Tricks and illusions, once the domain of magicians, are helping scientists unveil how the brain works.
Here’s one you can try using a tabletop mirror. Place your left hand
on the table in front of the mirror’s reflective surface and your right
hand behind the mirror, about six inches away, where you can’t see it.
Now tap the table surface with both hands while looking at your
reflection. Within a minute, you’ll feel as though the hand you see
reflected in the mirror is your right hand and it’s right next to the
mirror—even though the hidden hand did not move.
This classic “mirror box” illusion has been used in a number of
neuroscience studies, including with amputees as a possible therapy to
alleviate phantom limb pain, where it may help the brain re-map and
adapt to a missing limb.
Now, a new version of the mirror box illusion, developed by
University of Delaware brain scientist Jared Medina and doctoral student
Yuqi Liu, is pulling back more of the curtain on how the brain
processes multiple sensory inputs to perceive our bodies and the world
around us. Their study, which is supported by the National Science
Foundation, appears in Scientific Reports, a multidisciplinary, open access journal from the publishers of Nature.
In their novel illusion, study participants placed their hands in
opposite postures (one hand palm-up, the other palm-down), creating a
conflict between visual and proprioceptive feedback for the hand behind
the mirror. Proprioception is your so-called “sixth sense,” the sense of
where your body is in space, that comes from your muscles and joints.
It’s the sense that allows you to touch your nose with confidence even
with your eyes closed.
After synchronous opening and closing of the two hands, the study
participants felt that the hand behind the mirror rotated or completely
flipped to match the hand reflection.
“All of a sudden during our experiments, you’d hear a little laugh of
surprise when people experienced this neat sensation of feeling like
their hand flipped, even though it did not move,” Medina said.
Resolving battle of the senses
The illusion’s effectiveness was influenced by the perceived
difficulty of moving the hidden hand to the position viewed in the
mirror. Less illusion occurred for more difficult rotations requiring
more strain. Such biomechanical data, Medina said, is coded in the body
schema, a representation of your body position in space that takes into
account feedback from all the relevant senses, plus stored information
from muscles and joints about what your body can and can’t do.