This Week 
There's no such time as the present 13 Jan 01

THE perception of "now" varies from person to person, say researchers in Britain.

Jim Stone and his colleagues at the University of Sheffield looked at differences in the time it takes for audio and visual stimuli to reach people's conscious awareness. They showed volunteers a red light and played a tone anywhere up to a quarter of a second apart.

Sometimes the light came first, sometimes the sound and sometimes they occurred simultaneously. The test was repeated 1000 times for each of the 17 volunteers, who were asked to say whether the light and the sound happened at exactly the same time.

Stone was surprised to find that some people reported the events as simultaneous when the light actually preceded the sound by as much as 150 milliseconds. Others did so when the sound came before the light.

To find out if people take into account the time it takes distant sounds to travel to them, Stone repeated the experiment with sound coming from about 4 metres away, taking an extra 11 milliseconds to reach the volunteers.

None of them took the extra distance into account when reporting simultaneous events, Stone found. But he was astonished by how consistent each individual's judgements remained--exactly 11 milliseconds off their original judgements.

It doesn't seem to be important if different people have different ideas about whether events are simultaneous, Stone concludes--but personal consistency is vital. "It should be rock-solid stable," he says, "otherwise you wouldn't be able to play ping-pong."

Alison Motluk

From New Scientist magazine, 13/1/2001.