The Mystery of the Stopped Clock

Kielan Yarrow, a graduate student in neurophysiology at London's Institute of Neurology, is fascinated by the common perception that a clock appears to stop for a moment when you glance at it. To find out what happens during these hiccups in time, he and his colleagues asked a group of volunteers to look off to the side and suddenly shift their gaze to a digital counter. When the subjects tried to estimate how long they'd been looking at the digit that first appeared, they consistently guessed too high. While their eyes were sweeping toward the counter, Yarrow believes, their brains ignored the disorienting imagery and retroactively filled in the visual gap with the first thing they saw. As a result the subjects were certain they had been staring at the first digit up to a twentieth of a second before they even moved their eyes to look at it, creating the "stopped clock" illusion.