Cognitive illusions are optical illusions that capitalize on the difference between the way we see things and the way things actually are. Cognitive optical illusions include those that involve parallel lines that look uneven and those that involve impossible situations. Many drawings by the artist M.C Escher involve cognitive optical illusions. Illusions that involve a switch between two different possibilities are also cognitive optical illusions.
Literal optical illusions are illusions that make one thing appear to be something else. The photograph of birds flying that looks like the shape of a smiling face is a literal optical illusion. This is the most basic type of optical illusion because it simply makes one object appear to be a different one. These optical illusions can also appear in real life when you mistake one object for something entirely different due to the context of the situation.
Physiological optical illusions rely on the ways that our brains process information. They encourage us to see things that aren't there. Grids of squares that appear to have dots in the middle are physiological optical illusions as are circles that appear to move even though they are standing still. These types of optical illusions can include afterimages such as what happens when you stare at a brightly colored picture and then look at a white sheet of paper.
These are just a few of the categories of optical illusions. Other optical illusions include impossible situations such as one where water appears to run uphill. Another popular optical illusions is one where it is possible to see two different images but not at the same time. Illusions that fall into this category include the old woman, young girl illusion and the vase, girl illusions. A person's brain can switch back and forth between the two pictures but cannot process both of them simultaneously.