With the passage of time visual perception has been analyzed from two points of view: top-down processing and bottom-up processing. Top-down processing is the way the brain represents itself an object before the stimulus is processed. Bottom-up processing is given by the stimulation of the object. And this visual optical illusion seems to trick both!
What to see?
This image, also known as a blivet, appears to have three cylindrical spikes at one end which transform into two rectangular spikes at the other end. If you have doubts on the number of the spikes you can cover the end whit the base and you will see that the picture is made out of three cylinders.
How it works?
This visual optical illusion is a simple angle trick played on the visual system. Perception is usually supplemented by what we know from our life experience; for example when we look at an object we see of course only one angle of it, but in our mind it has a three-dimensional representation and we tend to see it as a hole.
In this image we have something that resembles to a fork, reason why is called Devil’s Fork. In the first image you have the fork simple and clear and you will probably wonder what happened with the middle spike. In the second image you have an extra object to help you see that the fork has all her spikes.