The power of invisibility has long fascinated
man and inspired the works of many great authors and philosophers. In a
study from Sweden's Karolinska Institutet, a team of neuroscientists
now reports a perceptual illusion of having an invisible body, and show
that the feeling of invisibility changes our physical stress response in
challenging social situations.
The history of literature features many well-known narrations of
invisibility and its effect on the human mind, such as the myth of
Gyges' ring in Plato's dialogue The Republic and the science fiction
novel The Invisible Man by H.G. Wells. Recent advances in materials
science have shown that invisibility cloaking of large-scale objects,
such as a human body, might be possible in the not-so-distant future;
however, it remains unknown how invisibility would affect our brain and
body perception.
In an article in the journal Scientific Reports, the
researchers describe a perceptual illusion of having an invisible body.
The experiment involves the participant standing up and wearing a set of
head-mounted displays. The participant is then asked to look down at
her body, but instead of her real body she sees empty space. To evoke
the feeling of having an invisible body, the scientist touches the
participant's body in various locations with a large paintbrush while,
with another paintbrush held in the other hand, exactly imitating the
movements in mid-air in full view of the participant.
"Within less than a minute, the majority of the participants started
to transfer the sensation of touch to the portion of empty space where
they saw the paintbrush move and experienced an invisible body in that
position," says Arvid Guterstam, lead author of the present study. "We
showed in a previous study that the same illusion can be created for a
single hand. The present study demonstrates that the 'invisible hand
illusion' can, surprisingly, be extended to an entire invisible body."
The study examined the illusion experience in 125 participants. To
demonstrate that the illusion actually worked, the researchers would
make a stabbing motion with a knife toward the empty space that
represented the belly of the invisible body. The participants' sweat
response to seeing the knife was elevated while experiencing the
illusion but absent when the illusion was broken, which suggests that
the brain interprets the threat in empty space as a threat directed
toward one's own body.
In another part of the study, the researchers examined whether the
feeling of invisibility affects social anxiety by placing the
participants in front of an audience of strangers.
"We found that their heart rate and self-reported stress level during
the 'performance' was lower when they immediately prior had experienced
the invisible body illusion compared to when they experienced having a
physical body," says Arvid Guterstam. "These results are interesting
because they show that the perceived physical quality of the body can
change the way our brain processes social cues."
The researches hope that the results of the study will be of value to
future clinical research, for example in the development of new
therapies for social anxiety disorder.
"Follow-up studies should also investigate whether the feeling of
invisibility affects moral decision-making, to ensure that future
invisibility cloaking does not make us lose our sense of right and
wrong, which Plato asserted over two millennia ago," says principal
investigator Dr. Henrik Ehrsson, professor at the Department of
Neuroscience.
This research was funded by the Swedish Research Council, and the Söderberg Foundation.
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