whose body is it anyway? 4: body swap

21 March 2009 | NewScientist | 37

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Under normal circumstances, your sense

of self is firmly anchored inside your body.

Sometimes, though, something goes

awry, the connection between body

and self breaks down and you have an

out-of-body experience.

Such moments occur when brain

function is disturbed, such as after a

stroke or epileptic seizure, or while on

drugs. In 2007, however, two research

teams independently reported ways of

inducing an out-of-body experience in

the lab in normal healthy people.

The techniques differ slightly, but both

involve feeding volunteers video images

of themselves from an unusual perspective

while applying tactile stimulation,

somewhat like the rubber hand illusion.

In the first set-up, a team led by Olaf

Blanke at the Swiss Federal Institute of

Technology in Lausanne got volunteers

to stand about 2 metres in front of a video

camera while wearing goggles, which

displayed the video images, converted

into a holographic-like 3D projection. This

meant that the volunteers could see a

version of their own backs. When they

stoked the volunteers’ backs, many

reported a weird feeling that they were

somehow inside the virtual body in front

of them ( Science, vol 317, p 1096 ).

The volunteers also experienced

“proprioceptive drift” towards the virtual

body: they felt as if they were standing in

the position of their virtual self. When the

researchers turned off the display, moved

their volunteers backwards and asked

them to return to their original position,

many overshot towards where they felt

their virtual body had stood.

Last year Blanke’s team also reported

inducing a feeling of out-of-body

levitation by repeating the experiment

with volunteers who were lying down

( Consciousness and Cognition, DOI:

10.1016/j.concog.2008.11.003 )

Nevertheless, Blanke points out that they

have not yet recreated the entire out-of-

body experience: “It remains an ‘as-if’

feeling, but we’re trying to refine it.”

Ehrsson’s team have done something

similar , with seated volunteers filmed

from behind while a researcher stands

to the side of them stroking the

volunteer’s chest and a space just in front

of the camera (illustrated above). The

volunteers see their own backs, feel the

stroking but also see somebody stroking

a position just behind them. This

strongly creates the illusion that they are

outside their own bodies, says Ehrsson

(Science, vol 317, p 1048).

What’s more, when Ehrsson tried

swinging a hammer at the previously

stroked airspace, it elicited a strong

stress response in the volunteer.

3

Leave your body behind

If leaving your own body behind or having an

extra or virtual arm is not enough for you, how

about switching bodies with someone else? In late

2008, Ehrsson and his colleague Valeria Petkova

described an experimental set-up that creates

this freaky illusion.

They got two people to stand facing one another,

one wearing a pair of head-mounted video cameras

and the other wearing goggles with the images

from the cameras projected inside. The pair were

then asked to reach out and shake each other’s

right hand and, as they held their grip, to apply

a synchronised series of squeezes.

For the goggle-wearer, this creates an uncanny

feeling of occupying the camera-wearer’s body:

where they would normally see their own torso and

their own arm and hand reaching out, they see their

partner’s. Within a couple of minutes about 70 per

cent of participants feel as if their partner’s arm –

and by extension the body attached to it – is their

own. And when the experimenters brandish a knife

over the participants’ wrists, the goggle-wearers

stress response was greatest when it is closer to

their partner’s wrist ( PLoS ONE, vol 3(12), p e3832 ).

Ehrsson says the illusion is highly convincing: “The

first time I experienced the body-swapping illusion

with Valeria I almost started screaming because it

was such a surreal and striking experience to shake

hand with myself using a different body.” ■

4 Body swap

To experience the rubber hand

illusion, you’ll need a fake hand

of some kind – an inflated rubber

glove will often do the trick – a flat

piece of cardboard and two small

paintbrushes. Place the hand on a

table in front of you and conceal your

real hand behind the cardboard.

Now get somebody to stroke

and tap the fake hand and real hand

using identical movements of the

paintbrushes. Look at the fake hand

for a while until the illusion kicks in.

HOW TO TURN A RUBBER HAND INTO A ‘REAL’ ONE