Amputees commonly experience the sensation that their missing limb is still there, and now a new study finds that the illusion of having a "phantom limb" can be evoked in those who did not have an amputation.
Researchers in Sweden said they hope their findings offer insight for more research on phantom limb pain in amputees.
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"Taken together, our results show that the sight of a physical hand is remarkably unimportant to the brain for creating the experience of one's physical self," study lead author, Arvid Guterstam of the Karolinska Institute, said in an institute news release.
Participants' hands were hidden away
The study involved 234 participants who sat at a table with their right arm hidden behind a screen.
To trigger the illusion of a phantom hand, the researchers touched the participants' right hand with a small paintbrush while mimicking the same motion with another paintbrush in the air in plain sight.
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"We discovered that most participants, within less than a minute, transfer the sensation of touch to the region of empty space where they see the paintbrush move, and experience an invisible hand in that position," Guterstam noted.
"Previous research has shown that non-bodily objects, such as a block of wood, cannot be experienced as one's own hand, so we were extremely surprised to find that the brain can accept an invisible hand as part of the body."
The researchers confirmed the phantom limb illusion worked by making a stabbing motion with a knife towards the place where the participants' phantom limb would have been. As their phantom limb was stabbed, the participants' sweat response was measured.
Participants' stress level was higher
The study revealed the participants' stress level was higher while experiencing the phantom limb illusion. The study authors noted this stress response was nonexistent when the illusion was broken.
In a separate experiment, when asked to close their eyes and point to their right and left hand, participants who experienced the phantom limb illusion pointed to their invisible hand instead of their real hand.
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The participants also underwent an MRI to assess their brain activity.
The illusion of a phantom limb was associated with increased activity in the parts of the brain that are typically active when people see their real hand being touched or when a prosthetic hand is viewed as real, according to the report published in the current issue of the Journal of Cognitive Neuroscience.
"This illusion suggests that the experience of phantom limbs is not unique to amputated individuals, but can easily be created in non-amputees," concluded the study's principal investigator, Dr Henrik Ehrsson, docent at the department of neuroscience.
"These results add to our understanding of how phantom sensations are produced by the brain, which can contribute to future research on alleviating phantom pain in amputees," he said in the news release.
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