Can you see a wobbling ellipse? Actually, it rotates! To see this clearly, try changing the axes ratio.

What is going on?

You visual system needs to reconstruct the “real” motion from the changes to the retinal image. Unfortunately, the same motion on the retina can correspond to infinite (yes, infinite!) motion trajectories in the outside world. To solve this problem, your brain relies on its knowledge about how the world works. In this case, it knows that you are more likely to encounter slowly moving objects than the fast moving ones (so-called “slow prior”). For a “fat” ellipse, slight deformations of a wobbling ellipse are smaller than changes that correspond to a rotation. But as you change the axes ratio, assumed deformations become too large and you see a simpler and slower rotation.

Which source of information does the brain trust more, retinal inputs or prior knowledge?

Information from your eyes is your main source, of course! However, how much prior knowledge influences what you see depends on how reliable the information from your eyes is. When you lower the contrast of an ellipse (try that slider!), it makes it harder to see, reduces reliability of retinal inputs and gives higher priority to prior knowledge. This means that “slow prior” becomes even more important, so that it is even easier to see a wobbling ellipse.