THERE鈥橲 more to balancing acts than poise and sharp reflexes. The secret to tasks such as balancing a stick on the end of your fingertips is random noise generated by the nervous system. The finding is changing scientists鈥� views about how the body controls movement.
According to physicist Juan Cabrera of the Institute for Scientific Research in Caracas, Venezuela, and neurologist John Milton of Chicago University, random hand movements correct a stick鈥檚 wobbles far faster than a person would be able to react to seeing them.
Cabrera and Milton developed the idea by capturing the balancing act on film. By watching light reflected from the ends of the stick they measured the size of the wobbles and correlated them with people鈥檚 hand movements (see Graphic). 鈥淲e couldn鈥檛 think of a better experimental paradigm to study how the nervous system controls balance on short timescales,鈥� says Milton.
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They found that although it takes 100 milliseconds for someone to react to a visual cue like a wobble, 98 per cent of the hand movements people used to keep the stick upright happened faster than that (Physical Review Letters, vol 89, p 158,702).
To understand that result, the researchers used mathematical equations to describe the motion of the stick, based on ones describing a similar system involving an upright rod pivoting at its base, called an inverted pendulum. They simulated the hand movements by adding a random 鈥渞estoring force鈥� to their equations. They found that this random force was enough to keep the stick upright, making it wobble just like the real thing 鈥� but only if the stick was on the verge of toppling over.
Cabrera and Milton suggest that the nervous system must therefore be tuned to keep the stick very close to the point of instability. Random hand movements in every direction cancel out the wobbles so the stick stays upright, enabling the body to do a task for which normal reactions would be too slow.
The finding is causing neuroscientists to rethink their ideas about motor control in general. 鈥淥ur experiment suggests that the control of movements is organised on the edges of stability,鈥� says Milton. 鈥淭his is about as big a change in how one should think about things that I can imagine.鈥�