You've probably thought about the idea of becoming invisible, so have movie-makers and authors for centuries (think Harry Potter and Bilbo/Frodo Baggins). But a Swedish team has now figured out how to trick the mind into believing your body is invisible -- and how you might react before a crowd of people or a real knife about to slice through your shapeless form.
Just as some amputees feel a sensation called "phantom limb pain," so too did subjects who donned a virtual reality headset in the experiment by researchers at Sweden's Karolinska Institute in Stockholm and published today in the journal "Scientific Reports."
After putting on the headset, which was connected to a live 3-D video feed, each of the 125 experimental subjects stood up and looked down at their bodies. But instead of seeing a real body, they saw empty space. To trick the person, a scientist touched the participant's body in various locations with a paintbrush, while another paintbrush held in another hand imitated the movements in mid-air in view of the participant.
Once the illusion of the invisible body was created, the researchers tried two different experiments. The first one measured the sweating of skin and increased heart rate in response to a knife threatening the empty space representing the body, said Arvid Guterstam, a former doctoral student in cognitive neuroscience who is now at the University of Washington.
"We were measuring that the illusion actually works," Guterstam said. "The brain should react with automatic stress response when sharp object is approaching. We showed you have an increased stress response and similar to illusion of having mannequin body."
In the second experiment, the researchers created an invisible body in front of a crowd of strangers in the room. In both instances, the body exhibited a stress reaction.
Guterstam said the experiments could help people who suffer from social anxiety disorder, a common ailment that affects millions of people who fear they are being criticized in social situations.
"It's a form of anxiety triggered by standing in front of a crowd and you start hearing your own heartbeat, sweating, tremors and stuttering," Guterstam said. "The leading treatment is cognitive behavioral therapy in which you slowly increase the stress level to habituate the person to stress. This illusion could be incorporated so you stand in front of a virtual crowd so you are invisible, then increase transparency so you feel you are standing in front of them."
Michael Graziano, professor of psychology at Princeton University, said the results of the experiment are revealing.
"Studies like this explore the hidden work the brain does to process the physical body," Graziano said in an e-mail to Discovery News. "And this study in particular highlights how the body sense is the anchor for our mental and emotional lives as well. Tricked by an illusion into the perception of invisibility, we change our attitudes and behavior. Our sense of self grows out of our sense of body."
Guterstam said further experiments may look at whether moral decision-making changes under invisibility, and whether people really do act differently when nobody is looking.