PEOPLE with busy lives don’t necessarily live longer, but they might feel as if they do. Our brains use the world around us to keep track of time, and the more there is going on, the slower time feels.

Brains were thought to measure time by using some kind of internal clock that generates events at a relatively regular rate.

To test whether external stimuli might also play a role in our ability to process time, Misha Ahrens and Maneesh Sahani at University College London showed 20 subjects a video of either a randomly changing stimulus – statistically modelled on the way that things naturally change randomly in the world around us – or a static image, for a set period of time.

When asked to judge how much time had passed, the volunteers who had been shown the moving stimulus were significantly more accurate. The subjects were also shown the video at two different speeds and asked to r ate the duration of each clip. They thought both clips lasted the same amount of time, even though the faster version was shorter (Current Biology, DOI: 10.1016/j.cub.2010.12.043).

The results suggest that the brain exploits changes in visual information, when it’s available, to judge time, says Sahani.