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). We have provided three pairs of movies to accomodate machines with different CPU speeds. You should first try the top pair which is the highest quality one. If that does not work, then try the pair below it etc. The movies on the left are the illusion demos and the ones on the right are 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 movies two beeps accompany the flash whereas in the right-hand-side ones 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 and again, and you shall be able to see the complete movie at least in one of the pairs provided below.
Double Beeps Single Beep