What happens to people when they think they're invisible?

It's an experiment ripped straight from the pages of H.G. Wells.

Using a 3D virtual reality headset, neuroscientists at the Karolinska Institute in Stockholm gave participants the sensation that they were invisible, and then examined the psychological effects of apparent invisibility.

It doesn't actually take much to make a person feel invisible. The scientists outfitted participants in 3D virtual reality headsets and asked them to look down, at where their bodies should be. But instead, the headsets projected an image of empty space. Then, the scientists stroked volunteers with a paintbrush. At the same time, in the headset display, the brush appeared to be stroking empty space. Almost immediately, the subjects began reporting feeling as though their bodies had become hollow or transparent.

"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," said Arvid Guterstam, lead author of the 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."

In fact, the illusion was so real that when researchers made a stabbing motion with a knife in empty space, participants showed elevated stress and sweat levels.

But perhaps more interesting was the effect invisibility appears to have on social anxiety.

In the study, the scientists sought to create a socially stressful situation by having the volunteers stand in front of an audience of "serious-looking" strangers. The participant who had first been rendered "invisible," reported lower stress levels and showed slower heart rates than their "visible" counterparts.

"These results are interesting because they show that the perceived physical quality of the body can change the way our brain processes social cues," says Dr. Guterstam

In other words, as Guterstam told Live Science, "Having an invisible body seems to have a stress-reducing effect when experiencing socially challenging situations,"

As such, invisibility may, in fact, have practical applications, such treating social anxiety disorders, he suggests. For example, a patient may be rendered "invisible" to lower anxiety in simulated stressful situations and then gradually be made more visible. 

But could the sensation of invisibility have an even greater effect on our minds?

This is actually one of the oldest philosophical questions. In Book II  of Plato's Republic, one of Socrates's interlocutors tells a story of a shepherd, an ancestor of the ancient Lydian king Gyges, who finds a magic ring that makes the wearer invisible. The power quickly corrupts him, and he becomes a tyrant. 

The premise behind the story of the Ring of Gyges, which inspired HG Wells's seminal 1897 science fiction novel, The Invisible Man, is that we behave morally so that we can be seen doing so. Justice, according to this view, is a social construct, one that vanishes as soon as the ring is slipped on. Socrates spends much of the remainder of the Republic arguing that justice is real, and not just a construct. 

With this new technique for inducing the sensation of invisibility, scientists may actually be able to test this hypothesis.  "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," said the report's co-author, Henrik Ehrsson.

Added Guterstam, in a report in the journal Nature: “We are planning to expose participants to a number of moral dilemmas under the illusion that they are invisible, and compare their responses to a context in which they perceive having a normal physical body.”

"Illusory ownership of an invisible body reduces autonomic and subjective social anxiety responses" appears in the current issue of Scientific Reports.