Focus: The Hand Is Quicker than the Eye

Published October 20, 2004  |  Phys. Rev. Focus 14, 16 (2004)  |  DOI: 10.1103/PhysRevFocus.14.16
Figure 1
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Eye on the ball. Catching a ball requires the brain to predict where it will be by the time the hand gets the signal. Experiments suggest that the brain has optimized its prediction system for following objects that change speed.

When reaching for a moving object, people tend to overshoot the object by a little bit. But overreaching is part of the brain’s strategy for catching erratically moving objects most efficiently, according to the 15 October PRL. The researchers asked people to use a computer mouse to track a moving target on a screen. A subject’s error in adjusting to changes in speed was lowest when they led the object slightly with the mouse. The result suggests one way the brain’s visual-motor system compensates for the lag time between perception and action.

Perceiving a stimulus and initiating an action take about 100 milliseconds each because of the time an electrical impulse needs to travel through nerve cells. By the time a person’s hand has reached to grasp an object, say a rabbit, the hand’s position is being guided by information that’s already 200 milliseconds old. The rabbit may have zigged. Researchers believe the visual-motor system accounts for this delay by predicting an object’s trajectory and moving the body in accordance with that prediction, like a quarterback throwing the ball ahead of a running receiver.

But the brain seems to add an additional lead time in some cases, beyond what is needed to compensate for nerve transmission times. Previous studies have found that people tapping along with rhythmic noises or flashes tap a split second ahead of the actual rhythm, for example. No one has found a useful purpose for this extra anticipation, says Yasuji Sawada of the Tohoku Institute of Technology in Sendai, Japan. He reasoned that it might be one way the brain copes with a changing environment.

Sawada and a coworker asked eight males in their twenties to follow a 6-millimeter-wide red dot that cycled left to right and back again on a computer monitor at a frequency ranging from 0.1 to 2.0 hertz (cycles per second). The subjects attempted to keep the dot inside a 6-millimeter-diamter white circle controlled by a mouse. Their hand motions fluctuated around the dot’s trajectory, sometimes leading and sometimes lagging. On average, if the frequency was higher than 0.5 hertz, the subjects led the dot, and they led it more the faster it was moving.

At random times the program changed the dot’s frequency up or down by up to a few tenths of a hertz. The researchers measured each subject’s error in responding to the frequency change by recording the distance he had to make up to catch the dot. The subjects produced the smallest errors when their dot-leading times just before the change were about average for the whole group. In other words, the average lead was the best one for following erratic motions. The result indicates the visual-motor system is not only predictive, it’s proactive, says Sawada. “If you are already a little bit in front of the real world, it is always easier to adjust to new surroundings,” he says.

“I think that’s quite plausible,” says J. A. Scott Kelso, a psychologist at Florida Atlantic University in Boca Raton. “Here it’s very clear that the brain is anticipating an upcoming event.” To be sure that the effect is a general one, however, he says the researchers should vary more of the conditions in the experiment and analyze the reasons for individual differences. Some people may just be more skilled at this sort of task, he points out.

–JR Minkel

JR Minkel is a freelance science writer in New York City.