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23 April 2015

Illusion allows people to feel what it is like to be invisible

Brushing up on our sense of self
Brushing up on our sense of self

(Image: Group Ehrsson Brain, Body and Self Laboratory/Karolinska Institute)

The perks of being invisible… a lower heart rate, less anxiety, looser morals? A whole body version of the classic rubber hand illusion is helping us find out what it’s like to be invisible by tricking people’s brain into embodying empty space.

Over 15 years ago, psychologists found they could convince people that a rubber hand was their own by putting the fake hand on a table in front of them and stroking it and the person’s own hand at the same time. More than just a party trick, the illusion revealed how easily our sense of ownership over our body can be manipulated.

In 2008, Henrik Ehrsson at the Karolinska Institute in Stockholm, Sweden, and his team extended the rubber hand illusion to the full body. They took a life-size mannequin with cameras for eyes, and pointed its head towards its abdomen. A human volunteer stood facing the mannequin, and wore virtual reality goggles displaying a video feed from the mannequin’s cameras.


Next, the experimenter took two brushes and stroked both the volunteer’s and the mannequin’s abdomen. If this was done simultaneously, eventually people reported that the mannequin’s body felt like their own.

Phantom body

But do you even need the mannequin for the illusion to work? To find out, the researchers pointed the cameras at a region of empty space. Again the experimenter stroked the volunteer’s abdomen, but this time they moved another brush in empty space in the cameras’ field of view, in a manner suggestive of a body there. Brushing empty space wasn’t easy. “We put a lot of effort into perfecting the actual brushstrokes,” says team member Arvid Guterstam.

The researchers tried the illusion on themselves first. “I am very susceptible to illusions, so for me it worked,” says Guterstam. “You have a vivid sensation of having a body, but it’s not directly visible. I don’t know what it would feel like to have a phantom body, but I imagine that’s what it’d feel like.”

The team carried out a series of experiments, each involving about 20 people. About 75 per cent experienced the illusion. “Most were surprised, and some started giggling. It’s a very unnatural, unexpected experience,” says Guterstam. “It’s the first time that anyone has shown that you can embody a full invisible body.”

The mannequin illusion works because it feeds the brain conflicting information, disrupting its integration of vision, touch and proprioception – the body’s sense of its own position. People who experienced it were also more susceptible to the invisibility illusion, suggesting a common underlying mechanism.

Centre of attention

Once the invisibility illusion had taken hold, the volunteers were asked to look up. Now the VR goggles showed them a group of people wearing serious expressions looking down at them. The same was done to volunteers in the grip of the mannequin illusion. When the team measured everyone’s heart rates, they found they were lower when people felt they were invisible than when they felt they were embodying the mannequin.

“If you are truly experiencing a non-physical body, it should also be represented in the brain as invisible to outside observers,” says Guterstam. “So, the others’ gaze should not fall upon your physical body but on the illusory body.” This was probably reducing the stress the subjects felt when being scrutinised by stern-looking faces.

Neuroscientist Olaf Blanke of the Swiss Federal Institute of Technology in Lausanne, an expert on full-body illusions, says he is impressed by the findings and thinks they could have clinical applications. We know virtual reality can be used to treat social phobias by gradually exposing people to virtual situations they would find threatening in real life. The invisibility illusion highlights the importance of body ownership in phobia treatments, something that should be investigated further, he says.

How far can it go?

The illusion also sheds some light on the mind-body connection and on our sense of self, says Guterstam. “It tells us that body representation in the brain is even more malleable than previously thought.”

The team now plan to use brain imaging to examine the neural processes at play as the illusion emerges. They also want to investigate if the feeling of invisibility affects the way we make moral choices and whether it is possible to elicit the illusion of having an invisible face.

“The invisible body illusion is definitely pushing the boundaries of what the brain can accept as part of the bodily self,” says Guterstam.

Future experiments could potentially push them even further by examining whether it is possible to dynamically transform the perceived invisible body. “Since there is no mannequin present and the experimenter is defining the contours of the invisible body with brushstrokes in mid-air, I have the feeling that changing the speed and direction of the brushstrokes in empty space could lead to interesting perceptual effects, such as the elongation of an invisible arm or leg.”

Journal reference: Scientific Reports, DOI: 10.1038/srep09831

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