An optical illusion is caused by the work between eye and brain complexity of the way images of transmitted from the eye to the brain, optical illusions are not as rare as one might consider.
The Anatomy of the Eye
The eye has two types of receptors on it (cones and rods) that pick up different bits of information on image. Around the retina, these cones and rods rest, waiting to pick up a stimuli and transfer it to the optic nerve. The optic nerve, in turn, transmits the information to the brain for processing.
Cone cells detect color and rod cells detect low-light contrasts. They work together to provide the necessary information to form an imag. However, at the edges of the retina, there are more rods and at the center of the retina, there are more cones. Because of this, based on how someone is looking at an image, they might see things differently. This is an optical illusion. For a better image, simply turning one's head and looking straight at something will provide the cones access and give a more detailed image.