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Invisible hand illusion. Credit: Arvid Guterstam

The Invisible Hand Illusion

Hold your hand up in front of your face. It is patently obvious that the five-fingered thing in front of you is your hand, and the empty space next to it is not. But this ability to recognise your own body is more complicated than it first appears, and can be fooled through a surprisingly simple trick.

Henrik Ehrsson from the Karolinska Institute is a master of such illusion. When I visited his lab in 2011, he used little more than virtual reality headsets, mannequins and batons to convince me that I had left my body, shrunk to doll-size, and gained a third arm. Now, his team member Arvid Guterstam has devised a way of convincing people that they’ve got an invisible hand.

The team’s work was inspired by the classic rubber-hand illusion, devised by two Princeton scientists in the 1990s. You sit with your real hand under a table and out of sight, while a rubber hand sits in front of you. By stroking the fake hand in time with your real one, an experimenter can convince you that the fake hand is yours. The trick proved popular at college parties, and when Ehrsson tried it, the experience was revelatory.

It suggested to him that even though we have a lifetime’s experience of owning our bodies, this seemingly ingrained feeling is actually very fragile. Our brain constructs it all the time using information from our senses.

Ehrsson’s team have proved the point with one spectacular illusion after another. The principles are always the same as the rubber hand one. You fool the eyes with fake body parts or headsets, and you fool the body with synchronous strokes and prods. If you see a doll’s feet through a headset, and those feet are stroked in time with your actual ones, you will think you’ve shrunk. Change the view, and you can convince people they have swapped bodies with someone else, or had an out-of-body experience.

The team stumbled upon the latest invisible hand illusion by accident. “Almost everybody that has worked with the rubber hand illusion has assumed that the actual rubber hand is kind of very important,” says Ehrsson. So when Guterstam stroked the hands of volunteers while also stroking empty space, he expected them to feel nothing. It was meant to be the control condition of another experiment. “But strangely enough the participants started talking about sensing touches on an invisible hand,” says Ehrsson.

Guterstam started investigating the effect more purposefully. He asked volunteers to sit with both hands out on a table. A partition blocked their view of their right hand, and a cloth covered their shoulder. Guterstam stroked the empty space in front of the cloth, along with the real hand behind the partition. This was enough to convince them that they owned an invisible hand in the empty space.

Guterstam asked the volunteers about their experiences in a questionnaire, but he also took more objective measures to check that the illusion worked. When he stabbed the empty space with a knife, the volunteers’ skin became sweaty and better at conducting electric currents. It was as if an occult hand had been stabbed instead of their own.

Guterstam also asked the volunteers to close their eyes and point to their right index finger. Those who were under the illusion’s spell pointed to the empty space on the left of their actual finger. And as with the team’s other tricks, this one only worked if the real and invisible hands were stroked in time with each other.

The illusion has its limits, though. The invisible hand needs to be facing in the same direction as the real one—if the volunteer can feel that their shoulder is sticking out one way, they won’t buy that they’ve got an invisible hand sticking out in another. The hand needs to be at the right distance away from the body—you can’t make someone believe they have an invisible hand at the end of a really long invisible arm.

This new illusion, much more than the rubber hand one ever did, tells us about the conditions you need to feel that you own a “limb”. These include, at a bare minimum, synchronous information from sight and touch.* Surprisingly, they do not include the presence of an actual hand. Even though the brain can patently see that there is nothing there, the visual and tactile signals that it gets are so aligned that it conjures up the sensation of a hand anyway.

Why does this work? Ehrsson thinks that it’s because we are “very used to feeling our hands without seeing them”. Our brain has to continually update its impression of where our limbs are based on what our senses are saying and “the empty space close to the body represents an array of potential locations for the limbs.”

Some people actually feel invisible limbs all the time. The majority of amputees still feel something in their missing limbs, often with serious pain. “This is a serious medical problem for many amputees and it is very hard to treat pain in a limb that no longer exists,” says Ehrsson. He has recently been trying to use his illusions to treat phantom pain by stroking or stretching the invisible limb, and he thinks that the new illusion will give him a way of testing these ideas.

* Guterstam also tried the illusion on volunteers who were having their brains scanned. When they felt the invisible hand, Guterstam saw more blood flow (an indirect reflection of brain activity) in parts of their brain involved in combining signals from different senses—the ventral premotor cortex and the intraparietal cortex. The connection between these areas also became stronger.

Reference: Guterstam, Gentile & Ehrsson. 2013. The Invisible Hand Illusion: Multisensory Integration Leads to the Embodiment of a Discrete Volume of Empty Space. J Cogn Neurosci

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