Moving in Symmetry May Be Our Natural Way
Perception contributes more to movement than once thought
WEDNESDAY, Oct. 31, 2001 (HealthDayNews) -- Think of yourself as a windshield wiper.
You might have trouble being the kind of wiper that swishes from side to side, moving in unison or parallel with its pair. That's considered asymmetrical movement. You'd probably find it easier to be part of a pair of wipers that sweep toward each other, moving symmetrically.
Humans, it seems, are better at performing high-speed symmetrical rather than asymmetrical movements, thanks to both their vision and sense of body positioning, says a new study.
In fact, the researchers say people are more likely to slip from making asymmetrical movements to patterns of mirror symmetry. The finding challenges existing scientific theory that matching sets of muscles move more easily in unison.
Instead, say researchers at the Max Planck Institute of Psychological Research, in Munich, Germany, it has more to do with spatial and perceptual symmetry. Details appear in the Nov. 1 issue of Nature.
In one experiment, researchers led by senior researcher Franz Mechsner asked eight volunteers to hold their hands in four ways: with either both palms down, both palms up, one palm down and the other up or vice versa.
The volunteers had to move their index fingers in symmetrical or parallel movements in time with a metronome. Then, they had to move their fingers in congruous patterns with both their palms facing up or down, or with one palm up and the other down -- an incongruous position.
At a slow pace, people had no difficulty with either parallel or symmetrical movements. But when the pace quickened, symmetrical movements were more stable, and people were more likely to slip from parallel into symmetrical movements, the study says.
In another experiment, the researchers asked eight volunteers to turn two flags on a table using hand cranks under the tabletop. The left flag moved in direct tandem with the left crank, while the right flag was attached to a gear system, forcing the volunteers to turn the crank at a different rate than the left crank.
The participants tried to turn both flags together, or they turned the right flag while the researchers controlled a motor turning the left crank. All volunteers were able to circle the flags in symmetry, at low or high speeds, while either controlling both cranks or turning only the right crank.
But when asked to turn the flags asymmetrically, the volunteers could perform the tasks only at low speeds.
"This means that perception guides your movement, and not motor tendency," Mechsner says. "If you see that one flag is a little bit behind, then you simply speed up your hand-circling a little bit."
Timothy Lee, professor of kinesiology at McMaster University in Ontario, Canada, says symmetrical and parallel movements are natural patterns performed well even by young children and people with neurological disorders such as Parkinson's disease.
He says that Mechsner's findings open new avenues of research in perception and coordination. "It adds a new dimension to how we conceptualize movement and the contribution of perception to movement," Lee says.
Mechsner says perception doesn't have to be visual. If your eyes are closed, or it you can't see you hands, something called proprioception -- perception relating to one's own posture or position -- guides your movement, he says.
Lee says the findings ultimately could improve the understanding of certain human illnesses.
"When we start to understand how action and perception become diminished with age and with particular neurological diseases, then it's important to understand how we perceive and act normally," he says.
"If we can take the next step and understand the neurological reasons underlying these effects, then we'll be in a better position to understand the neurological pathologies that accompany these disease states, too."