Your tennis partner whacks the ball, and in a split second you are lunging for it—but is it the sound of the hit or the sight of the ball that tells your brain when to react? Recent research indicates that each sense has its own clock for judging the timing and duration of fleeting stimuli, but it is unclear how these clocks interact. One new study suggests they can override and deceive one another.

Neuroscientists led by Virginie van Wassenhove of the California Institute of Technology found that a visual time-stretching illusion could alter volunteers’ perception of audio stimuli, whereas an audio illusion had no such power over visual perception. The researchers flashed five gray disks paired with uniform half-second tones and asked subjects if the fourth tone was longer or shorter than the others. When the tone was paired with an expanding disk, the subjects incorrectly perceived it as being longer than the other tones, which were paired with stationary disks. But when the team tried pairing uniform disks with even tones versus tones that were rising in pitch—an audio trick that by itself causes a similar illusion of time dilation—the subjects were not fooled. They correctly perceived all disks and tones as equal in duration.

The ability of the brain’s visual timekeeper to override its auditory timekeeper probably reflects our brain’s tendency to give more weight to signals that might represent a threat, according to Marc Wittmann, a time researcher at the University of California, San Diego, who was not involved in the study. The expanding disk resembles an approaching object, which “has emotional value, like a dog running toward you,” he says. Emotional events are stored in our memory in more detail and therefore seem to have transpired over a longer period.

It remains to be seen whether the visual clock will always trump the audio clock or whether the reverse could happen in certain situations. Nevertheless, the study is an important first step in the effort to understand the interactions among the sensory stopwatches in our brain.