Field of Science


Rubber Hand Experiment Shows Kids Have More Flexible Body Boundaries

Close your eyes. Do you know where all your fingers and toes are? Can you pinpoint the exact edges of your body in space?

You may think your knowledge of your body is unshakeable, but a simple trick with a rubber limb can sway you. In kids, the effect is even more extreme—a finding that gives intriguing hints about how our body sense develops.

The new research relies on the "rubber hand illusion," first published in 1998. To produce this illusion, an experimenter sits across a table from a subject. The subject rests one hand, let's say the left, flat on the table and keeps the other hand in his lap. A little wall blocks the left hand from the subject's sight. But the subject can see a rubber hand, also a left hand, sitting on the table just inside the wall.

Actually, hold on, I'll draw you a picture.

OK. The experimenter (or, oval with glasses) holds two paintbrushes and uses them to stroke the backs of the real hand and the rubber one simultaneously. The subject watches these paintbrush strokes that seem to match the ones he's feeling, and eventually the brain takes a shortcut: it decides the seen hand and the felt hand are one and the same. This gives the subject the eerie impression that the rubber hand is part of his body. (I wrote about trying the rubber hand experiment for my eighth-grade science fair—and someone else's kooky version of the illusion that uses entire bodies instead of hands—here.)

University of London psychologist Dorothy Cowie and her colleagues tested the rubber hand illusion on kids of varying ages to see how their response compared to adults. Like researchers before them, they measured the effect in two ways. The first was a questionnaire about whether the rubber hand felt like the subject's own (for kids, the scale went from "definitely not" to "lots and lots").

For the second measurement, subjects closed their eyes and slid the index finger of their right hand under the edge of the table until they believed it was aligned with the index finger of their left hand. After experiencing this illusion, subjects tend to get skewed in the direction of the rubber hand.

The researchers tested adults as well as 90 kids between the ages of 4 and 9. They saw that in the sliding-finger measurement, kids in all age groups drifted farther toward the rubber hand than adults did. The results are reported in Psychological Science.

To explain this, Cowie suggests that people rely on two different methods to figure out where their body parts are. One combines vision and touch: do the cues I'm feeling match what I see? The illusion worked best for adults when the paintbrush strokes on both hands were perfectly in sync.

But for kids, the illusion stayed strong even when the paintbrush strokes they saw were out of sync with the ones they felt. This suggests that a second system of perception simply asks whether something that looks like our arm appears in roughly the place we expect it. Kids overuse this system, Cowie says. "Seeing a 'hand-like thing' in front of them on the table was enough to sway their perception of where their own hands were." By adulthood, Cowie thinks, we learn to pay more attention to tactical cues. "Adults rely less on the visual stuff than kids."

The fact that the illusion works at all demonstrates that "we don't just rely on muscle info to tell us where our body is," Cowie says, whether we're kids or adults. "In fact vision is really important!" Her research group is conducting further studies to find out how perception changes with age. "The results are absolutely always that kids are more susceptible than adults" to the illusion, she says.

If you're feeling worried that you don't know your body very well, consider taking on a more childlike attitude. Cowie says the kids in her experiment enjoyed being tricked by the illusion. One kid reacted with "You seem to have painted my hand!" They were eager to check where their hands really were when the test was over.

"Kids are actually open to weird stuff more than adults are," she says.

Cowie, D., Makin, T., & Bremner, A. (2013). Children's Responses to the Rubber-Hand Illusion Reveal Dissociable Pathways in Body Representation Psychological Science DOI: 10.1177/0956797612462902

Images: St0rmz (via Flickr); me (via Post-It note).

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