Many people who have had a limb amputated report feeling sensations that appear to come from their missing arm or leg. Now researchers have found that anyone can experience having such a phantom limb.
"Previous research shows that you can convince a person that a rubber hand is their own by putting it on a table in front of them and stroking it in synchrony with their real hand," explains Arvid Guterstam at the Karolinska Institute in Stockholm, Sweden, who led the study.
The illusion does not work with a block of wood, he says. "But our study shows that if you take away this rubber hand, people will attribute sensations to an invisible entity."
Guterstam and his colleagues made volunteers sit at a table with their right arm hidden from view behind a screen
. An experimenter then applied brush strokes to the concealed hand and, simultaneously, to a portion of empty space in full view of each volunteer.
"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," says Guterstam.
Mock stabbing
Experimenters also mimicked stabbing the phantom hand with a kitchen knife, while monitoring volunteers' stress level. To minimise any effect related to seeing the knife for the first time, the volunteers were warned that it would be used at some point. The researchers found that during the mock stabbing, stress levels, measured using a type of sweat test, went up in about 75 per cent of the 234 participants.
Intriguingly, the phantom hand effect was weaker if the researchers brushed a region of empty space more than an arm's length away from the volunteer, or they brushed it in a different direction to what was happening behind the screen.
In another part of the study, volunteers had to close their eyes immediately before and after each bout of brushing, and then point at their right index finger with their left hand. Participants pointed to an area nearer the invisible hand when brushing was synchronised, than when the illusion was dimmed by unsynchronised brushing.
Brain scans showed that volunteers who watched their phantom hand being touched showed activity in areas that are active when they see their real hand being touched.
"The overall message is that viewing the body is not necessarily for the brain to link visual and tactile stimulation to the body," says Patrick Haggard from the Institute of Cognitive Neuroscience at University College London, who was not involved in the work.
Guterstam's team will next try to trick volunteers into perceiving other phantom body parts, work that should yield more insights into how the brain rewires itself following the loss of a limb.
Journal reference: Journal of Cognitive Neuroscience, doi.org/k6q
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