There’s nothing cooler for a perception researcher (or writer) than a new visual illusion. When I learned about this one, I spent half the day Thursday trying to recreate it, but I couldn’t get it to work. Finally, in five minutes on Friday morning, I think I figured it out. (Update: Actually, as it turned out, I didn’t figure it out at all. In the meantime, Chris from Mixing Memory pointed me to Shams’ web page with much better demos. So let’s try this again, using one of Shams’ demos.) Play the movie and watch for a dot flashing in the middle of the screen. Make sure you’ve got the sound turned up on your computer — the flash will be accompanied by beeps. How many times does the dot flash?
Let’s make this one a poll:
If this works, most respondents should say the dot flashed two times. Actually it only flashes once. You can convince yourself of this by moving the slider slowly back and forth.
This illusion was discovered by Ladan Shams, Yukiyasu Katmitani, and Shinsuke Shimojo, who claim that the three auditory beats cause the phenomenon to be perceived visually. But is the illusion really a visual phenomenon, or is there some other explanation? In a separate study, Shams’ team systematically explores the phenomenon. First, they showed viewers movies with different numbers of auditory stimuli accompanying the flash (they used beeps instead of drumbeats). Here’s the result:
As the number of beeps increases, so does the perceived number of flashes. Next, they tried the same test and actually varied the number of flashes, while playing just one or no beeps. Viewers were accurate in reporting how many flashes they saw.
In these tasks, the beeps and flashes were spaced about 60 milliseconds apart. Will the illusion still occur, no matter fast or slowly the beeps are played? In a second experiment, the viewers saw movies with one flash and two beeps. One beep was always synchronized with the flash, but the second beep was played at varying distances from the first beep, up to 250 milliseconds before or after. Here’s a chart of how likely viewers were to see multiple flashes
The phenomenon only occurs when the beeps are about 100 milliseconds or less apart. Here’s a movie with beats about 200 milliseconds apart:
Do you see the difference from the first movie?
Shams’s team also notes that this is a one-way phenomenon: You don’t perceive multiple beeps when there are many flashes, but you do perceive multiple flashes with multiple beeps. Fascinating stuff!
Shams, L., Kamitani, Y., & Shimojo, S. (2002). Visual illusion induced by sound. Cognitive Brain Research, 14, 147-152.