Optical illusions are caused when your brain pictures what it thinks the the world will look like in the immediate future. The part of the eye that processes images into perception is called the retina. the retina takes about one tenth of a second to translate the image into perception. So with optical illusions we see what we think will be there in the future because of that one tenth of a second delay.
DiSpezio, Michael A. Optical Illusion Magic: Visual Tricks & Amusements. New York: Sterling Pub., 1999. Print.
"Optical Illusions: Your Brain Is Way Ahead of You : DNews." DNews. N.p., n.d. Web. 17 Mar. 2015.