If you didn’t participate in last week’s Casual Friday study, you should definitely see what it’s like to experience the flash-lag effect:

When the blue rectangle flashes, it’s always precisely aligned with the gray bar, yet it appears to be behind the bar. Cool, isn’t it? You can stop the movie and check for yourself that there’s no funny business going on.

Another illusion we’ve discussed recently is a visual-sound illusion. Here, the dot flashes just once, but when it’s accompanied by two beeps, it appears to flash twice:

Clearly sound can affect what we see. So can sound affect the flash-lag effect? That’s what we were attempting to uncover with this week’s study. The short answer is yes, but not in the way you might expect. Read on for the rest of the story:

We created six different versions of the flash-lag movie and showed them to over 300 people. One of the movies was silent, like the demo above. The other five movies had beeps at different times relative to the flash. In one movie, the beep occurred just as the blue rectangle flashed. In two movies, the beep was just before or just after the flash. Finally there were two movies with two beeps: one with the first beep before the flash, and one with the first beep aligned with the flash. In every case, the flashing rectangle was aligned with the bar.

In each case, we measured the flash-lag effect by asking viewers whether the rectangle appeared to be aligned, behind, or ahead of the gray bar. Did we find a flash-lag effect? Definitely: Although most of the time they said the bar and rectangle were aligned, viewers indicated that the rectangle was behind the bar about 30 percent more often than they said it was ahead of the bar.

Whether the beep occurred before, after, or aligned with the flash had no effect on the results. Neither did double-beeps, regardless of when they occurred. You might think that this was because viewers weren’t experiencing the visual-sound illusion, but actually most viewers said that they did see the rectangle flash multiple times at least once during the study. Even when considering just these viewers, however, there was no difference in the flash-lag effect when comparing double- or single-beeps.

It’s tempting to argue that sound doesn’t affect the flash-lag effect in any way, but we did come away with one very curious finding. The movies were presented in three different orders — a third of viewers saw the silent movie first, a third saw a single-beep movie first, and a third saw a double-beep movie first. Here’s a graph of the flash-lag effect among these three groups:

When the single-beep movie was presented first, significantly more flash-lag was perceived than when the movies were presented in a different order. Sound has an influence on the flash-lag effect, but I’m baffled as to why it might be happening in this way. Do readers have any ideas?