsson, a research fellow from the Wellcome Institute of Neurology at the University College of London.
"The feeling that my hand is my hand is not a sixth sense, but an analysis of the correlation between the senses. If a hand both looks like my hand and feels like my hand, then the brain will say, 'Aha! It's my hand,'" he explained.
Ehrsson said this finding can shed light on disorders of self-perception, such as stroke, schizophrenia and phantom limb syndrome. "People with brain damage can misidentify their limbs and can't believe that the limb belongs to them," he said. People who've lost a limb, meanwhile, often experience the eerie feeling that the arm or leg is still there.
Ehrsson's team conducted what it calls the rubber hand experiment. In this test, a volunteer's own hand is hidden below a table, while a rubber hand is placed on a table in the same position as the hidden real hand.
The researchers then stroke both hands simultaneously with a paint brush. According to Ehrsson, eventually the volunteer senses the rubber hand is his or her own.
"By tricking the brain, the subject starts feeling the rubber hand is his hand," Ehrsson said. "Using our method, you can switch the feeling of ownership on and off."
To identify which parts of the brain were involved, the investigators used MRI imaging to monitor the subject's brain activity during these experiments.
This finding can lead to further research on disorders of self-perception, Ehrsson said. "The key thing we are looking at is how the brain distinguishes itself from the environment," he added.
"This study sheds new light on situations we see in our patients," said Dr. Hal Blumenfeld, an assistant professor of neurology and neurobiology at Yale University.
Some patients who have had a stroke in the right side of the brain are suddenly not able to recognize their hand as being their own, Blumenfeld said.
"I had a patient who woke up in the middle of the night and was shouting to his wife, 'What's this arm doing in my bed? Whose arm is this?'" he said.
Blumenfeld told of another case in which he held up the patient's arm to him and asked whose arm it was and the patient replied, "That must be my brother's arm. He must have left it here when he was visiting before."
"These patients have no ability to recognize their own arm as theirs," Bloomenfeld said. This study identifies the brain circuitry that is necessary for people to identify something as their own, he added.
The National Institute of Neurological Disorders and Stroke can tell you about brain basics.