WEDNESDAY, Nov. 14, 2001 (HealthDayNews) -- Did you ever notice that when you stare at a clock, the sweep hand occasionally takes its sweet time to mark the passage of a second?
A new study acquits the timepiece in a plot to make you linger in a place you'd rather not be. Rather, British researchers say, your brain is creating an illusion by filling in a perceptual gap involving your eyes.
Lead author Kielan Yarrow, a research fellow in neurophysiology at the Institute of Neurology in London, says that the researchers suspected that the size of the illusion was somehow related to size of the eye movement towards the clock.
They had a group of volunteers watch a small cross on one side of a computer screen while instruments monitored their eye movements. The computer timed each person's eye movements, or saccades, towards the seconds counter of a digital clock on the opposite side of the screen.
By the time each person's eyes shifted to the counter, the number had been shifted to one, which then increased just like a digital second counter.
However, the computer randomly varied the time that the "1" lasted -- from 0.5 seconds to 1.5 seconds -- in subsequent repetitions. Once the counter reached "2," it progressed at normal speed. The volunteer then reported whether the first interval was longer than subsequent intervals.
"It seems as if there's quite a precise mechanism whereby the amount of time that they're extending this subjective second by relates exactly to the amount of time it takes them to move their eyes over to the clock," says Yarrow.
For example, the shortest eye movements took about 70 milliseconds, while the longer ones averaged around 140 milliseconds. "We actually got the same increase in the size of the illusion," says Yarrow.
What's happening, says Yarrow, is that the brain's perception of what it sees once the eyes reach the clock has been extended back in time to when the eyes started to move towards the clock. The longer their eyes were moving, the longer that second counter seemed to freeze.
The key, he says, is a phenomenon called saccadic suppression. We move our eyes roughly three times a second, but we aren't actually seeing much while our eyes are in motion.
Yet we're unaware of these "blank spots" in our perception because our brain fills in the gaps, Yarrow says.
"The brain makes assumptions about the world, and when you don't have very good information or the things you see are ambiguous
the brain uses some kinds of short cuts and assumptions to make sense of that," he adds.
That's what's happening when we glance at a clock, he says. "Your brain is making the assumption that what you're seeing now has also been the case for all the time that your eyes were moving and you couldn't see anything," he says.
Bruce Bridgeman, a professor of psychology and psychobiology at the University of California, Santa Cruz, says that the stopped-clock illusion is a real-world example of saccadic suppression. "This process, presumably, is going on all the time," he says. "It's an involuntary thing that goes on whether you're aware of it or not."
Bridgeman, who has studied a similar phenomenon related to eye blinking, says that this study raises questions about how we synchronize physical movements with these sorts of stimuli.
"For instance, when you're grasping something or swatting a fly, and at the same time you make a saccadic eye movement towards the fly and the fly is moving, you have to somehow catch up the fact that your vision was absent for a 20th of a second during your movement," says Bridgeman.
The findings appear in the Nov. 15 issue of the journal Nature.
What To Do
Curious about clocks? Check out MSN Encarta or Horology.com, an index of the science of timekeeping, clocks and watches.
And the HowStuffWorks Web sites can tell you how time, atomic clocks, pendulum clocks and digital clocks work.
