“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,” said the lead author of the study, Arvid Guterstam of Sweden’s prestigious Karolinska Institute.
Phantom limbs can be distressing and painful for amputees, and drugs cannot help as the sensation is essentially a trick of the brain, which imagines the existence of a limb that is not there.
Guterstam said his team hoped the results of their study would help lead to future research on amputees’ phantom pain.
The researchers conducted 11 different experiments creating a perceptual illusion so that volunteers with two arms and hands experienced having an invisible hand.
In the experiments, participants sat at a table with their right arm hidden from their view behind a screen.
A scientist then touched the participant’s right hand with a paintbrush while imitating the exact movements with another paintbrush in mid-air within the participant’s full view.
“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 said.
“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,” he added.
In another experiment, researchers made a stabbing motion with a knife toward the empty space “occupied” by the invisible hand and measured the participant’s sweat response in their palms to the perceived threat.
They found that the participants’ stress responses were higher when they experienced the illusion, but absent when the illusion was broken.
And in a third experiment, the volunteers were asked to close their eyes and point with their left hand to their right hand. After having experienced the illusion for a while, they pointed to the location of their invisible hand instead of the real hand.
Researchers also measured brain activity, and found that the invisible hand illusion led to increased activity in the parts of the brain that are normally active when individuals see their real hand being touched.
Seventy-four percent of the 234 volunteers experienced a phantom limb during the experiments, Guterstam said.
The results were published Thursday in the US Journal of Cognitive Neuroscience.