Okay, do it again. This is super quick to do. Why?
The answer is – as you can tell – that we see the circles as 3d. In fact, the speed difference is pretty big. For that last picture, it takes longer to spot the different circle the more there are on the screen. For this one, it doesn’t matter how many there are. The different ones pops out. You see it just as fast, regardless of how many you have to hunt through.
One of the things it’s worth asking is: Why?
The world is made up of objects. 3d objects. We need to recognise objects, because objects tend to behave together. And we need to recognise things as the same regardless of their angle. But 3d information doesn’t come into our eyes.
Our eyes just pick up patterns of light. There are no objects in patterns of light, no size, and no shape. All of that has to be deduced.
What that’s deduced by are hard-wired assumptions about how the world works. One assumption is that 3d objects are shaded.
Actually, it’s finer than that. 3d objects are shaded at the bottom, because light comes from overhead.