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.
According to Medina, the brain does “optimal integration” of incoming
sensory information and then sorts out what the most reliable sense is.
“Vision is really precise,” Medina said, “but proprioception—the
sense of where your body is in space—is noisier. So if there is a
conflict between these senses, and vision is telling you that your hand
is right there, but proprioception says it isn’t, your brain is
optimally calculating. Vision, because it is more precise, typically
rules. However, in our study, the brain also appears to be considering
additional information—biomechanical constraints from the body schema—in
resolving this conflict between the senses.”
Medina and his students are now using functional magnetic resonance imaging (fMRI) in UD’s Center for Biomedical and Brain Imaging
to further uncover how the brain calculates and integrates the vast
inputs it receives from all the senses. This sophisticated tool can
illuminate which regions of the brain are at work when performing a
task. A better understanding of such brain processing could help advance
new treatments for patients with brain injuries such as strokes,
chronic pain and other disorders, and for developing artificial limbs
that feel like a part of the body.
“How do you embody an artificial limb? It has to respect the laws of
the body you’ve learned all your life,” Medina noted. “It’s quite
important to figure out how the brain solves the problem of multisensory
information, and how that relates to embodiment and our sense of self.
These cool tricks and illusions are a path to understanding how the mind
works.”
Article by Tracey Bryant; video by Robert DiIorio
