Swapping Bodies with a Child Makes Everything Seem Bigger

Remember revisiting your preschool or kindergarten classroom once you were older, and realizing all those tables and sinks that are normal-sized in your memory were actually miniature? And that the giant hill you used to struggle up is more of a mound? Adults can regain that feeling of living in an oversize world just by putting on a virtual-reality headset. (Large kid who used to budge you in line for the slide not included.)

This is the latest spinoff of the rubber-hand illusion, a phenomenon in which watching a rubber hand being stroked with a paintbrush, while you feel a matching sensation on your own hand, creates an eerie sensation that the rubber hand is your own. Another recent study found that kids experience the illusion more strongly than adults. Aside from being a neat party trick, the research may have implications for amputees who experience phantom limbs.

The illusion's newest incarnation, by Mel Slater at the University of Barcelona and others, didn't use any physical contact. Instead, subjects wore virtual-reality goggles that let them see through the eyes of a virtual body. The avatar's movements were matched to their own with motion tracking, and subjects could watch their virtual bodies in a mirror while they moved and stretched.

Although the virtual body matched a subject's movements, it didn't match his or her size. The avatars were all miniature people--either a child about four years old, or an adult scaled down to the same height.

With either kind of small body, subjects reported that they felt an illusion that the avatar's body was their own. Researchers quantified this by having subjects look at different-sized objects in the virtual world and hold out their hands to indicate how wide the objects were. (For this part of the experiment, they couldn't see their virtual hands.)

Size perception is always tied to the size of your own body, Slater says, so all subjects overestimated the size of the objects they saw. The same thing happened in an earlier rubber-hand study that had subjects inhabiting both tiny and giant bodies.

But with a child's body, the effect was significantly greater, the authors report in PNAS. People virtually inhabiting a four-year-old's body perceived objects as even larger than people inhabiting a small adult body did.

The authors think this may be because the experiment triggers specific, first-person memories of being in a child's body. Living in a miniature adult body, of course, is a less common experience. This is a "possible new discovery," Slater says—"that the brain codes for body type, not just for size."

Slater has experienced the illusion himself. "It is very powerful and strange to see yourself in a mirror as a small child," he says. Maybe stranger, even, than those tiny sinks.


Image: Slater et al. (from supplemental movie)

Domna Banakou, Raphaela Groten, & Mel Slater (2013). Illusory ownership of a virtual child body causes overestimation of object sizes and implicit attitude changes PNAS DOI: 10.1073/pnas.1306779110