A visit to the circus would not be the same without jugglers. But although the skill is used primarily for entertainment purposes, new research suggests juggling may provide clues as to how vision and sense of touch help control the way animals and humans repetitively move their limbs - a motion that is carried out when we run, for example.
This is according to a study recently published in the Journal of Neurophysiology.
The research team, led by investigators from the Johns Hopkins University in Baltimore, MD, says the findings could lead to the development of more effective prosthetic limbs and may help treat people who suffer with neurological conditions.
To juggle, a person must consistently repeat rhythmic motions in order to keep multiple balls in the air. The researchers note that this movement is not dissimilar to other body movements we use every day.
"It turns out that the art of juggling provides an interesting window into many of the same questions that you try to answer when you study forms of locomotion, such as walking or running," says Noah Cowan, associate professor of mechanical engineering at Johns Hopkins University and study leader.
"In our study, we had participants stand still and use their hands in a rhythmic way. It's very much like watching them move their feet as they run. But we used juggling as a model for rhythmic motor coordination because it's a simpler system to study."
The research team recruited participants to take part in a virtual juggling experiment. This was to determine how the brain uses vision and sense of touch to carry out the juggling motion.
Using a "real-world paddle" that was connected to a computer, participants were asked to repeatedly bounce an on-screen table tennis ball with an aim of reaching a target area situated between two lines on the screen.
The investigators say that in some of the experiments, participants were guided only by their vision.
But in others, when the virtual ball hit the paddle on the screen, the subjects felt a short impulse through their real-world paddle, which the researchers call "haptic feedback." This was to make them feel as if a real ball hit the paddle.
Cowan explains the experiment and findings in the video below:
Haptic sensation 'crucial' for good timing
The investigators found that when the participants could feel the virtual ball on their real-world paddle, they made around 50% fewer errors than the subjects who were only guided by their vision.
The research team notes that when it comes to juggling, getting the right rhythm is crucial. This means a person must become familiar with the movement of the balls.
Cowan believes that in this study, participants who felt the pulse of the ball were able to perform better because they could determine exactly when the ball would hit the paddle.
"It gives you a precise sense of the timing for the juggling pattern that you're trying to achieve," adds Cowan.
The researchers note that participants who felt the pulse of the ball on their paddle did not seem to be able to improve on any individual errors they made when attempting to the hit the ball into the target area, but they did make fewer overall errors.
"The haptic sensation is just a tiny bit of feedback that's provided once per juggling cycle.
Yet that tiny bit of information seems to be critical for people to improve their juggling performance. We think that's because while vision provides excellent spatial and positioning information, the haptic information provides very important timing information."
Sense of touch is very important to humans and animals when they run or walk.
Cowan explains that when feet touch the ground during running, this triggers the nervous system to change leg movement in order to make the body feel more comfortable with the running surface.
He adds that in order for the body to successfully carry out repetitive movements, it is crucial that the brain instantly recognizes information from both vision and sense of touch.
Future research 'could aid understanding of neurological disorders'
The investigators say that further research looking at the link between haptic feedback, timing and limb movements is warranted.
They add that future studies in this area could help health professionals gain a better understanding as to how certain neurological diseases, such as sensory ataxia, could interrupt timing in the brain, which impacts limb movements.
Additionally, the team says future findings may aid the development of touch-sensitive artificial limbs.
Medical News Today recently reported on a study detailing the world's first sensory-enhanced artificial hand that enabled an amputee to feel.