This simple motion illusion can completely throw us off our game. And why? All kinds of reasons, from our brains trying to play catch-up with the world to the conscious mind fooling itself about what it already saw. Apparently, anything is better than accuracy.

Take a quick look at this (silent) film. What you'll see is two boxes. One moves steadily across a plane, like a good box. The other flashes in and out of existence. While the second box always flashes when the moving box is close to it, it never seems to get the timing precisely right and flash just when the other box is right in line with it. It's kind of irritating, really.


But wait, don't post that angry comment just yet! Every single time the second box flashes, the first box is perfectly in line with it. No it isn't, you say. I can see it with my own eyes! But, once again, your eyes are wrong. This phenomenon is called the Flash Lag Effect. When two objects are displayed in line with each other, one constantly moving while the other flashes in and out of existence, the eye will see them as not properly lined up. In fact, the eye will see the moving object as jetting well past the position of the flashed object.

This puzzled a lot of people, as there doesn't seem to be any reason why the brain should process the two object differently. There are three main theories about why our minds are letting us down. The first is that the brain is doing its best to help us out. It has clocked the motion of the object, and with experience it knows that moving objects are already ahead of where we see them, what with the way light takes time to travel. So it lets us see the moving object as ahead of the place we actually saw it, to attempt to give us a more accurate picture of where it actually is when we try to reach out and grab it. It's the brain's way of telling us not to run towards the ball, but run towards the space where the ball is going.

The second theory holds that, for some reason, we process moving objects faster than we do flashed objects. The body can't do much about sudden bursts of motion other than blink and flinch. If we see steady motion, though, we might be able to react to it in a constructive way. Therefore the brain will try to deal with a moving object before it turns its attention to the flash. It's the inverse of the first theory. Instead of seeing the moving box in the future, we see the moving box in the present and see the flashing box in the past. The brain is trying to help us out by showing the most important phenomenon first, and then dealing with the flash-in-the-pan.


Lastly, we have the theory that our brain has turned on us completely and is covering its tracks. It sees the flash and it sees the moving box, but it considers its position before it'll tell us what happened when. In the fraction of the second after the flash, it decides to show our conscious mind the flash a bit later than it actually happened. Sounds silly - why would it do that - but the body has pulled this stunt on us before. We perceive the past depending on what happens in the present. This is demonstrated by an experiment which presents people with two colored dots. It's very simple. The person is shown a dot that appears and disappears on the left in one color, and then a dot that appears and disappears on the right in another color. Overwhelmingly people will interpret this simple sequence of images as a ball moving to the right and changing color. But why? All they're seeing is two dots. And to begin with, all they saw was one dot. Since they can't have had the perception of motion or color change before the second dot appeared, argue the proponents of this theory, clearly what we perceive now affects what we saw in the past. People saw a second dot, and, even though it was a different color and in a different place, made the assumption of movement.

The flash lag illusion is one of those things that no one can conclusively attribute to any specific failing (or accomplishment) of the brain. What do you think causes it?

Via BCM.