Lots of legs. Cognitive neuroscientists Arvid Guterstam, Henrik Ehrsson, and Björn van der Hoort (left to right) show off their mis-sized mannequin legs, which they fooled test subjects into thinking were their own (inset).

Staffan Larsson, © Staffan Larsson/Henrik Ehrsson; (Inset) B. van der Hoort et al., PLoS One 6, 5 (May 2011)

Honey, I Shrunk the Test Subjects

When Lemuel Gulliver, the protagonist in Jonathan Swift’s Gulliver’s Travels lands on Lilliput, he doesn’t think of himself as a giant. Instead, he assumes everyone around him is tiny. Now, a team of cognitive neuroscientists has shown that we’re all a bit like Gulliver. In a clever experiment, they tricked people into thinking that their size relative to other objects had changed and showed that subjects assumed it was the objects, not themselves, that had been transformed.

Do people use their own bodies as meter sticks to estimate the size of things? Or are our brains capable of making such estimates without making comparisons to our bodies? To find out, cognitive neuroscientist Björn van der Hoort and colleagues at the Karolinska Institute in Stockholm recruited 198 volunteers and had them lie down with their legs in front of them so that they could see them. Each volunteer donned a headset that played a closed-circuit video of the legs of an adjacent mannequin. Using either his finger or a stick, Van der Hoort touched each volunteer’s leg, which the volunteer couldn’t see, while simultaneously touching the leg of the mannequin, which the volunteer could see. This combination of touch and sight was enough to dupe subjects into thinking the fake legs were their own: Van der Hoort could even make a subject sweat by cutting the mannequin’s legs with a knife.

Then things got really weird. The researchers secretly replaced the mannequin legs either with huge legs 400 centimeters long, short ones only 80 centimeters long, or tiny doll legs 30 centimeters long. They dangled a block in front of the camera and asked the subjects to describe how big it was, using both words and gestures. If their new legs were tiny, subjects tended to overestimate the size of the block, whereas if their transformed legs were large, they underestimated the block’s size, typically erring by about 40%.

The researchers then pointed out an object, such as a chair, past the subjects’ fake feet. When subjects were asked to stand up and walk to the object with their eyes closed and on their real legs, those shown the long, fake legs took too few steps toward it. Those shown the tiny legs took too many, the researchers report in PLoS ONE this week. The subjects, it seemed, were using the size of their bodies to take the measure of the world. And instead of perceiving that the lengths of their legs had changed, they assumed that the things around them had grown or shrunk.

The researchers did the experiments on themselves and were surprised how easily they were tricked. “It’s an amazing experience to feel this little Barbie doll body’s yours, and then you see a scientist walk in and he’s a giant,” says lead author H. Henrik Ehrsson. Van der Hoort adds that the illusion is similar to the weird feeling you get when you pick up a toy from your childhood and realize it’s much smaller than you’d remembered.

The experiment emphasizes how malleable our perception of our own body size is, says Olaf Blanke, a neuroscientist at the Swiss Federal Institute of Technology in Lausanne. Van der Hoort says that the ability to trick the brain into thinking a disconnected body part is your own may prove helpful if you wanted to body swap with a humanoid robot and empathetically control it with your own body rather than a joystick while it does a dangerous task.

Next, Van der Hoort plans to use brain imaging to see how the brain reacts when it finds its body to be very small or big. He expects parts of the brain that perceive vision and position in space to both be active as people try to figure out how to adapt to their new size.