What?  This is  the first visual illusion that is caused by a non-visual cue, namely sound! The illusion is: when a single flash is accompanied with two beeps, the single flash is perceived as two flashes. The illusion is strongest when the flash is in the periphery
but it also works in the fovea.
By Whom? This illusion was discovered by Ladan Shams, Yukiyasu Kamitani, and Shinsuke Shimojo.
How? To see how it works first hand, you can run the Quicktime movies below.  Here is how it works: the eye icon is the fixation point (i.e., where you are supposed to look at). The movie on the left is the illusion demo and the one on the right is just there for comparison. In both the left and right hand side movies the visual stimulus is the same--a single flash. In the left-hand-side movie two beeps accompany the flash whereas on the right-hand-side the flash is accompanied with one beep. Since the visual and auditory stimuli are very brief, the flash and/or the beep may be skipped by Quicktime. If so, do not despair ;-), try again.
 
 
 

         Double Beep                                                            Single Beep

Yukiyasu Kamitani, and Ladan Shams, December 14, 2000