Flash Lag Effect

The flash lag effect is an illusion that demonstrates the perceptual difficulty in accurately detecting the position of an object at the time of another event. You should find that you generally estimate the position of the blue circle, ahead of its actual position at the time the red circle flashes (although this can be compensated for with practice). If you are a benign skeptic (and I hope you are), you might want to film the screen as you do the task (a 30fps digital camera movie will do) to verify the real position of the blue circle at the time of the flash is indeed at the red line.

In contrast to many illusions, this one may have catastrophic real-world consequences, and indeed affect the fate of entire nations. I'm referring to soccer, of course. It is estimated that FIFA assistant referees incorrectly call offsides 24% of the time. This error has been proposed to be caused by the the visual system's tendency to perceive an object (in the offside call case: the moving and most forward receiver) at the time of another event (in this case the pass), ahead of it's true position 1,2 .